Heat treatment : the best one

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Arvind Thekdi - E3M, Inc.Sales

Metal Heat TreatingTopics of Presentation

• What Is Metal Heat Treating?

• Where Is It Used?

• Why and How It Is Done?

• What Processes & Equipment Are Used

for Heat Treating?


Heatin

g

Holding (soak)

Time

Tem

per

atu

re

Cooling

What is Heat Treating ?

Controlled Heating And Cooling of Metal to Change Its

Properties and Performance.

Through:

• Change in Microstructure

• Change in Chemistry or Composition


A Few Heat Treating Facts

• Heat Treating of Metals Represents Approximately 100 BCF Gas Load Nationwide.

• Heat Treaters Use Natural Gas to Supply About 2/3 of the Energy Used for Heat Treating (induction, vacuum & commercial atmospheres main competition).

• Current Share of Gas Decisions is about 50 / 50 Between Gas & Electric.


Why Use Heat Treating ?

• Soften a Part That Is Too Hard.

• Harden a Part That Is Not Hard Enough.

• Put Hard Skin on Parts That Are Soft.

• Make Good Magnets Out of Ordinary

Material.

• Make Selective Property Changes Within

Parts.

In simple Terms….


Who uses Heat Treating ?

• Aircraft Industry• Automobile

Manufacturing• Defense Sector• Forging • Foundry • Heavy Machinery

Manufacturing • Powder Metal

Industries


What Industrial Sectors Use Heat Treating ?

SIC Industry

331 Steel Mills332 Iron and Steel Foundries34 Metal Fabrication

35 & 36Machinery and Electrical/Electronic Equipment

37 Transportation Equipment3398 Commercial Heat Treating5051 Steel Service Centers


Types of Heat Treaters

• Commercial Heat Treaters– Heat Treating of Parts As “Job-shop”.– Reported Under SIC Code 3398.– Approx. 10% of All Heat Treating Production Is by Commercial

Heat Treaters.– Usually There Are 4 to 5 Captive Heat Treaters for Each

Commercial Heat Treater Shop.

• Captive Heat Treaters– Usually a Part of Large Manufacturing Business.– They Usually Produce “Products” Rather Than Parts.– Captive Heat Treating Is Scattered Through All Manufacturing

SIC Codes (DEO has over 100 individual SIC’s for Heat Treaters).


Commonly Heat Treated Metals

• Ferrous Metals– Steel– Cast Iron– Alloys– Stainless Steel– Tool Steel

• Non-ferrous Metals– Aluminum– Copper– Brass– Titanium

Steel Is the Primary Metal Being Heat Treated.

More Than 80% of Heat Treating Is Done for Steel


Heat Treating Processes


• Heating • Preheating• Heating• Soak & diffusion • Pre-cooling

• Quenching (Cooling)• Post-wash

•Tempering• Surface coating

Steps in Heat Treating Operation

• Loading

• Unloading

• Cleaning • Pre-wash with coalescer• De-phosphate system• Spray rinse


Commonly Used Equipment for Heat Treating Operations

• Metal Cleaning (Wash-Rinse) Equipment • Gas fired furnaces

– Direct fired using burners fired directly into a furnace

– Indirect fired furnaces: radiant tube, muffle, retort etc.

– Molten salt (or lead) bath– Fluidized bed

• Electrically heated Furnaces– Induction heating– Electrical resistance heating – Other (i.e. Laser, electron-beam etc.)

• Quench or cooling equipment• Material handling system• Testing and quality control laboratory equipment


Gas Fired Metal Heat Treating Furnaces


Electrically Heated Equipment for Metal Heating

Induction Equipment

Electric Atmosphere Furnace Vacuum Furnace


55

330

50

200

100

60

150

100

150

5 130

2510Vacuum

13425Box

4395Car Bottom

4890Bell, hood, tip-up

4765Vertical Pit

4560Induction

7700Salt Bath

6000Pusher

3195Conveyor

Box

Carbottom

Bell, Hood, tipup

Vertical Pit

Vacuum

Lead pot (cont.)

Rolelr hearth

Barre-roller

Cont. stripline

Cont. induction

Conveyor

Pusher

Fluidized bed

Salt-bath

Rotary hearth, shaker hearth

Plasma

Induction

Laser

Electric beam

Flame heads

Types of Heat Treating Furnaces


Heat Treating Furnaces Two Primary Types

• Atmospheric– Operated at ambient (atmosphere) pressure.

– Load is heated and cooled in presence of air

or special gases (process atmospheres), in

liquid baths or in a fluidized bed.

• Vacuum – Operated at vacuum or sub-atmospheric

pressure.

– May involve high pressure gas cooling using

special gases.

– Includes ion or plasma processing

equipment.


Heat Source for Gas Fired Furnaces

• Direct Fired Burners *

• Radiant Tubes *

• Muffle or Retort Heated

by Outside

Burners/Electrical

Elements

• Hot Oil or Steam Heating

* These could be directly exposed to the

work or can be outside a muffler a

retort.

Direct fired muffle furnace

Muffle

Burners


Nonferrous Heat Treating Furnaces

Types of Furnaces

· Coil/foil Annealing Furnaces

· Rod/wire Annealing Furnaces

· Log Homogenizing Furnaces

· Ingot Preheating Furnaces

· Aging Furnaces

· Indirect Heating (Radiant Tubes or Electrical Resistance)

· Temperature Range 350°F to 1150°F

· Atmosphere With Dew Point Control

· May Includes Water Quench or Controlled Cooling


Why Use Protective Atmospheres?

• To Prevent Oxidation, Loss of Carbon (Decarburizing), and Avoid Corrosion.

– Most Gases Containing Oxygen (i.e. Air, Water Vapor [H2O], Carbon Dioxide [CO2] React With Iron, Carbon and Other Elements Present in Steel and Other Metals.

– Reactivity Depends on Temperature and Mixture of Gases in Contact With Steel.

• To Avoid and Eliminate Formation of Flammable or Explosive Mixtures


Types of Process Atmospheres

• Protective– To Protect Metal Parts From Oxidation or Loss of

Carbon and Other Elements From the Metal Surfaces.

• Reactive– To Add Non-metallic (i.e., Carbon, Oxygen,

Nitrogen) or Metallic (i.e., Chromium, Boron, Vanadium) Elements to the Base Metal.

• Purging – To Remove Air or Flammable Gases From Furnaces

or Vessels.


Importance of Protective Atmospheresin Heat Treating

• Proper composition and concentration in a furnace is required to give the required surface properties for the heat treated parts.

• Loss of atmosphere “control” can result in unacceptable parts and result in major economic penalty - it can cost a lot!

• Atmospheres contain potentially dangerous (explosive, life threatening) gases and must be treated with “respect”.

• New advances in measurement and control of atmospheres in heat treating allow precise control of atmospheres to produce quality parts.


Commonly Used Atmospheres in Heat Treating

Protective and Purging– Endothermic gases

• Lean – high and low dew point• Rich - high and low dew point

– Nitrogen– Mixture of N2 and small amount of CO

Reactive– Exothermic gases– Mixture (or

individual) of gases: Hydrogen, CO, CH4, Nitrogen and other hydrocarbons

– Dissociated Ammonia (H2 + N2)


Source of Atmospheres

• Natural Gas (Hydrocarbon) - Air Reaction

• Natural Gas - Steam Reaction

• Ammonia Dissociation or Ammonia-air Reaction

Or: • Mixture of Commercial Gases (N2, H2 and Hydrocarbons)

Requirement:A Mixture of Gases (CO, H2, CO2, H2O and N2) That Give the Required Composition for the Processing Atmosphere.


Use of Atmospheres in a Plant

• Most plants have an in-house, centrally located, atmosphere gas generators for different types of atmospheres required in the plant

• In some cases one or more generators may be located for each “shop” or production area

• In many cases other gases (i.e. N2, H2, NH3) are piped from storage tanks located within the plant premises and distributed by a piping system to furnaces.

• Gas flow is mixed, measured and controlled prior to its injection in the furnace.

Requirement:A Mixture of Gases (CO, H2, CO2, H2O and N2) That Give the Required Composition for the Processing Atmosphere.

ANNEALING• Annealing, involves heating to a predetermined

temperature, holding at this temperature, and finally cooling at a very slow rate.

• The temperature, to which steel is heated, and the holding time are determined by various factors such as the chemical composition of steel, size and shape of steel component and final properties desired.

Purpose of Annealingi. Relieve internal stresses developed during

solidification, machining, forging, rolling, or welding;

ii. Improve or restore ductility & toughness;

iii. Enhance machinability

iv. Eliminate chemical non-uniformity;

v. Refine grain size; and

vi. Reduce the gaseous contents in steel.

CLASSIFICATION

1. Heat treatment temperature

• Full annealing• Partial annealing• Sub-critical

annealing

2. Phase transformation

• First-order annealing

• Second-order annealing

3. Specific purpose

• Full annealing• Isothermal

annealing• Diffusion

annealing• Partial annealing• Recrystallization

annealing• Spheroidisation

annealing

Annealing treatment can be classified into groups based on the following:

1.1 In full annealing the steel is heated above the critical temperature(A3) and then cooled very slowly.

1.2 Partial annealing, also known as incomplete annealing or intercritical annealing, involves heating of steel to a temp. lying between lower critical temperature(A1) and upper critical temperature (A3 or Acm).

1.3 Subcritical annealing is a process in which the maximum temp. to which is heated is always less than the lower critical temperature(A1).

Classification based on phase transformation features.

2.1 First-order annealing is performed on steel with the sole aim of achieving some properties. Any change in the characteristics of steel achieved by this type of annealing is not correlated to phase transformation.it can be performed at a wide range of temperatures above or below the critical temperatures.

2.2 The second-order annealing differs from the former in the sense that the end results in the former are essentially due to phase transformation which takes place during the treatment.

Types of annealing based on specific purpose

3.1 Full annealing

• In this, steel is heated to its 50°C above the austenitic temperature and held for sufficient time to allow the material to fully form austenite or austenite-cementite grain structure. The material is then allowed to cool slowly so that the equilibrium microstructure is obtained.

• The austenitising temp is a function of carbon content of the steel and can be generalized as:

• For hypoeutectoid steels and eutectoid steel» Ac3+(20-40oC) [to obtain single phase austenite]

• For hypereutectoid steels» Ac1+(20-40oC) [to obtain austenite+ cementite]

•

Purpose of full annealing• To relieve internal stresses• To reduce hardness and increase ductility• For refining of grain size• To make isotropic in nature in mechanical aspects• For making the material having homogeneous

chemical composition• For making the material suitable for high machining

processes• To make steel suitable for undergoing other heat

treatment processes like hardening, normalizing etc.

• The grain structure has coarse Pearlite with ferrite or Cementite (depending on whether hypo or hyper eutectoid). The steel becomes soft and ductile.

• The formation of austenite destroys all structures that have existed before heating. Slow cooling yields the original phases of ferrite and pearlite in hypoeuetectoid steels and that of cementite and pearlite in hypereutectoid steels.

3.2 Isothermal Annealing• It is a process in which hypoeutectoid steel is

heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature. This method rids any temperature gradient.

• The prefix “isothermal” associated with annealing implies that transformation of austenite takes place at constant temperature.

The closer the temp of isothermal holding is to A1, coarser is the pearlite, softer is the steel, but longer is the time of isothermal transformation.

Advantages:

• Improved machinability.• Homogeneous structure and better surface finish.• Time required for complete cycle is comparably less.• The process is of great use for alloy steels as the steels

have to be cooled slowly. Limitation:It is suitable only for small-sized components. Heavy components cannot be subjected to this treatment because it is not possible to cool them rapidly and uniformly to the holding temperature at which transformation occurs. Thus structure wont be homogeneous mechanical properties will vary across the cross-section.

3.3 Diffusion Annealing

• This process, also known as homogenizing annealing, is employed to remove any structural non-uniformity like dendrites, columnar grains and chemical inhomogeneity which promote brittleness and reduce ductility and toughness of steel.

• Process: Steel is heated sufficiently above the upper critical temperature (say,

1000-2000oC), and held at this temperature for 10-20 hours, followed by slow cooling.

Segregated zones are eliminated, and a chemically homogeneous steel is obtained by this treatment as a result of diffusion.

Heating to such a high temp. results in considerable coarsening of austenitic grains & heavy scale formation. The coarse austenite thus obtained further transforms to coarse pearlite on cooling, which is not a desirable structure as mechanical properties are impaired.

The main aim of homogenising annealing is to make the composition uniform, i.e to remove chemical heterogeneity.

The impact energy and ductility of the steel increase as the homogenizing temperature increases and the hardness, yield strength and tensile strength decrease with an increase in the homogenizing temperature.

Homogenizing annealing has a few shortcomings as well. It results in:

Grain coarsening of austenite, thereby impairing the properties Thick scales on the surface of steels It is an expensive process

3.4 Partial Annealing

• Partial annealing, also known as inter-critical annealing or incomplete annealing, is a process in which steel is heated between A1 and Acm and is followed by slow cooling.

• Generally, hypereutectoid steels are subjected to this treatment. The resultant microstructure consists of fine pearlite and cementite instead of coarse pearlite and a network of cementite at grain boundaries.

• As low temperatures are involved in this process, it is less expensive than full annealing.

• Hupoeutectoid steels are subjected to this treatment to improve machinability. However, steels with coarse structure of ferrite and pearlite or with widmanstätten structure are not suitable for this treatment. This is because only a considerable amount of ferrite remains untransformed, and only a part of it along with pearlite transforms to austenite.

• This coarse or accicular untransformed ferrite results in poor mechanical properties.

3.5 Recrystallization Annealing

• The process consists of heating steel above the recrystallization temperature, holding at this temperature and cooling thereafter.

• It is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to undergo further cold working without fracturing.

• Recrystallization temp(Tr) is given by:

• Tr= (0.3-0.5)Tmp

• As little scaling and decarburization occurs in recrystallization annealing, it is preferred over full annealing.

• No phase change takes place and the final structure consists of strain-free, equiaxed grains of fine ferrite produced at the expense of deformed elongated ferrite grains.

• However It would produce very coarse grains if the steel has undergone critical amount of deformation. In such cases, full annealing is preferred.

Aims of Recrystallization Annealing

To restore ductility To refine coarse grains To improve electrical and magnetic

properties in grain-oriented Si steels.

3.5 Spheroidization annealing• Spheroidization annealing consists of heating, soaking and

cooling, invariably very slowly to produce spheroidal pearlite or globular form of carbides in steels.

• To improve the machinability of the annealed hypereutectoid steel spheroidize annealing is applied.

• Hypereutectoid steels consist of pearlite and cementite. The cementite forms a brittle network around the pearlite. This presents difficulty in machining the hypereutectoid steels.

• This process will produce a spheroidal or globular form of a carbide in a ferritic matrix which makes the machining easy.

• Prolonged time at the elevated temperature will completely break up the pearlitic structure and cementite network. The structure is called spheroidite.

Spheroidising Process:

• Heat the part to a temperature just below the Ferrite-Austenite line, line A1 or below the Austenite-Cementite line, essentially below the 727 ºC (1340 ºF) line. Hold the temperature for a prolonged time and follow by fairly slow cooling. Or

• Cycle multiple times between temperatures slightly above and slightly below the 727 ºC (1340 ºF) line, say for example between 700 and 750 ºC (1292 - 1382 ºF), and slow cool. Or

• For tool and alloy steels heat to 750 to 800 ºC (1382-1472 ºF) and hold for several hours followed by slow cooling.

• All these methods result in a structure in which all the Cementite is in the form of small globules (spheroids) dispersed throughout the ferrite matrix. This structure allows for improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion.

Spheroidizing process applied at a temperature below and above the LCT.

Aims Of Spheroidization Annealing:

minimum hardness maximum ductility maximum machinability maximum softness

– Martensite is a very strong phase, but it is normally very brittle so it is necessary to modify the mechanical properties by heat, treatment in the range 150—700°C.

– Essentially, martensite is a highly Supersaturated solid solution of carbon in iron which, during tempering, rejects carbon in the form of finely divided carbide phases.

– The end result of tempering is a fine dispersion of carbides in an α-iron matrix which often bears little structural similarity to the original as-quenched martensite.

TEMPERING

NORMALIZING

The normalizing of steel is carried out by heating above the UCT (Upper Critical Temperature) to single phase austenitic region to get homogeneous austenite, soaking there for some time and then cooling it in air to room temperature.

The austenitising temperature range are:For hypoeutectoid steels and eutectoid

steel

• Ac3 + (40-60oC)

For hypereutectoid steels

• Acm + (30-50oC)

During normalising we use grain refinement

which is associated with allotropic transformation upon heating γ→α

Parts that require maximum toughness and those subjected to impact are often normalized.

When large cross sections are normalized, they are also tempered to further reduce stress and more closely control mechanical properties.

The microstructure obtained by normalizing depends on the composition of the castings (which dictates its hardenability) and the cooling rate.

Figure below shows the normalizing temperatures for hypoeutectoid and hypereutectoid steels

AIMs OF NORMALIZING

• To produce a harder and stronger steel than full annealing

• To improve machinability• To modify and/or refine the grain structure• To obtain a relatively good ductility without

reducing the hardness and strength• Improve dimensional stability • Produce a homogeneous microstructure • Reduce banding • Provide a more consistent response when

hardening or case hardening

EFFECT OF SOAKING TIME ON THE

MICROSTRUCTURE:

NORMALIZING VS

ANNEALING

COMPARISON OF ANNEALING AND NORMALIZING

The metal is heated to a higher temperature and then removed from the furnace for air cooling in normalizing rather than furnace cooling.

In normalizing, the cooling rate is slower than that of a quench-and-temper operation but faster than that used in annealing.

As a result of this intermediate cooling rate, the parts will possess a hardness and strength somewhat greater than if annealed.

Fully annealed parts are uniform in softness (and machinability) throughout the entire part; since the entire part is exposed to the controlled furnace cooling. In the case of the normalized part, depending on the part geometry, the cooling is non-uniform resulting in non-uniform material properties across the part.

Internal stresses are more in normalizing as compared to annealing.

Grain size obtained in normalizing is finer than in annealing. Normalizing is a cheaper and less time-consuming process.

Comparison of temperature ranges in annealing and

normalizing

Comparison of time-temperature cycles for normalizing and full annealing

The slower cooling of annealing results in higher temperature transformation to ferrite and pearlite and coarser microstructures than does normalizing.

effect of annealing and normalizing on ductility of steels

Annealing and normalizing do not present a significant difference on the ductility of low carbon steels. As the carbon content increases, annealing maintains the % elongation around 20%. On the other hand, the ductility of the normalized high carbon steels drop to 1 to 2 % level.

effect of annealing and normalizing on the tensile strength AND YIELD POINT

of steels

The tensile strength and the yield point of the normalized steels are higher than the annealed steels.

Normalizing and annealing do not show a significant difference on the tensile strength and yield point of the low carbon steels.

However, normalized high carbon steels present much higher tensile strength and yield point than those that are annealed. This can be illustrated from the figures.

effect of annealing and normalizing on the hardness of steels

Low and medium carbon steels can maintain similar hardness levels when normalized or annealed. However, when high carbon steels are normalized they maintain higher levels of hardness than those that are annealed.

ADVANTAGES OF NORMALIZING OVER

ANNEALINGBetter mechanical properties Lesser time-consuming Lower cost of fuel and operation

ADVANTAGES OF ANNEALING OVER NORMALIZING

Greater softnessComplete absence of internal stresses

which is a necessity in complex and intricate parts

HARDENING• It is the process of heating the steel to

proper austenitizing temperature , soaking at this temperature to get a fine grained and homogeneous austenite , and then cooling the steel at a rate faster than its critical cooling rate.

OBJECTIVES OF HARDENINGThe aims of hardening are:

1. Main aim of hardening is to induce high hardness. The cutting ability of a tool is proportional to its hardness.

2. Many machine parts and all tools are hardened to induce high wear resistance higher is the hardness , higher is the wear and the abrasion resistance .For example ,gears, shaft.

3. The main objective of hardening machine components made of structural steel sis to develop high yield strength with good toughness and ductility to bear high working stresses.

Austenising Temperature for Pearlitic Steels

• The steel is first heated to proper austenising temperature to obtain a homogeneous and fine grained austenite. This temperature depends on the composition(carbon as well as alloying elements).

• The austenitising temperature of plain carbon steels depends on the carbon content of the steel and is generalised as :

• For hypo-eutectoid steels :Ac3 + (20 — 40°C) • For hyper-eutectoid steels and eutectoid steel:Ac1 + (20

— 40°C)


• Hypereutectoid steels, when heated in above temperature range, to obtain homogeneous and fine-grained austenite which on quenching transforms to fine-grained (very fine needles/plates), and hard martensite as is desired to be obtained.

• Heating these steels only up to critical range (between Ac3 and Ac1) is avoided in practice.

• Steel then has austenitic and ferrite. • On quenching, only austenite transforms to martensite, and

ferrite remains as it is, i.e., incomplete hardening occurs . • The presence of soft ferrite does not permit to achieve high

hardness, if that is the objective.


• If the aim is to get high strength by the process of tempering ferrite does not permit this as it has low tensile and yield strengths .

• In fact, ferrite forms the easy path to fracture. • Quenching of hypoeutectoid steels from temperatures

much above the proper temperatures , when austenite has become coarse, results in coarse acicular form of martensite.

• Coarse martensite is more brittle, and a unit or two lower in hardness. It lowers the impact strength even after tempering, and is more prone to quench-cracking.


• Hypereutectoid steels, when heated in the above range, i.e., just above Ac1 have fine grains of austenite and proeutectoid cementite.

• On quenching austenite transforms to fine martensite and cememtite remains unchanged.

• As the hardness of cementite (≈ 800 BHN) is more than that of martensite (650-750 BHN), its presence increases the hardness, wear and abrasion resistance as compared to only martensitic structure.

• If temperature of austenitisation is much higher than Ac1 but still below Acm temperature, a part of proeutectoid cementite gets dissolved to increase the carbon content of austenhlc(> 0.77%)


• On quenching as-quenched hardness is less, because :

1. Lesser amount of proeutectoid cementite is present.

2. Larger amount of soft retained austenite is obtained as the dissolved carbon of cementite has lowered the Ms and Mf temperature.

3. A bit coarser martensite has lesser hardness.


• Heating hypereutectoid steels to a temperature higher than Acm results in 100% austenite . It is very coarse austenite as very rapid grain-growth occurs due to dissolution of restraining proeutectoid cementite .

• The as-quenched hardness is low because of:1) Absence of harder cementite.2) As more carbon has dissolved in austenite, more retained

austenite is obtained.3) Coarser martensite is a bit less hard and more brittle.

• Thus, these temperatures are avoided in carbon steels

PROCESS OF QUENCHING• When a heated steel object (say at 840°C) is plunged into a

stationary bath of cold it has three stages as:

Stage A -vapour-blanket stage: • Immediately on quenching, coolant gets vapourized as the

steel part is at high temperature, and thus, a continuous vapour- blanket envelopes the steel part.

• Heat escapes from the hot surface very slowly by radiation and conduction through the blanket of water vapour.

• Since the vapour-film is a poor heat conductor, the cooling rate is relatively low (stage A in fig ). This long stage is undesirable in most quenching operations.

PROCESS OF QUENCHING

PROCESS OF QUENCHINGStage B-Intermittent contact stage (Liquid-boiling stage): • Heat is removed in the form of heat of vaporization in this

stage as is indicated by the steep slope of the cooling curve.

• During this stage, the vapour-blanket is broken intermittently allowing the coolant to come in contact with the hot surface at one instant, but soon being pushed away by violent boiling action of vapour bubble.

• The rapid cooling in this stage soon brings the metal surface below the boiling point of the coolant.

PROCESS OF QUENCHING• The vaporization then stops. Second stage

corresponds to temperature range of 500◦ to 100◦c , and this refers to nose of the CCT curve of the steel , when the steel transforms very rapidly ( to non martensite product ).

• Thus, the rate of cooling in this stage is of great importance in hardening of steels.

Stage C-Direct-Contact stage (Liquid-cooling stage):• This stage begins when the temperature of steel

surface Is below the boiling point of coolant. • Vapours do not form. The cooling is due to convection

and conduction through the liquid. Cooling is slowest here.

QUENCHING MEDIUMS• As the aim is to get martensite, the coolant should have

quenching power to cool austenite to let it transform to martensite. The following factors effect the quenching power of the coolant :

• The cooling rate decreases as the temperature of water and brine increases, i.e., it increases stage ‘A’, i.e., helps in persistence of the vapour blanket stage.

• The increased temperature brings it closer to its boiling point, and thus, requires less heat to form vapour, specially above 60°C.

• Good range of temperature for water as coolant is 20-40°C.• Oils in general, show increased cooling rates with the rise of

temperature, with optimum cooling rates in range 50°—80°C.

QUENCHING MEDIUMS• In oils, the increase of temperature increases the persistence of

vapour-blanket, but this resulting decrease in the cooling rate is more than compensated by the decrease of viscosity (with the rise in temperature) to result in increase of rate of heat removal through the oil.

• If the boiling point of a coolant is low, vapours form easily to increase the ‘A’ stage of cooling. ¡t is better to use a coolant with higher boiling point. A coolant with low specific heat gets heated up at a faster rate to form vapours easily.

• A coolant with low latent heat of vapourisation changes into vapour easily to promote ‘A’ stage, i.e., decreases the cooling rate.

• A coolant with high thermal conductivity increases the cooling rate. Coolants with low viscoity provide faster cooling rates and decrease the ‘A’ stage.

QUENCHING MEDIUMS• A coolant should be able to Provide rate of cooling fast

enough to avoid transformation of austenite to pearlite and bainite . Plain carbon steel invariably require çooling in water or brine. Whereby alloy steels are quenched normally in oils.

• But milder the cooling medium , lesser the internal stresses developed , and thus lesser the danger of distortion , or cracks . An ideal quenching medium is one which is able to provide very fast cooling rate near the nose of the curve ( 650 -550°C)and at the same time it should provide very considerable slower rate if cooling within the range of martensitic transformation( 300 - 200°C) to minimize internal stresses .

WATER• The oldest and still the most popular quenching medium, water meets the

requirements of low cost ,general easy availability, easy handling and safety. • The cooling characteristics change more than oil with the rise of

temperature, specially there is a rapid fall in cooling capacity as the temperature rises above 60°C, because of easy formation of vapour-blanket.

• The optimum cooling pover is when water is 2O-4O°C. • Thc cooling power of water is between brine and oils. • Water provides high cooling power to avoid the transformation of austenite

to pearlite/bainite, but the major draw back is that it also provides high cooling rate in the the temperature range of martensite formation.

• At this stage, the steel is simultaneously under the influence of structural stresses (non-uniform change in structure) and thermal stresses which increase the risk of crack formation.

BRINE• Sodium chloride aqueous solutions of about 10% by weight are

widely used and are called brines.• The cooling power is between 10% NaOH aqueous solution and

water.• These are corrosive to appliances.• The greater cooling efficiency of brines, or other aqueous solutions is

based as :• In brine heating of the solution at the steel surface causes the

deposition of crystal of the salt on hot steel surface .• This layer of solid crystals disrupts with mild explosive violence, und

throws off a cloud of crystals. This action destroys the vapour-film from the surface, and thus permits direct contact of the coolant with the steel surface with an accompanying rapid removal of heat.

• Brines are used where cooling rates faster than water arc requited.

OILS• Oils have cooling power between water at 40°C to water at 90°C. • In oil-quench, considerable variation can be obtained by the use of

animal, vegetable, or mineral oil, or their blends. • Oils should be used at 50- 80°C when these are more fluid, i.e less

VISCOUS, which increases the cooling power. • As the oils used generally have high boiling points, moderate increase of

temperature of oil does not very much increase the vapour blanket stage. However, oils in contrast to water, or brine, have much lower quenching power .

• Its this relatively slow cooling rate in the range of martensitic formation is atlvantageous as it helps in minimsing the danger to crack formation.

• Oils with high viscosity are less volatile, and thus have decreased vapour-blanket stage (increase thecooling rate). As lesser volatile matter is lost, their cooling power is not affected much with use.

POLYMER QUENCHANTS• polymer quenchants cool rapidly the heated steel to Ms

temperature, and then rather slowly when martensite is forming .

• Polymer quenchants are water-soluble organic chemicals of high ,molecular weights, and are generally polyalkylene glycol-based, or polyvinyl pyrolidene-based.

• Widely different cooling rates can be obtained by varying the concentration of Organic additives in water; higher the additions, slower is the cooling rate of solution.

• There are little dangers of distortions and cracks.

SALT BATHS• It is an ideal quenching medium for a steel of

not very large section but with good hardenabilty.

• Addition of O.3-O.5’% water almost doubles the cooling capacity. Normally holding time is 2-4 minutes/cm of section thickness.

• Salt baths used for austenitising keep the steel clean.

INTERNAL STRESSES S DURING QUENCHING• internal stresses are produced due to non-uniform plastic deformation.

In quenching of steels ,this may be caused by thermal stresses, structural stresses, or both, or even premature failure of part in service.

• Cooling during quenching lakes place non-uniformly, i.e., causes temperature gradient across the section.

• Surface layers contract more than the central portion. • Contraction of surface is resisted by the central portion, and this puts

the central portion under the compressive stresses, and the surface layers in tension .

• If the magnitude of stress becomes more than the yield stress of steel (at that deformation occurs.

• These stresses that develop in a quenched part as a result of unequal cooling are called thermal stresses.

INTERNAL STRESSES DURING QUENCHING• Structural stresses are the stresses which develop due to due to

phase change (mainly austenite to martensite), and at different times.• Structural stresses are developed due to two main reasons:

1. Austenite and its transformation products have unequal specific volume i.e. change in volume occurs when transformation occurs.

2. Phase changes occur at different times in the surface and in centre. • Under right conditions, both types of stresses get superimposed to

become larger than the yield strength to cause warping. but when the tensile internal stresses become larger than the tensile strength cracks appear.

• If an austenitised steel is quenched, it contracts thermally till Ms temperature is attained .

INTERNAL STRESSES S DURING QUENCHING

INTERNAL STRESSES DURING QUENCHING• figure(a) illustrates this in stage 1 As surface cools faster than

centre, i.e., contracts more than centre distribution of stresses across the section is illustrated in fig (b), i.e, the surface is under tensile nature of stress, while centre is under compressive stresses.

• Only thermal Stresses are produced in stage 2 , surface having attained Ms temperature, transforms to martenSite, and thus expands, while the centre is still contracting as it is getting cooled.

• In stage Il, centre may get plastically deformed ,as it is still ductile austenite.

• In stage 3, martensite of surface and austenite of centre continue contracting leading to slight increase in stress levels.

INTERNAL STRESSES DURING QUENCHING• In stage IV, centre has attained M5 temperature, and begins

to expand as it forms martensite, while surface is still contracting.

• The centre, as it expands, puts the surface in higher stress levels .

• The surface has little deformation as it consists of brittle martensie.

• It is during this stage, the greatest danger of cracking exists.• Thus, stress levels are highest not in the beginning of the

quench, but when the centre is transforming to martensite.

INTERNAL STRESSES DURING QUENCHING• However, higher is the Ms temperature of the

steel, lesser is the expansion, there is reduced danger of quench-cracking.

• Increase of carbon and alloying elements lower the Ms temperature making the steel more prone to quench cracking.

RETAINED AUSTENITE• Martensitic transfomiation is essentially an athermal transformation. • Austenite begins to transform to martensite at Ms, and the amount of

martensite formed increases as the temperature decreases to complete at Mf temperature.

• Less than 1 % of austenite may not transform because of unfavourable stress conditions.

• The Ms and Mf temperatures are lowered as the amounts of carbon content and alloying elements(except cobalt and aluminium) increase in the steel.

• In a quenched steel, the amount of martensite formed depends on the location of Ms and Mf and the temperature of the coolant (which is normally room temperature. As long as room temperature lies between Ms and Mf temperatures, austenite does not transform completely to martensite as it has not been cooled below Mf temperature.

• This untransformed austenite is retained austenite.

ADVANTAGES OF RETAINED AUSTENITE

• 10% retained austenite is normally desirable as its ductility relieves some internal stresses developed during hardening. This reduces the danger to distortion or cracks.

• The presence of 30-40% retained austenite makes straightening of components possible after hardening.

• Non distorting tools owe their existence to retained austenite . It tries to balance transformational volume changes during heating as well as cooling cycles of heat treatment to produce little overall change in size of the tools.

DISADVANTAGES1) The presence of soft austenite decreases the

hardness of hardened steels.

2) As retained austenite may transform to lower bainite, or martensite later in service ,increase in dimensions of the part occurs.

3) This creates problems in precision gauges or dies.

4) Stresses may develop in the part itself as well as in adjacent pans. Grinding-cracks are mainly due to retained austenite transforming to martensite.

5) Austenite is non-magnetic, decreases the magnetic properties of the steels.

ELIMINATION OF RETAINED AUSTENITE• Retained austenite is generally undesirable. It is eliminated by

one of the methods:

1. Sub-Zero treatment (cold treatment): • It Consists of cooling the hardened steel (having retained

austenite) to a temperature below 0°C or its Mf temperature. • There is no reason to cool a steel much below its Mf temperature. • Sub-zero treatment is more effective if it is done immediately after

the quenching operation (normally done to room temperature).• Sub-zero treatment is done in a low temperature-cooling unit,

which consists of double-walled vessel. • The interior is made of copper in which the parts to be deep-frozen

are kept, and the exterior is made of steel provided with good heat-insulation.

ELIMINATION OF RETAINED AUSTENITE

• The space in between the vessels is filled with some coolant.

• The sub-zero coolants could be, dry ice (Solid CO2) + acetone (— 78°C); Ice + NaCl (—23°C); liquid air (—183°C); liquid N2 (— 196°C); Freon (— 111°C).

• Total time of cooling in this unit is 1/2 to 1 hour.• As this treatment transforms austenite to

martensite, steels after sub-zero treatment have high hardness, wear and abrasion resistance, and have no danger of grinding-cracks.

ELIMINATION OF RETAINED AUSTENITE• The stresses are increased further and thus, tempering

should be done immediately after sub-zero treatment.• Carburised steels, ball-bearing steels, highly alloy tool steels,

are normally given cold treatment.

2. Tempering: • The second stage of tempering eliminates the retained

austenite in most steels.• In high alloy steels, multiple tempering is able to eliminate

the retained austenite during cooling from the tempering temperature.

DEFECTS IN HARDENING• The main defects produced during hardening are:

1. Mechanical properties not up to specifications: • The common defect in hardened tools, or component is too

low a hardness. • One or more of the followings could be the cause of such a

defect.• Insufficient fast cooling due to overheated or even polluted

coolant could be responsible for a defect. • The presence of scale, or oil, etc. on the surface also

decreases the cooling rate.• Circulation of coolant, or agitation of component may also

result in such defect.

DEFECTS IN HARDENING• A shorter austenitising time can also cause such a defect.

Lower austenitising temperature can also result in such a defect. • Decarburisation can also result in low surface hardness. If too

high temperature had been used, which produces larger amount of retained austenite can result in low surface hardness.

2. Soft Spots: • Soft areas on the hardened surface are called ‘soft spots’. • The adhering scale, or decarburisation of some areas or

prolonged vapour-blanket stage due to overheated coolant or insufficient agitation or circulation of coolant, or rough surface could cause presence of soft spots surface.

ELIMINATION OF RETAINED AUSTENITE3. Quench Cracks: • Quench cracks form as a result of internal stresses developed of tensile

nature exceeding the tensile strength of the steel. • Steel with lower Ms temperature due to higher contents of alloying

elements are more prone to quench cracks. Higher carbon also results in more brittle martensite.

• Quench cracks can form if there is more time lag between the process of quenching and tempering.

• Overheating of steel or a wrong coolant which gave much faster rate of cooling, or there is faulty design of the component with sharp corners and sharp transition between sections, or a wrong steel has been chosen.

• Presence of large amounts of retained austenite causes grinding cracks. • The other defects could be distortion and warpage; change in dimensions;

oxidation and decarburisation

Tempering of plain carbon steels

– In the as-quenched martensite structure,the laths or plates are heavily dislocated to an extent that individual dislocations are very difficult to observe in thin-foil electron micrographs.

– A typical dislocation density for a 0.2 wt% carbon steel is between 0.3 and 1.0 x 1012 cm cm-3. As the carbon content rises above about 0.3 wt%, fine twins about 5—10 nm wide are also observed.

– Often carbide particles, usually rods or small plates, are observed (Fig. 9.1).

Tempering of plain carbon steels

– These occur in the first-formed martensite, i.e. the martensite formed near Ms, which has the opportunity of tempering during the remainder of the quench.

– This phenomenon, which is referred to as autó-tempering, is clearly more likely to occur in steels with a high Ms.

Stages of Tempering

Stages of Tempering

– On reheating as-quenched martensite, the tempering takes place in four distinct but overlapping stages:

– Stage 1, up to 250°C — precipitation of E-iron carbide; partial loss of tetragonality in martensite.

– Stage 2, between 200 and 300°C — decomposition of retained austenite .

– Stage 3, between 200 and 350°C — replacement of &iron carbide by cementite; martensite loses tetragonality.

– Stage 4, above 350°C — cementite coarsens and spheroidizes; recrystallization of ferrite.

Tempering — stage 1

– Martensite formed in medium and high carbon steels (0.3—1.5 wt% C) is not stable at room temperature because interstitial carbon atoms can diffuse in the tetragonal martensite lattice at this temperature.

– This instability increases between room temperature and 250°C, when €-iron carbide precipitates in the martensite (Fig. 9.2).

– This carbide has a close-packed hexagonal structure, and precipitates as narrow laths or rodlets on cube planes of the matrix with a well-definedorientation relationship .


– At the end of stage 1 the martensite still possesses a tetragonality, indicating a carbon content of around0.25 wt%.

– It follows that steels with lower carbon contents are unlikely to precipitate €-carhide.

– This stage of tempering possess an activation energy of between 60 and 80 kJ mo1, which is in the right range for diffusion of carbon in martensite. The activation energy has been shown to increase linearly with the carbon concentration between 0.2 and 1.5 wt% C.

– This would be expected as increasing the carbon concentration also increases the occupancy of the preferredinterstitial sites, i.e. the octahedral interstices at the mid-points of cell edges, and centres of cell faces, thus reducing the mobility of C atoms.


• During stage 2. austenite retained during quenching is decomposed usually in the temperature range 230-300°C.

• In martensitiC plain carbon steels below 0.5 carbon. the retained austenite is often below 2%, rising to around 6 % at 0.8 wt C and over 30 % at 1.25 wt C.

• The little available evidence suggests that in the range 230-300°C, retained austenite decomposes to bainitic ferrte and cementite, but no detailed comparison between this phase and lower bainite has yet been made.


– During the third stage of tempering, cementite flrst appears in the microstructure as a Widmanstatten distribution of plates which have a well-defined orientation relationship with the matrix which has now lost its tetragonality and become ferrite.

– This reaction commences as low as 100°C and is fully developed at 300°C, with particles up to 200 nm long and 15 nm in thickness.

– Similar structures are often observed in lower carbon steels as quenched, as a result of the formation of Fe3C during the quench.


– During tempering, the most likely sites for the nucleation of the cementite are the €-iron carbide irterfaces with the matrix (Fig 9.2) and as the Fe3C particles grow, the €-iron carbide particles graduallydisappear.

– The twins occurring in the higher carbon martensites are also site for the nucleation and growth of cementite which tends to grow alongthe twin boundaries forming colonies of similarly oriented lath shaped particles (Fig. 9.3) which can be readily ditinguished from the normal Widmanstatten habit.


– A third site for the nucleation of cementite is the grain boundary regions (Fig, 9.4)of both the interlath boundaries of martensite andthe original austenite grain b0unjaries.

– The cementite can form as very thin films which are difficult to detect but which gradually sp1eroidise to give rise to welI-defined particles of Fe3C in the grain boundary regions.

– There is some evidence to show that these. boundary cementite films can adversely affect ductility. However it can be modified by addition of alloying elements.


– During the third stage of tempering , the tetragonality of thc matrix disappears and it is then, essentially, ferrite, not supersaturated withrespect to carbon.

– Subsequent changes in the morpriology of cementite particles occur by process where the smaller particles dissolve in the matrix providing carbon for the selective growth of the larger particles.


– It is useful to define a fourth stage of tempering in which the cementite particles undergo a coarsening process and essentially lose their crystallographic morphology, becoming spheroidized.

– It commences between 300 and 400◦C, while spheroidizatiun takes place increasingly up to 700◦C.

– At the higher end of this range of tempera.ture the martensite lath boundaries are replaced by more equi-axid fèrrite grain boundaries by a process which is best described as recrystallization.

– The final result is an equi-axed array of ferritegrains with coarse spheroidized particles of Fe3C (Fig. 9.5), partly, but not exclusively, by the grain boundaries.


– The spherodisation of the Fe3C is encouraged by the resulting decrease in surface energy.

– The particles which preferentially grew and spheroidize are located mainly at interlath boundaries and prioraustenite boundaries, although some particles remain in the matrix.

– The boundary sites are preferred because of the greater ease of diffusion in these regions. Also, the growth of cementite into ferrite is associated with a decrease in density so vacancies are required to accommodate the growing cementite.

– Vacancies will diffuse away from cementite particles which are redissolving in the ferrite and towards cementite particles which are growing, so that the rate controlling process is likely to be the diffusion of vacancies.


– The original martensite lath boundaries remain stable up to about 600°C, but in the range 350—600°C. there is considerable rearrangement of the dislocations within the laths and at those lath boundaries which are essentially low angle boundaries.

– This leads to a marked reduction in the dislocation density and to lath-shaped ferritic grains closely related to the packets of similarly oriented laths in the originalmartensite.

– This process, which is essentially one of recovery, isreplaced between 600 and 700°C by recrystallization which results in the formation of equi-axed ferrite grains with spheroidal Fe3C particles in the boundaries and within the grains.


– This process occurs most readily in carbon steels.

– At higher carbon content, the increaseddensity of cementite particles is much more effective in pinning the ferrite boundaries, so recrystallisation is much more sluggish.

– The final process is the continued coarsening of the cementite particles and gradual ferrite grain growth.

Role of carbon content

– Firstly, the hardness of the as-quenched martensite islargely influenced by the carbon content, as is the morphology of the martensite laths which have a up to 0.3 wt% C, changing at higher carbon contents.

– The Ms temperature is reduced as the carbon content increases, arid thus the probability of the occurrence of auto-tempering is less.

– During fast quenching in alloys with less than 0.2 wt % C, the majority (up to 90%) of the carbon segregates to dislocations and lath boundaries, but with slower quenching some precipitation of cementite occurs.

– On subsequent tempering of low carbon steels up to 200°C further segregation of carbon takes place. but no precipitation has been observed.


• Under normal circumstances it is difficult to detect any tetragonality in martensitic in steels with less than 0.2 wt% C, a fact which can explained by the rapid segregation of carbon during quenching.

• The hardness change; during tempering are also very dependent on carbon content, as shown in figure for steels up to 0.4 wt% C.

• Above this concentration, an increase in hardness has been observed in temperature range 50—150°C, as €-carbide precipitation strengthens the martensite.

• However, the general trend is an overall softening, as the tempering temperature is raised.

Mechanical properties of tempered plain carbon steels

– The absence of other alloying elements means that the hardenability of the steels is low, so a fully martensitic structure is only possible in thin sections.

– However, this may not be a disadvantage where shallow hardened surface layers are all that is required.

– Secondly, at lower carbon levels, the Ms temperature is rather high, so autotempering islikely to take place.

– Thirdly, at the higher carbon levels the presenceof retained austenite will influence the results.

– Added to these factors, plain carbon steels can exhibit quench cracking which makes it difficult to obtain reliable test results. This is particularly the case at higher carbon levels, i.e. above 0.5 wt% carbon.

Tempering of alloy steels

– The addition of allying elements to a steel has a substantial effect on the kinetics of the y →α transformation, and also of the pearlite reaction.

– Most common alloying elements move the TTT curves to longer times, with the result that it is much easier to miss the nose of the curve during quenching.

– This essentially gives higher hardenability, since martensite structures can be achieved at slower cooling rates and, in practical terms, thicker specimens can be made fully martensitic.

– Alloying elements have also been shown to have asubstantial effect in depressing the Ms temperature.

The effect of alloying elements on the formation of iron carbides

• It is clear that certain elements, notably siliconcan stabilize the €-iron carbide to such an extent that it is still present in the ‘microstructure after tempering at 400°C in steels with 1-2 wt% Si, and at even higher temperatures if the silicon is furtherincreased.

• The evidence suggests that both the nucleation and growth of the carbide is slowed down and that silicon enters into the €-carbide structure.

• It is also clear that the transformation of €-iron carbide to cementite is delayed considerably.

The effect of alloying elements on the formation of iron carbides• While the tetragonality of martensite

disappears by 300°C in plain carbon steels, in steels containing some alloying elements, e.g. Cr, Mo, W V, Ti, Si, the tetragonal lattice is still observed after tempering at 450°C and even as high as 500°C .

• It is clear that these alloying elements increase the stability of the supersaturated iron-carbide solid solution.

• In contrast manganese and nickel decrease the stability.


• Alloying elements also greatly influence the proportion of austenite retained on quenching.

• Typically a steel with 4% molybdenum, 0.2%C, in the martensitic state contains less than 2% austenite, and about 5% is detected in a steel with 1% vanadium and 0.2%C.

• The austenite can be revealed as a fine network around the martensite laths, by using dark field electron microscopy.


• On tempering each of the above steels at 300°C, the austenite decomposes to give thin grain boundaryfilms of cementite which, in the case of the higher concentrations of retained austenite, can be fairly continuous along the lath boundaries.

• It is likely that this interlath cementite is responsible for tempered embrittlement frequently encountered as a toughness minimum in the range 300—350°C, by leading to easy nucleation of cracks, which then propagate across the tempered martensite laths.

The effect of alloying elements on theformation of iron carbides

• Alloying elements can also restrain the coarsening of cementite in the range 400-700°C, a basic process during the fourth stage of tempering.

• Several alloying elements, notably silicon, chromium.molybdenum and tungsten, cause the cementite to retain its fine Widmanstatten structure to higher temperatures, either by entering into the cementite structure or by segregating at the carbide-ferriteinterfaces.

• Whatever the basic cause may be, the effect is to delaysignificantly the softening process during tempering.


• This influence on the cementite dispersion has other effects, in so far as the carbide particles, by remaining finer, slow down the reorganization of the dislocations inherited from the martensite, with the result that the dislocation substructures refine more slowly.

• In plain-carbon Steel cementite particle begin to coarsen in the temperature range 350 -400°C and addition of chromium, silicon, molybdenum or tungsten delays the coarsening to the range 500-550°C.

• It should be emphasized that up to 500°C the only carbides to form are those of iron.

The formation of alloy carbides -Secondary Hardening

• A number of the familiar alloying elements in steels form carbides, nitrides and borides which are thermodynamically more stable than cementite.

• It would therefore be expected that when strong carbide elements are present in a steel in sufficient concentration, their carbides would be formed in preference to cementite .

• Nevertheless during the tempering of all ahoy steels, alloy carbides do not form until the temperature range 500-600°C , because below this the metallic alloying elements cannot diffuse sufficiently rapidly to allow alloy carbides to nucleate.


• The metallic elements diffuse substitutionally, in contrast to carbon and nitrogen which move through the iron interstitially.

• With the result that the diffusivities of carbon and nitrogen are of several orders of magnitude greater in iron than those of the metallic alloying elements.

• Consequently higher temperatures are needed for the necessary diffusion of the alloying elements.

• It is this ability of certain alloying elements to form fine alloy carbide dispersions in the range 500—600°C, which remain very fine even after prolonged tempering, that allows the development of high strength levels in many alloy steels.


• Indeed, the formation of alloy carbides between 500 and 600°C is accompanied by a marked increase in strength, often in excess to that of the as-quenched martensite.

• This phenomenon, which is referred to as

secondary hardening, is best shown in steels containing molybdenum, vanadium, tungsten, titanium, and also in chromium steels at higher alloy concentrations.


• This secondary hardening process is a type of age-hardening reaction, in which a relatively coarse cementite dispersion is replaced by a new and much finer alloy carbide dispersion.

• On attaining a critical dispersion parameter, the strength of the steel reaches a maximum, and as the carbide dispersion slowly coarsens, the strength drops.

The formation of alloy carbides -Secondary Hardening• The process is both time and temperature

dependent, so both variables are often combined in a parameter:

P = T(k + log t)

where: T is the absolute temperature and t the tempering time in hours, while k is a constant which is about 20 for

alloy steels, usually referred to as the Holloman-Jaffe parameter.

137

Martempering

Interrupted quench from the A1

Delay cooling just above martensitic transformation for a length of time to equalize Tthroughout the pieceReduce thermal gradient

btw surface & center Minimize distortion, cracking,

and residual stress.

CASE HARDENING

• Low carbon steels cannot be hardened by heating due to the small amounts of carbon present.

• Case hardening seeks to give a hard outer skin over a softer core on the metal.

• The addition of carbon to the outer skin is known as carburising.

Different stages of Case Hardening

• Case hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface. For steel or iron with low carbon content, which has poor to no hardenability of its own, the case hardening process involves infusing additional carbon into the case

• Case hardening is usually done after the part has been formed into its final shape, but can also be done to increase the hardening element content of bars to be used in a pattern welding or similar process. The term face hardening is also used to describe this technique, when discussing modern armour

• Because hardened metal is usually brittler than softer metal, through-hardening (that is, hardening the metal uniformly throughout the piece) is not always a suitable choice for applications where the metal part is subject to certain kinds of stress. In such applications, case hardening can provide a part that will not fracture (because of the soft core that can absorb stresses without cracking) but also provides adequate wear resistance on the surface

http://en.wikipedia.org/wiki/Hardening_(metallurgy)

http://en.wikipedia.org/wiki/Hardness

http://en.wikipedia.org/wiki/Carbon

http://en.wikipedia.org/wiki/Pattern_welding

http://en.wikipedia.org/wiki/Armour

http://en.wikipedia.org/wiki/Brittleness

http://en.wikipedia.org/wiki/Wear

Case Hardened ChiselChisel:

- cutting edge is hard and wear-resistant- tang is tough and elastic If the chisel would be hard throughout, it could break when the hammer is striked onto it!

Figure - Cut through a hardened chisel - 1 cutting edge (hard), 2 twig (tough)

METHODS OF SURFACE HARDENS

CARBURIZING The highest hardness of a steel is obtained when its carbon

content ishigh, around 0.8 weight % C (Figure 1). Steels with such high carbon content are hard, but also brittle, and therefore cannot be used in machine parts such as gears, sleeves and shafts that are exposed to dynamic bending and tensile stresses during operation. A carbon content as high as 1% C also makes the steel difficult to machine by cutting operations such as turning or drilling. These shortcomings can be eliminated by using a low carbon content steel to machine a part to its final form and dimensions prior to carburizing and hardening. The low carbon content in the steel ensures good machinability before carburizing. After carburizing and quenching the part will have a hard case but a softer core that will assure wear and fatigue resistance. The martensitic case attains a hardness corresponding to its carbon content. The case is typically 0.1–1.5 mm (0.004- 0.060 inches) thick. The core of the part maintains its low carbon concentration and corresponding lower hardness.

A carburizing atmosphere must be able to transfer carbon – and also nitrogen in the case of carbonitriding – to the steel surface to provide the required surface hardness. To meet hardness tolerance requirements this transfer must result in closely controlled carbon or nitrogen concentrations in the steel surface. The carbon concentration, as indicated in Figure 3, can be controlled by the ratio (vol% CO)2/(vol% CO2) in the furnace atmosphere. The atmosphere nitrogen activity, which plays an important role in carbonitriding

PROPERTIES OF CARBURISED STEEL

The gas-carburized (carbonitrided) part can be said to consist of a composite material, where the carburized surface is hard but the unaffected core is softer and ductile. Compressive residual stresses are formed in the surface layer upon quenching from the carburizing temperature. The combination of high hardness and compressive stresses results in high fatigue strength, wear resistance, and toughness.

Maximum hardness for unalloyed steels is obtained when the carbon concentration is about 0.8%C, as was shown. Above that carbon concentration the hardness decreases as the result of an increased amount of retained austenite. The hardness curve therefore often exhibits a drop in hardness close to the surface, where the carbon concentration is highest. Carbon, nitrogen and almost all alloying elements lower the Ms-temperature (see reference [2] for the definition of Ms temperature). This leads to a retained austenite concentration gradient that increases towards the surface after carburizing and quenching.

• To compensate for this effect, the surface carbon concentration after carburizing that provides maximum surface hardness has to be lowered as the alloy content of the steel increases. Carbide forming elements, such as chromium and molybdenum, can counteract this effect and raise the surface carbon concentration that provides maximum hardness. This is because the formation of carbides leads to a lowered carbon concentration in the austenite, although the average carbon concentration is high. Table 1 gives some examples of the relation between maximum hardness and carbon concentration for different types of steels. Mo-alloyed steels obtain the highest surface hardness and Ni-alloyed steels the lowest. Mn-Cr steels obtain an intermediate surface hardness

Case and Carburizing depth

According to European standards [6], the case depth is abbreviated to CHD (case hardened depth) and defined as the depth from the surface to the point where the hardness is 550HV, as shown . Sometimes a hardness other than 550HV is used to define the case depth

Attained case depth depends not only on carburizing depth, but also on the hardening temperature, the quench rate, the hardenability of the steel and the dimensions of the part. This is illustrated in the schematic CCT diagrams. The hyperbolic temperature/ time-dependent parts of the transformation curves depict the transformation from austenite to ferrite/pearlite. For a high hardenability steel these curves are located far to the right in the diagram, ensuring that the cooling curves do not cross the ferrite/pearlite transformationcurve

Hardenability increases not only with base steel alloy content but also with increased carbon and nitrogen concentrations. The carburized or carbonitrided case therefore has higher hardenability than the base steel

In Figure “a” the cooling curves for both “surface” and “center” cross the transformation line for the base steel, the core. This means that the core will transform to ferrite/pearlite upon cooling from hardening temperature. If the cooling curves are related to the “case” instead, it can be seen that the cooling line for the surface passes to the left of the ferrite/pearlite transformation curve. Thus the“surface” cooling line first crosses the Ms (case) line, meaning that the austenite will transform to martensite, as is the intention in case hardening. The hardenability of steel number 1 in Figure “b “is too low to result in martensite transformation even for the carburized case

– In Figure “c” carbonitriding is a method for achieving high enough hardenability to form a martensitic case. (The “surface” cooling line passes to the left of the carbonitrided transformation curve.) Carbonitriding is a way to make water-quench steels become oil hardening steels.

– In Figure “d” schematically shows the effect of part dimensions on cooling rate. The bigger the dimensions, the slower the cooling rate. Therefore there is a certain maximum diameter for a certain steel grade that can be hardened to form a martensitic case. When a martensitic case is formed the case depth will decrease with increasing diameter

Carburizing depth is not standardized but is nevertheless used in practice, and is defined as the depth from the surface to the point corresponding to a specified carbon concentration. As a guideline, the case depth (CHD) for common steels and part dimensions is approximately equal to the carburizing depth to the point where the carbon concentration is about 0.35%. The carburizing depth depends on treatment time and temperature. With prolonged carburizing time carbon can diffuse to a greater depth into the steel. Increasing the temperature increases the rate of diffusion and thus increases the carburizing depth

CHEMISTRY OF CASE HARDENING

Carbon itself is solid at case-hardening temperatures and so is immobile. Transport to the surface of the steel was as gaseous carbon monoxide, generated by the breakdown of the carburising compound and the oxygen packed into the sealed box. This takes place with pure carbon, but unworkably slowly. Although oxygen is required for this process it is re-circulated through the CO cycle and so can be carried out inside a sealed box

http://en.wikipedia.org/wiki/Carbon_monoxide

– The sealing is necessary to stop the CO either leaking out, or being oxidised to CO2 by excess outside air.

– Adding an easily decomposed carbonate "energiser" such as barium carbonate breaks down to BaO + CO2 and this encourages the reaction

– C (from the donor) + CO2 <—> 2 CO

http://en.wikipedia.org/wiki/Barium_carbonate

Increasing the overall abundance of CO and the activity of the carburising compound.[1] It is a common knowledge fallacy that case-hardening was done with bone, but this is misleading. Although bone was used, the main carbon donor was hoof and horn. Bone contains some carbonates, but is mainly calcium phosphate (as hydroxylapatite). This does not have the beneficial effect of encouraging CO production and it can also introduce phosphorus as an impurity into the steel alloy.

http://en.wikipedia.org/wiki/Common_knowledge

http://en.wikipedia.org/wiki/Hydroxylapatite

http://en.wikipedia.org/wiki/Phosphorus

Carbon transfer from gas to surface

Possible carbon transfer reactions are2CO → C+CO2

CH4 → C + 2H2

CO+H2→ C+H2O

It has been shown that the last of these reactions,is by far the fastest and is therefore the rate-determining reaction in carburizing atmospheres with CO and H2

as major gas components . The slowest carburizing reaction is from methane, with a rate that is only about 1% of the rate of carburizing from CO+H2

.

Interaction between Furnace Atmosphere and SteelIn the above reaction, carbon monoxide (CO) and hydrogen(H2) react so that carbon (C) is deposited on the steel surface and water vapor (H2O) is formed. The furnace atmosphere must contain enough carbon monoxide and hydrogen to allow the carburizing process to proceed in a uniform and reproducible fashion. The supply of fresh gas must compensate for the consumption of CO and H2. A higher gas flow is required in cases where the furnace charge area is high, resulting in a high rate of carbon transfer from gas to surface. In the initial part of a carburizing cycle, there is also a high carbon transfer rate, which may be compensated for by increasing the gas supply.

According to the fundamental principles of chemistry, the equilibrium condition for the carburizing reaction 1 is described by an equilibrium constant expressed by:K1 = (ac· PH2O)/(PCO · PH2)

The value of K1 is dependent on the temperatureand can be calculated from the relationship:log K1 = –7.494 + 7130/Twhere T is the absolute temperature in Kelvin. ac is termed carbon activity and is a measure of the “carbon content” of the gas. We see that ac can be calculated if K1 and the gas composition are known. When the carbon activity of the gas, ac

g, is greater than that of the steel surface, ac

s, there is a driving force to transfer carbon as expressed by the following equation:dm/dt = k · (ac

g – acs)

dm/dt = k’ · (ccg – cc

s)

where: m designates mass, c concentration per unit volume, t time,dm/dt expresses a carbon flow in units of kg/cm2 · s or mol/m2 · s, and k or k’ is a reaction rate constant dependent on temperature and gas composition

The gradient dc/dx has its highest value at the beginning of the cycle when carbon has only diffused to a thin depth. This results in a high driving force for carbon flux by diffusion into the steel. The rate of the carbon transfer from gas to surface will therefore initially be the limiting step. At the start of a carburizing cycle, the term cc

g – ccs has its highest

value, and accordingly the driving force for carbon transfer from gas to steel has its highest value. The surface carbon Concentration cc

s will increase with increasing carburizing time. The driving force for carbon transfer, cc

g – ccs, will thus decrease. The carbon

concentration gradient, dc/dx, will decrease concurrently as carbon diffuses into the steel. In conclusion, these limitations will lead to a continuous reduction of carbon flux into the steel

Carburizing Atmospheres

EndogasA carburizing atmosphere can be achieved by means of incomplete combustion of propane or methane with air in accordance with one of the reactions:C3H 8 + 7.2 air → 5.7 N2 + 3CO + 4H2

CH4 + 2.4 air → 1.9 N2 + CO + 2H2

The mixing and combustion of fuel and air takes place in special endothermicgas generators

Nitrogen/Methanol AtmospheresIntroducing nitrogen and methanol directly into the furnace chamber is a common way of creating the furnace atmosphere. Upon entering the furnace, methanol cracks to form carbon monoxide and hydrogen in accordance with the following reaction:CH 3 OH → CO + 2H 2

complete cracking of methanol into CO and H 2 only occurs if the temperature is above 700-800°C (1292-1472°F),which is why methanol should not be introduced into a furnace at a lower temperature. The cracking of methanol into CO and H2 requires energy. This energy is taken from the area surrounding the point of methanol injection. There must therefore be sufficient heat flux towards the injection point to ensure proper dissociation

A high gas flow is desirable in the following cases:– At the beginning of a cycle when the furnace is originally airfilled or has been contaminated with air after a door opening. The higher the gas flow is, the faster the correct gas composition will be obtained.– When carbon demand is great, i.e. at the beginning of a process or in cases with a large charge surface area.

Low gas flow can be used in the following cases:– When the furnace is empty.– When the carbon demand is low, i.e. at the end of a process or in cases with a small charge surface area

Purging of a furnace with inert gas.

DIFFERENT TYPES OFCARBURISING

– Solid or pack carburising– Liquid carburising– Gas carburising

Pack carburising

– The component is packed surrounded by a carbon-rich compound and placed in the furnace at 900 degrees.

– Over a period of time carbon will diffuse into the surface of the metal.

– The longer left in the furnace, the greater the depth of hard carbon skin. Grain refining is necessary in order to prevent cracking.

The gas-carburizing process

In gas carburizing the workpieces are heated in contact with carbon containing gases such as the hydrocarbons, methane, ethane, and propane. The carburizing gases are diluted with an endothermic carrier gas which consists mainly of nitrogen (N2) and carbon monoxide (CO) along with smaller amounts of carbon dioxide (CO2), hydrogen (H2), and water (H2O). Of these gases, N2 is inert and acts only as a dilutent. The carrier gas serves to control the amount of carbon supplied to the steel surface and prevents the formation of soot residue.

The reactions involved in carburizing are as below. First, the methane or propane enrichment of the carburizing-gas mixtures provides the primary source for the carbon for carburizing by slow reactions such as CH4 + CO2 →2CO + 2H2 (14-1) CH4 + H2O →CO + 3H2 (14-2) These reactions decrease the concentrations of CO2 and H2O and increase the amounts of CO and H2. Then the CO breaks down to deposit and allow the carbon to diffuse into the steel surface by the following overall reversible reactions:2CO ↔C (in Fe) + CO2 (14-3)CO + H2 ↔C (in Fe) + H2O (14-4)

The carbon-potential control during carburization is attained by maintaining a steady flow of the carrier gas and varying the flow

of the hydrocarbon enrichment gas.

Induction: Why Induction Heat Treatment?

Greatly shortened

heat treatment cycle Highly

selective Highly energy

efficiency Less-pollution

process

Advantages

Practical Problems • Lack of systematic heating time and temperature distribution control inside WP.

• Nonlinear effect of material properties.

• Lack phase transformation data inside WP for hardness and residual stress determination.

• Evaluate combination effect of AC power density, frequency and gap on final hardness pattern.

• Trial and error, cost and design period.

Numerical modeling may provide better prediction

Research content: FEM based electromagnetic/thermal analysis + quenching analysis + hardening analysis

Research objective: (1) Provide T field, time history inside WP(2) Determine formed content of martensite, pearlite and bainite.(3) Determine hardness distribution in WP. (4) Guidance for induction system design.

•Martensite content determines the hardness

Introduction: Induction Hardening Process• Induction heating: metal parts heated to austenite Phase

•Fast quenching process transforms austenite to martensite phase

•Martensitic structure is the most hardest microstructure

workpiece Inductor/coil

Heating process

Electromagnetic field

Inductioncoil

High freq. AC power

Joule heat byeddy current

Principle: Electromagnetic and Thermal Analysis

Calculation ofmagnetic vector potential (A)

Calculation ofmagnetic flux density (B)

Calculation ofmagnetic field intensity (H)

Calculation ofelectric field intensity (E)

Calculation ofelectric field density (D)

Calculation ofcurrent density (J)

Calculation ofInducting heat (Qinduction)

B = A

H = B /

D = E

Qinduction = E J = J2/

Output: Heat generation Qinduction in WP

(Gauss’ Law for magnetic field)

(Ampere’s Circuital Law)

C r

dlIA

40

t

BE

(Faraday’s Law)

Jt

DH

Input AC power to coil

Electromagnetic Analysis

CoilWP

QCQE

t

QN

QW

QS

QBQE

QN

QR+ QCV

QS

(Outside)

(a) WP geometry (b) FEA model

(c) Interior element (d) Surface element

Thermal Analysis with finite element model

inductionQTkt

Tc

2

airairinduction TThATTFAQTkt

Tc

442

Induced Joule heat Heat radiation Heat convection

Heat conduction

Case Study: Complex Surface Hardening

concave

convex

Geometry Model

Automotive parts from Delphi Inc., Sandusky,Ohio

Material: Carbon Steel, AISI 1070

Real spindle to be hardened

FEA model and B.C. Mesh generated by ANSYS

•Concave and convex on surface of workpiece make the heating process not easy to control.

•ANSYS system is employed for the analysis.

•Mesh should be much finer at locations of convex and concave in both coil and workpiece.

Case Study: Material Properties -- AISI 1070

Emissivity

conductivity

WP relative permeability

ElectricalResistivity

Specific heat

Convection coefficient

(a) Electromagnetic Properties

(b) Thermal Properties

Case Study: Magnetic Field Intensity Distribution

Effect of current density distribution

• Constant current distribution in coil can not result in good heating pattern, especially at concaves of workpiece

• Better hardened pattern resulted from modification of Finer coil mesh and enhanced coil current density at area neighboring to surface concaves of workpiece.

• Enhanced coil current density suggests utilization of magnetic controller at those area in coil design process. Physically this can be fulfilled by magnetic controller.

(a1) Constant current distribution in coil (a2) heated pattern

(b1) Adjusted current distribution in coil (b2) heated pattern

Case Study:Temperature Variation with Time in Induction Heating Process

Total heating time th = 7.05s

t=0.5s

f=9600Hz s=1.27mm J=1.256e6 A/m2

t=2s

t=4s

Case Study: Heating Curves

Summary

• A finite element method based modeling system is developed to analyze the coupled electromagnetic/thermal process in induction heating and implemented in ANSYS package, with following capabilities.

• Provide electrical and magnetic field strength distribution. • Provide instantaneous temperature field data in workpiece. • Provide Temperature history at any location in heating process. • Provide guidance for inductor/coil design based on adjustment of

current density distribution and desired heating patterns.

Quenching of carburized parts

Carburized parts are usually quenched from the austenitic condition to produce a hard case with a martensitic structure, as shown

Most gas-carburized parts are directly quenched from the carburizing temperature of about 925°C or from about 845°C without being cooled to room temperature. The decrease in temperature from about 925 to 845°C can be accomplished by allowing the temperature of the carburizing to decrease, moving the workpiece to a lower temperature zone of the furnace, or by transferring the workpiece to another furnace

Tempering of carburized parts

Many carburized and hardened parts are placed into service without tempering, especially if the applications for the parts are not critical with respect to cracking and chipping. On the other hand, many hardened carburized parts are given a low-temperature temper treatment, usually in the 150 to 190°C range, since in this temperature range, hardening is not greatly reduced and toughness and resistance to cracking is slightly increased

The diffusion of nitrogen into the surface layers of low carbon steels at elevated temperature. The formation of

nitrides in the surface layer creates increased mechanical properties.

•Benefits of Nitriding

•Types of Nitriding

•Future of Nitriding

•Process Determination• CVD Reaction• Deposition Process• Diffusion Depth

•Process Results

•Principal Reasons for Nitriding are:

• Obtain High Surface Hardness

• Obtain a Resistant Surface

• Increase Wear Resistance • Increase Tensile Strength and Yield

Point

• Improve Fatigue Life • Improve Corrosion Resistance

(Except for Stainless Steels)

•Improves Mechanical Properties• Surface Hardness• Corrosion Resistance

•Chemical Reaction• Nitrogen & Iron• Core Properties Not Effected

•Temperature Range• 495 - 565 ºC• Below Tempering Temperature

• White Layer By-Product• Thin• Hard Iron Nitride

•Process methods for nitriding include:

• Gas• Liquid• Plasma

• Bright• Pack

***Lots of more nitriding methods for specific applications***

http://www.nitriding.co.uk/np01.htm

•Gas methods:

• Case-Hardening Process

• Nitrogen Introduction• Surface of a Solid Ferrous Alloy

• Suitable Temperature• Between 495 and 565°C (for Steels)

• Nitrogenous Gas• Ammonia

Liquid nitriding:

•Thermo-chemical Diffusion Treatment

•Hardening Components With Repeatability.

•Salt Bath, at Less Critical Temperatures.

•Preserves Dimensional Stability•Corrosion Protection

•Exhibit Long-Term Resistance to Wear, Seizure, Scuffing, Adhesion and Fatigue.

• Vacuum Chamber• Pressure = 0.64 Pa

• Pre-Heat Cycle• Surface Cleaning

• Ion Bombardment

• Control Gas Flow• N, H, CH4

• Ionization by Voltage• Blue-Violet Glow• Wear Resistant Layer

http://www.milwaukeegear.com/nitrid.htm

Mechanism of Nitriding• Fe- N equilibrium diagram can be used to study

nitriding process.• The solubility limit of nitrogen in iron is temperature

dependent, and at 450 °C the iron-base alloy will absorb up to 5.7 to 6.1% of N.

• Beyond this, the surface phase formation on alloy steels tends to be predominantly ε-phase.

• This is strongly influenced by the C-content of the steel; the greater the carbon content, the more potential for the ε phase to form

• As the temperature is further increased to the gamma prime (γ′-nitride) phase temperature at 490 °C , the “window” or limit of solubility begins to decrease at a temperature of approximately 680 °C.

Effect of alloying elements

• Plain carbon steel form Iron nitride (brittle & low hardness) on nitriding due to absence of nitriding forming element.

• Strong nitriding elements- Al, Mo, Cr, Ti, V etc form nitrides causes internal precipitation of nitrides resulting high surface hardness.

• Steel containing several alloying elements have higher hardness than by a single element

Effect of alloying element on hardness after nitriding

•Replacing Liquid Nitriding • Environmental Effects• Ease of Control• More Complex Substrates• Performed at Lower

Temperatures

• Creates Higher Residual Stress

< http://www.northeastcoating.com/

Gas Nitriding

•How do the variables of nitriding steel affect the process and the mechanical properties of the surface?

• The following variables were investigated:

• Time

• Temperature

• Gas Velocity

• Develop Process Model

CVD Process

CVD Equations

CVD Reaction

CVD Process

The Following Variables Were Used in The Calculation of C surface η(T) ρ(T) D(T) δ(T,v) hmass(T,v)

Surface Composition

Case Depth

• Time Effects• Increase Diffusion Depth

• Temperature Effects• Surface Composition• Deposition Efficiency• Diffusion Rate• Diffusion Depth

• Gas Velocity Effects• Surface Composition• Replenishes Nitrogen Gas• Minimizes Stagnant Layer Thickness

• Microstructural Effects• Processing Temperature• Surface Microstructure

• Mechanical Property Effects• Improves

• Surface Hardness• Wear Resistance• Corrosion Resistance• Fatigue Life• Yield Strength

• Lowers• Ductility• Fracture Toughness

Disadvantages

– It is an expensive process.– Much more time is required to develop

the requisite case depth (due to low temp)

– Expensive gas ammonia is used in nitriding.

– Expensive alloy steels can only be nitrided and are used.

– Nitriding is more expensive than carburising and carbonitriding

Carbonitriding of SteelsCarbonitriding is a modified form of carburizing and is not a form of nitriding. The modification in carbonitriding consists of adding ammonia (NH3) to the carburizing gas so that nitrogen diffuses in the steel case along with carbon. Carbonitriding is usually carried out at a lower temperature and for a shorter time than gas carburizing, and so a thinner case is usually produced than by carburizing. Carbonitriding is principally used to produce a hard, wear-resistant case in steels, normally from 0.075 to 0.75 mm thick. Nitrogen increases the hardenability of steel, and so a carbonitrided case has higher hardenability than a carburized case on the same steel. Also, since nitrogen is an austenite stabilizer, high nitrogen levels can result in retained austenite, particularly in alloy steels. Maximum hardness and less distortion can be attained by carbonitriding since less drastic oil quenching than for carburizing can be used.

Laser Hardening

Introduction• LASER beams are invisible electromagnetic

radiation in infra-red portion of spectrum.• Used for surface hardening of ferrous

material• Laser used for hardening:

– YAG- Solid state type– CO2 Gas type

• CO2 Laser is commonly used for surface hardening when the power required is more than 500W.

Hardening Mechanism-Hypoeutectoid

• Consider the microstructure of a hypoeutectoid steel containing 0.35% carbon. It consists of pearlite colonies surrounded by proeutectoid ferrite.

• On heating, – Pearlite Austenite (dissolution of the cementite), – Growth of the Austenite transformation front into

regions of high carbon concentration, at a rate controlled by carbon diffusion between the lamellae.

– Ferrite transforms by nucleation and growth of austenite at internal ferrite grain boundaries, (rate controlled by carbon diffusion

• The phase diagram shows that under equilibrium conditions, pearlite begins to transform to austenite at 723◦C(Ac1), and that transformation of ferrite is complete at about (Ac3) temperature.

• The high heating rate experienced during laser heating (on the order of 1000 K s−1) results in superheating of Ac1 and Ac3, typically by about 30 and 100◦C, respectively.

• The heat is conducted to the bulk at a very fast rate which results in surface quenching and martensitic hardening.

• By selecting power density and the speed of the laser spot a desired case depth can be hardened.

• Case depth also depends on hardening response of ferrous material (not more than 2.5mm)

Why LASER Hardening?

• Local surface hardening exactly where required• Low distortion and no rework• Short wavelength enabling superior absorption• Closed-loop temperature control• Highest process efficiency of all laser types• Extremely reliable for production processes• Highest level of process safety and

reproducibility

Advantages of LH vs other hardening techniques

• Selective areas hardened without affecting surrounding material

• Possible automation and integration with other in-line production processes

• Quick turn-around time

• Treatment depth accurately controlled and highly reproducible due to direct temperature control

• Superior hardness can be obtained compared to conventional processes • (typically 20% higher hardness)

• No external quenching required → eliminates complex quench equipment

• Minimal heat input → limited distortion → no need for post treatment machining → final machined components for laser hardening

• Phase transformation + volume expansion → residual compressive stresses into surface → improves mechanical properties, e.g. wear and fatigue resistance, lowers crack sensitivity

Laser hardening applications

• Steering gear assemblies

• Turbine blades

• Cutting edges and edges of dies for sheet metal forming

• Cam followers, Gear teeth and Shafts

• Rim geometries, e.g. Piston rings

• Plastic injection moulds at highly loaded areas on the surface

• Cylinder liners in diesel engines

• Heavy duty and ball bearing steels

• Tool steels

Disadvantages

• High initial cost• Laser use 10% of the input energy• Depth of case is very limited• Working cost is high• Difficult to surface harden high alloy

steel• Extra care is needed to avoid fusion

TOOL STEELS

INTRODUCTION– TOOL STEEL are high quality steels made to

controlled chemical composition and processed to develop properties useful for working and shaping of other materials.

– The Carbon content in them is between 0.1 -1.6% . Tool steel also contain alloying elements like, Chromium, Molybdenum and Vanadium.

– Tool steel offers better durability, strength, corrosion resistance and temperature stability, as compared to the Construction & Engg. Steel.

– These are used in applications such as Blanking, die forging, forming, extrusion and plastic molding etc..

OBJECTIVES OF HEAT TREATMENT OF TOOL STEELS

– To obtain a desired microstructure and properties suitable for machining or cold deformation.

– To release residual stresses accumulated during previous thermal and mechanical treatments.

– To homogenize the microstructure with globular carbides by a spheroidization treatment.

– To dissolve by a normalizing treatment the intergranular carbides that are detrimental to the mechanical properties of tool steels.

CHARACTERISTICS OF TOOL STEELS– Require special heat treatment

process.– Higher cost than alloy steels.– Better hardenability than most carbon

and alloy steels.– Higher heat resistance.– Easies to heat treat.

Carbides in Tool steels• The

HOT-WORKED

TOOL STEELS

COLD-WORKED

TOOL STEELS

SHOCK-RESISTING

TOOL STEELS

WATER-HARDENED

TOOL STEELS

HIGH-SPEED TOOL STEELS

TYPES

Oil-hardened

Air-hardened

High Carbon, High Chromium

HOT-WORKED

TOOL STEELS

COLD-WORKED

TOOL STEELS

SHOCK-RESISTING

TOOL STEELS

Chromium-based

WATER-HARDENED

TOOL STEELS


Tungsten-based

Molybdenum-based

HOT-WORKED

TOOL STEELS

COLD-WORKED

TOOL STEELS

SHOCK-RESISTING

TOOL STEELS

WATER-HARDENED

TOOL STEELS


Tungsten-based

Molybdenum-based

SHOCK RESISTING TOOL STEELS– Carbon content = 0.5-0.6%. Alloying elements – Cr,

W , Mo.– These are characterized by good toughness,

hardness and improved hardenability. These steels are generally, water or oil- hardened.

– “Low temperature Tempering” is carried out where, toughness and hardness of the tool steel are of prime importance, otherwise “High temperature Tempering” is preferred.

– Silicon-manganese steels (0.55% C, 2.0% Si, 1.0 % Mn) are included in this group. Due to their high Si-content, decarburization and grain coarsening takes place in these type of steels.

• HEAT TREATMENT PROCEDURE (in general) :-– Annealing : Slow & uniform heating in the

range of 790-800°C followed by furnace cooling at rate of 8-15°C/hr.

– Stress relieving : Heat to 650- 675°C and furnace cooling.

– Hardening : Preheating – warming to about 650°C &

holding for 20 minutes/ 25mm. Austenitizing – heating to 900-950°C &

holding again for 20minutes/25mm.

– Tempering : Heating to 205-650°C, holding for 30 minutes/25mm and then, air cooling.

• Applications: – Chisels– Pneumatic chisels– Punches– Shear blades– Scarring Tools– River sets – Driver bits.

ROTARY PIPE

CUTTER

SWANSTORM DISC CUTTER

GOLD SET MAGNETIC NUT

SETTER

HOT WORKED TOOL STEELS– Carbon content = 0.3-0.5% . These steels are used for

high temperature metal forming operation (except cutting), where the temperature is around 200-800°C.

– These are characterized by high hot yield strength, high red hardness , wear resistance, toughness, erosion resistance, resistance to softening at elevated temperatures, good thermal conductivity

– These are divided into 3 groups depending on the principle alloying elements: – Chromium based [H11- H19]– Tungsten based [H20- H26]– Molybdenum based [H41- H43]

CHROMIUM BASED– Contains Chromium (>=3.25%), and small amounts of

Vanadium, Tungsten and Molybdenum.

– These are characterized by high red hardness & high hardenability.

– Oil quenching is reqd. when dimensional stability is not of prime importance. Tempering temperature for these steels varies from 550-675°C.

– Applications: – Hot dies for extrusion, forging, mandrels,

punches. – Highly stressed structural parts of supersonic

aircrafts.– Hot work steels.

– Contains tungsten (=9.00%) & Chromium (2.0 -12.0%), and low Carbon %.

– These are characterized by resistance to high temperature softening.

– Tempering temperature for these steels varies from 550-675°C.

– Applications: – Punches . – Mandrels .– Extrusion dies for Brass, Steel & Nickel alloys.

TUNGSTEN BASED

– Contains Molybdenum (8.00%) & Chromium (4.0 -12.0%), and some tungsten and Vanadium.

– These are characterized by high toughness & high heat check resistance.


MOLYBDENUM BASED

Microstructure of H-13 TOOL STEEL

(1000x)

Microstructure of Annealed H-13 TOOL

STEEL

– These steels are used for making tools for cold work applications, when the tool surface temperature does not rise more than 200°c.

– These are characterized by high abrasion & wear resistance, higher toughness and high impact resistance.

– These steels are also called “Non- distorting Steels”, as they show little change in dimension during heat treatment.

– These are divided into 3 groups:– Oil hardening Steels [GRADE ‘O’]– Air hardening Steels [GRADE ‘A’]– High Carbon, High Chromium Steels

[GRADE ‘D’]

COLD WORKED TOOL STEELS

– These are hardened by oil-quenching & contain high carbon with manganese, chromium & molybdenum.

– These are characterized by high machinability, wear resistance & non-distorting properties.


– Applications: – Taps . – Blanking & forging dies.– Threading dies.– Expansion reamers.

OIL HARDENING STEELS

– These are hardened by air-quenching and contain Carbon (1.0%) with manganese, chromium & molybdenum & tungsten.

– These are characterized by high wear resistance & high hardenability, fair red hardness, good toughness & resistance to decarburization.


– Applications: – Knives . – Blanking & trimming dies.– Coining dies.

AIR HARDENING STEELS

– These are hardened by oil- or air- hardening & contain Carbon (1.4-2.3%) & Chromium (12-14%), with molybdenum, cobalt, vanadium.

– Vanadium prevents these steels form showing Grain coarsening (upto 1040°C). Chromium imparts non-deforming properties. Tempering of these steels results in high hardness, wear & abrasion resistance.


– Applications: – Mandrel for tube rolling by Pilger rolls . – Blanking & piercing dies, Coining dies,

Drawing dies.

HIGH CARBON HIGH CHROMIUM STEELS

Microstructure of As-Tempered COLD WORK TOOL STEEL

(1000x)

COINING DIES

EXPANSION REAMERS

– These are high alloyed tool steels developed initially to do high speed metal cutting. Now, they used in a wide variety of machining operations.

– These are characterized by high hardness (60-65 HRC at 600-650°C), high red hardness, wear resistance, reasonable toughness and good hardenability.

– They contain 0.6 % carbon, 4% Chromium, 5-12% Cobalt.

– Carbon imparts hardness of at-least 60 HRC of martensite formed. Chromium increase hardenability & corrosion resistance. Cobalt increases the thermal conductivity, melting point, red hardness & wear resistance of high speed steels.

High Speed Tool Steels

These are divided into two groups depending upon the principal alloying elements & the composition:

Molybdenum High speed steel [GRADE ‘M’] (contain Molybdenum, Tungsten, Chromium,

Vanadium & sometimes cobalt).

Tungsten High Speed steels [GRADE ‘T’] (contain high amount of tungsten with chromium,

vanadium and some cobalt.)

Applications : End mills, drills, lathe tools, planar tools. Punches, reamers, Routers, taps, saws. Broaches, chasers, and hobs.

2 minutes (1200°C)

20 minutes

Tempering 1 (1 hr) Tempering

2 (1 hr)

4-5 hrs(850°C)

Tem

pera

tur

e (

°C)

Time

Typical heat treatment cycle for HSS

MILL CUTTERSMicrostructure of

oil- quenched HSS (500x)

Microstructure of oil- quenched

18-4-1 type HSS

– These steels contain carbon in the range of 0.9-1.0% along with Cr, V, Mo.

– These are characterized by high tensile strength & hardness levels but low ductility & toughness values.

– In order to improve machinability, these steels are given “Spheroidizing annealing treatment”.

– Presence of Cr improves both hardness & hardenability and Vanadium checks the tendency of grain coarsening.

– Tempering temperatures are in the range 170-220°C.

Water hardened tool steels

– Applications :– Heavy forging hammers, hand hammers. – Forging dies, bending dies, cutting dies.– Large blanking tools, boring tools.– Chisels, scissors, knife blades.– Milling cutters, lathe centre.– Watch maker’s tools.– Engraving tools.

ENGRAVING TOOL

WATCH MAKER’S TOOLS

DRILL ROD OF W1 TOOL STEEL

– Conventional low carbon steels have low yield strength but good weldability.

– However when carbon content is increased to increase strength, toughness and weldability decreases.

– Microalloyed steels are mild steels with carbon content between 0.03-0.15 % , manganese content around 1.5 % and less than 1% of niobium,

– Titanium, vanadium and also aluminium, which have been given controlled rolling and controlled cooling.

– Hence we obtain ultra fine ferrite grains of size below 5 and attain yield strength of 290-550 Mpa along with tensile strength of 415-700 Mpa.

– The alloying elements are intended to change the microstructure of carbon steels from a pearlite mix to that of an almost pure ferrite having fine dispersion of carbides.

– Presence of low carbon i.e. no pearlite with ultra fine ferrite grains induces high strength and toughness and also good weldability.

– Precipitation strengthening plays a minor role also in increasing the strength.

High strength low alloy steels

Conventional high strength sheet steel for automobiles used to be solid solution-hardened steel or precipitation-hardened steel with micro-alloy added.

Currently, high strength steel products whose microstructure is reinforced for greater strength have been used.

(DP steel, TRIP steel)

Relation between tensile strength and elongation of

HSS

Microstructure of a cold rolled HSLA 340 steel;Courtsey - www.arcelormittal.com

Conventional thermo-mechanical processing technique diagram.

High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined

steel)• Addition of micro-alloy (carbide, nitride or carbo-nitride forming elements) such as Nb, V, Ti in structural steel and strip steel grades, the materials are known as “High Strength Low Alloy (HSLA) steel”

• At slab soaking temperature ~ 1200 ºC- undissolved particles (such as TiN, NbC and

AlN) restricts the size of austenite grain (affect to inhibit recrystallization during hot rolling → produces fine austenite grain size → induces fine ferrite grain size)

- a proportion of micro-alloys are dissolved to solid solution (affect to precipitate in later process in form of fine carbide/carbonitride/nitride at austenite-ferrite interface on cooling to room temperature)

• Hot rolled materials can be strengthened by separate mechanisms of grain refine & precipitation strengthening

• Magnitude of effects depend on:- type and amount of elements added- base compositions- soaking temperatures- finishing and coiling temperatures- cooling rate to room temperature

• Strength increment up to 300 N/mm2 and Y.S. ~ 500-600 N/mm2 can be produced in hot rolled state

• Y.S. ~ 350 N/mm2 are produced in cold-rolled strip containing 0.06-0.10 %Nb


steel)


steel)• Precipitate Ti growth austenite > 1250 ºC• Precipitate Nb growth austenite 1150 ºC

• Precipitate V growth austenite 1000 ºC

• Precipitate Al growth austenite 1100 ºC

HSLA steel คื�อ precipitation strengthening และ ferrite grain refining


steel)


steel)Precipitation-Time-Temperature (PTT) Diagram Nb(CN) austenite 50%

• %Mn precipitation (shift PTT curve

• Nb(CN) dynamic precipitation ~ 900 ºC

• Ps : Precipitation start

• Pf : Precipitation finish


steel)

• Ti(CN) เกิดdynamic precipitation ~ 1025 ºC No-recrystallization temperature (Tnr) Nb(CN))

• %Mn precipitation (shift PTT curve HSLA steel Nb)

Precipitation-Time-Temperature (PTT) Diagram Ti(CN) austenite

High Strength Low Alloy Steel (HSLA)

a) recystallization rate ใน Nb microalloyed steel และ plain carbon steel

Recystallization-Time-Temperature (RTT) Diagram Nb microalloyed steel และ plain carbon steel

Rs: Recystallization start, Rf: Recystallization finish

Ps: Precipitation start, Pf: Precipitation finish

(C): for plain carbon steel(S): for Nb microalloyed steel (solute effect only)(Nb): for Nb microalloyed steel (precipitation effect)

b)Nb solute atom (solute effect only) ที�ecystallization rate (recystallization plain carbon steel

c) precipitation Nb(CN) recystallization ให้�ช้�าลง

High Strength Low Alloy Steel (HSLA) (Precipitation strengthened/Grain refined steel)

• Nb มี�อทีธิพลต่�ออ�ณห้ภู�มีที�� ไมี�มี�กิารต่กิผล!กิให้มี� (No-

recrystallization temperature; Tnr) มีากิที��สุ�ด

Grain size refinement– Fine grain sizes in HSLA steels are possible by control of austenite grain

size by the precipitation of carbonitrides during hot rolling as the temperature of the steel falls.– The austenite grains may recrystallize several time during during hot

controlled rolling resulting in marked refinement of austentite grains until transformation temperature.

– Fine precipitation of carbonitrides must occur in rolling range of 1300 0C - 925 0C when the recrystallization of austenite could occur and the volume of the precipitates formed must be large.

– It is essential to use high soaking temperatures to dissolve as much of the elements niobium, vanadium, titanium so that these could precipitate later during rolling while the temperature continuously drops.

– A fine grain boundary precipitate of TiN does not dissolve completely at the highest soaking temperature(1200-1300 0C) and controls austenite grain growth.

– Main refinement during hot rolling is achieved in range of 1300 – 925 0C when the temperature continuously falls and fine carbonitrides are precipitated from austenite.

– Niobium effectively precipitates carbides and carbonitrides during hot rolling (1050 – 8000C) and modifies the recrystallization behaviour of austenite.

– Niobium does not allow recrystallization to occur even at a high finishing temperature of 925 0C.

– Vanadium (C,N) precipitate during transition or in the ferrite itself.

Refining the ferrite grain size(Grain size effect)

Refining the ferrite grain size (Grain size effect)

Effect of microalloying on recrystallization temperature

Effect of micro-alloying on austenite grain coarsening

Controlled Rolling– Controlled hot rolling is done to obtain ultra fine grains of ferrite and

precipitation hardening.– Fine austenite grains or very thin unrecrystallized grains of austenite

before − transition is a prerequisite to obtain fine ferrite grains.𝛾 𝛼– Presence of TiN and Nb(C,N) during hot rolling of austenite at high

temperatures does some refinement.– This demands heavy deformation and low finishing temperatures below

950 0C so that austenite is unable to recrystallize.– Heavy reductions (more than 50 %) is done during the finishing stages in

no- recrystallization range of austenite to obtain, thin, elongated and flattened austenite grains.

– Normally finishing temperature is above the − transition temperature 𝛾 𝛼and nature of transformation is changed by using water sprays during rolling to enhance cooling rate.

– The subcritical transformation produces still finer ferrite grains and this may produce widmanstatten ferrite with still higher dislocation density.

Controlled rolling/Thermo-mechanical processing (TMCP)

1. Outline process

SRT ~ 1200-1250 ºC

FT ~ 1000 ºC

normalizing ~ 920 ºC

Hold/Delay

Roughing rolling

Finishing rolling(Below Tnr) Austenite-elongated grain

(pancake structure)

No-recystallization temperature (Tnr)

2. Slab Reheating

• Importance of slab reheating stage- control amount of micro-alloying element taken into solution- starting grain size

• Re-solution temperature of micro-alloy precipitates- VC: complete solution ~ 920 ºC (normalizing temp.)- VN: at somewhat higher temperature- Nb(CN), AlN and TiN: around 1150-1300 ºC- TiN (most stable compound) little dissolution at normal slab reheating temperature (SRT)


2. Slab Reheating

• Un-dissolved fine carbo-nitride (CN) particles- maintain fine austenite grain size at slab reheating stage

• Micro-alloying elements taken into solution (which can be influence in later stage in process)- control of recrystallization- precipitation strengthening

• Multiple micro-alloy additions for above dual requirements


3. Rolling• Three distinct stages during controlled rolling.

- Deformation in the recrystallization (austenite phase) temperature range just below SRT- Deformation in temperature range between recrystallization temperature and Ar3

- Deformation in 2 phase (austenite-ferrite) temperature range between Ar3 & Ar1

• At temperature just below SRT- rate of recrystallization is rapid- provided the strain per pass exceeds a minimum critical level- recrystallization is retarded by presence of solute atom Al, Nb, Ti, V (solute drag) → strain induced precipitation → form fine carbonitride during rolling process


3. Rolling- rolling temperature decrease, recrystallization more difficult and reach a stage “recrystallization stop temperature (Trs or No-recrystallization temperature; Tnr)” (the temperature at which recrystallization is complete after 15 s. after particular rolling sequence)- Nb is powerfull retardation effect which depend on solubilities in austenite

- Nb lease soluble- largest driving force for precipitation- creating greater effect in increasing of

recrystallization temperature than Al and V• At temperature between recrystallization

temperature & Ar3

- temperature below 950 ºC


3. Rolling- strain induced precipitation of Nb(CN) or TiC is sufficient rapid to prevent recrystallization before the next pass (deformed-austenite providing nucleation sites of carbo-nitride precipitation and pins the substructure which inhibits recrystallization)- finishing rolling below recystallizaion stop temperature- can be obtain elongated-pancake morphology in the austenite structure

• At temperature between Ar3 & Ar1

- further grain refinement- mixed structures of polygonal-ferrite (transformed from deformed-austenite) and deformed-austenite during rolling process


4. Transformation to ferrite• Mean ferrite grain size relate to:

- thickness of pancake-austenite grain- alloying elements depress the austenite to ferrite transformation which decrease ferrite-grain size- cooling rate from austenite or austenite-ferrite region (accelerate cooling)

→ increase strength→ achieve strength level by lower alloy

content- direct quenching

→ refine ferrite-grain→ formation of bainite and martensite

(required tempering)


Sketches of microstructural changes in low-carbon steels that develop as afunction of finishing temperature in austenite and cooling to initiate ferriteformation.

Precipitation Hardening

• Precipitate strengthening occur by those particles that form :-i. In austenite at low temperatures at low temperatures.ii. At the interface at low temperatures.iii. In ferrite during further cooling.• Main contribution to precipitation strengthening is due to precipitation of carbides of Nb, Ti, V which occurs during transformation of austenite to ferrite progressively at interface boundaries called interphase precipitation.• It occurs on a very fine scale between 850 – 650 0C with size about 5 nm.• Solid solution strengthening due to presence of elements like Mn, Si, and even uncombined N takes place.• The most effective factor is the ultra fine ferrite grain size as ferrite size is 5-10 in thin plates though a size of 1-2 has been obtained under laboratory conditions.

Yield strength as a function of grain size in various low carbon steels.

Advantages of HSLA steels

• HSLA steels are designed to handle large amount of stress and possess a good strength to weight ratio. They are usually 20-30 % lighter than a carbon steel of same strength.

• HSLA steels are also more resistant to rust then most carbon steels. Although the material quickly becomes covered with surface rust, this is superficial and rust takes a long time to threaten the integrity of a structure made from the material.

• Good formability at room temperature. Hot forming requires careful control to avoid strength degradation.

• Good weldability with most conventional welding methods and lower preheat requirements.

• They are further characterised by a good resistance to fatigue and impact.

HSLA steels are used in sectors such as :• Automotive: pressed chassis and reinforcement parts, beams or welded tubes.• Seats: tubes, rails and mechanical elements.• Industrial vehicles, tractors, trailers and skips as chassis components (resistance to fatigue).• Lifting and handling equipment (cranes, fork lifts, platforms, warehouse shelves, lifts).• The agricultural sector for chassis and protective elements.• Roll bars, buildings, containers, urban lighting masts, concrete Mixers.

Uses of HSLA steels

Uses of HSLA steels

Dual phase steels• Dual phase steels have microstructures consisting of islands of

martensite (normally 10-20 %), or dispersion of martensite in a ferrite matrix.

• DPS starts as a low or medium carbon steel and is quenched from a temperature above A1 but below A3 on a continuous cooling transformation diagram.

• This results in a microstructure consisting of a soft ferrite matrix containing islands of martensite as the secondary phase (martensite increases the tensile strength).

• The soft ferrite phase is generally continuous, giving these steels excellent ductility.

• When these steels deform, strain is concentrated in the lower-strength ferrite phase surrounding the islands of martensite, creating the unique high work-hardening rate exhibited by these steels.

• The desire to produce high strength steels with formability greater than microalloyed steel led the development of DPS in 1970s.

• Dual phase steels are gaining importance among auto makers as they provide excellent combination of strength and ductility while at the same time they are widely available due to relative ease of manufacture.

(a) Dual phase 600 (b) Dual phase 980 Y700

Methods of obtaining DP steels

• A normalized steel is heated to intercritical temperature (e.g. 790 0C) and held there for several minutes till equilibrium amounts of austenite and ferrite are obtained.

• On quenching it ( or air cooling it if it has been alloyed with 0.2-0.4 % Mo and 1.5 % Mn), austenite transforms to fine martensite.

• Nowadays this extra reheating is avoided. The required structure is obtained during cooling itself after controlled rolling.

• Such steels normally have 0.4 % Mo and 0.5 % Cr.• The hot rolling is completed at around 870 0C when the

steel has around 80 % ferrite on the water cooled run out tables from the mills.

• This steel is cooled bypassing the pearlitic nose to 550-620 0C, where the steel is cooled ,when it is in metastable state.

• On subsequent cooling, transformation of austenite occurs to martensite.

Iron-Carbon phase diagram with thermal treatment for dual phase steels

TTT diagram for obtaining dual phase steel

Advantages of DP steels

• Low cost of manufacturing.

• Excellent formability.

• High strength (500-700 MN/m2).

• Low yield stress (300-350 MPa).

• Rapid rate of work hardening.

• No yield point elongation.

• A high strain rate sensitivity (the faster it is crushed the more energy it absorbs).

.

• Good fatigue resistance.

Comparison between dual phase and hsla steel mechanical properties

Dual Phases (DP) Steel

Maraging steels• Maraging steels, a portmanteau of martensitic and aging, are iron alloys,

are a special class of low carbon ultra high strength steels which derive their strength not from carbon, but from precipitation of inter metallic compounds.

• The maraging steels offer a good combination of strength and ductility with a yield strength as high as 400 MPa and an elongation of 6 %.

• These are essentially high alloy steels with C < 0.03 %, upto 25 % Ni, 7-10 % Co, 3-5 % Mo,1.75 % Ti & 0.2 % Al.

• Presence of Ni in large amount is necessary to form ductile and soft martensite based on Fe-Ni system.

• The soft ductile and tough martensite is strengthened by precipitation hardening resulting in a fine dispersion of Ni3(X,Y) intermetallic phases along dislocations left by martensitic transformation, where X and Y are solute elements added for such precipitation.

• Maraging steels have good weldability and can be cold rolled to 80-90 % before aging.

• Maraging steels also have good machinability and undergo little dimensional changes onheat treatment.

• Due to the higher cost maraging steels are mainly used for rocket casings and other aerospace applications.

• They are suitable for engine components, such as crankshafts and gears, and the firingpins of automatic weapons that cycle from hot to cool repeatedly while under substantial load.

Properties of maraging steels

Maraging steels

A series of iron base alloys capable of attaining

yield strengths upto 300,000 psi. in combination with excellent fracture toughness.

Low carbon (0.03%),18-25 % Nickel and other hardening elements.

Yield strength: The stress at which a material exhibits a specified deviation from proportionality of stress and strain.

Fracture toughness:resistance to crack propagation.

Maraging steels

As annealed, these steels are martensitic.

They achieve high strength on being aged in the annealed or martensitic condition.

This martensite is soft and tough as compared to the hard and brittle martensite formed in conventional low alloy steels.

This ductile martensite has a low work hardening rate and so can be cold worked to a high degree.

Maraging steels

There are two groups, based on hardening element used.

1.The 18% nickel grades use cobalt molybdenum additions

2.The 20% nickel grades use titanium-aluminium-columbium additions.

Maraging steels

18% NickelHeated to 1500 deg F , held for 1 hour .

Soaked to anneal austenite and dissolve hardening elements Co and Mo.

Air cooled to 100 deg F, forms martensite of 28 – 32 RC

Reheated to 900 deg F, held for 3 hrs- aged to harden.

stress relief also occurs at the same time.

Hardness of 52 RC can be achieved.

300300300

300300

3 hrs

1 hr

300

Deg F

900

1500

Ms

10028 RC 52 RC

Maraging steels

18% Nickel

•The interest is in achieving high strength at room temperature.

•Simple heat treatment carried out at moderate temperature is enough to achieve good properties.

•Section size, heating and cooling rates are not important.

•Very low in carbon content and so no problem of decarburization.

•Protective atmosphere is not required.

•Low aging temperatures means less distortion.

•So,no deformation on hardening and not much machining is required after heat treatment.

Maraging steelsEffect of additives on maraging strength development

Solution treated 110,000 psiAfter maraging 300,000 psi

Iron nickel martensite 25Rc

A weak response to maraging is seen after addition of 7% cobalt.

The addition of molybdenum alone gives a slight increase in annealed hardness and good maraging response.

When Mo is added in presence of 7% Cobalt, an increase in hardness greater than the combined effect of both the elements is seen.

Co

Co7Co+Mo

Mo

Co

Solution treated

24

52

Rockwell C

2 4 6 8

%Mo or %Co

Maraging steels

25%Nickel

Largely austenitic after annealing.

The conversion to martensite is done by ausaging or cold working.

Ausaging-conditioning treatment at 1300 deg F.Reduces the stability of austenite by causing nickel titanium compounds to precipitate from the austenitic solid solution.

It raises the Ms temperature so that martensite will start forming at room temperature.

Cold worked to 25% to start the transformation to martensite and is completed by refrigeration at minus100 deg F.

Maraging steels

20% NickelLower alloy content Freedom from cobalt and molybdenum. Useful in certain environments and applications.

Compared to the 25% grade, this does not require a conditioning treatment to become martensite.Ms temperature is above room temperature.

Disadvantages:

Lower intoughness, resistance to stress corrosion cracking and in dimensional stability during heat treatment.

Maraging steels

Applications for maraging steels

Hulls for hydrospace vehiclesPressure vehicles

Motor cases for missilesMortar and rifle tubing

Hot extrusion dies

Low temperature structural parts

Cold headed bolts

(Complex shapes that need to be strong after shaping)

STAINLESS STEELS

WHAT IS STAINLESS STEEL

• 1913, Sheffield, U.K.

• Harry Brearley experimenting with alloy steels for gun barrels

• Samples with 14% chromium were discarded as they proved to be unsuitable for this application

• Some months later he noticed that low alloyed samples had rusted

• The chromium alloyed samples were still bright

In general terms we can say that a stainless steel is an alloy with:

a minimum 12% Cr

IMPORTANCEStainless steel are versatile for the following properties– Good corrosion and oxidation resistance

Cr2O3 protective layer formed– Good creep strength– High resistance to scaling and oxidation at

elevated temperatures– Wide range of strength and hardness– High ductility and formability– Excellent pleasing appearance– Good weldability and machinability– Good low temperatur properties

PHASE DIAGRAMS AND TYPICAL PHASES

– Stainless steels contain large amounts of chromium, and an important starting place to understand the phase relationships and microstructures in stainless steels is the iron-chromium (Fe-Cr) equilibrium phase diagram.

– Phase diagrams of importance are Fe–Cr, Fe–Ni, Cr–Ni, Fe–Mo, Fe–Ti, Ni–Ti, Fe–Nb, Fe–Mn, Fe–Si, Fe–Cr–Ni and Fe–Cr–Mo ternary diagrams and the quaternary Fe–Cr–Ni–Mo

– three main features of the Fe–Cr diagram which are relevant to stainless steels, are the ferrite-stabilizing character of Cr and the presence of sigma (σ) and alpha prime (α’) phases

• The Fe–Ni diagram clearly shows the strong austenite-stabilizing effect of Ni. The intermetallic compound Ni3Fe is not normally observed in stainless steels. Also the CrNi2compound present in the Cr–Ni diagram does not form in stainless steels

– The Fe–Mo diagram shows that Mo is a strong ferrite stabilizer and also that it forms four intermetallic compounds with iron. Of these, the sigma (σ) phase and the Laves phase, Fe2Mo, often occur in stainless steels. The mu (µ) phase, Fe7Mo6, occurs less frequently in stainless steels

– In the Fe–Ti diagram one can also see the very strong ferrite-stabilizing character of the Ti and also the presence of a Laves phase, Fe2Ti, that can occur in stainless steels, particularly in those in which the relationship Ti:C is high, the so-called over-stabilized steels.

– The Ni–Ti diagram shows the presence of the Ni3Ti that can be used to produce precipitation strengthening in Ni-containing stainless steels. The Fe–Nb diagram shows the ferrite-stabilizing character of the Nb and also the presence of a Laves phase, Fe2Nb. Fe2Nb, similar to Fe2Ti, can occur in stainless steels in which the relationship Nb:C is high, which are also called over-stabilized steels.

– As to the Fe–Mn diagram the most relevant characteristic is the austenite-stabilizing character of Mn. Finally, the Fe–Si diagram shows a strong ferrite-stabilizing effect of Si but none of the five intermetallic compounds is found in commercial stainless steels.

– The ternary Fe–Cr–Ni diagram is the basic diagram for stainless steels. It shows the presence of only three solid phases: austenite, ferrite, and sigma phase. For a high Cr/Ni ratio delta ferrite may occur during solidification and sigma phase may occur during aging at temperatures between 550oC and 900oC

• The Fe–Cr–Mo diagram shows the presence of six phases: (Fe,Cr)=solid solution; (Cr,Mo)=solid solution; Fe7Mo6=mu (µ) phase;σ=sigma phase; x=chi phase; the Laves phase Ƞ-Fe2Mo

Binary iron–chromium equilibrium diagram. (From J.R. Davis (Ed.): ASM Speciality Handbook: Stainless Steels, ASM International, Materials Park, OH, 1994

Gamma loops formed in various binary systems of iron

Three-dimensional view of the Fe–Cr–Ni equilibrium diagram

The Fe-Ni phase diagram

Projections of the liquidus and solidus surfaces of the Fe-Cr-Ni ternary system

The Schaeffler constitution diagram (1949) for stainless steel weld metal

The Delong constitution diagram (1974) with Welding Research Council ferrite number system for weld metal

Austenitic Stainless Steel

GENERAL FEATURES

– Basic composition of it is 16-25% Cr & sufficient amount of austenite stabilisers

– Austenite at room temperature– Most commonly used stainless steel– Austenite is the primary phase– These are not hardenable by heat treatment– These have high ductility, low yield strength,

relatively high ultimate tensile strength – Basic crystal structure is FCC

Characteristics– As austenitic steels have FCC structure

non magnetic in naturetough at low temperatures as no

ductile brittle transition good ductility with elongation about 50%

– Excellent corrosion resistance in organic acid , industrial & marine environment

– Excellent formability, fabricability, cleanability and hygiene characteristics

– Cr/Ni austenitic steels are resistant to high temperature oxidation

– Excellent weldability since single phase materials– Austenitic steels do not have 475o C embrittlement but

show reduced ductility and toughness due to formation of brittle intermettalic compounds called sigma phase. Sigma phase increaseswith increase in chromium amount & presence of Mo, Ti & Si. Manganese helps reduce the formation of σ-phase

–

– These steels are not strong materials because these are single phase materials. Austenitic steels are not hardenable by heat treatment but they are hardened by cold work solid solution strengthening

Cold work– Work hardening rate is high due to low stacking fault energy of

0.002 J/m2

– Upon cold working by 60% increases yield strength from 240 Mpa to 1035 Mpa & tensile strength of 585 Mpa gets doubled

– Loss of strength at temperatures above 600 OC & also in HAZ of a weld

solid solution strengthening– Substitutional solids show little increase of strength but

interstitial solutes are very effective

Microstructure of austenitic steels

Microstructure of annealed type 316L austenitic stainless steel. Etched in 20% HCl, 2% NH4FHF,0.8% PMP

Microstructure of annealed type 316L austenitic stainless steel. Etched in waterless Kalling’s reagent

Limitations There are two limitations of austenitic stainless steel

Cracking due to stress corrosionIntergranular corrosion

Stress corrosion– Austenitic stainless steel is prone to stress corrosion

cracking in the presence of chloride ions even present in ppm levels

– Failure can occur due to the presence of small stresses or even residual stresses

– Fracture ids transgranular with little or no plastic deformation

– Can reduced inhigh nickel (>30%Ni) austenitic steels or reduce the stress or eliminate chloride ions

Intergranular carbides in austenitic stainless steel– Austenitic steels on being heated in the range of 500 OC to 800

Ocfor any reasonable time leads to the chromium present in stainless steel to react with carbon to form Cr23C6 . This reduces the the Cr available(below 12 %) to provide the passive film & leads to preferential corrosion which can be severe. This is also known as sensitization.

Microstructure of type 304 stainless steel with chromium carbideprecipitation on grain boundaries. ASTM A262 Practice A oxalicacid etch. Scanning electron micrograph

Grain boundary M23C6 precipitates in an austenitic stainless steel observed using transmissionelectron microscopy ISIJ International (Japan)

– High-angle grain boundaries are preferred sites for precipitation and diffusion because of the relatively high atomic disorder where grains of different orientations meet. Thus, M23C6 particles readily nucleate and grow, severely depleting the adjacent austenite of chromium, from 19 wt% to about 10 wt%.

– Twin boundaries have much better atomic matching than most high-angle boundaries and therefore are not as favourable for nucleation and growth of M23C6 particles. The carbides are largest on the high-angle grain boundaries, quite small on the incoherent twin boundaries, and absent on the coherent twin boundaries

Chromium depletion as a function of distance from various typesof grain boundaries in type 304 stainless steel. Courtesy of M.G.Burke, Westinghouse Electric Corp., Pittsburgh

Chromium carbide precipitation on various types of boundariesin type 304 stainless steel. Arrows in upper left point to largecarbides on a high-angle grain boundary, and IT and CT refer to incoherent andcoherent twin boundaries, respectively. Transmission electron micrograph

M23C6 carbide precipitation kinetics in type 304 stainless steelcontaining 0.05% C and originally quenched from 1250 C(2282 F)

Heat Treatment in Austenitic Steel

Main thermal treatments and transformations that occur in austenitic stainless steelsbetween room temperature and the liquid state ISIJ International (Japan)

Solution Annealing– main objective of this treatment is to dissolve the phases that

have precipitated during the thermo mechanical processing of the material, especially the chromium-rich carbides of the M23C6-type where M =Cr, Fe, Mo

– lower temperature limit for solution annealing should be over 900 oC

– Carbides should be completely dissolved but they dissolve slowly. Grain growth limits the maximum solution-annealing temperature

– abnormal grain growth, also known as secondary recrystallization, must be avoided

– Cooling from heat treatment temperatures should be sufficiently fast to avoid chromium carbide precipitation. On the other hand, too fast cooling rates cause component distortions

• During solidificationor welding, the formation of d-ferritemay occur, whichmay be difficult to eliminate completely during the thermomechanical treatment and itmay be present before the solution-annealing heat treatment or even may survive it. If the material has d-ferrite it may be even more susceptible to the precipitation of the intermetallic phases

– In the case of non stabilized grades such as AISI 201, 202, 301, 302, 303, 304, 305, 308, 309, 316, and 317, if distortion considerations permit, water quenching may be utilized. In the case of the AISI 309 and 310 types that contain maximum allowed carbon content and are susceptible to carbide precipitation, water cooling is mandatory. In the case of stabilized AISI 321, 347, and 348 types, water cooling is not needed and air cooling is sufficient to avoid sensitization. Molybdenum-containing steels, such as AISI 316, 316L, 317, and 317L, present an intermediate tendency toward sensitization when compared to non stabilized conventional and the stabilized types, i.e., they do not require water cooling from the solution-annealing temperature

– In the case of molybdenum-containing steels, such as AISI 316, 316L, 317, and 317L types, long exposure times at temperatures in the 650 to 870 8C (1200 to 1600 8F) temperature range should be avoided, to avoid the precipitation of intermetallic phases such as sigma (s), chi ( x), and Laves (Fe2Mo) phases.

Grain boundary M23C6 precipitates in an austenitic stainless steel observed using transmissionelectron microscopy

Optical micrograph showing secondary recrystallization start in a titanium-stabilizedaustenitic stainless steel after solution annealing. Etched with V2A-Beize

Stabilize Annealing– Stabilize annealing is used for stabilized austenitic stainless

steels in order to assure maximum intergranular corrosion resistance

– After the solution-annealing treatment, only part of the carbon is bound in the form of primary phases, such as carbides, MC, carbo-nitrides, M(C,N), nitrides MN, or carbo-sulfides M4C2S2, where M=Ti, Nb, or V. The remaining carbon stays in solid solution and may precipitate as secondary carbides MC or M23C6 at lower temperatures, since the carbon solubility in austenite under 900 oC is very low

– exposing the steel, after solution annealing, to temperatures in the 845 to 955 oC temperature range for up to 5 h (depending on component size), favors MC precipitationin detriment to M23C6. Furnace atmosphere control, avoiding carburizing or excessively oxidizing conditions, should be employed and the sulfur content in oil- or gas-fired furnaces should be kept at low levels.

Stress Relief Annealing– cool the component slowly from the solution annealing

temperature. during slow cooling, some M23C6 precipitation may occur with consequent sensitization. On the other hand, fast cooling may reintroduce residual stresses and make the component susceptible to stress-corrosion cracking (SCC). In general, a small amount of intergranular corrosion is preferable to a failure in few weeks due to SCC. Moreover, the selection of a low carbon or of a stabilized steel would be a more appropriate solution. The selection of a lower working temperature range, say in the 925 to 1010

oC range, would allow longer time exposure without significant grain growth.

– stress relieve at a lower temperature range, between 425 and 550 oC where M23C6 precipitation is very slow, allowing the material to be exposed for some hours without sensitization occurrence. This treatment may not be very efficient to reduce residual stresses, nevertheless it may be sufficient to reduce residual stresses sufficiently to significantly reduce the risk of SCC

Bright Annealing

All austenitic stainless steel types can be bright annealed in a pure hydrogen or dissociated ammonia atmosphere, provided that its dew point is kept below 508 oC and that the components are dry and clean before entering the furnace. If the dew point is not kept sufficiently low, some thin green oxide film may be formed, which will be difficult to remove.

Martensite FormationAustenite in stainless steel is generally not a stable phase.In the solution-annealed condition, the Ms temperature is normally below room temperature. For the majority of these steels, the Md temperature (the temperature below which martensite will form under deformation) is above room temperature.Two kinds of martensite can occur in stainless steels: á (bcc, ferromagnetic) and € (hcp,nonferromagnetic). The transformation of austenite to martensite can be also induced in austenitic stainless steels by cathodic charging with hydrogen. Typical lattice parameters areá=0.2872nm &a€ =0.2532nm C€ =0.4114nm

Transformation during cooling– many steels when cooled to cryogenic

temperatures,will form alpha prime (a’) martensite. The ability to form alpha prime (a’) martensite becomes more significant during cooling after sensitization. M23C6 precipitation at grain boundaries causes depletion of chromium, carbon, and other alloying elements in the vicinity of the grain boundaries. This leads to a higher Ms temperature, making the material more susceptible to the formation of alpha prime (a’) martensite close to grain boundaries during cooling

– formation of epsilon (e) martensite increases with a decrease in the stacking fault energy of austenite

– Ms ( C) =1302-42 (% Cr)-61 (% Ni)-33 (% Mn)-28 (% Si)-1667 (% [C+N])

– Residual nitrogen contents of austenitic stainless steel are usually in the range of 300 to 700 ppm (0.03 to 0.07 wt%) and thus when combined with carbon may have a strong effect on stabilizing austenite with respect to martensite formation

• The epsilon martensite forms on close-packed (111) planes in the austenite and, except for size, is morphologically very similar to deformation twins or stacking fault clusters, which also form on (111) planes. The á martensite forms as plates with (225) habit planes in groups bounded by faulted sheets of austenite on (111) planes. The nucleation of á martensite and its relationship to € martensite has been difficult to resolve; evidence for á formation directly from austenite and with € as an intermediate phase

Strain Induced Transformation– Strain-induced martensite forms at higher temperatures than

does martensite, which forms on cooling, and the parameter Md, the highest temperature at which a designated amount of martensite forms under defined deformation conditions, is used to characterize austenite stability relative to deformation

– Carbon and nitrogen have a very strong effect on austenite stability, and the extra-low carbon grades are quite sensitive to strain-induced martensite formation, a characteristic that may render them susceptible to reduced performance in high-pressure hydrogen

– Deformation-induced martensite, however, significantly enhances strength generated by cold work, and types 301 and 302 stainless steels are designed to have lower chromium and nickel contents in order to exploit this strengthening mechanism

– The extent of strain-induced transformation of austenite to martensite is dependent on temperature, strain rate, and strain, in addition to composition. Large amounts of martensite form at low strains during low-temperature deformation, and the amount of strain-induced transformation becomes negligible above room temperature.

– The strong effect of strain-induced martensite formation at lower temperatures is marked by noticeable inflections in the stress-strain curves. The strain hardening associated with these inflections produces very high ultimate tensile strengths, and as the strain-induced transformation decreases, the ultimate tensile strength also decreases.

Stress-strain curves for types 304 and 301 austenitic stainlesssteels

Strain-induced martensite formation as a function of strain at various temperatures. Solid lines are original data of Angel,dashed lines are data of Hecker et al., and dotted extrapolations are from Olson’s analysis

Engineering stress-strain curves for type 304 stainless steels atvarious temperatures

Ferritic Stainless SteelCharacteristics– Chromium content is usually between 17 to 26%– Carbon content is kept as low as 0.08 to 0.2% to improve

toughness and reduce sensitization– Ferrite stainless steel follow relationship of (Cr-17xC)>12.7– They have ferritic structure upto melting point– They have slightly high yield strength but lower strain

hardening – These have BCC crystal structure & are less ductile than

austenitic steels– These are not hardenable by heat treatment

Body Centred Cubic“Ferrite”

Basic Properties– Moderate to good corrosion resistance increases with Cr

content– Not hardenable by heat treatment and always used in the

annealed condition– Magnetic in nature– Formability not as good as the austenitic stainless steel– Moderate ductility - not easy to deep draw 20%

elongation– Low impact strength - brittle at low temperatures– Poor weld ductility due to grain growth in HAZ– Moderate strength– These steels work harden less– Yield strength is 275-415 Mpa ,tensile strength is 500-

600Mpa &yield elongation is 30%

Advantages of ferritic stainless steel

– Ferritic stainless steels are cheaper than austenitic stainless steels because expensive nickel is not used

– High corrosion resistance, increases with increase in Cr content

– Immune to chlorides hence no failure due to stress corrosion

– Reasonable cold formability– Excellent hot ductility– Cr rich ferritic steelshave good oxidation resistance at

high temperatures .thus used as furnace components– Good machinability, higher thermal conductivity lower

thermal expansion – Magnetic in nature

Limitations of ferritic steels– These steels get corroded in Cl- & SO4

2- containing industrial and marine atmospheres. These are less corrosion resistant than austenitic stainless steels in reducing atmospheres

– Grain refinement is difficult because no phase change occurs on heating. Grain growth is rapid on heating as diffusion is faster in BCC structure. Grains start coarsening at 600oC.

– Due to its BCC structure, they show ductile to brittle transition & it is considerably higher for mild steel due to embrittlement effect of Cr . Presence of coarse grains raises the transition temperature.

– Also due to its BCC structure ,these steels have lower general ductility and higher yield strength

– These show stretcher strains during drawing or stretching– These steel suffer from two types of embrittlement effects

other than the natural brittleness– Suffer from intergranular corrosion in the HAZ of the weld due

to precipitation of chromium carbides. Sensitisation occurs rapidly in ferritic as diffusion is faster due to bcc structure.

Microstructure of ferritic steels

Microstructure of annealed ferritic stainless steel (E-Brite 26-1containing 26% Cr and 1% Mo). Etched electrolytically in 60%HNO3-H2O.

Intermetallic Phases – Various phases found are SIGMA Phase ,CHI phase &

LAVES Phase– Various intermetallic phases form by the arrangement of

iron, chromium, molybdenum, and other transition metal atoms into crystal structures that accommodate atomic size and electronic differences that limit the low-temperature solid solubility of alloying elements in the bcc ferritic structure

– The formation of the intermetallic phases follows “C” curve kinetics, which are influenced by alloy composition The “C” curves are useful in that they define temperature ranges that can be used to dissolve the intermetallic phases and through which specimens must be rapidly cooled to avoid reprecipitation of the phases. The “C” curves also identify operating temperatures that should be avoided for application of ferritic stainless steels.

Estimated time-temperature-transformation curves for ferritic stainless

SIGMA (σ)-PHASE– Treischke and Tamman studied the Fe–Cr system and

proposed the existence of an intermetallic compound containing Cr in the 30 to 50 wt% range. In 1927, Bain and Griffiths studied the Fe–Cr–Ni system and observed a hard and fragile phase, which they called constituent B, from brittle. In 1936, Jett and Foote called it sigma(σ)phase and in 1951, Bergmann and Shoemaker determined through crystallography its structure in the Fe–Cr system.

– The precipitation of sigma phase in stainless steels can occur in the austenitic, ferritic, and ferritic–austenitic phases with duplex structure types. The precipitation of this Fe–Cr–Mo intermetallic, of tetragonal structure with 30 atoms per unit cell, causes loss in toughness and results in the matrix becoming depleted of Cr & Mo

Intermetallic Phase ie. ( Sigma)

Sigma

Ferrite

Austenite

Time

Tem

pera

tur

e

-Phase

1000°C

600°C

“Cooling Curve”

No - Sigma Phase Formation

Time

Tem

pera

tur

e

-Phase

1000°C

600°C

“Cooling Curve”

Sigma Phase Formation

Sigma Phase

Sigma Phase

Cr

MoFe

– In the austenitic steels, precipitation generally requires hundreds or even thousands of hours and the precipitated volumetric fraction is generally smaller than 5 vol% [58]. Precipitation can be represented by a common precipitation reaction: ¥→ ¥* +σ where ¥* is a chromium- and molybdenum-depleted austenite, if compared to the original austenite. Precipitation occurs predominantly at grain boundaries, especially at triple points.

– In the case of duplex stainless steels, precipitation can be complete in a few hours and consumes all ferrite of the microstructure [59]. Precipitation in this case can be represented by a eutectoid-type reaction: α→ ¥* +σ where ¥* is a chromium- and molybdenum-depleted austenite if compared to a non transformed austenite. Precipitation starts at the a–g interface and moves into the ferrite grain.

– The quantity, speed, and probably the mode of the sigma-phase precipitation in ferritic stainless steels strongly depend on the steel composition, especially on the chromium and molybdenum contents. Increasing chromium and molybdenum levels displace precipitation start to shorter times and to higher temperatures.

Microstructures of ferritic stainless steel containing 24.5% Cr, 3.54% Mo, 3.90% Ni, 0.17% Nb,and 0.32% Al annealed at 850 C (1560 F). (a) Annealed 100 min. Arrow points to chi phase. (b)Annealed 300 min. Dominant second phase (etched gray) is sigma

Sigma-phase precipitation in aged samples (8508C for 30 h) of a superferritic stainlesssteel X 1 CrNiMoNb 28 4 2 (W. Nr. 1.4575). Etched with V2A-Beize

Chi Phase– Chi (x) phase was identified, for the first time, by Andrews in

1949 in residues extracted from the Cr–Ni–Mo steel. Later, Kasper synthetically produced the chi (x) phase with the Fe36Cr12Mo10 composition and studied its crystal structure in detail.

– Chi ( x) phase, for example, may occur also in austenitic, ferritic, and duplex (ferritic– austenitic) stainless steels and its precipitation is associated with negative effects on properties. While sigma phase is present in the binary Fe–Cr system, chi phase appears only in the Fe–Cr–Mo ternary and in the Fe–Cr–Ni–Mo and Fe–Cr–Ni–Ti quaternary systems. Still in comparison with the sigma phase, chi ( x) phase is richer in molybdenum and poorer in chromium.

– The occurrence of (x)-phase in ferritic stainless steels is conditioned to a minimum in the molybdenum content

Effect of molybdenum on the sigma (s)- and chi (x)-phase formation in the Fe–28 wt% Cr–Mo system

The 475 oC Embrittlement – It is caused by the presence of the alpha prime phase in the

300 to 550 oC temperature range. This phase contains mainly chromium and iron, richer in chromium than in iron, as shown in the Fe–Cr The alpha prime phase has a bcc structure and is coherent with ferrite. The alpha prime precipitates are small, in the range of 20 to 200 A ° . They have a high coarsening resistance, even for long exposure times. The alpha prime precipitates contain essentially chromium and iron. These two atoms show very similar atomic sizes, x-ray, and electron scattering amplitudes

– Hardness, yield, and tensile strength are increased, while elongation and impact resistance are decreased by the presence of alpha prime. Ferrite without alpha prime presents wavy glide lines because of the numerous gliding systems in the bcc structure and its high stacking fault energy facilitating dislocation cross-slip. The presence of alpha prime changes this situation, it makes the dislocation movement difficult, restricting slip to a few crystal planes. This causes some straight slip lines, typical of fcc and low stacking fault energy alloys, such as in austenitic stainless steels or brass

– Alpha prime containing ferrite predominantly deforms by twinning and that the straight deformation lines mentioned above are, in reality, deformation twins. Ferrite embrittled due to the presence of alpha prime, in general, presents a cleavage type brittle fracture at room temperature.

– Corrosion resistance is also affected by the presence of alpha prime. Bandel and Tofaute observed that the presence of alpha prime significantly reduced the corrosion resistance in a solution of boiling nitric acid. Pitting corrosion resistance, determined by cyclic polarization tests, in a solution of 3.5 wt% NaCl, is also significantly reduced by the presence of alpha prime. It is important to remember that super ferritic stainless steels have as one of their main features an excellent resistance to pitting corrosion in seawater.

– The magnitude of the effects of alpha prime on the properties depends chiefly on the chromium content of the alloy and it increases with an increase in chromium content

Martensitic Stainless SteelCharacteristics– Main alloying elements are Cr(12-17%),Mo(0.2-

1%),no Ni except for two grades & 0.1-1.2%C– Follows relationship %Cr-17%(C)<=12.7%– These steels are austenite at a temp of 950-1000 oC

but transform to martensite on cooling– Increasing the C content increases the strength &

hardness but decreases ductility & toughness

Basic Properties– Less resistant to corrosion in comparison to the other

grades of steel– Can be hardened by heat treatment– High strength & hardness levels can be achieved – Poor weldability– Magnetic in nature– Yield strength of 550-1860Mpa– Poor machinability– As Cr content increases hardenability increases– Have improved toughness

Microstructure of annealed martensitic stainless steels. Fine particles are spheroidized carbides. (a) Type 403 stainless steel etched in 4% picral-HCl. (b) Type 416 stainless steel etched with Vilella’s reagant. Arrows point to sulfide particles for machinability

Projection of the ternary Fe–Cr–C diagram on the temperature x% Cr (in wt%) plane

Influence of nickel on the extent of the g-loop in ternary Fe–Cr–Ni alloys (in wt%).

Effect of carbon and nitrogen on gamma loop in Fe-Cr alloys

Martensitic stainless steels may be subdivided into three subgroups: (a)low-carbon steels for turbines(b) medium-carbon steels for cutlery(c) high-carbon wear-resistant steels

The microstructure of each group is also characteristic(a)Martensitic needle-like microstructure(b)Very fine martensitic microstructure(c) Ultrafine martensitic microstructure containing

primary carbides,

Low Carbon High Strength– C content is kept low ~ 0.1% to give good weldability,

formability & impact strength– Quenched in oil or air from around 1050 oC when fully

austenitic & then tempered. The tempering temperature is kept low for high tensile as well as yield strength with low toughness.

– Tempering range of 440-540 oC is avoided as it causes reduction in impact strength

– ApplicationsPetrochemical & chemical plant constructionGas turbine enginesTurbine blades Electrical generation plantsCompressor & discsAircraft structural & engine applicationsPropeller shafts in ships sailing in fresh water

High carbon High Strength– Strength & hardness can be increased by increasing

the C content of steels but decreases weldability, toughness & even corrosion resistance

– Increasing C increases the amount of carbides & thus higher austenising temperatures

– ApllicationsKnives, needle valves, gears, razor bladesSurgical instrumentsBall bearings for high temperature applicationsStainless steel bearings

AISI 410 martensitic stainless steel, quenched and tempered to 20 HRC. Microstructure of tempered martensite with fine-carbide precipitates

AISI 420 martensitic stainless steel. Microstructure of tempered martensite with intergranular and intragranular precipitates. Scanning electron microscopy using secondary electrons. Etched with Villela.

Duplex Stainless SteelCharacteristics– Main alloying elements Cr(23-30%) & Ni(2.5-7%) ,Mo

(2.5-4%) & Ti– These steels contain ferrite and austenite in its

microstructure. Thus combining the toughness & weldability of austenite with strength & resistance to localized corrosion of ferrite.

– Austenite phase as islands surrounded by ferrite phase

– On melting it solidifies from liquid phase to a completely ferritic structure

– At room temperature microstructure has roughly 50% of ferrite & asutenite each

Basic Properties– Stronger than austenitic steels due to

two phase structurepresence of delta ferrite causes grain refinement causing

strengtheningrefinement in grain structure due to thermo-mechanical

treatment– Good corrosion resistance– Freedom from transgranular stress corrosion cracking– Good weldability but micro duplex structure destroyed in HAZ

which decreases strength as well as stress corrosion resistance– Duplex steels have ductile brittle transition temperature due

the presence of ferrite – Suffer from embrittlement effect

Duplex Applications

– Chemical & Petrochemical industry

– Pharmaceutical industry

– Pulp & paper industry

– On/offshore industry– Marine chemical tankers

– Food & Beverage Industry– Architecture, Building, Construction

Combining Ferrite & Austenite forms the “Duplex” structure

FERRITIC AUSTENITIC DUPLEX

Solidification of DSS

Correct balance of Ferrite & Austenite

Austenite

Ferrite

Microstructure of duplex steels

Ferrite (F) and austenite (A) grains in duplex stainless steel Al 2205 (UNS 531803). Transmission electron micrograph

Three-dimensional composed micrograph of rolled duplex stainless steel. Optical microscopy. Ferrite is the darker phase. Etched with Behara II.

Embrittlement EffectsDuplex stainless steels are susceptible to three types of embrittlement :

1. Embrittlement caused by the presence of a carbide network, particularly in the austenite, in alloys with higher carbon content

2. Embrittlement caused by precipitation of the α ’- phase, 475 oC embrittlement of ferrite

3. Embrittlement caused by precipitation of the σ-phase, particularly in the ferrite

In the case of duplex steels with higher carbon content, the first phase that solidifies is also ferrite. The residual liquid enriched in carbon solidifies forming austenite and a chromiumrich M23C6-type carbide network. This carbide network within the austenite leads to an improved wear resistance. During cooling, austenite islands can also form in the ferrite grains.Some secondary carbide may also precipitate in austenite in the solid state. With the application of mechanical stresses, these carbides initiate fractures and cracks that propagate along the carbide network

Crack propagation along carbide network in the high-carbon duplex stainless steel

The schematic TTT diagram shows the regions in which alpha prime ( α’) and sigma ( σ) precipitation can occur. These precipitates increase the hardness and decrease ductility and the toughness. It must be pointed out that σ-phase precipitates within the ferrite . In comparison to austenitic and ferritic stainless steels, precipitation of σ-phase in duplex alloys occurs at shorter times, at higher temperatures and larger volume fractions may be formed

Schematic TTT diagram showing precipitation of sigma (σ), alpha prime (α’), and other phases in duplex stainless steels

Sigma phase precipitation in duplex stainless steels observed using scanning electron microscopy with secondary electrons. Etched with V2A-Beize. A¼austenite; F=ferrite; S=sigma phase. (a) Low-carbon duplex stainless steel (b) High-carbon duplex stainless steel

Time

Tem

pera

tur

e

+N -Phase

Influence of Nitrogen on Sigma Phase

600º C

950ºC

Sigma nose

Too slow cooling rate

• Risk for intermetallic phase formation– Reduces pitting and impact properties

The role of Nitrogen

Nitrogen is a very important alloying element in DSSImproves corrosion resistanceImproves austenite reformation

During TIG welding operation, the loss of nitrogen is compensated for by using Ar + 1 - 2%N2 as a shielding gas

Phase Imbalance

• High ferrite (> 70%):– low ductility– loss of corrosion

resistance– H2 cracking susceptibility

• High austenite (> 80%):– low SCC resistance– low strength

Avoid imbalance

The importance of the “Duplex Road”

Must stay on the road !!!

Intermetallics when Welding DSS

– Sigma Phase formation will occur as a result of the interpass temp exceeding it’s limit of 150º C - 250º C for prolonged periods.

– Secondary Austenite is a result of TIG “HOT” pass - second layer over heating the root weld

– This will result in loss of corrosion resistance & lowers ductility in the weld

Chrome Nitrides

– Result from rapid cooling

– Can result in excessive ferrite content and chromium nitrides– Which will reduce pitting resistance

Chromium Nitrides

Precipitation Hardenable Steels

Precipitation hardenable stainless steel is the special class Martensitic or austenitic type, modified by addition of alloying elements like Al to form hard intermetallic compound during temperCharacteristics– Can be supplied in a solution treated condition in

which they are machinable & can be hardened– Used for high strength to weight ratio applications– Matrix in precipitation hardenable stainless steel

could be austenite or martensite– Hardening is obtained after adding one or more

alloying elements

Types of precipitation hardenable steels

Types of steelsmartensitic ,semi-austenitic & austenitic

Lath martensite microstructure of hardened type 403 stainlesssteel. 4% picral-HCl etch

Solution-treated and aged microstructure of martensitic precipitation- hardening stainless steel PH 13-8 Mo. Etched in Fry’s reagent

Microstructure of 17-7 PH. (a) Surface tilting caused by martensite formation on refrigeration to-730C (100 F). (b) Refrigerated and aged at 4800C (896 F). Electropolished and etched in chrome acetic acid electrolyte

Fine, disc-shaped ý precipitates in an aged austenitic precipitation- hardening stainless steel

Martensitic alloysPredominantly austenitic structure at annealing temperature of around 1040-1065 0C. Upon cooling to room temperature austenite changes to martensite

Semi –Austenitic alloysThese are soft enough to be cold worked. They retain their austenitic structure at room temperature but will form martensite at very low temperature.

Austenitic alloysRetain their austenite structure after annealing & hardening by ageing. Precipitate hardening phase is soluble. Hardness is lower than the other two & remain non magnetic

Typical P-H Stainless Steel

– 17-7 Stainless Steel. – Low carbon (<0.09%C).

17% Cr. 7% Ni.– Quenched and Aged.

Sometimes Cold-worked aged.

– Yield: 1590 Mpa. 231 Ksi.

– UTS: 1650 Mpa, 239 Ksi.– Ductility: %EL = 1%.– Uses: High strength high

temperature applications. Chemical processing equipment, heat exchangers, boiler tubes.

http://www.superior-ny.com/mk41.htm

http://www.superior-ny.com/mk41.htm

Grade 630 martensitic precipitation hardening stainless steel has a combination of high hardness and strength after suitable heat treatment. It also has similar corrosion and heat resistance to Grade 304. The terms "Type 630" and "17-4PH" refer to the same grade.

The great benefit of this grade (and of other precipitation hardening grades of stainless steel) is that they are generally supplied in the solution treated condition, in which they are just machinable, and then can be age hardened to achieve quite high strengths. This aging treatment is so low in temperature that there is no significant distortion. These grades are therefore well suited to production of long shafts, which require no re-straightening after heat treatment.

http://www.atlasmetals.com.au/files/ASM%20Grade%20Datasheets/Atlas%20Grade%20datasheet%20630%20rev%20May%202008.pdf



Mechanical properties as a function of tempering temperature of type 410 stainless steel. Data on left is for specimens austenitized at 925 C (1697 F) and data on right is for specimens austenitized at 1010 C (1850 F). All specimens oil quenched between 65 and 95 C (149 and 203 F), stress relieved at 175 C (347 F), and tempered for 2 h

The effect of adding alloying elements

“Austenite”

Cr

Ni

Mo

Fe

Interstitial ElementsC, N, B etc.

Substitutional Elements

Influence of Alloying Elements

Carbon - C - austenite (¥) former

– Low carbon (<0.03%) improves weldability – High affinity with Chromium - Forms chrome

carbides (Cr23C6) resulting in risk of intergranular corrosion

– Improves creep strength in high temperature alloys (this can also be achieved with Nitrogen)

– In martensitic stainless steels high C (0.15-1.20%) makes them hardenable

Chromium - Cr - Ferrite (α) former

– Main alloying element in Stainless Steel– >12% for self healing passive oxide

film– Improves pitting resistance– risks formation of sigma (d) phase

embrittlement

Nickel - Ni - Austenite (¥) former

– >8% Nickel in a Chromium steel, structure usually becomes austenitic

– Great effect on – mechanical properties - toughness

– Improves resistance to– general corrosion– stress corrosion cracking

– Beneficial for strength at high temperatures

Molybdenum - Mo - Ferrite (α) former

– Greatly improves the resistance to general corrosion in most media

– Greatly improves resistance to pitting– PRE = %Cr + (3.3 x %Mo) + (16 x

%N)– Beneficial for strength at elevated

temperatures– Promotes formation of sigma (σ) phase

Titanium - Ti, Niobium - Nb - Ferrite ( formers

– Stabilising elements (5 x %C for Ti, 10 x C for Nb)

– Higher affinity with C than Cr– Obstructs formation of Cr carbides– T321 - Ti, T347 - Nb, T316Ti

Copper - Cu - Austenite () former

– Reduces Hardness and Ultimate Tensile Strength

– Lowers rate at which austenite hardens with cold work & and the temperature to soften it.

– In Molybdenum alloyed steels, improves general, pitting and crevice corrosion resistance

– Improves resistance to sulphuric & phosphoric acids

Nitrogen - N - Austenite () former

– Added in contents of 0.15%-0.3%– Improves strength and corrosion resistance of

austenitic and duplex stainless steels– Improves pitting resistance– Reduces tendency to form sigma phase

embrittlement– Duplex (weldability) and high temperature alloys

– Reduces grain growth in high temperature alloys (4C54) at elevated temperatures

Silicon - Si - Ferrite (α) former

– Positive effect on resistance to high temperature corrosion

– Increases the tendency to form sigma (§)phase embrittlement

– Increases risk of hot cracking during welding– In welding improves the fluidity of the

weldmetal

Manganese - Mn - Austenite (¥) former

– Little influence on corrosion resistance– Improves dissolution of N2 (in steelmaking)– Combines with sulphur to form Manganese

Sulphide (MnS)– MnS improves chip forming properties in

free machining grades e.g. T303– MnS has a negative effect on pitting

resistance

Sulphur - S

– Usually below 0.03%– Increases risk of hot cracking during welding– Improves machinability - refer Manganese

Phosphorus - P

Usually below 0.040% Increases risk of hot cracking during welding

Aluminium - Al - Ferrite (¥) former

– Improves oxidation resistance at high temperatures– Sanicro 31HT, Kanthal APM, Inconel 601

– Forms aluminium carbonitrides in precipitation hardening steels to increase hardness and tensile at elevated temperatures

Rare Earth MetalsCereium - Ce, Lanthanum - La

– Increase oxide strength and oxidation resistance

– Strong affinity with sulphur– Refines grain boundary structure by

removing sulphur and dissolved gases– Improves resistance to high temperature

sulphur containing atmospheres forming oxy-sulphides instead of Ni-NiS (650oC)

– Causes arc instability during welding

Tungsten - W - Ferrite (α) former

– Strong carbide forming tendency– Strong tendency to form sigma § phase

embrittlement in super duplex stainless steels (Zeron 100)

– Contributes significantly to hardenability and high temperature creep strength

Stress strain curvesAustenite, ferrite and duplex

0,0 0,2 0,4 0,6 0,80

200

400

600

800

1000 austenite duplex (2205) ferrite

Str

ess

[MP

a]

Strain

ferrite

duplex

austenite

• Strength at temperature

Impact Resistance

442

Objective

The objective of this lecture is to describe the relationship between precipitation and hardness as an example of a key microstructure-property relationship.

443

Age Hardening Curves

– The most quoted age hardening curve is that for Al-Cu alloys performed in the late 40s. Keep in mind that age hardening was known empirically (Alfred Wilm) as a technologically useful treatment from the early days of aluminum alloys.

– Higher Cu contents result in higher maximum hardnesses because larger volume fractions of precipitate are possible.

444

Al-Cu precipitation sequence• The sequence is:

a0 a1 + GP-zones a2 + q“ a3 + q’ a4+ q

• The phase are: an - fcc aluminum; nth subscript denotes each equilibriumGP zones - mono-atomic layers of Cu on (001)Al

q“ - thin discs, fully coherent with matrix q’ - disc-shaped, semi-coherent on (001)q’ bct. q - incoherent interface, ~spherical, complex body-centered tetragonal (bct).

445

Al-Cu ppt structures

GP zone structure

446

Al-Cu microstructuresThis tableau shows which of the different ppt types are associated with which part of the hardening curve.

GP zones

q”

q’

q

447

Al-Cu driving forces

• Each precipitate has a different free energy curve w.r.t composition. Exception is the GP zone, which may be regarded as continuous with the alloy (leading to the possibility of spinodal decomposition, discussed later).

• P&E fig. 5.27 illustrates the sequence of successively greater free energy decreases and also successively greater ∆G*.

• P&E fig. 5.28 illustrates the point that the nucleation barriers are much smaller for each individual nucleation step when the next precipitate nucleates heterogeneously on the previous structure.

448

Al-Cu phase relationships

The explanation of age hardening depends on understanding the metastable phases that can appear.

449

Nucleation sites, reversion

• The nucleation sites vary depending on circumstances.• q“ most likely nucleates on GP zones by adding additional

layers of Cu atoms.• Similarly, q’ nucleates on q“ by in-situ transformation.• However, q’ can also nucleate on dislocations, see P&E fig.

5.31a.• The full sequence is only observable for annealing

temperatures below the GP solvus. Any of the intermediate precipitates can be dissolved, reverted, by increasing the temperature above the relevant solvus, fig. 5.32.

450

Al-Ag: example 2

The age hardening curve has the same double peak as for the Al-Cu series, but the separation is more pronounced.

Shewmon

451

Al-Ag, contd.• GP zones are spherical (Ag atom is larger than Al).• g’ is hcp with OR (0001)g//(111)a and [1120]g //[110]a;

heterogeneously nucleated on the stacking faults of dislocations which provide sites of local hexagonal packing.

• g is also hcp with the same OR; forms plate-like precipitates. A cellular mechanism can also occur.

Shewmon

452

Age hardening in steel: example 3

• It is important to understand that age hardening occurs in almost any system in which the solid solubility decreases appreciably with decreasing temperature. Ferrite has a very low solubility for carbon and therefore age hardening (also called quench hardening) occurs here too. To avoid it, the soluble carbon levels must be reduced, which is a common objective of the IF or interstitial-free steel grades. These have additions of carbide formers such as Ti or Nb to sequester the C.

Shewmon

453

0 10 20 30 40 50wt% Cu

L+La

+a q q

+L

300

400

500

600

700

(Al)

T(°C)

composition range needed for precipitation hardening

CuAl2

A

Adapted from Fig. 11.24, Callister 7e. (Fig. 11.24 adapted from J.L. Murray, International Metals Review 30, p.5, 1985.)

PRECIPITATION HARDENING

• Particles impede dislocations.• Ex: Al-Cu system• Procedure:

Adapted from Fig. 11.22, Callister 7e.

--Pt B: quench to room temp.--Pt C: reheat to nucleate small q crystals within a crystals.

• Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al

Temp.

Time

--Pt A: solution heat treat (get a solid solution)

Pt A (sol’n heat treat)

B

Pt B

C

Pt C (precipitate )

HEAT TREATING ALUMINUM

Solution Treat

Quench

Age

f11_22_pg403

F11_22_PG403

f11_23_pg404

F11_23_PG404

457

• 2014 Al Alloy:

• TS peaks with precipitation time.• Increasing T accelerates process.

Adapted from Fig. 11.27 (a) and (b), Callister 7e. (Fig. 11.27 adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, 1979. p. 41.)

PRECIPITATE EFFECT ON TS, %EL

precipitation heat treat timetensi

le s

trength

(M

Pa)

200

300

400

1001min 1h 1day 1mo1yr

204°C

non-

equi

l.

solid

sol

utio

nm

any

smal

l

prec

ipitat

es

“age

d”

few

er la

rge

prec

ipitat

es

“ove

rage

d”149°C

• %EL reaches minimum with precipitation time.

%EL

(2 in s

am

ple

)10

20

30

0 1min 1h 1day1mo1yr

204°C 149°C

precipitation heat treat time

AGING AND OVERAGING

After quenching, there is thermodynamic motivation for precipitate to form.

Precipitates initiate and grow due to diffusion, enhanced by higher temperatures.

To get significant strengthening the precipitate should be coherent

When the precipitates get too large, they lose coherence and strengthening decreases (overaging)

f11_27_pg406

F11_27_PG406

f11_25_pg405

F11_25_PG405

462

Cutting versus Bowing• At small sizes, the dislocation cuts through the particle at a

lower stress than the Orowan bowing stress (and so this is what is observed). Larger particles mean higher cutting stresses.

• At large sizes, the dislocation bows around the particle more easily than it cuts through it (so no cutting is observed). Larger particles mean fewer particles (via coarsening) hence lower flow stresses.

Particles becoming strongerFewer and fewer particles, further apart

463

Breaking Angle: fc

Strong Obstacles: f ~ 0° Weak Obstacles: f ~ 180°

Gb

L cos

c

2

Courtney

464

Hardness -microstructure relationships• In order to understand the relationship between

microstructure and hardness, we need to delve into the subject of hardening mechanisms.

• The central concept is that the strength of a ductile material is governed by dislocation flow past obstacles. Therefore strength can be designed by controlling the density and nature of the obstacles to dislocation motion. Most technological (metallic) alloys rely on precipitation hardening in one form or another to achieve high strengths. Ceramics, on the other hand, are intrinsically harder and therefore the main objective of strengthening is to increase their fracture toughness and thereby increase their (reliable) load carrying capacity. The objective of this discussion is therefore to bring your attention to a number of ways in which we can understand and predict the contributions to strength of different types of obstacle.

465

Strengthening Methods

• Microstructural Feature: strength dependence.

• Dislocations: strain/work hardening (discussed in 301): (dislocation spacing)1/2.

• Internal Boundaries: grain boundaries can have a strong strengthening effect, i.e. the Hall-Petch effect (discussed in 301): (grain size)-1/2.

• Dislocation Boundaries (low angle boundaries): (subgrain size)-1.

• Second Phase Particles: particle spacing.• Solutes: (concentration)1/2.

466

Mechanisms of particle strengthening

1) Coherency Hardening: differences in density between the particle and the matrix give rise to elastic stresses in the vicinity of the particle.

2) Chemical Hardening: creation of new surface when a particle is sheared increases the area of the interphase boundary, which increases the energy associated with the interface and hence an additional force must be exerted on the dislocation to force it through the particle.

3) Order Hardening: passage of a dislocation through an ordered particle, e.g. Ni3Al in superalloys, results in a disordered lattice and the creation of antiphase boundaries.

4) Stacking-fault Hardening: a difference in stacking fault energy between particle and matrix, e.g. Ag in Al, increases flow stress because of the different separation of partial dislocations in the two phases.

5) Modulus Hardening: a large difference in elastic modulus results in image forces when a dislocation in the matrix approaches a particle. Consider, e.g., the difference between silver particles (nearly the same shear modulus) and iron particles (much higher shear modulus) in aluminum.

467

Dislocations

A re-statement of the governing equation for strength controlled by obstacle spacing:

M 0 Gb / Parameter Description Comments

flow stress Experimentally accessible through mechanicaltests

<M> Average Taylor factor Magnitude ~3 for tension or compression;depends on the nature of the deformation, the

texture and the crystal structure, e.g. <M>~1.73for torsion (cubic metals)

Athermal stress Contributions from grain size hardening,solutes, etc.

Geometrical factor This term accounts for both geometrical factors,and for thermal activation

G Shear Modulus Must choose appropriate shear modulus for theslip plane used;

Temperature dependentb Burgers vector Derived from the force on a dislocation (Peach-

Koehler Eq.)¦ dislocation density Equivalent to the reciprocal of a mean obstacle

spacing; depends on work hardening obstacle spacing Given a number density of particles, the mean

spacing, =N1/2

468

crss versus density

Courtney

469

Dislocation Boundaries• At large strains and higher temperatures, low angle

boundaries appear as a subgrain network forms. We distinguish this microstructural feature from the first two categories because the [lattice] misorientations are much smaller (2-5°) than grain boundaries (15°+) and they are distinct from statistically stored dislocations. This strengthening method is most important at high temperatures where other microstructural features such as solutes are weak.

• The contribution to the flow strength is typically found to be proportional to (grain size)-1; this is in contrast to the 1/√d dependence of the Hall-Patch effect.

470

Solutes

Solutes in a crystal act as obstacles to dislocation motion through their elastic and/or chemical interactions with dislocations. Most solutes are weak hardeners except for the (technologically) important class of interstitial solutes that induce anisotropic distortions of the lattice, e.g. tetragonal distortions of C in Fe.

471

Substitutional solutes• Most Solutes have only a rather weak effect on strength. In

other words, even if you put several per cent of a soluble atom into another element, you will not see a dramatic increase in flow stress. These remarks can be quantified by going back to the Orowan equation, i.e. the force balance between the forward motion and the resisting force:

tcrss = µb/l.

• For substitutional solutes, the numerator in the RHS, i.e. the reaction force from the solute atoms is of order Gb2/120, which is a small number. This is so because the small differences in size between solute and matrix atoms results in a small interaction energy with dislocations. Thus they are weak obstacles and dislocations remain nearly straight when interacting with solutes (“weak obstacles”, 7slides before this).

472

Interstitial solutesInterstitials in bcc, however, can exert forces on the order of Gb2/5 to Gb2/10, which are large values. In this case, the dislocations bow out significantly between the atoms, and the breaking angle deviates significantly from 180°. In this case, the concentration dependence is easy to obtain. The spacing between interstitials is inversely proportional to the (square root of the) concentration, and so we can insert a spacing into the standard (Orowan bowing) formula to get the following, where A is a constant of order unity:

t = AGb(√c/b) = AG√c.

473

Strength vs. solute content: examples

Examples:

a) Substitutional solutes in Cu

b) Interstitial solutes in Nb, Fe

c) Ca in NaCl

[Courtney]

474

Second Phase Particles

• Whether introduced as insoluble particles in powder compaction, or as precipitates in a solid state reaction, second phase particles are generally the most potent strengthening agent in practical high strength engineering materials. Iron-base, aluminum, nickel, titanium alloys all employ second phases to achieve high strength.

• Quantitative relationships: from previous stereological analysis (301 - lecture 4):

L3 4r

3; L3

1 VV()

VV( ) ;

4r

3

1 VV()

VV()

4r 1 f 3 f

4r

3 f

475

Coherency hardeningDifferences in density between the particle and the matrix give rise to elastic stresses in the vicinity of the particle. This has been analyzed on the basis of the elastic stresses that exist in the matrix adjacent to a particle that has a different lattice parameter than the matrix. Ignoring differences in modulus for now, and setting a parameter, e, that approximates a strain to characterize the magnitude of the effect. For

e = (aparticle – amatrix )/ amatrix

t = 7|e|3/2 G(rf/b)1/2

• This mechanism applies to the early stages of precipitation, e.g. strengthening by GP zones.

476

Chemical hardeningCutting through a particle with a dislocation displaces one half relative to the other by b, thereby creating new interfacial energy of 2πrbg, where g is the interfacial energy between the matrix and the particle. The distance over which this energy has to be created occurs at the entry and exit points and so the characteristic distance is of order b. Thus the force is dE/dx, or,

F = 2πrbg/2b = πrg

477

Chemical hardening, contd.• If the dislocations are straight, we can approximate the spacing

between particles as L=2r/f. Dividing the force by bL to find the stress,

t = πfg/2b.

• A more realistic approach produces the following relationship.

t = 2G{g/Gr}3/2(fr/b)1/2

• Courtney defines a chemical hardening parameter, ech = g/Gr, related to the interfacial energy, modulus and particle size. This parameter is precisely analogous to the same parameter used, e.g. in APB hardening. Chemical hardening applies only in the early stages of precipitation.

478

Order Hardening• The hardening depends on the product of the antiphase-

boundary energy (APBE) and the area swept by a dislocation in a particle. Thus the increase in flow stress is given by:

t = πf{APBE}/2b• In general, low values of the APBE not only predict small

increments in hardness, but also the result that the dislocations can move through the particles independently of one another. A more detailed analysis, not presented here, shows a square root dependence on volume fraction, with particle size,

t = 0.7 Ge3/2 √(fr/b)

eord= APBE/Gb

• Important for Ni-based superalloys

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Modulus hardening• The line length in the particle is 2r and the change in tension is

(Gparticle-Gmatrix)b2/2, assuming the same Burgers vector in matrix and particle. Multiplying the two together and dividing by the distance, i.e. the radius, we get:

F= b2(Gparticle-Gmatrix) = Gb2e,

where e = (Gparticle-Gmatrix)/Gmatrix, a measure similar to that used in solution hardening.

• More realistic estimates of modulus hardening lead to the following formula:

t = 10-2 G e3/2 √{fr/b}

• Think of modulus hardening as being caused by a temporary increase in dislocation line energy while it resides within a particle.

Hardenability--Steels

• Ability to form martensite• Jominy end quench test to measure hardenability.

• Hardness versus distance from the quenched end.

24°C water

specimen (heated to g phase field)

flat ground

Rockwell Chardness tests

Har

dnes

s, H

RC

Distance from quenched end

• The cooling rate varies with position.

Why Hardness Changes with Position

distance from quenched end (in)Ha

rdn

ess

, H

RC

20

40

60

0 1 2 3

600

400

200A ® M

A ®

P

0.1 1 10 100 1000

T(°C)

M(start)

Time (s)

0

0%100%

M(finish) Martensite

Martensite + Pearlite

Fine Pearlite

Pearlite

Hardenability vs Alloy Composition

• Jominy end quench results, C = 0.4 wt% C

• "Alloy Steels" (4140, 4340, 5140,

8640) --contain Ni, Cr, Mo (0.2 to 2wt%) --these elements

shift the "nose". --martensite is

easier to form.

Cooling rate (°C/s)

Har

dne

ss, H

RC

20

40

60

100 20 30 40 50Distance from quenched end (mm)

210100 3

4140

8640

5140

1040

50

80

100

%M4340

T(°C)

10-1 10 103 1050

200

400

600

800

Time (s)

M(start)M(90%)

shift from A to B due to alloying

BA

TE

Equivalent distance and Bar diameter

(Quenched in water) (Quenched in oil)

Radial hardness profile

(Quenched in water) (Quenched in oil)

Heat treatment : the best one

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