TTT Diagram & Heat Treatment

Associate ProfessorDept. of Mech. Engg.

RajaRajeswari College of Engineering

Thanuj Kumar M.

TTT Diagram &

Heat Treatment

Experimental study of getting TTT diagram for 08% C in Steel

7270CQuenching in

Water

Molten Salt Bath

7000C

Heat Treatment

Time Temperature Transformation (TTT) Diagram: TTT diagram is a plot of temperature versus the logarithm of

time for a steel alloy of definite composition.

It is used to determine whentransformations begin andend for an isothermal heattreatment of a previouslyaustenitized alloy.

TTT diagram indicateswhen a specifictransformation starts &ends.

It also shows what % oftransformation of austeniteat a particular temperatureis achieved.

Depending on the type of heat treatment, time and temperature,final microstructure of the steel, or any Iron-Carbon will bechanged . The properties also does change .Ex: Strength and Ductility

Stre

ngth

Duc

tilit

y

Martensite

Tempered Martensite

Bainite

Fine Pearlite

Coarse Pearlite

Spheroidite

GENERAL TRENDS

Water Cooled Oil Cooled

Normalizing

Annealing

Bainite

Pearlite

Martensite

Austenite

Iron-Carbon Alloy with Eutectoid (0.8 % Carbon) composition.

M (start)

M (Finish)

TTT-Diagram for

25

Iron-carbon alloy witheutectoid composition.

Alloy begins to cool from760˚C and has been heldlong enough to achieve acomplete andhomogeneous austeniticstructure.

Nature of the finalmicrostructure of Austenite,Pearlite, Bainite &Martensite seen at variousTime-Temperature &Transformation Diagram.

Austenite

Pearlite

Bainite

Martensite

100% Bainite

Process: Alloy is held at 760˚C to

achieve complete and

homogeneous austenite

microstructure.

Rapidly cool to 350˚C, where

Austenite is unstable.

Hold for 104 seconds for

change in microstructure.

Quench to room temperature

To get 100% Bainite.

Formation of 100% Bainite in TTT Diagram

25

100% Martensite

100% Austenite

25

Formation of 100% Austenite & 100% Bainite in TTT Diagram

Process:

Alloy is held at 760˚C to

achieve complete and

homogeneous austenite

microstructure.

Rapidly cool to 250˚C, where

100% Austenite (unstable) is

formed

Hold for 102 seconds for

change in microstructure.

Quench to room temperature

to get 100% Martensite.

Austenite, 100%

Almost 50% Pearlite,

50% Austenite

Bainite, 50%

Final: 50% Bainite, 50% Pearlite

Formation of 100% Austenite, 50% Pearlite & 50%Process: Alloy is held at 760˚C to

achieve complete andhomogeneous Austenitemicrostructure.

Rapidly cool to 650˚C, where100% Austenite (unstable) isformed.

Hold for 20 seconds to get50% Austenite & 50%Pearlite.

Rapidly cooled to 400˚C and Hold for 103 Seconds to get

50% Bainite. Quench to room temperature

to get 50% Bainite, 50%Pearlite.

25

Complete Isothermal

Transformation diagram for

EUTECTOID composition

With

Continuous Cooling

Transformation diagram-CCT

Superimposing of TTT Diagram with CCT Diagram

AVERAGE OF TEMP. IS TAKEN IN TO

ACCOUNT

A- fine Pearlite

B- M + little AuC D

Transformation starts/begins

Transformation ends

A - 50C/sec(normalising)

B - Rapid 4000C/sec

(water quench)

C - 1400C/sec- Martensitic

D - 500C/sec- P + M + Au

EFFECT OF DIFFERENT COOLING RATES

Continuous Cooling Transformation Diagram

Continuous cooling Transformation

Heat TreatmentIntroduction: Heat treatment involves the use of heating or

chilling, normally to extreme temperatures, to

achieve a desired result such as hardening or

softening of a material.

Heat treating is a metalworking processes used to

alter the physical, and sometimes chemical,

properties of a material.

The most common application is metallurgical.

Improvement in Ductility

Relieving Internal Stresses

Grain Size Refinement

Increase of Strength & Hardness

Improvement in Machinability

Improvement Toughness

Purpose of Heat Treatment:

Factors Affecting:

Temperature up to which material is

heated.

Length of time that the material is held at

the elevated temperature.

Rate of cooling.

The surrounding atmosphere under the

thermal treatment.

These differ mainly in the way material is cooled froman elevated temperature.

Note:

HEAT TREATMENTHEAT TREATMENT

BULK SURFACE

ANNEALING

Full Annealing

Recrystallization Annealing

Stress Relief Annealing

Spheroidization Annealing AUSTEMPERING

THERMAL THERMO-CHEMICAL

Flame

Induction

LASER

Electron Beam

Carburizing

Nitriding

Carbo-nitriding

NORMALIZING

HARDENING &

TEMPERING

MARTEMPERING


Spheroidization


Full Annealing

Normalization

723

EFFECTS OF HEAT TREATMENT

Annealing & Normalizing Hardening or Quenching

Furnace Cooling Air

Cooling Oil

Quenching Water

Quenching

Softer, less strong Harder and stronger

More ductile More brittle

Less internal stress More internal stress

Less distortion, cracking More distortion, cracking

Medium

Air

Oil

Water

Severity of Quench

Small

Moderate

Large

Hardness

Small

Moderate

Large

Effect of Quenching Medium

The severity of quench: Water > Oil > Air

During annealing, material is cooled in air and/or heating furnace itself.

For hardening, material is immersed in water / oil quench bath.

ANNEALING METALS

Annealing process of heating the material to apredetermined temperature and hold thematerial at the temperature and cool thematerial to the room temperature slowly so thatits particles arrange into a defined lattice.

The process involves:

1) Heating of the material at the elevated orpredetermined temperature

2) Holding the material (Soaking) at thetemperature for longer time.

3) Very slowly cooling the material to the roomtemperature.

This treatment alters the microstructure of amaterial causing changes in properties such asstrength, hardness, and ductility.

ANNEALING

The various purpose of Annealing are:

1) Relieve Internal Stresses developed during

solidification, machining, forging, rolling or

welding,

2) Improve or restore ductility and toughness,

3) Enhance Machinability,

4) Eliminate chemical non-uniformity,

5) Refrain grain size,

6) Reduce the gaseous contents in steel.

ANNEALING

ANNEALING

TYPES OF ANNEALING

Full Annealing

Process Annealing


Re Crystallization Annealing

Spheroidise Annealing

Full Annealing

Heating the steal to a temperature at or near thecritical point, holding there for a time period andthen allowing it to cool slowly in the furnace itself.

Ex: In full annealing of hypo eutectoid steels lessthan 0.77% is heated to 723 to 9100 C above A3line convert to single phase austenite cooledslowly to room temperature .

Resulting structure is coarse Pearlite with excessof Ferrite.

It is quite soft and more ductile.

It has low hardness,

Cooling rate of full annealing is 30-400 C

A1

A3

Acm

T

Wt% C0.8 %

723C

910C

Full Annealing

Full Annealing

During any cold working operation (say cold rolling), the

material becomes harder (due to work hardening), but

loses its ductility. This implies that to continue

deformation the material needs to be re-crystallized

(wherein strain free grains replace the ‘cold worked

grains’).

Hence, re-crystallization annealing is used as an

intermediate step in (cold) deformation processing.

To achieve this the sample is heated below A1 and held

there for sufficient time for re-crystallization to be

completed.


A1

A3

Acm

T

Wt% C0.8 %

723C

910C


PROCESS ANNEALING In this treatment, steal

(or any material) isheated to a temperaturebelow the lower criticaltemperature, and isheld at this temperaturefor sufficient time andthen cooled.

Cooling rate is of littleimportance as theprocess is being done atsub criticaltemperatures.

The purpose of this treatment is to reduce hardness and to

increase ductility of cold-worked steel so that further

working may be carried easily. This process is extensively

used in the treatment of sheets and wires.

Parts which are fabricated by cold forming such as

stamping, extrusion, upsetting and drawing are frequently

given this treatment as an intermediate step.

Scaling or oxidation can be prevented or minimized by

this process specially if, annealed at lower temperatures or

in non-oxidizing areas.

Stress Relieving As the name suggests, this process is employed to relieve

internal stresses.

Internal stresses are those stresses which can exist within a

body in the absence of external forces. These are also

known as residual stresses are locked-in stresses.

No micro-structural changes occur during the process.

These stresses are developed in operations like:

Solidification of castings, welding, machining, grinding,

shot peening, surface hammering, cold working, case

hardening, electroplated coatings, precipitation and phase

transformation.

A1

T

Wt% C0.8 %

723C

910C


These internal stresses under certain conditions can haveadverse effects:ex: Steels with residual stresses under corrosive environmentfail with stress corrosion cracking.

Causes of Residual Stresses

1. Thermal factors

(e.g., thermal stresses caused by temperaturegradients within the workpiece during heating orcooling)

2. Mechanical factors

(e.g., cold-working)

3. Metallurgical factors

(e.g., transformation of the microstructure)

How to Remove Residual Stresses….?

Residual Stresses can be reduced only by a plastic deformation

in the microstructure.

This requires that the yield strength of the material be lowered

below the value of the residual stresses.

The more the yield strength is lowered, the greater the plastic

deformation and correspondingly the greater the possibility or

reducing the residual stresses.

The yield strength and the ultimate tensile strength of the steel

both decrease with increasing temperature.

Spheroidization Annealing Process:1. This is a very specific heat treatment given to

high carbon steel requiring extensive machiningprior to final hardening & tempering.

2. The main purpose of the treatment is to increasethe ductility of the sample.

3. Heat to just below Lower Critical Temperature.(about 650-7000C)

4. Cool very slowly in the furnace.5. Structure will now be spheroidite, in which the

Iron Carbide has ‘Balled up’.6. Used to improve the properties of medium and

high carbon steels prior to machining or coldworking.

Application of Annealing Process

1. Casting

2. Forging

3. Rolled stock

4. Press work ….etc.

Recovery, Recrystallization & Grain Growth

The phenomena intimately associated with the annealing of

a plastically deformed crystalline material

Plastic deformation increases the density of point

imperfections in crystalline materials.

This leads to an increase in internal strain energy.

On annealing, the material tends to lose the extra strain

energy and revert to the original condition.

This is achieved by the processes of recovery and

Recrystallization.

Recovery

Takes place at low temperatures of annealing,

The point imperfections created during plastic deformations

are absorbed at grain boundaries,

Dislocations of opposite sign come together and mutually

destroy each other,

Some of the stored internal strain energy is relieved by virtue

of dislocation motion, as a result of enhanced atomic diffusion

at the elevated temperature.

Recrystallization The process of nucleation and growth of new strain free crystals,

which replace the deformed crystals,

There is no change in crystal structure,

These equi-axed grains will have low dislocation densities and

have characteristics of the pre cold-worked condition.

Grain Growth Increase in the average grain size on further annealing after all

the cold worked material has recrystallized.

Larger grains grow at the expense of smaller grains

As the grains grow larger, rate of grain growth decreases.

Larger grains will reduce the strength and toughness of the

material.

NORMALIZING

Normalizing is similar to full annealing, exceptsteel is generally cooled in still air.

The normalizing consists of heating steel to about 40-55 oCabove critical temperature (Ac3 or Accm), and holding forproper item and then cooling in still air or slightly agitated airto room temperature.

In some special cases, cooling rates can be controlled by eitherchanging air temperature or air volume.

After normalizing, the resultant micro-structure should bepearlitic.

Since the temperature involved in this process is more than thatfor annealing , the homogeneity of austenite increases and itresults in better dispersion of ferrite and Cementite in the finalstructure.

Results in better dispersion of ferrite and Cementite in thefinal structure.

The grain size is finerin normalized structurethan in annealedstructure.

Normalized steels aregenerally stronger andharder than fullyannealed steels.

Steels are soft in annealedcondition and tend to stick duringmachining. By normalizing, anoptimum combination of strengthand softness is achieved, whichresults in satisfactory level ofMachinability in steels.

Normalizing is the effective way toeliminate the carbide network.

Normalized treatment is frequently applied to steel in order to achieve any one or more of the objectives,

To refine the grain structure

To obtain uniform structure

To decrease residual stresses

To improve Machinability

HARDENING: Hardening and Hardness are two very different things.

Hardening means: Heat treatment processHardness means: Extrinsic property of a material.

Hardening treatment generally consistsof heating to hardening temperature (30-50°C above upper critical temp.)

Holding at that temperature, followed byrapid cooling i.e. quenching (oil / water /salt baths)

The hardening temperature depends on chemical composition.1. Plain carbon steels, hardness depends on the carbon content alone.2. Hypo-eutectoid steels - heated to about 30-50oC above the upper

critical temperature,3. Eutectoid and Hypereutectoid steels - heated to about 30-50oC above

lower critical temperature.

Hardening temperature is same as thatfor Normalising

Hardening is applied to cutting tools and machine parts where highhardness and wear resistance are important.

The Process Variables:

Hardening Temperature:The steel should be heat treated to optimum austenitising temperature.A lower temperature results lower hardness due to incompletetransformation to austenite. If this temperature is too high will alsoresults lower hardness due to a coarse grained structure.

Soaking Time:Soaking time at hardening temperature should be long enough totransform homogenous austenite structure. Soaking time increases withincrease in section thickness and the amount of alloying element beingtreated.

Delay in Quenching:After soaking, the steel is immediately quenched. Delay in quenchingmay reduce hardness due to partial transformation of austenite.

Hardenability

Hardenability (not “hardness”): Qualitative measure of rate at whichhardness decreases with distance from surface due to decreasedmartensite content.

High hardenability means the ability of the alloy to produce a highmartensite content throughout the volume of specimen

Hardenability dependent upon the chemical composition of the steel alloy.

Most heat treatable steels are alloys rather than plain carbon steels.

Addition of Nickel, Chromium and Molybdenum will slow thetransformation to other phases and allow more martensite to form.

Quenching Medium:

Cools faster in water than air/oil.

Fast coolingWarping and cracks

(since it is accompanied by large thermal gradients)

Shape and Size:

Cooling rate depends upon extraction of heat to surface.Greater the ratio of surface area to volume, deeper thehardening effect.

Geometry:

Spherical objects cools Slower

Irregular objects cools Faster

Factors Affecting or Influencing Hardenability are Quenching Medium, Specimen Size, and Geometry

Hardenability Test: using Jominy End Quench Test

The most simple and convenient method of determining the Hardenability is

the Jominy End - Quench Test.

The Jominy test involves heating a

standard test piece of dia. 25 mm and

length 100mm to the austenite state,

fixing it to a frame in a vertical

position and then quenching the

lower end by means of a jet of water

as shown in figure.

Holder to support

specimen

Specimen

Slow air quench

Raid water quench

water quench medium

Φ25

100m

m

The mode of quenching results in different rate of cooling along the length of

the test piece.

After a quenching, a flat of 0.38 mm deep is ground along one side of thetest price, and hardness measurements are made along the length of thetest piece.

A bar of steel having good hardenability shows higher hardness readingsfor greater distance from the quenched end.

Distance from Quenching end (inches)

Rock

wel

l C s

cale

Har

dnes

s

Cooling rate at 13000F, 0F/Sec

TEMPERING (formerly called Drawing)

Tempering is a process where steel is reheated after it is beenquenched, to a suitable temperature below the transformationtemperature for an appropriate time (for an hour or more, dependingon its size) and cooling back to room temperature.

Tempering

Freshly quenched martensite ishard but not ductile.

Tempering is needed to impartductility to martensite usually ata small sacrifice in strength.

Tempering is a sub-critical heattreatment process used toimprove the toughness(elongation) of hardened steel.

Example:If the head of a hammer were quenched to a fully martensitic structure, itprobably would crack after the first few blows.Tempering during manufacture of the hammer imparts shock resistancewith only a slight decrease in hardness.

Tempering temperatures are usually identified by the colors.

Tempering temperatures for tools and shafts along with tempercolors.

Low- temp. tempering (150 – 250oC)

Medium – temp. tempering (350 – 450oC)

High – temp. tempering (500 – 650oC).

MARTEMPERING OR STEPPED QUENCHING

The holding time in thequenching bath should besufficient to enable a uniform

After heating the steel to ahardening temperature, it isquenched in the mediumhaving a temperature, from150°C to 300°C.

The specimen is held until itreaches the temperature ofmedium and then its cooledfurther to room temperaturein air and sometimes in oil,

temperature to be reached throughout the cross section but long enough to cause austenitic decomposition.

Austenite is transformed into martensite during the subsequent

period of cooling to room temperature.

This treatment will provide a structure of martensite and

retained austenite in the hardened steel.

AUSTEMPERING or ISOTERMAL QUENCHING

This is the second method that

can be used to overcome the

restrictions of conventional

quench and tempering.

The quench is interrupted at a

higher temperature than for

martempering to allow the metal

at the center of the part to reach

the same temperature as the

surface.

By maintaining that temperature, both the center and the surface are

allowed to transform to Bainite and are then cooled to room temperature.

1. Less distortion and cracking than martempering.

2. No need for final tempering (less time consuming and more

energy efficient).

3. Improvement of toughness (impact resistance is higher than

the conventional quench and tempering).

4. Improved ductility.

Advantages of Austempering:

In many situations hard and wear resistance surface is requiredwith the tough core (tough core can withstand impact load).

SURFACE HARDENING:

1. Hardening and tempering the surface layers (surface hardening)

(i) Flame Hardening (ii) Induction Hardening

2. Changing the composition at surface layers (chemical heat

treatment or case hardening)

Carburizing Carburizing & Cyaniding Nitriding

The combination of the these properties can be achieved by thefollowing methods:

The typical applications requiring these conditions include gearteeth, cams shafts, bearings, crank pins, clutch plate, tools anddies.

The flame hardening involves heating the surface of a steel to atemperature above upper critical point (850oC) with aoxyacetylene flame and then immediately quenched the surfacewith cold water.

Heating transforms the structure of surface layers to austenite,and the quenching changes it to martensite.

Flame Hardening

The surface layers are hardened to about 50 – 60 HRC. It is lessexpensive and can be easily adopted for large and complexshapes.

Flame hardened parts must be tempered after hardening. Thetempering temperature depends on the alloy composition anddesired hardness.

The flame hardening methods are suitable for the steels withcarbon contents ranging from 0.40 to 0.95% and low alloy steels.

Induction hardening involves placing the steel componentswithin a coil through which high frequency current is passed.

The current in the coil induce eddy current in the surfacelayers, and heat the surface layers up to austenite state.

Then the surface is immediately quenched with the cold waterto transfer the austenite to martensite.

Induction Hardening

Advantages of induction hardening over flame hardening is itsspeed and ability to harden small parts; but it is expensive.

Like flame hardening, it is suitable for medium carbon and lowalloy steels.

Typical applications for induction hardening are crank shafts,cam shafts, connecting rods, gears and cylinders.

Carburizing is carried out on a steels containing carbon less than0.2%.

It involves increasing the carbon contents on the surface layersupto 0.7 to 0.8%.

CARBURIZING or CASE HARDENING

2 CO C + CO2

In this process, the steel is heated in contact with carbonaceousmaterial from which it absorbs carbon. This method is mostlyused for securing hard and wear resistance surface with toughcore.

Carburizing is used for gears, cams, bearings and clutch plates.

The Following methods are used to diffuse carbon into surfacelayers:1) Pack (Solid) Carburizing

2) Liquid Carburizing

3) Gas Carburizing

Gas Carburizing

Liquid Carburizing

Nitriding involves diffusion of nitrogeninto the product to form nitrides. Theresulting nitride case can be harderthan the carburized steel.

This process is used for alloy steelscontaining alloying elements(Aluminum, Chromium andMolybdenum) which form stablenitrides.

Nitriding consists of heating acomponent in a retort to a temperatureof about 500 to 600oC.

NITRIDING

2 NH3 2N + 3H2 Through the retort the ammonia gas is allowed to circulate. At this

temperature the ammonia dissociates by the following reaction.

Nitriding increase wear and corrosion resistance and fatiguestrength of the steel. Since nitriding is done at low temperature, itrequires more time than carburizing, and also the capital cost if theplant is higher than carburizing.

The depth of nitride case ranges from 0.2 to 0.4mm andNO MACHINING is done after nitriding.

The atomic nitrogen diffuses into steel surface, and combines withthe alloying elements (Cr, Mo, W, V etc) to form hard nitrides.

The depth to which nitrides areformed in the steel depends on thetemperature and the time allowedfor the reaction.

After the nitriding the job isallowed to cool slowly. Since there isno quenching involved, chances ofcracking and distortion of thecomponent are less.

Similar to carbonitriding, cyaniding also involves the diffusion ofcarbon and nitrogen into the surface of steel. It is also called liquidcarbonitriding.

The components are heated to the temperature of about 800-900oCin a molten cyanide bath consisting of sodium cyanide, sodiumcarbonate and sodium chloride.

After allowing the components in the bath for about 15-20mins,they are quenched in oil or water.

Cyaniding is normally used for low-carbon steels, and case depthsare usually less than 0.25 mm.

Produces hard and wear resistance surface on the steels. Because ofshorter time cycles, the process is widely used for the machinecomponents subjected to moderate wear and service loads.

The process is particularly suitable for screws, small gears, nutsand bolts.

CYANIDING

AGE HARDENING / PRECIPITATION HARDENING

Foreign particles can also obstruct movement of dislocations

and increase strength of the material.

Hardening over a period of time.

Occurs in dur-aluminium which is an aluminium alloy that

contains 4% copper.

The metal alloy is heated and soaked (solution treatment)

then cooled and left.

This makes this alloy very useful as it is light yet reasonably

hard and strong.

Al Alloy is used in the space industry.

THANK YOU

TTT Diagram & Heat Treatment

Engineering

Transcript of TTT Diagram & Heat Treatment