Heat treatment part 2

Prepared by

Prof. Naman M. Dave

Assistant Professor,

Mechanical Engg. Dept.

Gandhinagar Institute of Technology

MATERIAL SCIENCE &

METALLURGY

2131904

Chapter 8

Heat Treatment Processes

Please do not blindly follow

the presentation files only, refer

it just as reference material.

More concentration should

on class room work and text

book-reference books.

Steels Can be heat treated to alter properties by either ...

B) Heating and rapid cooling (Quenching)

1. Hardening

1. Full hardening / Through Hardening

2. Surface Hardening / Case

Hardening / Selective Hardening

(i) Nitriding

(ii) Induction Hardening

(iii) Carburising – (a) Liquid

Carburising (b) Gas Carburising (c) Pack

Carburising

(iv) Cyaniding

(v) Electron Beam Hardening

(vi) Flame Hardening

(vii) Laser Beam Hardening

2. Tempering

1. Low Temperature Tempering

2. Medium Temperature Tempering

3. High Temperature Tempering

3. Aus-tempering

4. Mar-tempering

A) Heating and slow cooling

1. Annealing

1. Stress-relief annealing

2. Process annealing

3. Spheroidize annealing

4. Full annealing

5. Bright and Black

Annealing

6. Iso-thermal annealing

2. Normalising

Prof. Naman M. Dave

Prof. Naman M. Dave

Objectives / Purposes

Softening the steel

Refining grain structure

Improve mechanical, physical, electrical and

magnetic properties

Relieving Internal Stresses

Improve machinability

Heat Treatment Processes for

Steels

Annealing

Prof. Naman M. Dave

Heating Temperature >

Normally heating temperature for

annealing is as under

• Hypo-eutectoid Steels

Upper Critical Temp. :

(723-910OC) + 30-50OC

• Eutectoid Steel

Critical Temp.

(723OC) + 30-50OC

• Hyper-eutectoid Steels :

Lower Critical Temp.

(723OC) + 30-50OC

However, some special annealing

cycles (like spheroidise annealing

and process annealing) are carried

out below lower critical

temperature also.

Annealing

Prof. Naman M. Dave

Holding Time

1 minute per mm of maximum thickness cross

section OR

1/2 hr. Per 1 inch of maximum thickness cross

section

Cooling Rate : Very slow e.g. 25-30OC per

hour. Refer CR1 shown in the T.T.T.

Diagram here. >

Cooling Medium : Cooling in furnace or heap

of ashes or in an insulated chamber

Final Micro-Structure

Hypo-eutectoid Steel : Coarse Pearlite + Ferrite

Eutectoid Steel : Coarse Pearlite

Hyper-eutectoid Steel : Coarse Pearlite+Cementite

Annealing

Prof. Naman M. Dave

Types of Annealing

There are different types of Annealing

Processes. Some of the most widely used

processes are…

1.Stress Relief Annealing

2.Process Annealing

3.Spheroidize Annealing

4.Full Annealing

5.Bright Annealing and Black Annealing

6.Isothermal Annealing

Prof. Naman M. Dave

Stress Relief Annealing • Stress relief annealing process consists of three steps.

• The first step is heating the cold worked steel to a temperature between 5000C

and 5500C i.e. below its recrystallization temperature.

• The second step involves holding the steel component at this temperature for

1-2 hours.

• The final step is to cool the steel component to room temperature in air.

• The stress relief annealing partly relieves the internal stress in cold worked

steels without loss of strength and hardness i.e. without change in the

microstructure.

• It reduces the risk of distortion while machining, and increases corrosion

resistance. Since only low carbon steels can be cold worked, the process is

applicable to hypo eutectoid steels containing less than 0.4% carbon. This

annealing process is also used on components to relieve internal stresses

developed from rapid cooling and phase changes.

Annealing

Prof. Naman M. Dave

Spheroidise Annealing • Spheroidise annealing is one of the variant of the annealing process that

produces typical microstructure consisting of the globules (spheroid) of cementite or carbides in the matrix of ferrite. The following methods are used for spheroidise annealing.

• Holding at just below A1:Holding the steel component at just below the lower critical temperature (A1) transforms the pearlite to globular cementite particles. But this process is very slow and requires more time for obtaining spheroidised structure.

Spheroidise Annealing • Thermal cycling around A1: In this method, the thermal cycling in the narrow

temperature range around A1 transforms cementite lamellae from pearlite to spheroidal. Figure depicts a typical heat treatment cycle to produce spheroidised structure. During heating above A1, cementite or carbides try to dissolve and during cooling they try to re-form. This repeated action spheroidises the carbide particles. Spheroidised structures are softer than the fully annealed structures and have excellent machinability. This heat treatment is utilized to high carbon and air hardened alloy steels to soften them and to increase machinability, and to reduce the decarburization while hardening of thin sections such as safety razor blades and needles.

Recrystallization or Process Annealing

• Recrystallization annealing process consists of heating a steel component below A1 temperature i.e. at temperature between 6250C and 6750C (recrystallization temperature range of steel), holding at this temperature and subsequent cooling.

• This type of annealing is applied either before cold working or as an intermediate operation to remove strain hardening between multi-step cold working operations. In certain case, recrystallization annealing may also be applied as final heat treatment.

• The cold worked ferrite recrystallizes and cementite tries to spheroids during this annealing process. Recrystallization annealing relieves the internal stresses in the cold worked steels and weldments, and improves the ductility and softness of the steel. Refinement in grain size is also possible by the control of degree of cold work prior to annealing or by control of annealing temperature and time.

Annealing

Prof. Naman M. Dave

Full Annealing

• Full annealing process consists of three steps. First step is heating the steel component to above A3 (upper critical temperature for ferrite) temperature for hypoeutectoid steels and above A1 (lower critical temperature) temperature for hypereutectoid steels by 30-500C (Figures 1 and 2). In Figure 2, the terms α, γ and Fe3C refer to ferrite, austenite and cementite phases.

• The second step is holding the steel component at this temperature for a definite holding (soaking) period of at least 20 minutes per cm of the thick section to assure equalization of temperature throughout the cross-section of the component and complete austenization. Final step is to cool the hot steel component to room temperature slowly in the furnace, which is also called as furnace cooling. The full annealing is used to relieve the internal stresses induced due to cold working, welding, etc, to reduce hardness and increase ductility, to refine the grain structure, to make the material homogenous in respect of chemical composition, to increase uniformity of phase distribution, and to increase machinability.

Annealing

Isothermal Annealing • Isothermal annealing consists of four steps. The

first step is heating the steel components similar as in the case of full annealing. The second step is slightly fast cooling from usual austenitizing temperature to a constant temperature just below A1. Third step is to hold at this reduced temperature for sufficient soaking period for the completion of transformation and the final step involves cooling the steel component to room temperature in air. Fig. depicts heat treatment cycles of full annealing and isothermal annealing. The terms α, γ, P, PS and PF.

• Reduced annealing time, especially for alloy steels which need very slow cooling to obtain the required reduction in hardness by the full annealing.

• More homogeneity in structure is obtained as the transformation occurs at the same time throughout the cross section.

• Improved machinability and surface finish is obtained after machining as compared to that of the full annealed components.

• Isothermal annealing is primarily used for medium carbon, high carbon and some of the alloy steels to improve their machinability.

Annealing

1. Stress Relief Annealing

• Heating up to a temperature of 500-550 OC

• Iso-thermal holding

• Cooling in still air up to room temperature

2. Process Annealing

• Heating up to a temperature of 600-650 OC


• Cooling in still air up to room temperature

3. Spheroidize Annealing

• This Annealing process is used to convert the carbides in a steel

into globular form

• Heating & cooling alternately in the temperature range of 650-

700 OC


• Very Slow Cooling (furnace cooling) up to room temperature

Annealing

4. Full Annealing

• Heating up to UCT (723-910 OC) + 30-50 OC for hypo-eutectoid

steels (less than 0.8% C) &

• LCT (723 OC) + 30-50 OC for eutectoid and hyper-eutectoid

steels (more than 0.8% C)


• Very Slow Cooling (furnace cooling or insulated chamber

cooling) up to room temperature

5. Bright Annealing and Black Annealing

• Annealing process carried out in protective atmosphere

prevents dis-colouration of the steel. This process is called

as Bright Annealing.

• When components to be annealed are surrounded by

reducing agents like charcoal and annealed in a box then

oxidation of these components is reduced. This process is

Box or Black annealing.

Prof. Naman M. Dave

Annealing

6. Isothermal Annealing

• Medium or High Carbon Steels have their T-T-T curves

positioned substantially away from temperature axis. This

demands pro-longed annealing cycles.

• The steels are fast cooled up to the subcritical i.e. below A1 and

then held isothermally there until the transformation is

completed. Subsequently these steels are cooled to room

temperatures. This saves cycle time substantially.

Prof. Naman M. Dave

Annealing

Tem

pera

ture

(OC)

Time

550

500

(1) Stress Relief Annealing Range

(2) Process Annealing Range

(3) Spheroidize Annealing Range

25

960

LCT

600

UCT-1 910

(4) Full Annealing Range 763

723

700 650

UCT-2 1130

Prof. Naman M. Dave

Annealing


Softening the steel

Eliminate carbide network at grain boundaries

Relieving Internal Stresses

Refining grain structure

Improve mechanical, physical, electrical and

magnetic properties


Steels

Normalizing

Prof. Naman M. Dave


• Hypo-eutectoid Steels:

Upper Critical Temp. (723-910OC) + 50-70OC

• Eutectoid Steel:

Critical Temp. (723OC) + 50-70OC

• Hyper-eutectoid Steels:

Upper Critical Temp. (723-1130OC) + 50-70OC

Normalizing

Prof. Naman M. Dave

Holding Time

About 15 minutes

Cooling Rate : Slow e.g.

50-100OC per hour.

Refer CR3 shown in

the T.T.T. Diagram

here. >

Cooling Medium :

Cooling in still

ambient air

Final Micro-Structure:

• Hypo-eutectoid Steel:

Fine Pearlite + Ferrite


Fine Pearlite

• Hyper-eutectoid Steel:

Fine Pearlite +

Cementite

Prof. Naman M. Dave

Normalizing

910

Tem

pera

ture

(OC)

Time

763

25

LCT

UCT-1

1130 Normalising Range

UCT-2

723

1180

Prof. Naman M. Dave

Normalizing


1. To increase the hardness

2. To increase the wear resistance


• Hypo-eutectoid Steels:

Upper Critical Temp.

(723-910OC) + 10-30OC


Critical Temp.

(723OC) + 10-30OC

• Hyper-eutectoid Steels:

Lower Critical Temp.

(723OC) + 10-30OC


Steels

Hardening

Hardening

Holding Time

• 1 minute per mm of maximum

thickness cross section OR

• 1/2 hr. Per 1 inch of maximum

thickness cross section

Cooling Rate >

• Faster than Critical Cooling

Rate. For e.g. If CCR is 250OC

per hour then we

• have to cool at say 250 + 30 to

50 i.e. 280-300OC. In other

words the

• temperature drop per hour

should be more than 250 OC.

Prof. Naman M. Dave

Hardening

Cooling Medium :

Quenching medium are many. For e.g.

1) 5-10% Caustic Soda (Very Drastic Quench)

2) 5-20% Brine (NaCl)

3) Cold Water

4) Warm Water

5) Mineral oil

6) Animal oil

7) Vegetable oil

8) Air

Normally water is recommended for quenching of plain carbon

steels and oil is

recommended for quenching alloy steels

Prof. Naman M. Dave

Final Micro-Structure >

Steels with C% below 0.3% do

not harden.

For Steels with C% higher than

0.3%, the final microstructure

will be Martensite + Retained

Austenite + Carbides.

Relative amount of these

phases will depend on

composition of Steel,

hardening temperature, soaking

time and temperature of

quenching medium.

For e.g. If the cooling rate

becomes slower than critical

cooling rate then it can result

in formation of pearlite and

bainite also.

Needle type micro-structure indicating Martensite,

white coloured structures are a mixture of

carbides and retained austenite

Prof. Naman M. Dave

27

910

Te

mp

era

ture

(OC

)

Time

763

25

LCT

UCT-1

1130

Hardening Range

UCT-2

723

960

Prof. Naman M. Dave

Hardening

Hardenability of Steels

Hardness is a measure of resistance to wear.

Hardening is done to improve the hardness of steel i.e. To improve the

wear resistance of steel.

Hardenability of steel means the depth and distribution of hardness

attained by quenching. It is defined in following ways :

1. It is ease with which the steel can be hardened or ability of steel to get

hardened.

(Qualitative definition)

2. It is the length of the test bar up to which 50% martensite is achieved

OR

It is the length of the test bar up to which 50 RC hardness is achieved.

(Quantitative definition).

It is noted by a notation like this : J50 = 5. It means 50 RC hardness can

be obtained in the test bar upto 5/16 inch length from the quenched end.

For testing the hardenability of a steel, there is a test called Jominy

Quench Test. This will be explained in lab.

All steels are not hardenable. For e.g. low C steels are not hardenable.

This is due to their low Carbon content.

Hardenability

Prof. Naman M. Dave

29

Note :

Only the alloys highlighted in blue colour can

be full hardened

Hardenability

Prof. Naman M. Dave

Factors affecting hardenability are as under...

Alloying content : Higher alloying means good

hardenability

Homogeneity of Austenite : Higher homogeneity

means good hardenability

Grain size of Austenite : Higher the size better is the

hardenability

Presence of un-dissolved carbides : Lesser carbides

better hardenability

Section of steel : Smaller the section better is the

hardenability

Quenching medium : Selecting the quenching

medium as per the size of the casting will give better

results for full hardening.

Hardenability


1. To reduce hardness

2. To eliminate retained austenite

3. To relieve internal stresses induced by

quenching

4. To improve toughness and ductility

5. To improve other mechanical properties


Low temperature tempering : 150-250OC

Med. temperature tempering : 350-450OC

High temperature tempering : 500-650OC


Steels

Tempering

Tempering

Prof. Naman M. Dave

Holding Time :

• Low temperature tempering cycle :

Heating results in formation of low C martensite from Martensite. Hence holding time

depends on Martensite content in the micro-structure (after hardening). Holding should be

done untill C% in Martensite decreases and reaches the value of 0.3%.

Tempering

Cooling Rate : ->

Slower than hardening See the curve shown on T.T.T. Dig. Here

• Medium temperature tempering cycle :

Heating results in formation of Bainite

from retained austenite. Hence holding

time depends on Retained Austenite (RA)

content in the micro-structure (after

hardening). The holding should be done

untill all the RA converts to Bainite.

• High temperature tempering cycle :

Heating results in formation of ferrite

from Martensite. Hence holding time

depends on Martensite content in the

micro-structure (after hardening). The

holding should be done untill all

Martensite converts to ferrite by losing

carbon.

Tempered Martensite (Black colour)

+

Carbides (White colour)

+

Retained Austenite (White colour).

Cooling Medium : Cooling in air

Final Micro-structure : ->

Tempered Martensite (having lower C% than

Martensite formed after hardening) +

‘ε’ or Epsilon Carbide (Fe2.4C) (having higher C%

than Cementite i.e. Fe3C +

Retained Austenite. The quantity of retained austenite

after tempering is lesser than before.

Tempering

<-Tempering Colors and

Temperatures

Surface Hardening Concept

Surface

- Pearlite +

Cementite

- High Hardness

- Highly Wear

Resistant Core

- Ferrite

- Low hardness

- Ductile

- Capable of

withstanding

stress

Cross section of a case

hardened gear teeth

The Carbon content in the steel determines whether it can be directly hardened or

not. If the Carbon content is low (for example less than 0.3%) then an alternate

means exists to increase the Carbon content of the surface. The part then can be

heat-treated by either quenching in liquid or cooling in still air depending on the

properties desired.

Note that this method will only allow hardening

on the surface, but not in the core, because the

high carbon content is only on the surface. This

is sometimes very desirable because it allows

for a hard surface with good wear properties (as

on gear teeth or knife), but has a tough core that

will perform well under impact loading.

It is also possible to add additional carbon or

chromium or boron to the outer surface of a

component low in carbon which will make the

surface sufficiently hard.

This process is known as case hardening or

surface hardening or selective hardening.

Prof. Naman M. Dave

I. Thermal Treatments

1) Induction Hardening

2) Flame Hardening

3) Laser Beam

Hardening*

4) Electron Beam

Hardening*

II. Thermo-Chemical Treatments

1) Nitriding

a) Gas Nitriding

b) Liquid Nitriding

2) Carburizing*

a) Solid / Pack Carburizing*

b) Gas Carburizing*

c) Liquid Carburizing

(Cyaniding)

3) Carbo-nitriding*

4) Chroming*

5) Boronizing*

Surface Hardening Concept

Prof. Naman M. Dave

36

Thermal Treatment

Induction Hardening

In Induction hardening, the steel part is

placed inside a electrical coil which has

alternating current through it. This induces

Eddy Current on the outer surface of the

component and heats it up.

Depending on the amperage of current,

frequency of current and heating time

depth of hardening can be controlled .

Temperature is around 750-800OC.

The heated region is then quenched by

water jets to achieve the desired hardness.

Tempering can be done to eliminate

brittleness.

Maximum 6 mm of depth can be achieved

in about 6 seconds time.

Example of application : Gear teeth

hardening

Coil

Work

Piece

Work Piece

(Gear)

Coil

Prof. Naman M. Dave

Flame Hardening A high intensity oxy-acetylene flame is applied to the selective region. The

temperature is raised high enough to be in the region of Austenite transformation.

The "right" temperature is determined by the operator based on experience by

watching the color of the steel.

Flame hardening of a flat cross section work piece Flame hardening of a circular cross section work piece using lathe

(MOVING)

(STATIONARY)

(Stationary)

Work piece

(Rotating)

Thermal Treatment

Prof. Naman M. Dave

The overall heat transfer is limited by the torch and thus the interior never

reaches the high temperature.

The heated region is quenched to achieve the desired hardness.

Tempering can be done to eliminate brittleness.

The depth of hardening can be increased by increasing the heating time.

Maximum 6 mm of depth can be achieved.

A gear teeth being surface hardened

using flame hardening method

Thermal Treatment Flame Hardening

Prof. Naman M. Dave

Flame Hardening Large parts, which will not normally fit in a furnace, can be heat-treated using

this method.

Example of application : Lathe Bed Hardening, Large Gears’ Teeth hardening,

Large Sprockets’ teeth hardening, etc.

Thermal Treatment

Prof. Naman M. Dave

Thermo-chemical Treatment

Nitriding

Introduction of nitrogen into the outer surface of steel parts in

order to give an extremely hard, wear resisting surface is called

as nitriding or nitrogen hardening.

Nitride compounds precipitate out during one of the following

processes

a) Gas nitriding - heat in ammonia

b) Liquid nitriding - dip in molten cyanide bath

A schematic layout of gas nitriding plant is shown in the next

slide.

Prof. Naman M. Dave

41

Gas Nitriding

The part to be nitrided is placed in a box made of

heat resistant steel. This box also has provision

for inlet and outlet of ammonia gas (2 holes

drilled on the walls – one for inlet of ammonia

gas and another for outlet of ammonia gas)

The box (along with the component) is then

placed in the furnace and heated to around 450-

550OC.

Then a steady flow of Ammonia gas is injected

in the box.

On coming in contact with the steel, the

ammonia vapour gets dissociated and nascent

nitrogen is released. This nitrogen combines with

other elements in outer surface of steel like C, Si,

Mn, Ni, Cr, Mo, etc and forms nitrides which are

extremely hard. After that, quenching is done.

Case thicknesses are between 0.5 to 0.8 mm with

hardness up to 70 RC.


Prof. Naman M. Dave

Solid Carburizing or Pack carburizing

The component is placed in a Cast Iron box

along with a carbon rich material (e.g.

charcoal). The box is placed in a furnace and

heated to a temperature around 900-950 oC and

maintained at that temperature for some period

(1 hr for 0.1 mm depth).

The carbon is absorbed into the austenite on the

surface of the component.

The depth of penetration of carbon into the

surface depends on the temperature and the

time spent in the furnace.

After enriching the surface with carbon, oil

quenching is done.

Carburizing

Low-carbon steel is heated in a carbon-rich environment and then

quenched.

Depth achieved by carburizing is 0.025 to 4 mm with hardness around 60

RC.


43

Gas carburizing

1. A mild carburizing gas is generated from LPG

(Liquified Petroleum Gas). This gas is fed into the

furnace containing the heated steel component.

2. Another method of gas carburizing is called liquid

feed or drip feed method. In this method, an organic

liquid such as a mixture of iso-propyl or methyl

alcohol with benzene is dripped directly into the

furnace for generating a specific furnace atmosphere.

• In any case, the gas mixture breaks to form an

atmosphere containing N2, Ha, CH4, CO2, and H2

(due to heat). The component to be carburized is

heated externally and made to travel through this

atmosphere of the furnace and thus their surface

becomes rich in Carbon content. It is then quenched

to form a hard external surface whereas the core

remains soft and ductile. The depth depends upon

time and temperature of exposure to carbon rich

atmosphere.

HIGH FREQUENCY ELECTRIC

CURRENT GAS-CARBURISING

UNIT

Gas Carburizing is conceptually the same as pack carburizing, except that gas is

supplied to a heated component through the furnace.


44

Liquid Carburizing (Cyaniding) The steel parts are immersed in a molten carbon

rich bath maintained at 850-950OC for periods

ranging from 15 minutes to 3 hours.

Sometimes the heating / holding can be intermittent

i.e. there can be two baths – one at 850-950OC and

another at 760OC. This done so as to minimize the

distortion of shape of product during heat

treatment. After that quenching is done.

This process produces a thin, hard shell that is

harder than the one produced by other carburizing

methods, and can be completed in 3 hours

compared to several hours as in other methods so

the parts have less opportunity to become distorted.

Sketch on the left illustrates an electric salt bath

furnace used for cyaniding.

It is typically used on small parts such as bolts,

nuts, screws and small gears.

The major drawback of cyaniding is that cyanide

salts are poisonous. Therefore, safety concerns

have led to non-toxic baths that achieve the same

result. ELECTRIC SALT BATH FURNACE


Carbo-nitriding

Use both carbon and nitrogen – hardness around 70 RC –

maximum depth of hardened surface 0.07 – 0.5 mm

Chromizing

Pack or dip in chromium-rich material - adds heat and

wear resistance

Boronizing

Pack or dip in boron-rich material - Improves abrasion

resistance, coefficient of friction


Prof. Naman M. Dave

Sr.

No.

Thermo-

chemical

Treatment

Hardness of

Surface

Depth of Hardened

Layer from Surface

1. Nitriding 70 RC 0.5 – 0.8 mm

2. Carburizing 60 RC 0.025 - 4 mm

3. Carbo-

Nitriding

70 RC 0.07 – 0.5 mm

4. Chromizing 70 RC -

5. Boronizing 70 RC -

Comparison of different thermo-chemical Treatment


Prof. Naman M. Dave

Thank

You

Heat treatment part 2

Engineering

Transcript of Heat treatment part 2