Heat Treatment

Thirugnanam K

SEA Materials Engineering

Fundamentals of Heat Treatment for Metallic Materials

IntroductionThe purpose of this presentation is to provide a basic understanding of the metallurgical processes associated with the heat treatment of metallic materials.

Thermal Processes:

1. Shape change of materials

Such as forging, forming, extrusion, rolling, welding

and casting (foundry).

2. No shape change of materials

Such as heat treatment and coating.

Liquid water and ice are familiar examples of how a material can exist in various forms. Steel also exists in

various forms, including several different solid forms.

Various forms of material

What is Heat treatment?

• Answer: The controlled heating and cooling of

materials for the purpose of altering their structures

and properties.

Purposes of performing a heat treatment process on a metallic component

•Modification of the microstructure for improvement of machining, cold forming processes.

•Obtaining the required mechanical properties, such as strength, toughness, hardness, wear resistance and fatigue life based on application.

•Reduction in brittleness, residual stress, dimensional instability of components.

•Surface protection from environment, such as oxidation, corrosion medium and stress corrosion.

Fundamentals of Heat Treatment Processing Development for Metallic Components

When a material is selected per design requirements and application, the next step is how to have the designed parts satisfy these requirements, such as mechanical properties (strength, hardness,toughness, residual stress state and fatigue strength) and microstructure. They are achieved through a proper heat treatment process.

How to develop a heat treatment process???

Tool one: Fe-Carbon phase diagram

For heating above the austenitizing temperature

Tool two: T-T-T or IT curve of selected material

Determination of cooling rate (reduction of cracking risk and distortion) to obtain the required microstructure.

Iron-Carbon phase Diagram

Since steel is a Fe-C ally, the carbon is a key element in any grades of steel.

Most alloying elements in the steels are Cr, Mo, Ni, Si, Mn, as well as V and W.

•All of them change the positions of the A1, A3 and Am boundaries and the eutectoid composition will be changed.

•All important alloying elements decrease the eutectoid carbon content. For example, 1080 (0.8% C) steel is called hyper-eutectoid steel. However, H19 (0.3% C) is hyper-eutectoid steel due to addition of Cr (2%), W (8%) and V (1%). Eutectoid point shifts toward to left in the Fe-C diagram.

•Austenite-stabilizing elements (Mn and Ni) decrease A1 temperature, i.e., expend γ gamma zone.

•Ferrite-stabilizing elements (Cr, Si, Mo , W, V and Ti) increase A1 temperature, i.e., shrink γ gamma zone.

•The effect of combination of alloying elements on the Fe-C diagram is very complicated (may expand or shrink gamma zone).

Effects of alloying elements on the Fe-C phase diagram

1. 1035 steel – 1570 F (855 C)2. 1040 steel – 1555 F (845 C)3. 4340 steel – 1570 F (855 C) 4. 5130 steel – 1570 F (855 C)5. 8620 steel – 1600 – 1700 F (870 – 925 C for carburizing)6. H19 steel – 2005-2200 F (1095 – 1205 C)7. D2 steel – 1795-1875 F (980-1025 C) 8. T2 steel – 2300-2375 F (1260-1300 C)

Examples of austenitizing temperature development for steel hardening

Full TTT Diagram

The complete TTT diagram for an

iron-carbon alloy of

eutectoid composition.

A: austenite

B: bainite

M: martensite

P: pearlite

Effect of the carbon content on the T-T-T- curve profile

1. Move the nose of T-T-T curve toward the lower-right direction.

2. Lowering Ms point temperature (martensitic transformation start temperature).

3. Increasing hardening ability or hardenability using same cooling rate.

Effect of the Alloying elements on the T-T-T- curve profile

A

0.42C 0.20Mn 0.21Mo

B

0.48C 0.94Mn 0.25Mo

C

0.36C 0.17Mn 0.82Mo

F

E

0.23C 0.82Mn 1.22Cr 0.53Mo 0.22V

1. Most of them move the nose of T-T-T curve toward the lower-right direction.

2. Some of them promote the formation of two noses in T-T-T curves

3. Some of them lower Ms temperature point (martensitic transformation start temperature).4. Most of them increase hardening ability or hardenability using same cooling rate.

F

1.5C 12Cr 1Mo 1V

D

0.39C 0.56Mn

0.34Ni 0.74Mo

So What’s a CCT Diagram?

• Phase Transformations and Production of Microconstituents takes TIME.

• Higher Temperature = Less Time.

• If you don’t hold at one temperature and allow time to

change, you are “Continuously Cooling”.

• Therefore, a CCT diagram’s transition lines will be

different than a TTT diagram.

Slow Cooling

Time in region

indicates amount of microconstituent!

Medium Cooling

Cooling Rate, R, is

Change in Temp / Time °C/s

Fast Cooling

This steel is very

hardenable… 100% Martensite in ~ 1 minute of cooling!

Heat Treatment can be considered in terms of three aspects

1. In crystallographic change

2. In microstructure change

3. In mechanical and physical property change

•In crystallographic change

From BCC to FCC to BCT

BCC

FCC

BCT

BCC – Body centered cubic

FCC – Face centered cubic

BCT – Body centered tetragonal

•In microstructure change

From pearlite to austenite to martensitethrough quenching (fast cooling)

In mechanical properties, such as hardness

SAE 1050 SAE 4147

Heat Treatment Process Tree

Heat Treatment (Heating, Holding & Cooling)

Annealing Normalizing Through hardening Case hardening

•Stress relief

annealing

•Recrystallization

annealing

•Spheroidizedannealing

•Isothermal annealing

•Quenching &

tempering

•Interrupted

quenching

•Isothermal

•Austempering

•Martempering

•Vacuum

•Induction hardening

•Flame hardening

•Laser hardening

•Carburizing

•Carbonitriding

•Nitrocarburizing

FULL ANNEALING

• It consists of heating steel to

austenitic region (790-900°C),

followed by slow cooling, preferably in

the furnace itself or in any good heat-

insulating material.

Full annealing

8620 ASR

AS Received

Objectives of the Full Annealing

• To improve ductility

• To facilitate cold working or machining

• To remove internal stresses completely

• To get enhanced magnetic and electrical properties

• To promote dimensional stability

• To refine grain structure

• Disadvantage:– The prolonged heat treatment cycle, involved in this process, makes it quite

expensive.

ISOTHERMAL ANNEALING

• In this process, hypoeutectoid steel is heated

above the upper critical temperature (A3 -750-

900°C)and held for some time at this

temperature.

• The steel is then cooled rapidly to a temperature

less than the lower critical temperature (i.e. 600 -

700 °C)

• After all the austenite is transformed into

lamellar pearlite, steel is cooled in air.

Adv. of Isothermal Annealing

• The time required is less compare to Full Annealing.

• Hence cheaper than full annealing process.

• Improves Machinability and also results in a better surface finish by machining.

• Widely used for alloy steels.

Disadv. of Isothermal Annealing

• Used for hypoeutectoid steels only

• 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.

PROCESS ANNEALING

• Steel is heated to a temperature below the lower

critical temperature (670-720°C), and is held at

this temperature for sufficient time and then

cooled.

Process Annealing

• To reduce hardness and to increase ductility of

cold-worked steel so that further working may be

carried out easily.

• It is an intermediate operation and is sometimes

referred to as in-process annealing.

• Mostly used in sheet and wire industries

Spheroidise Annealing

• Spheroidising is a heat treatment process which

results in a structure consisting of globules or

spheroids of carbide in a matrix of ferrite.

Spherodised Annealing

• Spherodisation can take place by the following

methods– Prolonged holding at a temperature just below the lower critical line

(727°C)

– Heating and cooling alternately between temperature that are just above

and just below the lower critical line.

– Heating to a temperature above the lower critical line and then either

cooling very slowly in the furnace or holding at a temperature just below

the lower critical line.

•Spheroidizing annealing

8620 ASRAs Received

Purpose of Spheroidising

• The majority of all spheroidising activity is

performed for improving the cold formability of

steels.

• The spheroidised structure is desirable when

minimum hardness, maximum ductility, or (in

high carbon steels ) maximum machinability is

important.

Spheroidise Annealing

• Low-carbon steels are seldom spheroidised for

machining, because in the spheroidised

condition they are excessively soft and gummy.

• The cutting tool will tend to push the material

rather than cut it, causing excessive heat and

wear on the cutting tip.

Stress Relieving

• It is used to relieve stresses that remain locked

in a structure as a consequence of a

manufacturing sequence.

• No microstructural changes occur during the

process.

Stress Relieving

• The process of stress relieving consists of

heating steel uniformly to a temperature below

the lower critical temperature (less than 600°C),

holding at this temperature for sufficient time,

followed by uniform cooling.

Stress Relieving

• Sources of internal stresses– solidification of castings

– welding

– machining

– grinding

– shot peening

– surface hammering

– cold working, bending

– electroplated coatings

Stress Relieving

• Adverse effect of internal stresses– steels with residual stresses under corrosive environment fail by stress-

corrosion cracking.

– Residual stresses will enhance the tendency of steels towards warpage

and dimensional instability.

– Fatigue strength is reduced considerably when residual tensile stresses

are present.

NORMALISING

• WHAT IS NORMALISING?– Normalising is an austenitising heating cycle followed by cooling

in still air or slightly agitated air.

– Typically, the job is heated to a temperature about 50°C above the upper critical line of the iron-iron carbide phase diagram prior to cooling. (830 - 925°C)

NORMALISING

PURPOSE OF NORMALISING

• To improve Machinability

• To refine the grain structure

• To homogenise the microstructure in order to

improve the response in hardening operation.

• To modify and refine cast dendritic structure

• To reduce banded grain structure due to hot

rolling.

NORMALISING Vs ANNEALING

• Normalised steels are harder than annealed one.

• Prolonged heat treatment time and higher energy

consumption make the annealing treatment more expensive than normalising.

• Cooling rates are not critical for normalising as in the

case of annealing.

• Annealing improves the machinability of medium carbon

steels, whereas normalising improves machinability of low carbon steels.

HARDENING

• Certain applications demand high hardness

values so that the components may be

successfully used for heavy duty purposes.

• High hardness values can be obtained by a

process known as Hardening.

HARDENING

• Hardening treatment consists of heating to austenitising temperature(815 - 870°C), holding at that temperature,

followed by rapid cooling such as quenching in water, oil,

or salt baths.

• The high hardness developed by this process is due to

the phase transformation accompanying rapid cooling.

• The product of low temperature transformation of

austenite is martensite, which is a hard microconstituent of steel.

•Conventional quenching & tempering

HARDENING

• Successful hardening usually means achieving

the required microstructure, hardness, strength,

or toughness while minimising residual stress,

distortion, and the possibility of cracking.

Selection of Quenching Medium

• Selection of a quenching medium depends on

the hardenability of the particular alloy, the

section thickness and shape involved and the

cooling rates needed to achieve the desired

microstructure.

– Hardenability: It is the ability of the steel to be transformed partially or

completely from austenite to martensite while quenching.

Various Quenching Mediums

• Gaseous Quenchants– Helium, Argon and Nitrogen

• Liquid Quenchants– Oil

– Oil with some additives

– Polymer Quenchants

– Water

– Brine Water

Factors affecting Hardening

• Chemical composition of steel

• Size and shape of the steel part

• Hardening cycle (heating rate, hardening

temperature, holding time and cooling rate)

• Homogeneity and Grain size of austenite

• Quenching Media

• Surface condition of steel part.

Hardening

• High hardness developed by hardening enables

tool steel to cut other metals.

• It also improves wear resistance.

• Tensile strength and Yield Strength are

improved by hardening.

• This process is frequently used for chisels,

sledge, hand hammers, centre punches, shafts,

collars and gears.

Tempering

• Tempering consists of heating hardened steel

below the lower critical temperature, followed by

cooling in air or at any other desired rate.

Tempering

• In the as-quenched martensitic condition, the

steel is too brittle for most applications.

• The formation of martensite also leaves high

residual stresses in the steel.

• Therefore, hardening is almost always followed

by tempering.

Tempering

• The purpose of tempering is to relieve residual

stresses and to improve the ductility and

toughness of the steel.

• This increase in ductility is usually attained at the

sacrifice of the hardness or strength.

• Hardness decreases and toughness increases

as the tempering temperature is increased.

Tempering

• Dimensional Changes– Martensite transformation is associated with an increase in volume.

– During tempering, martensite decomposes into a mixture of ferrite and

cementite with a resultant decrease in volume as tempering

temperature increases.

Sub-Zero Treatment

• Retained Austenite:– In practice, it is very difficult to have a completely martensitic structure

by hardening treatment.

– Some amount of austenite is present in the hardened steel.

– This austenite existing along with martensite is referred to as Retained

Austenite.

Sub-zero Treatment

• Retained austenite is converted into martensite

by this treatment.

• The process consists of cooling steel to sub-zero

temperature which should be lower than the Mf

temperature of the steel (-30 to -70°C)

• Tempering is done immediately to remove the

internal stresses developed by Sub-zero

treatment.

Sub-zero Treatment

• Increase in hardness

• Increase in wear resistance

• Increase in dimensional stability

Martempering

• In the conventional hardening process, the surface and centre cool at different rates and transform to martensite

at different times.

• In Martempering, the steel is quenched into a bath kept

just above Ms. After allowing sufficient time for the

temperature to become uniform throughout the cross-section, it is air-cooled through the martensitic range.

• The transformation to martensite occurs more or less simultaneously across the section.

Adv. Of Martempering

• Residual stresses developed during martempering is lower.

• It also reduces or eliminates susceptibility to cracking.

Austempering

• This is a heat-treating process developed to obtain a structure which is 100 percent bainite.

• It is accomplished by first heating the part of the proper austenitising temperature (790 - 915°C) followed by cooling rapidly in a salt bath, held in the

bainite range (250-400°C).

• The piece is left in the bath until the transformation

to bainite is complete.

Bainite

• Upper (550-350°C)

– Rods of Fe3C

• Lower (350-250°C)– Fe3C Precipitates in Plates of

Ferrite

• It is still Ferrite and Cementite! It’s just acicular.

Austempering

• Increased ductility, toughness and strength

• Reduced distortion, which lessens subsequent

machining time, stock removal, sorting,

inspection and scrap.

• The shortest overall time cycle to through

harden within the hardness range of 35 - 55

HRc, which results in savings in energy and

capital investment.

Austempering

• Limitation– Limitation on size is necessary since the part is required to attain

uniform temperature of the quenching bath rapidly.

– Therefore, only comparatively thin sections can be austempered

successfully.

•Austempered Ductile Iron (ADI)

Aus-ferrite

Actually, the definition is Isothermal temperature heat treatment of ductile iron (spheroidized iron)

ADI Microstructures(a) As-received ductile iron.(B) Aus-ferrite at high T.(C) Aus-ferrite at low T.

B

A

C

•ADI grade and mechanical properties

CASE HARDENING

• There are situations in which the requirement is

such that the outer surface should be hard and

wear resistant and the inner core more ductile

and tougher.

• Such a combination of properties ensures that

the component has sufficient wear resistance to

give long service life and at the same time has

sufficient toughness to withstand shock loads.

CASE HARDENING

• CARBURISING

• CYANIDING

• CARBONITRIDING

• NITRIDING

• PLASMA NITRIDING

• FLAME HARDENING

• INDUCTION HARDENING

CARBURISING

• This is the oldest and one of the cheapest

methods of case hardening.

• It is carried out on low carbon steels which

contain from 0.10 - 0.25% carbon.

• Carburising is carried out in the temperature

range of 900 - 930 °C

• The surface layer is enriched with carbon upto

0.7 - 0.9 %

CARBURISING

• In this process, carbon is diffused into steel by

heating above the transformation temperature

and holding the steel for sufficient time in contact

with a carbonaceous material which may be a

solid medium, a liquid or a gas.

• Followed by Quenching and Tempering.

GAS CARBURISING

• The Steel is heated in contact with carbon

monoxide and/or a hydrocarbon which is readily

decomposed at the carburising temperature.

• Temperature : 870-950°C

• Gas carburising may be either batch or

continuous type.

Air

Natural gas

Mixer

Retort

En

do

ga

s

Natural gas

Endo-gas Generator Carburizing furnace

N2

GAS CARBURISING

• Gas atmosphere for carburising is produced

from liquid (methanol, iso-propanol) or gaseous

hydrocarbons (propane and methane)

• An endo-thermic gas generator is used to supply

endothermic gas.

GAS CARBURISING

• Approximate composition of the gas inflow into

the furnace is– Nitrogen 40%

– Hydrogen 40%

– Carbon Monoxide 20%

– Carbon Dioxide 0.3%

– Methane 0.5%

– Water vapour 0.8%

– Oxygen in traces

GAS CARBURISING

• CHEMICAL REACTIONS– C3H8 ---> 2CH4 + C (Cracking of hyd.carbon)

– CH4 + Fe ---> Fe(C) + 2H2

– CH4 + CO2 ---> 2 CO + 2H2

– 2CO + Fe ---> Fe(C) + CO2

1. Heat and soak at carburizing temperature to ensure temperature uniformitythroughout steel.

2. Boost step to increase carbon content of austenite.

3. Diffusion step to provide gradual case/core transition.

4. Gas pressure or oil quench

CH4 + Fe=Fe(C) +2H2

Atmosphere carburized surface

profile, showing the IGOVacuum carburized surface profile, showing a clear surface (no IGO)

LIQUID CARBURISING

• Popularly known as Salt bath carburising.

• In this process, carburising occurs through

molten cyanide (CN) in low carbon steel cast pot

type furnace heated by oil or gas.

• Bath temperature : 815 - 900°C

• Salt mixture consists of– Sodium or Potassium Cyanide

– Barium chloride

LIQUID CARBURISING

• CHEMICAL REACTION– BaCl2 + 2NaCN ---> Ba(CN)2 + NaCl

– Ba(CN)2 + Fe ---> Fe(C) + BaCN2

SOLID CARBURISING

• This method of carburising is also known as

pack carburising.

• In this process, steel components to be heat

treated are packed with 80% granular coal and

20% BaCO3 as energizer in heat resistant boxes

and heated at 930°C in electric chamber furnace

for a specific period of time depends on case

depth.

SOLID CARBURISING

• CHEMICAL REACTION– Energizer decomposes to give CO gas to the steel furnace

– BaCO3 ---> BaO + CO2

– CO2 + C ---> 2CO

– Carbon monoxide reacts with the surface of steel

– 2CO + Fe ---> Fe(C) + CO2

CARBONITRIDING

• The surface layer of the steel is hardened by

addition of both carbon and nitrogen.

• This process is carried out a lower temperatures

(in the range 800 - 870°C) in a gas mixture

consisting of a carburising gas and ammonia.

• A typical gas mixture contains about 15% NH3,

5% CH4 and 80% neutral carrier gas.

Air

Natural gas

MixerRetort

En

do

ga

s

Natural gas

Endo-gas Generator Carbonitriding furnace

NH3

N2

Air

Natu

ral ga

MixerRetort

End

o g

as

NITRIDING

• Nitriding is most effective for those alloy steels which contain stable nitride forming elements such as Aluminium, Chromium, Molybdenum, Vanadium and Tungsten.

NITRIDING

• Nitriding is carried out in a ferritic region below 590°C.

• So there is no phase change after nitriding.

• Before nitriding, proper heat treatment should be given to steel components.

• All machining and finishing operations are finished before nitriding.

• The portions which are not to be nitriding are covered by

thin coating of tin deposited by electrolysis.

NITRIDING

• Anhydrous ammonia gas is passed into the

furnace at about 550°C, where it dissociates into

nascent nitrogen and hydrogen.

• Thus,

2NH3 ----> 2[N]Fe + 3H2

• The surface hardness achieved varies from 900

to 1100 HV.

PLASMA NITRIDING

• Plasma nitriding is also known as ion nitriding process.

• In this process, the steel component to be nitrided is kept

at 450°C in vacuum at a negative potential of the order of 1000 volts with respect to chamber.

• Then an appropriate mixture of N2 and H2 is passed at a

pressure of 0.2-0.8 m bar.

• As a result, plasma formation of these gases takes

place.

Ion (Plasma) Nitriding Equipment

(Photos Courtesy of Surface Combustion)

Applications: T/C stamped components

FLAME HARDENING

• Flame hardening is done by means of

oxyacetylene torch.

• Heating should be done rapidly by the torch and

the surface quenched before appreciable heat

transfer to the core occurs.

• Application– For large work pieces

– Only a small segment requires heat treatment

– When the part requires dimensional accuracy

Induction Hardening

• Here, an alternating current of high frequency

passes through an induction coil enclosing the

steel part to be heat treated.

• The induced emf heats the steel.

• Immediately after heating, water jets are

activated to quench the surface.

Induction hardening of camshaft

20

25

30

35

40

45

50

55

60

65

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Distance from surface, inch

Har

dn

ess,

HR

C (

con

ver

ted

by

mic

ro)

Induction hardeningInduction hardening of crankshaft

Adv. of Induction Hardening

• Provides energy savings

• Provides much higher heating rates

• Ease of automation and control

• Reduced floor space requirements

• Quiet and clean working conditions

• Suitability for integration in a production line

PS-1: PS-1<S> HEAT TREATMENT - QUENCH AND TEMPER AND AUSTEMPERD

PS-2: PS-2<S> HEAT TREATMENT - GAS CARBURIZINGDPs-3: PS-3<S> HEAT TREATMENT - LIQUID BATH CASE HARDENING

PS-4: PS-4<S> HEAT TREATMENT - MISCELLANEOUSPs-5: PS-5<S> SELECTIVE HEATING SPECIFICATIONS - HEAT STAKING, INDUCTION BONDING, INDUCTION HARDENING & TEMPERING PROCESSES, LASER HEAT TREATING

PS-6: PS-6<S> HEAT TREATMENT – ALUMINUM ALLOYSPS-7: PS-7<S> HEAT TREATMENT - FLAME HARDENING

PS-8: PS-8<S> HEAT TREATMENT - CARBONITRIDINGPS-9: PS-9<S> HEAT TREATMENT-AUSTEMPERED NODULAR AND MALLEABLE IRON

http://adress2.tcc.chrysler.com/adress/

•Chrysler Engineering Standards Related to Heat Treatment Process

Q & A

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