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Heat Treatment of Steels :Recrystallization annealing

• The carbon and alloy steels were treated at a temperature of about 700°C, which is about 20 °C below the eutectoid temperature. The holding time should be long enough for recrystallization to be proceed.

• Recrystallization time depends on the degree of cold deformation. The higher the degree of cold deformation, the lower

recrystallization time is.

Ref: Linde booklet: Furnace Atmosphere No.2

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Heat Treatment of Steels : Spheroidize annealing

• The purpose of this process is to improve machinability by changing the morphology of cementite from continuous network or large plate into globular. Therefore, machining become easier.

• Spheroidize annealing shall be applied only to medium and high carbon steels.

3/21/2012 Kitkamhorn, U.

Ref: http://www.astonmet.com/ss/images/win_ht1207_23.html

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Heat Treatment of Steels : Spheroidize annealing

• The steel is heated to temperature just below or slightly above the eutectoid temperature. Holding for 2-6 hr followed by slow cooling

3/21/2012 Kitkamhorn, U.

Modified from William D., and Jr. Callister, 2007

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Heat Treatment of Steels : Normalizing

• The purpose of this process is to refine grain size or to produce uniform grain size.

• It is also used for softening the heavily cold deformed materials. It is quite similar to full annealing but the parts are cooled in air. Thus the cooling rate is faster compare to annealing.

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Heat Treatment of Steels : Normalizing

Normalizing is considered to be out of equilibrium since the

air-cooling is considerably fast.

Eutectoid temperature and composition become lower as

cooling rate increase.

Ar3 and Ar1 merge and full pearlitic microstructure is obtained

in steels with >0.4%C

3/21/2012 Kitkamhorn, U. A1 Acm A3

Modified from William D., and Jr. Callister, 2007

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Heat Treatment of Steels : Quenching and Tempering

The purpose of quenching and tempering is to improve

hardness, strength, and wear resistance of the parts.

The desired microstructure is martensite or bainite or mixture

of both depending on the hardness, strength, and toughness

required in service.

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[Ref: Heat Treater’s Guide, 1995]

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Heat Treatment of Steels : Quenching and Tempering

The hardness of martensite is a

function of carbon contents only.

The maximum hardness (fully

matensite) is then a function of

carbon contents in steels.

Ref: Linde booklet: Furnace Atmosphere No.2

3/21/2012

This point is important for materials selection

Kitkamhorn, U.

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Heat Treatment of Steels : Quenching and Tempering

• The steel parts are heated to a temperature above A3for hypoeutectoid steel and above A1 for hypereutectoid steel.

• Soaking time is about 30 minutes per 1 inch cross section.

• The parts are rapidly cooled (so called “quench) in either brine, water, hot oil, cold oil, forced air, and still air.

te m p er a tu re time austenitizing q u en ch in g tempering 3/21/2012 Kitkamhorn, U.

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Heat Treatment of Steels : TTT and CCT

Metallurgists use TTT and CCT diagrams to design the quenching

severity required to achieve the target hardness value.

TTT (time-temperature transformation diagram) indicates the

isothermal transformation versus holding time.

CCT (continuous cooling transformation diagram) indicates the

transformation by the different cooling rate.

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Heat Treatment of Steels : TTT diagram

Ref: William D., and Jr. Callister, 2007

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3/21/2012 Kitkamhorn, U.

Heat Treatment of Steels : TTT diagram

Modified from William D., and Jr. Callister, 2007

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Heat Treatment of Steels : TTT diagram

Curves on TTT

• Upper curves are of diffusion

transformation e.g. • γ→ α +γ

• γ→ Fe3C +γ

• γ→ α +Fe3C

• Lower curve are of bainitic

transformation

• Horizontal lines are of

martensitic transformation

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Heat Treatment of Steels : TTT diagram

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Heat Treatment of Steels : TTT diagram

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Heat Treatment of Steels : TTT diagram

TTT diagram of AISI 4340 steel (JIS-SNCM 439, DIN 20CrMo5) (0.4%C, 1.0%Mn, 0.8%Cr, 0.8%Mo, 1.85%Ni)

Ref: William D., and Jr. Callister

3/21/2012 Kitkamhorn, U.

Additions of alloying elements result in

• Separation of diffusion

transformation and bainitic transformation curves.

• The position of the nose of

diffusion transformation curve (upper curves).

• The position of the nose of

bainitic transformation curve

• The position of Ms.

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Heat Treatment of Steels : TTT diagram

Important information of TTT needed to be considered

• The position of the nose

which indicates the critical cooling rate to suppress diffusion transformation

• Mstemperature indicating that

austenite start to transform into martensite at such temperature.

• Fraction of martensite at a

temperature below Ms. This

indicates that after quenching to room temperature, how much retained austenite exist

in the steel. Modified from William D., and Jr. Callister, 2007

3/21/2012 Kitkamhorn, U.

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Heat Treatment of Steels : CCT diagram

Modified from William D., and Jr. Callister, 2007

3/21/2012 Kitkamhorn, U.

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Heat Treatment of Steels : CCT diagram

CCT curves are slightly on

the lower right of TTT

curves

Better to follow CCT since

most heat treatment

processes are based on

continuous cooling except

martempering and

austempering.

Ref: William D., and Jr. Callister, 2007

3/21/2012 Kitkamhorn, U.

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Heat Treatment of Steels : CCT diagram

TTT and CCT Diagram of AISI 4340 steel (0.4%C, 1.0%Mn, 0.8%Cr, 0.8%Mo, 1.85%Ni)

Ref: William D., and Jr. Callister, 2007

3/21/2012 Kitkamhorn, U.

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Heat Treatment of Steels : How to read CCT

3/21/2012 Kitkamhorn, U. Hardness %bainite %proeutectoid ferrite %pearlite

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Heat Treatment of Steels : Quenching and Tempering

Alloying elements except cobalt shift TTT to the right.

This increases hardenability of the steels.

Ac1 Ac3 F+C F+C A+F+C A+F+C A Ms ½” 1” 2” T em p er at u re Time H R C

Distance from specimen core

Critical cooling rate Ac1 Ac3 F+C F+C A+F+C A+F+C A 1” ½” 2” Ms T em p er at u re Time H R C

Distance from specimen core

Critical cooling rate

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Heat Treatment of Steels : Quenching and Tempering

• The hardenability of high alloy steels is much better than that of carbon steels.

• Air-cooling is fast enough to quench the high alloy steel.

• Alloying elements also decrease Msand Mf.

• Carbon strongly influences Ms

• In alloy steels with >0.4%C, some retained austenite exists after quenching.

Ref: Linde booklet: Furnace Atmosphere No.2

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Heat Treatment of Steels : Quenching and Tempering

As-quenched martensite is brittle because of strain

accompanied with phase transformation.

There is a volume expansion when austenite transforms into

martensite.

Tempering is a process used for stress-relieving of fresh

martensite by heating to a temperature at a range of 150-650°C

for 40-60 minutes or longer.

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Heat Treatment of Steels : Quenching and Tempering

• At high temperature Some carbon atoms are rejected from martensite and form carbide. The BCT- martensite become closer to BCC-ferrite and the hardness decreases.

• Retained austenite can also decompose into ferrite and carbide.

• These phenomena are also accompanied with the volume change.

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Heat Treatment of Steels : Quenching and Tempering

Tool steels such as hot-work

tool steels, cold-work tool

steels, and high speed tool

steels, the tempering curves

exhibit secondary hardening at

tempering temperatures in a

range of 450-600°C.

Such steels need to be tempered

twice or more.

Carbon steels Low to medium alloy steels

high alloy steels with high carbon contents

high alloy steels with medium carbon contents

Ref: George E. Totten

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Heat Treatment of Steels : Quenching and Tempering

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Heat Treatment of Steels : Quenching and Tempering

• Sub-zero quenching is an option treatment for high alloy steel quenching.

• The purpose of this method is to remove the retained austenite.

• After oil or air quenching to room temperature, the parts are dipped into cryogenic media such as liquid nitrogen.

• The parts are then tempered afterward. te m p er a tu re time austenitizing q u en ch in g tempering Sub-zero quenching 3/21/2012 Kitkamhorn, U.

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Heat Treatment of Steels : Martempering

• The transformation of austenite into pearlite or bainite is delayed by addition of some alloying elements.

• The high alloy steels can be isothermal holding at a temperature above Msfor long time without undergoing phase transformation

• Large or complex high alloy steel parts can then be treated by martempering to avoid quench- cracking te m p er a tu re time austenitizing tempering 3/21/2012 Kitkamhorn, U.

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Heat Treatment of Steels : Martempering

martempering quenching 3/21/2012 Kitkamhorn, U.

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Heat Treatment of Steels : Austempering

te m p er a tu re time austenitizing

• Austempering produces bainite microstructure by quenching to the temperature above Msand

isothermally holding until the bainitic transformation is completed.

• The austempered steel exhibit high strength, high tougness, and high ductility.

• Coil spring and disc spring are examples of steel parts treated by austempered

Ref: George E. Totten, 2006

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Heat Treatment of Steels : Austempering

- Not all steels can be austempered practically.

- Their kinetics of bainitic transformation determined whether austempering shall be applied to such steels.

Ref: George E. Totten, 2006

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Heat Treatment of Steels : Summary of Metallurgical Factors during heat treatment

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Understand the concept of microstructural change due to thermal treatment

Case 1

When crystallization of austenite and its transformation are the principles behind then what needed to be considered

• Austenitizing temperature must be proper selected depending on the chemical compositions of steels, the methods of cooling, and the final microstructure required.

• When austenitizing treatment is in the single austenite region such as hardening of medium carbon steels, treatment temperature and time should not be too high since austenite grain coarsening may occur.

• Proper cooling rate (quenching method) is required. • The final microstructure required

• There is a limitationof miximum hardness of steel, which can be reached depending on the carbon conceration

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Case 2

When recrystallization of the same phase (α in α) is the principle behind

• Treatment temperature must be high enough to drive nucleation of

new strain-free grains.

• Proper time is required but too long time has no benefit or may cause

to large grain size.

• Only heavily cold-worked low carbon steels are treated with this

method since large cold deformation is required as a drivig force for necleation.

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Heat Treatment of Steels : Summary of Metallurgical Factors during

heat treatment

Heat Treatment of Steels : External Factors during heat treatment

3/21/2012 Kitkamhorn, U. • Oxidation • Discoloration • Carburization/decarburization • Effectiveness of quenchant

• Stability of quechant temperature and its thermo-physical properties

• Transport of heat into/out of steel parts

• Parts configuration

• Jig Design

• etc

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References:

• George E. Totten. Steel Heat Treatment Handbook : Metallurgy and Technologies, CRC Press, USA 2006

• George Krauss. S T E E L S :Processing, Structure, and Performance . ASM International, USA 2005

• Heat Treater’s Guide: Practice and Procedure for Iron and Steel. ASM International 2ndedition, USA 1995

H. Eallentowitz et al. “Materials for future automotive body structure” Business Briefing:GlobalAutomotive Manufacturing & Technology 2003

• Karen Connery and Len Switzer. “High Quality Heat Treatment: Atmosphere choice of critical” Heat Treating Progress, September 2008

• Linde booklet: Furnace Atmosphere No.1 Gas carburizing and carbonitriding. Special Edition available from http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage

• Linde booklet: Furnace Atmosphere No.2 Neutral hardening and annealing. Special Edition available from http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage

• Linde booklet: Furnace Atmosphere No.3 Nitriding and nitocarburizing. Special Edition available from http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/docbyalias/homepage

• William D., and Jr. Callister MATERIALS SCIENCE AND ENGINEERING: AN INTRODUCTION

John Wiley & Sons, 7th 2007

• Dissemination of IT for the Promotion of Materials Science (DoITPoMS), Universiy of Cambridge, http://www.doitpoms.ac.uk/about/dissemination.html

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References

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