Heat Treatment of Steels :

Metallurgical Principle

Usanee Kitkamthorn

Email: k_usanee@sut.ac.thhttp://www.heattreatment.sut.ac.thhttp://www.sut.ac.th/engineering/Metal/ru/indexhttp://personal.sut.ac.th/usanee

**The materials was prepared for non-commercial purpose such as teaching and learning. It may not be reproduced for commercial use but may be copied for educational purposes.

Outlines:

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• Fe ad Fe-Fe3C system

• Phases and Microstructure

• Fe-Fe3C Phase Diaram

• General Physical and Mechanical Properties of each Microstructure

• Transformation of austenite to final microstructure

• Transformation in out of equilibrium

• Effect of cooling/heating rate on critical temperature

• Heat treatment processes

• Metallurgy principles

• Stress-relief annealing, full annealing, spheroidize annealing,

• Normalizing, quenching and tempering

• TTT and CCT Diagram

• Martempering and Austempering

• Factors influence microstructure after heat treatment2

Heat Treatment of Steels: Phases in Fe-Fe3C System

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1. Ferrite(α) is an iron solid solution in which small amounts of carbon atoms can dissolved. The maximum solubility at 727 °C is ∼ 0.022wt%C. The crystal structure is BCC with lattice parameter of 2.87 A° for pure Fe at 295 K.

2. Austenite(γ) is an iron solid solution in which small amounts of carbon atoms can dissolved. The maximum solubility at 1147°C is ∼ 2.14 wt%C. The crystal structure is FCC with lattice parameter of 3.57 A° for pure iron.

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Heat Treatment of Steels: Phases in Fe-Fe3C System

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3. Delta ( δ) is an iron solid solution in which small amounts of carbon atoms can dissolved. The maximum solubility at 1495°C is ∼0.09 wt%C. The crystal structure is also BCC but the lattice parameter is 2.93 A° for pure Fe.

4. Cementite (Fe3C) is an intermetallic compound. Its crystal structure is orthorhombic having 12 Fe-atoms and 4 C-atoms. This is equivalent to 6.67 wt.%C

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Heat Treatment of Steels: Phases in Fe-Fe3C System

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5. Martensite(α′) is a supersaturated solid solution of iron in which high amounts of C-atoms (or N-atoms) are trapped. It is metastable phase and its crystal structure is BCT having lattice parameter depending on the C content.

FCC BCT

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Heat Treatment of Steels: Fe-Fe3C Phase Diagram

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Reference:William D., and Jr. Callister, 20076

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Transfomation temperature or “Critical Temperature”

• A1 – below this temperature, 727°C, γ is unstable and transform to α + Fe3C

• A3 – the critical temperature below which being single phase γ is unstable. Proeutectoid ferrite must form.

• Acm– is the critical temperature below which being single phase γ is unstable. Proeutectoid cementite ferrite must form.

A1

Acm

A3

Heat Treatment of Steels: Phases in Fe-Fe3C System

Adapter from William D., and Jr. Callister, 2007 without pemission

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General Physical and Mechanical Properties of each Microstructure

hypoeutectoid steelhypereutectoid steel

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

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General Physical and Mechanical Properties of each Microstructure

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Austenite- Soft and good ductility- Non-magnetic- Strain harden-able- Its molar volume is less than

that of ferrite at the same temperature

- Stable only at high temperature except a sufficient high level of alloying elements is added.

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

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General Physical and Mechanical Properties of each Microstructure

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Ferrite + Pearlite- Ferrite is soft whereas pearlite is harder and higher in strength- Overall mechanical properties

depends on - Fraction of ferrite and

pearlite- Grain sizes of ferrite and

pearlite- Lamellar spacing between

ferrite and cementite within the pearlite.

Cementite + Pearlite- High strength but brittle- High hardness and good wear

resistance- Overall mechanical properties

depends on - Fraction of cementite and

pearlite- Grain sizes of pearlite and

lamellar spacing between ferrite and cementite within the pearlite.

- Morphology of pro-eutectoid cementite

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General Physical and Mechanical Properties of each Microstructure

Bainite- High strength and toughness- Considerably high ductility- Moderate high hardness- Tempering is not required- Not stable at high temperature

Martensite- Hard but brittle- High wear resistance- Martensite hardness depends on

%C- Not stable at high temperature

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Heat Treatment of Steels : Transformation of Austenite

1) γ→ α +γ

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- Transformation proceeds by diffusion process, which means time is required

- Low carbon austenite is easier to transform to austenite

- Defects such as grain boundaries, foreign particles and dislocations can act as heterogeneous nucleation sites for ferrite.

Reference:William D., and Jr. Callister, 200712

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Heat Treatment of Steels : Transformation of Austenite

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- Grain boundary is the most effective nucleation site (intergranularnucleation) Ferrite growing faster along austenite GB is called “allotriomorph ferrite”)

- Foreign particles in austenite grain can act as heterogeneous nucleation site (intragranular nucleation). Ferrite nucleates and grow within grains is called “idiomorph ferrite”)

[Dr. R F Cochrane, Micrograph No. 213, DoITPoMS: Fe, C 0.15(wt%) steel, normalised, Nital etched]

[Dr. R F Cochrane, Micrograph No. 230, DoITPoMS: Fe, C 0.5(wt%) steel, normalised, Nital etched]

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Heat Treatment of Steels : Transformation of Austenite

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- Widmanstätten ferriteis the lath ferrite which forms from the austenite grain boundary or from the allotriomorphic ferrite.

Widmanstätten ferrite forms at temperatures below that forallotriomorphic ferrite. Therefore mostly found in steel weld and hypoeutectoid hardened steels.

Ref: George E. Totten, 2006

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Widmanstättenferrite

Allotriomorphferrite

Heat Treatment of Steels : Transformation of Austenite

2) γ→ Fe3C + γ

- Transformation proceeds by diffusion process.

- Cementite nucleates on austensitegrain boundaries and can grow faster along the boundaries.

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[Dr. R F Cochrane, Micrograph No. 243, DoITPoMS: Fe, C 1.3(wt%) steel, annealed at 1100°C, Nital etched]

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Heat Treatment of Steels : Transformation of Austenite

3) γ→ α +Fe3C (pearlite)

- Transformation proceeds by diffusion process.

- Grow by cooperative growth of α

and Fe3C

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Ref: George E. Totten, 2006 16

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Heat Treatment of Steels : Transformation of Austenite

4) γ→ bainite (α +Fe3C)

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Ref: Bhadeshia and Honeycombe, 200617

Heat Treatment of Steels : Transformation of Austenite

5) γ→ martensite (α′-BCT)

- Diffusionless transformation

- Require fast cooling

- Large strain accompanied with

the transformation

- Level of strain is proportional to

%C in austenite

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http://www.threeplanes.net/martensite.html

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Heat Treatment of Steels : Evolution of Microstructure and dilatation

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Cooling ad heating in real world are faster

than that equilibrium can be reached.

• Ar is denoted for critical temperature upon cooling

• Ac is denoted for critical temperature upon heating

Heat Treatment of Steels: Out of equilibrium

θθθθ

A

heating rate or cooling rate

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As cooling rate increased, Ar3 become lower much faster than Ar1.

At a certain cooling rate, these two points merge at Ar′ which indicates the formation of fine pearlitic microstructure with out ferrite grain.

Heat Treatment of Steels: Out of equilibium

′

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Heat Treatment of Steels : Process Classificatio

Heat Treatment of Steels

Full Treatment

- Annealing- Normalizing- Quenching and tempering- Martempering- Austempering

Surface Treatment

- Carburizing- Nitriding- Carbonitriding- Nitrocarburizing- Boronizing- Induction hardening- Flame hardening- Laser hardening

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Heat Treatment of Steels : Metallurgy Principles

Many metallic materials can be heat treated in order to improve their properties such as hardness, fatigue strength, and wear resistance.

Steel heat treatment processes are based on

1) The recrystallization of new grains.

- recrystallization of ferrite

- recrystallization of austenite

2) The different transformation of austenite into the final microstructures.

3) Precipitation hardening

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The recrystallization of new grains.

- recrystallization of ferrite

- new strain-free of ferrite grain nucleate

- crystallization of austenite in ferrite + pearlite

- austenite nucleate from ferrite within pearlite

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Heat Treatment of Steels : Recrystallization

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Heat Treatment of Steels : Transformation of Austenite

Steels are iron alloys containing some other elements such as C, Mn, Cr, Si, etc.

At high temperature, austenite is the stable phase. It can then be transformed into different microstructure depending on the cooling rates and its composition.

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Austenite

Ferrite + Pearlite/ Cementite + Pearlite

Bainite Martensite

Tempered Martensite

cooling rate

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Heat Treatment of Steels

Austenite

NormalizingAnnealing- Stress-relief annealing- Recrystallization annealing- Spheroidize annealing

Quenching & Tempering

Austempering and martempering

Approach equilibrium microstructure Non-equilibrium microstructure

Induction HardeningFlame Hardening Laser Hardening

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TTT and CCT are useful tools for treatment design

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Heat Treatment of Steels : Stress-relief annealing

• The purpose of this process is to reduce residual stress caused by cold forming, thermal strain, machining, and transformation of the microstructure.

• In case of low alloy steels, the steels were treated at a temperature up to 650°C for 1 hr or longer and then cooled in still air.

Ref: Linde booklet: Furnace Atmosphere No.2Modified from William D., and Jr. Callister, 2007

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Heat Treatment of Steels :Recrystallization annealing

• The purpose of this process is to change a microstructure of a heavily cold-deformed steel. The microstructure changes as a results of recrystallization of new strain-free grain. The parts become soft enough to undergo further cold deformation without fracturing.

Ref: Linde booklet: Furnace Atmosphere No.2

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