Heat Treatment of Steels : Metallurgical Principle

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1 Heat Treatment of Steels : Metallurgical Principle Outlines: Fe ad Fe-Fe 3 C system Phases and Microstructure Fe-Fe 3 C Phase Diaram General Physical and Mechanical Properties of each Microstructure Usanee Kitkamthorn k_usanee@sut.ac.th **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. 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 treatment 2 Heat Treatment of Steels: Phases in Fe-Fe 3 C System 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. Heat Treatment of Steels: Phases in Fe-Fe 3 C System 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. 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. 4. Cementite (Fe 3 C) 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 3 4

2 Heat Treatment of Steels: Phases in Fe-Fe 3 C System 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. Heat Treatment of Steels: Fe-Fe 3 C Phase Diagram FCC BCT 5 6 Reference:William D., and Jr. Callister, 2007 Heat Treatment of Steels: Phases in Fe-Fe 3 C System General Physical and Mechanical Properties of each Microstructure Transfomation temperature or Critical Temperature A 1 below this temperature, 727 C, γ is unstable and transform to α + Fe 3 C A 3 the critical temperature below which being single phase γ is unstable. Proeutectoid ferrite must form. A 3 A cm A 1 A cm is the critical temperature below which being single phase γ is unstable. Proeutectoid cementite ferrite must form. Adapter from William D., and Jr. Callister, 2007 without pemission hypoeutectoid steel hypereutectoid steel 7 Reference:William D., and Jr. Callister,

3 General Physical and Mechanical Properties of each Microstructure General Physical and Mechanical Properties of each Microstructure 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 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 9 10 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 1) γ α+γ - 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 α Reference:William D., and Jr. Callister,

4 - Grain boundary is the most effective nucleation site (intergranular nucleation) 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. 230, DoITPoMS: Fe, C 0.5(wt%) steel, normalised, Nital etched] - Widmanstätten ferrite is the lath ferrite which forms from the austenite grain boundary or from the allotriomorphic ferrite. Widmanstätten ferrite forms at temperatures below that for allotriomorphic ferrite. Therefore mostly found in steel weld and hypoeutectoid hardened steels. Widmanstätten ferrite Allotriomorph ferrite 13 [Dr. R F Cochrane, Micrograph No. 213, DoITPoMS: Fe, C 0.15(wt%) steel, normalised, Nital etched] Ref: George E. Totten, ) γ Fe 3 C + γ - Transformation proceeds by diffusion process. - Cementite nucleates on austensite grain boundaries and can grow faster along the boundaries. 0 3) γ α+fe 3 C (pearlite) - Transformation proceeds by diffusion process. - Grow by cooperative growth of α and Fe 3 C 0 [Dr. R F Cochrane, Micrograph No. 243, DoITPoMS: Fe, C 1.3(wt%) steel, annealed at 1100 C, Nital etched] 3 15 Ref: George E. Totten, 2006 α 3 16

5 4) γ bainite (α +Fe 3 C) 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 Ref: Bhadeshia and Honeycombe, martensite.html Heat Treatment of Steels : Evolution of Microstructure and dilatation Heat Treatment of Steels: Out of equilibrium Cooling ad heating in real world are faster than that equilibrium can be reached. Ar is denoted for critical temperature upon cooling θ A Ac is denoted for critical temperature upon heating heating rate or cooling rate 19 20

6 Heat Treatment of Steels: Out of equilibium As cooling rate increased, Ar 3 become lower much faster than Ar 1. Heat Treatment of Steels : Process Classificatio Heat Treatment of Steels At a certain cooling rate, these two points merge at Ar which indicates the formation of fine pearlitic microstructure with out ferrite grain. Full Treatment - Annealing - Normalizing - Quenching and tempering - Martempering - Austempering Surface Treatment - Carburizing - Nitriding - Carbonitriding - Nitrocarburizing - Boronizing - Induction hardening - Flame hardening - Laser hardening 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 23 Heat Treatment of Steels : Recrystallization 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 24

7 Steels are iron alloys containing some other elements such as C, Mn, Cr, Si, etc. Heat Treatment of Steels Austenite At high temperature, austenite is the stable phase. It can then be transformed into different microstructure depending on the cooling rates and its composition. Ferrite + Pearlite/ Cementite. + Pearlite Austenite Bainite cooling rate Martensite Tempered Martensite 25 Approach equilibrium microstructure Annealing - Stress-relief annealing - Recrystallization annealing - Spheroidize annealing Non-equilibrium microstructure Normalizing Quenching & Tempering Austempering and martempering Induction Hardening Flame Hardening Laser Hardening TTT and CCT are useful tools for treatment design 26 Heat Treatment of Steels : Stress-relief annealing Heat Treatment of Steels :Recrystallization 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. 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. Modified from William D., and Jr. Callister, 2007 Ref: Linde booklet: Furnace Atmosphere No.2 27 Ref: Linde booklet: Furnace Atmosphere No.2 28