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Anandh Subramaniam & Kantesh Balani
HEAT TREATMENT With focus on Steels Bulk and Surface Treatments Annealing, Normalizing, Hardening, Tempering Hardenability MATERIALS SCIENCE & ENGINEERING Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur URL: home.iitk.ac.in/~anandh AN INTRODUCTORY E-BOOK Part of A Learner’s Guide Principles of Heat Treatment of Steels Romesh C Sharma New Age International (P) Ltd., Publishers, New Delhi, 1993.
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Heat Treatment of Steels
We have noted that how TTT and CCT diagrams can help us design heat treatments to design the microstructure of steels and hence engineer the properties. In some cases a gradation in properties may be desired (usually from the surface to the interior- a hard surface with a ductile/tough interior/bulk). The general classes of possibilities of treatment are: (i) Thermal (heat treatment), (ii) Mechanical (working), (iii) Chemical (alteration of composition). A combination of these treatments are also possible (e.g. thermo-mechanical treatments, thermo-chemical treatments). The treatment may affect the whole sample or only the bulk. A typical industrial treatment cycle may be complicated with many steps (i.e. a combination of the simple step which are outlined in the chapter). Thermal (heat treatment) Or a combination (Thermo-mechanical, thermo-chemical) Treatments Mechanical Chemical Bulk Surface
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An overview of important heat treatments
BULK SURFACE ANNEALING NORMALIZING HARDENING & TEMPERING THERMAL THERMO- CHEMICAL Full Annealing Carburizing MARTEMPERING Flame Recrystallization Annealing Induction Nitriding AUSTEMPERING Stress Relief Annealing LASER Carbo-nitriding Spheroidization Annealing Electron Beam
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Stress Relief Annealing T
Ranges of temperature where Annealing, Normalizing and Spheroidization treatment are carried out for hypo- and hyper-eutectoid steels. Full Annealing 910C Acm Normalization Normalization A3 Full Annealing 723C A1 Spheroidization Stress Relief Annealing T Recrystallization Annealing Wt% C 0.8 %
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Coarse Pearlite has ↓ Hardness, ↑ Ductility
Full Annealing The steel is heated above A3 (for hypo-eutectoid steels) | A1 (for hyper-eutectoid steels) → (hold) → then the steel is furnace cooled to obtain Coarse Pearlite Coarse Pearlite has ↓ Hardness, ↑ Ductility Not above Acm → to avoid a continuous network of proeutectoid cementite along grain boundaries (→ path for crack propagation)
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Recrystallization Annealing
Heat below A1 → Sufficient time → Recrystallization Cold worked grains → New stress free grains Used in between processing steps (e.g. sheet rolling)
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Annihilation of dislocations, polygonization
Stress Relief Annealing Annihilation of dislocations, polygonization Residual stresses → Heat below A1 → Recovery Welding Differential cooling Machining and cold working Martensite formation
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Spheroidization Annealing
Heat below/above A1 (long time) Cementite plates → Cementite spheroids → ↑ Ductility Used in high carbon steel requiring extensive machining prior to final hardening & tempering Driving force is the reduction in interfacial energy
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NORMALIZING Heat above A3 | Acm → Austenization → Air cooling → Fine Pearlite (Higher hardness) Purposes Refine grain structure prior to hardening To harden the steel slightly To reduce segregation in casting or forgings In hypo-eutectoid steels normalizing is done 50oC above the annealing temperature In hyper-eutectoid steels normalizing done above Acm → due to faster cooling cementite does not form a continuous film along GB
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HARDENING Heat above A3 | Acm → Austenization → Quench (higher than critical cooling rate) Quench produces residual strains Transformation to Martensite is usually not complete (will have retained Austenite) Martensite is hard and brittle Tempering operation usually follows hardening; to give a good combination of strength and toughness
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Typical hardness test survey made along a diameter of a quenched cylinder
Schematic showing variation in cooling rate from surface to interior leading to different microstructures
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Brine quench (saturated Salt water)
Severity of quench values of some typical quenching conditions Process Variable H Value Air No agitation 0.02 Oil quench 0.2 " Slight agitation 0.35 Good agitation 0.5 Vigorous agitation 0.7 Water quench 1.0 1.5 Brine quench (saturated Salt water) 2.0 5.0 Ideal quench Severity of Quench as indicated by the heat transfer equivalent H f → heat transfer factor K → Thermal conductivity Note that apart from the nature of the quenching medium, the vigorousness of the shake determines the severity of the quench. When a hot solid is put into a liquid medium, gas bubbles form on the surface of the solid (interface with medium). As gas has a poor conductivity the quenching rate is reduced. Providing agitation (shaking the solid in the liquid) helps in bringing the liquid medium in direct contact with the solid; thus improving the heat transfer (and the cooling rate). The H value/index compares the relative ability of various media (gases and liquids) to cool a hot solid. Ideal quench is a conceptual idea with a heat transfer factor of ( H = )
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Schematic of Jominy End Quench Test
Jominy hardenability test Variation of hardness along a Jominy bar (schematic for eutectoid steel)
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Tempering Heat below Eutectoid temperature → wait→ slow cooling
The microstructural changes which take place during tempering are very complex Time temperature cycle chosen to optimize strength and toughness Tool steel: As quenched (Rc 65) → Tempered (Rc 45-55)
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To avoid residual stresses generated during quenching
MARTEMPERING To avoid residual stresses generated during quenching Austenized steel is quenched above Ms for homogenization of temperature across the sample The steel is then quenched and the entire sample transforms simultaneously Tempering follows Austenite Pearlite Pearlite + Bainite Bainite Martensite 100 200 300 400 600 500 800 723 0.1 1 10 102 103 104 105 Eutectoid temperature Ms Mf t (s) → T → + Fe3C Martempering Austempering AUSTEMPERING To avoid residual stresses generated during quenching Austenized steel is quenched above Ms Held long enough for transformation to Bainite
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ALLOY STEELS Various elements like Cr, Mn, Ni, W, Mo etc are added to plain carbon steels to create alloy steels The alloys elements move the nose of the TTT diagram to the right → this implies that a slower cooling rate can be employed to obtain martensite → increased HARDENABILITY The ‘C’ curves for pearlite and bainite transformations overlap in the case of plain carbon steels → in alloy steels pearlite and bainite transformations can be represented by separate ‘C’ curves
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