HEAT TREATMENT With focus on Steels  Bulk and Surface Treatments  Annealing, Normalizing, Hardening, Tempering  Hardenability Principles of Heat Treatment of Steels Romesh C Sharma New Age International (P) Ltd., Publishers, New Delhi, 1993. 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) Treatments Mechanical Chemical Bulk Surface Or a combination (Thermo-mechanical, thermo-chemical) An overview of important heat treatments HEAT TREATMENT BULK ANNEALING SURFACE NORMALIZING HARDENING & THERMAL TEMPERING Full Annealing MARTEMPERING Recrystallization Annealing Stress Relief Annealing Spheroidization Annealing Flame Induction AUSTEMPERING LASER Electron Beam THERMOCHEMICAL Carburizing Nitriding Carbo-nitriding Ranges of temperature where Annealing, Normalizing and Spheroidization treatment are carried out for hypo- and hyper-eutectoid steels. Ful l 910C An ne A3 alin g No rma liz atio n N li a orm on i t za Acm Full Annealing 723C A1  T Stress Relief Annealing Spheroidization Recrystallization Annealing Wt% C 0.8 % 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) 910C Ful lA nne alin g A3 No rma liza tion N a liz a orm n tio Acm Full Annealing 723C A1  T Stress Relief Annealing Spheroidization Recrystallization Annealing Wt% C 0.8 % Recrystallization Annealing Heat below A1 → Sufficient time → Recrystallization Cold worked grains → New stress free grains Used in between processing steps (e.g. sheet rolling) 910oC No rma Ful liza lA tion nne alin A3 g on i t a l iz Acm a m r No Full Annealing 723oC A1 Spheroidization Recrystallization Annealing Stress Relief Annealing T 0.8 % Wt% C Stress Relief Annealing Annihilation of dislocations, polygonization Residual stresses → Heat below A1 → Recovery Welding Differential cooling 910oC Machining and cold working No rma Ful liza lA tion nne alin A3 g Martensite formation on i t a l iz Acm a m r No Full Annealing 723oC A1 Spheroidization Recrystallization Annealing Stress Relief Annealing T 0.8 % Wt% C 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 910oC No rma Ful liza lA tion nne alin A3 g on i t a l iz Acm a m r No Full Annealing 723oC A1 Spheroidization Recrystallization Annealing Stress Relief Annealing T 0.8 % Wt% C NORMALIZING Heat above A3 | Acm → Austenization → Air cooling → Fine Pearlite (Higher hardness) 910oC Ful l A3 No rma liza tion An nea ling ion zat i l Acm a rm No Full Annealing 723oC A1 Spheroidization Recrystallization Annealing Stress Relief Annealing Purposes T 0.8 % Wt% C 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 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 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 Severity of quench values of some typical quenching conditions Process Variable H Value Air No agitation 0.02 Oil quench No agitation 0.2 " Slight agitation 0.35 " Good agitation 0.5 " Vigorous agitation 0.7 Water quench No agitation 1.0 " Vigorous agitation 1.5 Brine quench (saturated Salt water) No agitation 2.0 " Vigorous agitation 5.0 Ideal quench Severity of Quench as indicated by the heat transfer equivalent H H f K [m 1 ] 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 = ) Schematic of Jominy End Quench Test Jominy hardenability test Variation of hardness along a Jominy bar (schematic for eutectoid steel) Tempering  ' ( BCT ) Martensite  Temper  ( BCC ) Fe3C (OR) Ferrite  Cementite  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) 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 800 Eutectoid temperature 723 Austenite Pearlite 600 Martempering T → AUSTEMPERING Pearlite + Bainite 400 300 Austempering  + Fe3C 500 Bainite Ms 200 Mf 100 Martensite 0.1  To avoid residual stresses generated during quenching  Austenized steel is quenched above Ms  Held long enough for transformation to Bainite 1 10 102 t (s) → 103 104 105 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