Bulk and Surface Treatments Annealing, Normalizing, Hardening, Tempering Hardenability HEAT TREATMENT With focus on Steels 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). Treatments Thermal (heat treatment) Mechanical Chemical Or a combination (Thermo-mechanical, thermo-chemical) Bulk Surface
An overview of important heat treatments HEAT TREATMENT BULK SURFACE ANNEALING NORMALIZING HARDENING & TEMPERING THERMAL THERMO- CHEMICAL MARTEMPERING Flame Carburizing Recrystallization Annealing Induction Nitriding Stress Relief Annealing AUSTEMPERING LASER Carbo-nitriding Spheroidization Annealing Electron Beam
Ranges of temperature where Annealing, Normalizing and Spheroidization treatment are carried out for hypo- and hyper-eutectoid steels. 910 C A 3 A cm 723 C A 1 T Spheroidization Recrystallization Annealing Stress Relief Annealing 0.8 % Wt% C
The steel is heated above A 3 (for hypo-eutectoid steels) A 1 (for hyper-eutectoid steels) (hold) then the steel is furnace cooled to obtain Coarse Pearlite Coarse Pearlite has Hardness, Ductility Not above A cm to avoid a continuous network of proeutectoid cementite along grain boundaries ( path for crack propagation) 910 C A 3 A cm 723 C A 1 T Spheroidization Recrystallization Annealing Stress Relief Annealing 0.8 % Wt% C
Recrystallization Annealing Heat below A 1 Sufficient time Recrystallization Cold worked grains New stress free grains Used in between processing steps (e.g. sheet rolling) 910 o C 723 o C A 3 Spheroidization Recrystallization Annealing Stress Relief Annealing A cm A 1 T 0.8 % Wt% C
Stress Relief Annealing Residual stresses Heat below A 1 Recovery Annihilation of dislocations, polygonization Welding Differential cooling Machining and cold working Martensite formation 910 o C 723 o C A 3 Spheroidization Recrystallization Annealing Stress Relief Annealing A cm A 1 T 0.8 % Wt% C
Spheroidization Annealing Heat below/above A 1 (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 910 o C 723 o C A 3 Spheroidization Recrystallization Annealing Stress Relief Annealing A cm A 1 T 0.8 % Wt% C
NORMALIZING Heat above A 3 A cm Austenization Air cooling Fine Pearlite (Higher hardness) Purposes 910 o C 723 o C T A 3 Spheroidization Recrystallization Annealing Stress Relief Annealing A cm A 1 Refine grain structure prior to hardening 0.8 % Wt% C To harden the steel slightly To reduce segregation in casting or forgings In hypo-eutectoid steels normalizing is done 50 o C above the annealing temperature In hyper-eutectoid steels normalizing done above A cm due to faster cooling cementite does not form a continuous film along GB
HARDENING Heat above A 3 A cm 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
Schematic showing variation in cooling rate from surface to interior leading to different microstructures Typical hardness test survey made along a diameter of a quenched cylinder
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 1 [ m ] 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 ) Temper ( BCC) Fe3C ( OR) Martensite 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 (R c 65) Tempered (R c 45-55)
MARTEMPERING To avoid residual stresses generated during quenching Austenized steel is quenched above M s for homogenization of temperature across the sample The steel is then quenched and the entire sample transforms simultaneously Tempering follows Martempering T 800 723 600 500 400 Austenite Eutectoid temperature + Fe 3 C Pearlite Pearlite + Bainite Bainite 300 M s AUSTEMPERING Austempering To avoid residual stresses generated during quenching Austenized steel is quenched above M s Held long enough for transformation to Bainite 200 100 M f Martensite 0.1 1 10 10 2 10 3 10 4 10 5 t (s)
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