1Introduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering1Chapter 11 Thermal Processing of Metal Alloys¾ Annealing, Stress Relief¾ More on Heat Treatment of Steels¾ Precipitation Hardening • Designer Alloys: Utilize heat treatments to design optimum microstructures and mechanical properties (strength, ductility, hardness….)• Strength in steels correlates with how much martensite remains in the final structure• Hardenability: The ability of a structure to transform to martensite• Precipitation hardeningIntroduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering2Annealing - A heat treatment process in which a material is heated to an elevated temperature, allowed to dwell there for a set amount of time and then cooled with a controlled rate. Stages of annealing:• Heating to required temperature• Holding (“soaking”) at constant temperature• CoolingThe time at the high temperature (soaking time) is long enough to allow the desired transformation (diffusion, kinetics) to occur.Cooling is done slowly to avoid warping/cracking of due to the thermal gradients and thermo-elastic stresses within the or even cracking the metal piece.Purposes of annealing:• Relieve internal stresses• Increase ductility, toughness, softness• Produce specific microstructureAnnealing2Introduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering3Process Annealing - used to revert effects of work-hardening (by recovery and recrystallization) and to increase ductility. Heating is usually limited to avoid excessive grain growth and oxidation.Stress Relief Annealing – used to eliminate/minimize stresses arising fromo Plastic deformation during machiningo Non-uniform coolingo Phase transformations between phases with different densitiesStress relief annealing allows these stresses to relax. Annealing temperatures are relatively low so that useful effects of cold working are not eliminated. Examples of Heat TreatmentIntroduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering4• Lower critical temperature A1below which austenite (γ) does not exist• Upper critical temperature lines, A3and Acmabove which all material is austenite (γ)Annealing of Fe-C Alloys (I)eutectoid point3Introduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering5Normalizing: an annealing heat treatment just above the upper critical temperature to reduce the AVERAGE grain sizes (of pearlite and proeutectoid phase) and make more uniform size distributions. After complete transformation to austenite (austenitizing -γ) the treatment is completed by cooling to the required microstructure.Annealing of Fe-C Alloys (II)eutectoid pointIntroduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering6Full annealing: austenizing (γ) and slow cooling (several hours). Produces coarse pearlite -large grains (and possible proeutectoid phase) that is relatively soft and ductile. Full annealing is used to soften pieces which have been hardened by plastic deformation, and which need to undergo subsequent machining/forming.Spheroidizing: prolonged heating just below the eutectoid temperature, which results in the soft spheroidite structure discussed in Sect. 10.5. This achieves maximum softness needed in subsequent forming operations.Annealing of Fe-C Alloys (III)eutectoid point4Introduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering7Martensite has the strongest microstructure and can be made more ductile by tempering. Therefore, the optimum properties of quenched and tempered steel are realized if a high content of martensite is produced.Problem: It is difficult to maintain the same conditions throughout the entire volume of steel during cooling: the surface cools more quickly than interior, producing a range of microstructures throughout. The martensitic content, and the hardness, will drop from a high value at the surface to a lower value in the interior of the specimen.Production of uniform martensitic structure depends on• composition• quenching conditions• size + shape of specimenHeat Treatment of SteelsIntroduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering8• Martensite is the hardest / strongest and most brittle of the steel microstructures• Hardness is a function of carbon content• Hardening mechanism is solid solution hardening from interstitial C• Enhance ductility by tempering.Anneal to equilibrium ferrite plus cementite phases. Formation by this route called tempered martensiteMartensiteTempered martensite(tempered at 371 °C)Fine PearliteBrinell Hardness NumberRockwell Hardness, Scale CTempering - Hardness5Introduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering9Hardenability is the ability of the Fe-C alloy to be hardened by forming martensite.Hardenability is not “hardness”. It is a qualitative measure of the rate at which hardness decreases with distance from the surface because of decreased martensite content.High hardenability means the ability of the alloy to produce a high martensite content throughout the volume of specimen.Hardenability is measured by the Jominy end-quench test, performed for standard cylindrical specimen, standard austenitization conditions, and standard quenching conditions (jet of water at specific flow rate and temperature).Hardenability (I)Introduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials Science and Engineering10The “Hardenability Curve” is the dependence of hardness on distance from the quenched end.Jominy end-quench test of HardenabilityHardenability (II)6Introduction to Materials Science, Chapter 11, Thermal Processing of Metal AlloysUniversity of Tennessee, Dept. of Materials