Thermal Processing of Metal Alloys

Thermal Processing of Metal Alloys

Annealing Processes 11.1 Introduction  Annealing is a heat treatment where the material is taken to a high temperature, kept there for some time and then cooled.  High temperatures allow diffusion processes to occur fast.  The time at the high temperature (soaking time) is long enough to allow the desired transformation to occur.  Cooling is done slowly to avoid the distortion (warping) of the metal piece, or even cracking, caused by stresses induced by differential contraction due to thermal inhomogeneities.  Benefits of annealing are: relieve stresses increase softness, ductility and toughness produce a specific microstructure 11.2 Process Annealing  Deforming a piece that has been strengthened by cold working requires a lot of energy.  Reverting the effect of cold work by process annealing eases further deformation.  Heating allows recovery and recrystallization but is usually limited to avoid excessive grain growth and oxidation.  11.3 Stress Relief Stresses resulting from machining operations of non-uniform cooling can be eliminated by stress relief annealing at moderately low temperatures, such that the effect of cold working and other heat treatments is maintained. 11.4 Annealing of Ferrous Alloys Normalizing (or austenitizing) consists in taking the Fe-C alloy to the austenitic phase which makes the grain size more uniform, followed by cooling in air.  Full anneal involves taking hypoeutectoid alloys to the austenite phase and hypereutectoid alloys over the eutectoid temperature (Fig. 11.1) to soften pieces which have been hardened by plastic deformation, and which need to be machined. Spheroidizing consists in prolongued heating just below the eutectoid temperature, which results in the soft spheroidite structure discussed in Sect. 10.5. This achieves maximum softness that minimizes the energy needed in subsequent forming operations.  Heat Treatment of Steels 1.5 Hardenability  To achieve a full conversion of austenite into hard martensite, cooling needs to be fast enough to avoid partial conversion into perlite or bainite.  If the piece is thick, the interior may cool too slowly so that full martensitic conversion is not achieved.  Thus, the martensitic content, and the hardness, will drop from a high value at the surface to a lower value in the interior of the piece.  Hardenability is the ability of the material to be hardened by forming martensite.  Hardenability is measured by the Jominy end-quench test (Fig. 11.2).  Hardenability is then given as the dependence of hardness on distance from the quenched end.  High hardenability means that the hardness curve is relatively flat.  11.6 Influence of Quenching Medium, Specimen Size, and Geometry The cooling rate depends on the cooling medium.  Cooling is fastest using water, then oil, and then air.  Fast cooling brings the danger of warping and formation of cracks, since it is usually accompanied by large thermal gradients.  The shape and size of the piece, together with the heat capacity and heat conductivity are important in determining the cooling rate for different parts of the metal piece.  Heat capacity is the energy content of a heated mass, which needs to be removed for cooling.  Heat conductivity measures how fast this energy is transported to the colder regions of the piece.  Precipitation Hardening Hardening can be enhanced by extremely small precipitates that hinder dislocation motion.  The precipitates form when the solubility limit is exceeded.  Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time.  11.7 Heat Treatments Precipitation hardening is achieved by:    a) solution heat treatment where all the solute atoms are dissolved to form a single-phase solution. b) rapid cooling across the solvus line to exceed the solubility limit. This leads to a supersaturated solid   solution that remains stable (metastable) due to the low temperatures, which prevent diffusion. c) precipitation heat treatment where the supersaturated solution is heated to an intermediate temperature to induce precipitation and kept there for some time (aging). If the process is continued for a very long time, eventually the hardness decreases.  This is called overaging.  The requirements for precipitation hardening are:  ·         appreciable maximum solubility ·         solubility curve that falls fast with temperature ·         composition of the alloy that is less than the maximum solubility 11.8 Mechanism of Hardening  Strengthening involves the formation of a large number of microscopic nuclei, called zones.  It is accelerated at high temperatures.  Hardening occurs because the deformation of the lattice around the precipitates hinder slip.  Aging that occurs at room temperature is called natural aging, to distinguish from the artificial aging caused by premeditated heating.  11.9 Miscellaneous Considerations  Since forming, machining, etc. uses more energy when the material is hard, the steps in the processing of alloys are usually: solution heat treat and quench do needed cold working before hardening do precipitation hardening Exposure of precipitation-hardened alloys to high temperatures may lead to loss of strength by overaging.