ABSTRACT
Heat treatment (HT) embraces many processes, employing combinations of heating and cooling operations, so applied to solid state metal or alloy to induce desired properties. It can alter the mechanical properties of the metal by changing the size and shape of grains of which it is composed. The various HT processes may be broadly classied into
• Softening processes such as annealing, stress relieving, normalizing, and spheroidizing • Tempering processes used for reheating quench-hardened or normalized steel to a tem-
perature below the transformation range and then cooling at any desired rate • Hardening processes such as full-depth (through) hardening, and anticrack hardening
(martempering and austempering) • Case hardening processes such as carburizing, nitriding, cyaniding, and induction and
¨ame hardening
Specically, HT processes have the following objectives:
1. Rening grain size 2. Improving machinability 3. Relieving induced stresses due to cold working 4. Relieving induced stresses due to welding operations 5. Improving mechanical properties such as strength, toughness, hardness, shock resistance,
fatigue strength, etc. 6. Improving magnetic and electrical properties 7. Increasing wear resistance, heat resistance, and resistance to corrosion 8. Producing extra-hard surface on a ductile interior
Consider again the Fe-Fe3C equilibrium diagram illustrated in Figure 3.23. It is the basic diagram for analyzing ferrous alloys. It is useful in studying the equilibrium and nonequilibrium changes that iron carbon alloys undergo during heating and cooling. Such usefulness stems from the fact that this diagram is relatively unaffected by the small concentrations of Mn, Si, S, and P ordinarily present in conventionally rened steels. Focusing on steels (2% C), the Fe-Fe3C equilibrium diagram can be simplied considerably. The portion of the phase (peritectic), which is totally ignored for heat treatment purposes, and that greater than 2% C are of little signicance and are deleted. The resulting diagram (Figure 5.1) focuses therefore on the eutectoid reaction, and is quite useful in understanding the properties of steels and their heat treatment.
