Heat treatment is a process in which a metal is heated to a certain temperature and then cooled in a manner to alter its internal structure for obtaining desired degree of physical and mechanical properties such as brittleness, hardness and softness. Heat treating can make a metal softer and more ductile.
Heat Treatment, annealing, and tempering are three of the most well-known methods for treating metals. Depending on the treatment used, a material may become brittle, harder or softer, or stronger or weaker.
The usual methods of heat-treating ferrous metals (metals with iron) are annealing, normalizing, hardening, and tempering. Most nonferrous metals can be annealed, but never tempered, normalized, or case-hardened.
The purpose of Heat treatment is to:
- Relieve internal stress developed during solidification, machining, forging, rolling or welding.
- Improve or restore ductility and toughness
- Enhance Machinability
- Eliminate Chemical non-uniformity
- Refrain Grain size
- Reduce the gaseous content in steel
Hardening: Hardening involves heating of steel, keeping it at an appropriate temperature until all pearlite is transformed into austenite, and then quenching it rapidly in water or oil. The temperature at which austenitizing rapidly takes place depends upon the carbon content in the steel used.
Tempering: Tempering involves heating steel that has been quenched and hardened for an adequate period, so that the metal can be equilibrated. The hardness and strength obtained depend upon the temperature at which tempering is carried out. Higher temperatures will result into high ductility, but low strength and hardness and vice versa.
Annealing: Annealing involves treating steel up to a high temperature, and then cooling it very slowly to room temperature, so that the resulting microstructure will possess high ductility and toughness, but low hardness. Steel is annealed before being processed by cold forming, to reduce the requirements of load and energy, and to enable the metal to undergo large strains without failure.
Normalizing: Normalizing involves heating steel, and then keeping it at that temperature for a period, and then cooling it in air. Normalizing is performed on structures and structural components that will be subjected to machining, because it improves the machinability of carbon steels