Heat treating is the process used to impart special qualities to metals and alloys (hardness, strength, ductility, etc.). When heat and subsequent cooling are applied to metals in their solid state, the physical and structural properties (but not the chemical composition) of these metals are changed. Different metals respond to treatment at different temperatures. Each metal has a specific chemical composition, so changes in physical and structural properties take place at different, critical temperatures. Depending on the temperature used, these changes can represent a change in grain size, increase in toughness, removal of internal stresses, or formation of a hard surface on a ductile core. In addition to changes induced by heat, the rate and method of cooling have an effect on the structural properties.
The most common reasons that metals undergo heat treatment are to improve their strength, hardness, toughness, ductility, and corrosion resistance. Common techniques for heat treatment include the following:
- Annealing: This is a form of heat treatment that brings a metal closer to its equilibrium state. It softens metal, making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.
- Quenching: It is a heat treatment method that quickly returns metal to room temperature after it is heated above its upper critical temperature. The quenching process stops the cooling process from altering the metals microstructure. Quenching, which can be done with water, oil, and other media, hardens steel at the same temperature that full annealing does. It is less expensive than annealing.
- Precipitation hardening: It is also known as age hardening. It creates uniformity in a metals grain structure, making the material stronger. The process involves heating a solution treatment to high temperatures after a fast cooling process. Precipitation hardening is usually executed in an inert atmosphere at temperatures ranging from 900 degrees Fahrenheit to 1,150 degrees Fahrenheit. It can take anywhere from an hour to four hours to carry out the process. The length of time typically depends on the thickness of the metal and similar factors.
- Tempering: It is commonly used in steelmaking today. This heat treatment used to improve hardness and toughness in steel as well as to reduce brittleness. The process creates a more ductile and stable structure. The aim of tempering is to achieve the best combination of mechanical properties in metals.
- Stress relieving: This is a heat treatment process that decreases stress in metals after they have been quenched, cast, normalized, and so on. Stress is relieved by heating metal to a temperature lower than that required for transformation. After this process, the metal is then slowly cooled.
- Normalizing: It is a form of heat treatment that eliminates impurities and improves strength and hardness by altering the grain size to be more uniform throughout the metal. This is achieved by cooling the metal by air after it has been heated to a precise temperature.