Heat Treatment
Heat treatment is a process used to alter the physical, chemical and mechanical properties of a metal to improve its desired characteristics. By adjusting the amount of time, temperature and other parameters it can be used to achieve a wide range of results. The most common types of heat treatment are annealing, hardening and tempering, precipitation hardening and carburizing.
Annealing
Annealing is a process used to reduce the hardness of a material and increase its ductility. It is achieved by heating material in a uniform manner to just above its recrystallization temperature and then allowing it to cool slowly. This ensures that the entire part is uniformly heated throughout and then cooled at the same rate. During this process, any existing dislocations are removed, and the atomic spacing gradually returns to its original geometry. This allows for the continued growth of crystals at the same time that it removes dislocations.
Hardening
Hardening is a process used to increase the hardness of a material and reduce its ductility. It is achieved by heating a material above its transformation temperature and allowing it to cool at a slower rate than annealing. This slower cooling rate allows the material to partially form larger grains than annealing and also allows for the formation of new grain boundaries that are relatively more resistant to deformation. Hardening will also cause the material to become brittle, as the grain boundaries are unable to deform freely.
Tempering
Tempering is a process used to reduce the brittle properties of a material that has undergone hardening. It is achieved by reheating the material to below its transformation temperature and then allowing it to cool at a rate that is greater than annealing, but slower than hardening. Dislocations remain in the material, but the material’s grain boundaries are redistributed by the relatively slower cooling rate. This redistribution of grain boundaries makes the material less brittle and more able to deform without fracturing.
Precipitation hardening
Precipitation hardening is a process used to increase the strength and/or corrosion resistance of a material. It is achieved by introducing a specific concentration of alloying elements into the material and then allowing it to age at elevated temperatures. During this aging process, the precipitates form a uniform dispersion throughout the material, causing an overall increase in the strength and corrosion resistance of the material.
Carburizing
Carburizing is a process used to increase the surface hardness and wear resistance of a material. It is achieved by introducing a carbon rich atmosphere into the material for a specified amount of time at elevated temperatures. During this time the surface of the material absorbs carbon from the atmosphere, which then increases the surface hardness of the material. Carburizing can be used to create wear resistant surfaces, while still allowing the material to retain its ductility.
Heat treatment is a powerful tool used to alter the properties of a wide variety of materials. By adjusting the temperature and cooling rate, it is possible to create many different types of materials with varying levels of strength, hardness, corrosion resistance and ductility. Knowing how to properly apply these treatments can be invaluable to those who are attempting to produce an optimal material for a given application.
