Heat treatment is commonly used to strengthen metals. Through proper heat treatment hardening and tempering, the strength and toughness of the metal can be improved. Improper heat treatment can cause it to become too brittle and prone to cracking when exposed to stress.
Heat treatment is the process of controlled heating and cooling of metals to change their physical and mechanical properties without changing its shape. Heat treatment is used to increase the strength, to harden or soften the material, to induce certain physical properties, to combine one or more metallurgical stages and to produce a desired microstructure.
The most common heat treatment process is hardening and tempering. This process involves heating the metal to a specific temperature to reduce chemical composition and then cooling it quickly to form a hardened surface layer. After hardening, the metal can be tempered at a lower temperature to prevent brittleness. The process is usually done in three stages: austenitizing, hardening, and tempering.
Austenitizing is the process of heating a metal to its austenitizing temperature. This temperature is usually above the metal’s melting point. This allows the metal to reach its maximum austenite microstructure, which has the highest strength and ductility in comparison to all other microstructures.
The hardening process then follows. When heated, the austenite transforms into martensite. Typically, the metal is quenched by submerging the heated metal in an oil or water bath. The baths disperse the heat quickly, maximizing the conversion of austenite to martensite. The heat can also be dispersed by air, gas, or an immersion quench.
The tempering process follows hardening. Tempering is performed to reduce the brittleness of the metal. Tempering is done at a lower temperature than the hardening temperature. After tempering, hardness and ductility are increased, but the strength is slightly reduced.
Heat treatment can also be used to remove stress from the metal. This process is called stress relieving. This process is usually done after hardening or after a welding process. Stress relieving is often done by heating up the metal to a temperature lower than its hardening temperature, and then cooling it with a slow-acting cooling medium like oil or air.
Heat treatment can also be used to improve the wear resistance of a metal or an alloy. Surface hardening is done by heating up the metal’s surface to a temperature low enough to cause a desired microstructure. This microstructure will be harder than the underlying material and increase the wear resistance.
Heat treatment operations are important in a variety of industries. The car industry, aerospace industry, and oil and gas industries all depend on properly manipulated metals. Heat treatment should be performed carefully, as the wrong temperature or incorrect cooling can cause the metal to become too brittle.
