Metal Heat Treatment
Heat treatment is an important manufacturing process that is often used to manipulate the structure, physical and chemical properties of a metal working component. By varying temperatures, holding times, rate of cooling and other parameters, the microstructure of metals, such as steel, aluminum, titanium and copper alloys, is altered to modify their properties as necessary for a particular application. Heat treatment, when properly performed, provides manufacturing parts with desired characteristics such as improved hardness, strength, ductility, fatigue and corrosion resistance, improved dimensional stability and wear resistance.
At its most basic, metal heat treating is the process of heating and cooling metals to change their physical and mechanical properties without changing the product shape. Heat treatment of metals is usually used to alter the hardness, strength, ductility, toughness, wear resistance, fatigue strength and corrosion resistance of a metal object.
Heat treatment techniques include annealing, hardening, tempering, quenching, normalizing, solution annealing and austempering. Annealing involves heating a metal component to a specific temperature, holding it at that temperature for a specified amount of time, and cooling at a controlled rate. The purpose of annealing is to soften metal components, improve their machinability, reduce hardness to improve workability and ductility, or reduce stress caused by welding or machining.
Hardening is a process to increase the hardness of a component by heating the metal above its critical temperature, which is the temperature at which it must be held while cooling to reach a desired hardness. The component is then cooled rapidly so that the temperature decreases rapidly. Heat treatment of steel components is generally done in one of two ways: by oil or water quenching. The advantage of oil quenching is that it cools more slowly, which helps to reduce cracking, as well as to evenly cool the component. Water quenching, however, cools more quickly, which may be desirable when components need to be quickly cooled to preserve the strength and wear resistance of the metal.
Tempering, or drawn tempering, is a common heat treating process used to increase the toughness of a hardened steel component without decreasing its overall strength. The tempering process is done by heating the component to a much lower temperature than the hardening process and then cooling it slowly and evenly. This heat treatment process helps to eliminate any internal stresses and break up grains of harder material within the metal to produce a better toughness overall.
Normalizing is a heat-treating process used to raise a metal’s strength and hardness. Normalizing involves heating metal above its critical temperature and then cooling by air. This combination results in increased softness and ductility that is particularly beneficial for parts designed to be machined and formed.
Solution annealing is a process that both softens metal and helps it to return to its original shape after a forming process. In this process, a metal component is heated above its critical temperature and then cooled abruptly in a quenching process, typically with water or oil.
Austempering is a form of heat treatment that requires heating a metal part above its critical temperature and then cooling in a salt bath or solution to change its properties. Austempering involves heating a metal part so that it is above the austenite stage and then cooling to just below the martensite stage. This process of cooling produces a bainitic microstructure that provides components with superior strength and ductility compared to those treated by other heat-treating processes.
In summary, heat treating is an important process that can be used to manipulate the physical and chemical properties of metals and alloys in order to change their properties and performance during manufacture and service. With the right application of heat treatment, metal components can have improved hardness, strength, ductility, fatigue and corrosion resistance, improved dimensional stability and wear resistance.
