Heat treating is used to alter and improve the physical properties of a given material using a heat treat furnace. Typical heat treatment techniques applied to steel forgings include annealing, normalizing, quenching, and tempering. Precipitation hardening applies to superalloys, titanium and some PH stainless steels.
Annealing is a rather generalized term that consists of heating a metal to beyond the upper critical temperature and then cooling very slowly and at a rate that will produce a refined microstructure. The rate of cooling for annealing is typically slow. Annealing is most often used to soften a metal to improve machinability and to produce a uniform microstructure.
Normalizing is a technique used in a heat treating forge to provide uniformity in grain size throughout an alloy metal. When normalizing, the metal is heated to a temperature just above its upper critical point and then held long enough for smaller and more uniform metal grains to form. This transformation is called grain refinement and leads to the formation of a more uniform piece of metal improving strength and toughness. After a steel piece is heated to a temperature above its critical point, it is air-cooled until it drops to room temperature.
The more uninformed and smaller metal grains can also improve a forging’s response to further heat treatment.
Stress relieving is a forging technique to remove or reduce the internal stresses in a metal. These stresses may be caused by a number of reasons, ranging from cold working to non-uniform cooling after forging. Stress relieving in forging is usually accomplished by heating a metal below the lower critical temperature and then cooling uniformly.
In quenching, a metal is heated above the upper critical temperature and then quickly cooled. Depending on the alloy and other considerations, such as concern for maximum hardness versus cracking and distortion, cooling may be done by different cooling methods. Cooling speeds for the various quench methods, from fastest to slowest, are brine, polymer, freshwater, oil and forced air. The proper quench media is important because quenching certain steels too fast can result in cracking.
Upon being rapidly cooled, the alloy transforms to martensite, a hard, brittle crystalline structure. The quenched hardness of a metal depends on its chemical composition and quenching method. Typically, parts are tempered soon after quenching.
Untempered martensitic steel is very hard but is too brittle to be used for almost any application. Tempering is used to develop the required combination of hardness, strength, and toughness or to relieve the brittleness of fully hardened steels. The combination of quenching and tempering in forging is important to make tough parts.
Tempering is effective in relieving stresses caused by quenching in addition to lowering hardness to a specified range. It’s also used for meeting certain mechanical property requirements for certain steels. Tempering is the process of reheating a steel at a relatively low temperature while time is controlled to produce the final property requirements of a particular steel. The result is a component with the appropriate combination of hardness, strength, and toughness for the intended application.