Unlike other processing techniques, metal heat treatment typically modifies the workpiece's internal microstructure or surface chemical composition to change or enhance its performance and internal quality-changes that are generally invisible to the unaided eye. The workpiece's shape and overall chemical composition, however, remain unchanged.
Steel is the most widely used material in the mechanical industry. Because its microstructure is complex and can be controlled through heat treatment, steel heat treatment is a primary focus. Additionally, metals and alloys like aluminum, copper, magnesium, and titanium can also be heat-treated to modify their mechanical, physical, and chemical properties and thus achieve different performance characteristics.
4 Types Of Heat Treatment
Annealing
Annealing involves heating the workpiece to an appropriate temperature, holding it for a specified time determined by the material and part size, and then cooling it slowly.
The goal is to achieve or approach an equilibrium state of the metal's internal structure or to relieve internal stresses generated by prior processes, thereby improving its workability and performance or preparing the microstructure for subsequent quenching.
Normalizing
Normalizing involves heating the workpiece to the appropriate temperature and then cooling it in air. Normalizing produces effects similar to annealing but results in a finer microstructure; it is often used to improve machinability or as the final heat treatment for parts with less stringent requirements.
Quenching
Quenching involves heating the workpiece, holding it at the specified temperature, and then rapidly cooling it in water, oil, inorganic salt solutions, or organic aqueous solutions. Quenching increases the hardness of steel but also raises its brittleness.
Tempering
To reduce the brittleness introduced by quenching, the workpiece is reheated to a temperature above room temperature but below 650°C, held for an appropriate duration, and then cooled. Tempering is closely related to quenching and is frequently performed together with it.
Annealing, normalizing, quenching, and tempering are known as the "four major heat treatments." Quenching and tempering are often used as a paired (inseparable) process.
