Heat treatment is a broad term encompassing a number of thermally involved methods and processes used to modify the physical, mechanical and even chemical properties of industrial materials with the goal of adding hardness or ductility.
Through a series of timed heating and cooling cycles, heat treatment processes such as case hardening and annealing cause a realignment of internal atomic structures and in so doing create application specific parts that are softened or hardened as needed. Both formed parts and raw materials may be treated in this manner. Automotive, construction, military, tool and die, shipping, transportation and aerospace industries among others frequently utilize heat treating to produce parts with heightened capabilities required for harsh industrial applications. Not only hardness and ductility, but also tensile, impact and yield strengths are significantly improved or reduced by variable heat treating procedures. Post-production heat treating also relieves stress and tension that may have been an incidental byproduct of manufacturing processes such as cold-rolling, forging and welding. Although heat treating metals and their alloys such as aluminum, steel, copper and stainless steel are the most predominant applications for heat treatment, specialized technologies are commonly employed to alter the physical and mechanical properties of glasses, ceramics and polymers as well. This energy intensive process is extremely versatile, but must be carefully conducted to reduce the opportunity for mechanical failure due to insufficient heating or cooling.
Heat treatment providers often start by considering the end goal of heat treatment. Rather than request a certain procedure, clients dictate the desired results or requirements for a given part or material. Metallurgists and other heat treatment professionals then determine the operations necessary to produce hardened or softened, flexible or rigid components. Often more than one process is used. With the end goal in mind, clients and engineers look at the microstructure, or internal atomic infrastructure of the material to be treated. Grains or crystallites form a complex lattice, the structure of which is reflected in the properties of the pre-treated material. Heat treatment raises the temperature of these grains to what is known as the critical temperature, the point at which the lattice begins to come undone. The temperature, rate and duration of heating as well as the speed, rate and degree of cooling are then manipulated so as to realign the atoms. In general, fast cooling produced via cooled gas or liquids engulfing the material results in coarse grain which provides excellent strength and rigidity, but may also become brittle. Slow cooling, such as that used in annealing produces small grain structures that have impressive strength but are flexible. While heating and cooling are essential to all heat treatments, additional considerations include chemical restructuring.