Metal Heat Treating Glossary
Heat treating is the intentional heating and/or cooling of a material to change the properties of the material for a desired purpose, normally in the case of metals to either harden or soften. Metallurgical Heat Treating processes include case hardening, through hardening, annealing, precipitation strengthening, quenching, tempering and aging, among many others.
Normally applied to ferrous alloys, there are basically two types of hardening: case hardening and through hardening.
Case hardening is a heat treatment in which the outer surface of a mild grade steel is made substantially harder than the interior by altering its composition. The steel part is heated in a furnace with a chemically reactive source of carbon such as an endothermic gas atmosphere to above its critical transformation temperature for a sufficient length of time, and then cooled rapidly, usually in a quench medium such as oil. Through hardening is a heat treatment in which the whole structure of a medium or high carbon steel is heated to above its critical transformation temperature, not necessarily in the presence of chemically reactive carbon, and then cooled rapidly (quenched).
Tempering and Drawing
Drawing is a common term for tempering; both mean the same thing. Tempering is a method used to increase the toughness of ferrous alloys after hardening and quenching by reheating to below the critical transformation temperature for a requisite amount of time, and then cooling, usually in still air. This process reduces the hardness of the alloy and increases its ductility so that it becomes tougher, or more resistant to fracture. Precise time and temperature control is necessary to balance the dual objectives of sufficient hardness with the necessary ductility.
Annealing and Stress Relieving
Annealing should not be confused with Stress Relieving; these are two separate processes. Annealing is intended to alter the microstructure of the alloy being processed, whereas stress relieving is intended to relieve residual stresses in the material built up from cold working, flame cutting or welding without altering the underlying microstructure.
Annealing is the process of heating an alloy to an elevated temperature to achieve a homogenous chemistry, equiaxed microstructure and uniform mechanical properties to produce what is essentially stress free, or “soft” material. There are three stages in annealing: recovery, recrystallization, and grain growth. Recovery happens at a lower temperature and involves softening the material by reducing any internal stresses. At higher temperatures recrystallization occurs, whereby new stress-free grains are formed and replace the crystals that were deformed by stress. If the annealing process is extended then grain growth can occur, reducing the strength of the material – which can be subsequently regained by hardening. Stress relieving is the process of heating a stressed part to below the critical temperature (around 1200 °F for most steels) in a furnace or oven, and letting it cool slowly to room temperature. This reduces the residual stresses so that the part can be further processed without unwanted dimensional changes.
Forging is the process of forming metal parts by means of localized compressive force. It can be done at low temperature, which is a type of cold working, but normally is done with work pieces that have been heated in a specific type of furnace, or forge. If the work temperature is below the recrystallization temperature of the material, the process is known as warm forging, and hot forging is done above the recrystallization temperature. The higher the temperature, the less force needs to be brought to bear on the work piece to accomplish the desired form.
Heat Treating Magnetic Steel
Magnetic steel is an alloy designed to produce magnetic properties and is used to fabricate such electrical devices as motors and transformers. The steel is usually produced in thin (less than 3/32”) strips which are cut to a desired shape and then stacked together to form what is called a core. The alloy is rich in silicon, and must be heat treated specifically to produce proper grain sizes inside the crystalline microstructure. The time and temperature cycle must be carefully controlled, and a protective atmosphere is used to avoid oxidation.
Weld Preheating is the application of heat to the base metal prior to welding. This may be necessitated by welding code, or may be indicated from past experience with weld fractures. Furnaces can be used to preheat the work piece in situations where the work piece is small enough to fit inside the chamber. Normally the temperature does not have to be precisely controlled, but must be held below the critical temperature to prevent changes in the base metal microstructure.