A heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time, and then allow slow cooling.
Black oxide is a conversion coating for ferrous materials, stainless steel, copper, zinc, and powdered metals. It is used to add mild corrosion resistance, for appearance and to minimize light reflection. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax. One of its advantages over other coatings is its minimal buildup.
Carburizing is a case-hardening process to improve surface hardness for low-carbon steels. This is achieved by adding free carbon to the processing atmosphere and allowing it to diffuse into the surface of the part. The carbon-infused surface layer has superior wear capabilities and high Rockwell hardness specifications.
Carbonitriding is a case-hardening process similar to carburizing. It is generally performed at lower temperatures with the addition of nitrogen via ammonia gas. This process increases wear resistance and surface hardness through the creation of a hardened surface layer, which is diffused at these lower temperatures. The diffusion of nitrogen at these temperatures provides a more resistant case and increases the hardenability of low-carbon steels. Carbonitriding is well suited for parts that require wear-resistant properties with reduced distortion. Case depths for carbonitriding are generally much lower than those of carburized parts, varying from a low of 0.003 to 0.030 inches (0.08mm to 0.76mm). Parts are commonly quenched in oil and tempered at low temperatures to increase toughness and reduce brittleness.
Everything begins with neutral hardening. Also known as "quench and temper," this process improves the strength and toughness of medium- to high-carbon steels evenly through the entire part. This result is achieved by heating parts above the phase-transformation temperature for a set period of time to change their crystalline structure from body-centered cubic to face-centered cubic. The part is then plunged rapidly into a quenchant to change the structure to a tetragonal arrangement, thus giving it a remarkably high hardness. Oil and water are both common quenchants, with oil being the less aggressive of the two. The choice to use water or oil is determined by the material being used and the desired metallurgical properties of the finished part. After quenching, parts must be reheated (i.e., tempered or drawn down) to the desired hardness to create a tempered martensitic microstructure. Tempering of quenched steel is a critical final process to relieve microstructural stresses in the part and prevent distortion / cracking while simultaneously improving toughness and ductility. After the tempering process, the part is tested for hardness using the Rockwell scale, and several material properties can be extrapolated from that measurement.
Normalization is an annealing process applied to ferrous alloys to give the material a uniform fine-grained structure and to avoid excess softening in steel. It involves heating the steel to 20–50 °C above its upper critical point, soaking it for a short period at that temperature and then allowing it to cool in air. Heating the steel just above its upper critical point creates austenitic grains (much smaller than the previous ferritic grains), which during cooling, form new ferritic grains with a further refined grain size. The process produces a tougher, more ductile material, and eliminates columnar grains and dendritic segregation that sometimes occurs during casting. Normalizing improves machinability of a component and provides dimensional stability if subjected to further heat treatment processes.
In wheel blasting, a spinning wheel propels the abrasive against an object. It is typically categorized as an airless blasting operation because there is no propellant used. A wheel machine is a high-power, high-efficiency blasting operation with recyclable abrasive (typically steel or stainless steel shot, cut wire, grit, or similarly sized pellets).
Heat straightening involves applying controlled heat to a deformed part of a steel specimen in heating and cooling cycles until the metal gradually straightens. Heat straightening does not symmetrically straighten the metal and is a gradual process. Cold straightening is utilized if too much heat can damage the metal. Mechanical forces can also be applied to the heated metal to straighten it. The metal specimen is laid on a flat surface and a metal peening hammer is used on the concave surface to create depressions in the metal. The act of creating these depressions causes the material on that side to stretch or elongate. This, in principle, tends to straighten the material. Other tools used to straighten metal include presses, vices and clamps, which hold the metal specimen in place so the operator can move a part of the metal until it straightens.
Stress relieving is carried out on metal products in order to minimize residual stresses in the structure thereby reducing the risk of dimensional changes during further manufacturing or final use of the component.
Cryogenic hardening is a treatment process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is used for tool steels, high-carbon, high-chromium steels and in some cases to cemented carbide to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides in the matrix during this treatment which imparts very high wear resistance to the steels.
