Heat Treating Terms

The following list includes the majority of processes and explanations utilized in today’s heat treating industry. For more information on any of these processes, please contact Phoenix Heat Treating’s metallurgist, Peter Hushek.

The change that occurs in the properties of metals and alloys at room temperature or slightly elevated temperatures after heat treating or cooling. This process can be applied to numerous alloys, including aluminum, nickel based super alloys, stainless steels and copper alloyed with beryllium.

A process used for steels that contain sufficient quantities of carbon and other alloys that will harden during air cooling or in controlled cooling conditions using other gasses. The steel is cooled from a specified heat-treated temperature to meet the hardening specs required on the job order. Air hardening is generally used for distortion critical tool steel components which cannot handle the distortion induced from oil quenching.

When maximum ductility and high strength is not needed, ductile iron castings and parts are processed with a full anneal. The metal is heated to a required temperature range and held at that temperature for the specified amount of time, then cooled at a controlled rate. The microstructure is converted to ferrite and spheroidal graphite. Elements that retard annealing, such as manganese and phosphorus, and alloying elements including chromium, nickel, copper and molybdenum are kept low to produce optimum machinability. Annealing will soften metals to a specific microstructure or change the molecular properties of the part. Annealing will also remove stress and align the molecular properties to increase strength. Annealing is often performed on a variety of metals to make magnetic properties more uniform. The Magnetic Anneal requires tight control of descending ramp rates and temperature uniformity.

Austenitizing is the process of forming austenite (the solute is usually carbon), by heating a ferrous alloy above the transformation range. Austenitizing is not a complete process, but is the first step used prior to normalizing, annealing of ferrous alloys or quench-hardening of ferrous alloys.

The hardness of a material is determined by forcing a hard steel or carbide ball of a specified diameter into the material. The Brinell Hardness number is the value derived by dividing the applied load in kilograms by the specified area of the resulting impressions in square millimeters. The Brinell test is mostly performed on softer material including cast irons, aluminum alloys and annealed steel alloys.

The absorption and diffusion of carbon into solid ferrous alloys by heating to a temperature range above the transformation temperature of the alloy in a controlled gas atmosphere to cause the absorption of carbon and nitrogen at the surface. Temp ranges required are 1650 to 1900-degrees-F (900 to 1040-C). Heating takes place in a carbon-rich environment of liquid, solid or gas to produce a carbon gradient that extends from the surface into the material. Carbonitrided parts are then cooled at a rate required on the job order to produce the desired properties in the material. Carbonitriding is used for thin, hard, wear-resistant cases on hardware parts manufactured via mass production. Carbonitriding is very similar to the cyaniding process which was performed in a salt bath many years ago. The resulting case structure is nearly identical to cyaniding, though not as economical due to environmental cost process.

This process replaces the carbon lost in the surface layer through previous heat treating or hot working of the metal by carburizing this layer to replenish the original carbon content. The process is also called recarburizing. This process is typically applied to alloy steel forgings or casting which must retain a uniform structure over the entire surface of a part.

A process that causes absorption and diffusion of carbon into solid ferrous alloys by heating the material to temperatures usually above the transformation temp of the alloy, 1650 to 1900-degrees F. (900 to 1040-C). Processing takes place in a carbonaceous environment consisting of gas, liquid or solid. The result is the diffusion of carbon into the material surface to the specified depth, resulting in a hardened surface that is completed by quenching from the carburizing temp or by cooling to room temp; then reaustenitizing and quenching. After quenching, the high-carbon exterior becomes harder, while the low-carbon core remains softer. This process is used for processing gears and requires tight controls on gas composition, temperature uniformity and atmosphere circulation.

This process can result from a chemical composition change of the surface of steel by absorption of carbon and/or nitrogen, and through diffusion, a concentration gradient is created. The processes commonly used are carburizing, carbonitriding, ferretic nitrocarburizing or nitriding. There is another type of case hardening where a localized area is quickly heated to the austenitizing temperature and rapidly cooled. The heating is typically performed by induction or flame in this process.