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A Quick Look at Heat Treating Processes for Metals

Steels and other metals have been boon to engineering. Engineers, in turn, have learned how to improve many metals and tailor their properties to the task at hand. Here are some of the heat treating processes metalworkers rely on to ensure the metals they turn out have the properties customers need.

Annealing. Heating and slowly cooling metal (usually steel) to remove stress, make the metal softer, refine the structure, or change its ductility.

Carburizing. Adding carbon to the surface of iron-based alloys by heating the metal to below its melting point (usually 1,560 and 1,740°F) and putting it in contact with carbon-rich solids, liquids, or gasses. This increases the metal’s strength and makes the surface harder and more abrasion-resistant, but also decreases it toughness. Carburizing is usually done on finished parts.

Case hardening. This hardens an iron alloy’s surface by first carburizing and then quenching it to fix the process, so that the center is relatively soft. Thus, the part consists of a hardened case around a soft core.

Cyanide hardening. This is a variant on case hardening that brings the metal surface in contact with molten cyanide salt. The part is then quenched.

Decarburization. This involves the removal of carbon from the surface of steel, either by applying heat or through the normal aging process of oxidation.

Drawing (tempering). Metal is reheated after being hardened, then held at a specific temperature, and finally quenched. This process reduces hardness and increases toughness.

Nitriding. This is a hardening process whereby nitrogen is added to the metal surface through contact with ammonia gas. It produces case hardening without quenching. 

Precipitation hardening. This is a hardening process (also known as age hardening) in which certain metals are held at elevated temperatures without quenching. The process increases the yield strength of malleable materials, including most structural alloys of aluminum, magnesium, nickel, titanium, and some stainless steels. In superalloys, it can improve high-temperature strength.

Quenching. Steel is rapidly cooled by immersing it into oil or water, fixing its structure in a hardened state.

Sphereoidizing (anneal). This describes any process of heating and cooling steel for the purpose of increasing the amount of rounded or globular carbide in the metal. It softens the steel and improves its ductility.

Cold working. While not a heat treating process, it is used to change a metal’s characteristics. This is accomplished by deforming the metal at room temperature without fracturing it. This changes its shape, giving the metal more tensile strength and better machinability.

David Zimmermann is president of Pivot Point Inc., Hustiford, Wis.

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