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Heat Treating Terms

AGE - Second step in precipitation hardening. A treatment consisting of heating to a moderate temperature and remaining there for a sufficient period to produce the optimal precipitate size and distribution to give the desired mechanical properties.

ANNEAL - Heating uniformly to a temperature, within or above some critical range, and cooling at a controlled rate to a temperature under the critical range. This treatment is used to produce a definite microstructure, usually one designed for best machinability and/or to remove stresses, induce softness, and alter ductility toughness or other mechanical properties.

CARBONITRIDE - A modified form of carburizing, consisting of introducing ammonia into the carburizing atmosphere to add nitrogen to the carburized case as it is produced. A carbonitrided case has better hardenability than a carburized case, and thus allows lower alloy grades to attain required hardness at greater depth.

CARBURIZE - In carburizing, a high-carbon surface layer is imparted to low-carbon steel by heating it in contact with carbonaceous materials. On quenching after carburizing, the high-carbon "case" becomes very hard, while the low-carbon core remains comparatively soft. The result is very wear-resistant exterior combined with an interior possessing great toughness. Particularly suitable for gears, camshafts, etc.

NITROCARBURIZE - A low temperature case hardening process that involves the introduction of carbon and nitrogen into a steel to produce a thin layer of iron carbonitrides and nitrides, the "white layer" or compound layer, with an underlying diffusion zone. The diffusion zone increases the fatigue properties, especially in carbon and low alloy steels. Unlike carbonitriding, the process is performed below the critical temperature range and therefore is relatively distortion free. Case depth obtained is less than .003" thick.

NORMALIZE - A special type of annealing for steel heating uniformly to a temperature at least 100° F above the critical range and cooling in still air at room temperature. The treatment produces a recrystallization and refinement of the grain structure and gives uniformity in hardness and structure to the product.

QUENCH - Heating uniformly to a predetermined temperature and cooling rapidly in air or liquid medium to produce a desired crystalline structure.

SOLUTION TREATMENT - A treatment consisting of heating to an elevated temperature and remaining there for a sufficient period of time to dissolve precipitates and create a solid solution. The treatment, which is actually an anneal, is the first step in precipitation hardening. It is also used to dissolve chromium carbide precipitates, which are detrimental to corrosion resistance, in austenitic stainless steels (300 series). Quenching is required for certain materials.

SPHEROIDIZE ANNEAL - A special type of annealing for steel that requires an extremely long cycle. This treatment is used to produce globular carbides and maximum softness for best machinability or to improve cold formability.

STRESS RELIEVE TEMPER - A thermal treatment to restore elastic properties and to minimize distortion on subsequent machining or hardening operations. This treatment is usually applied to material that has been subjected to thermal or mechanical forces that induced residual stress. Ordinarily, no straightening is performed after the stress relieve temper.

SUB-CRITICAL ANNEAL - Actually a high temperature tempering process for steel that produces many of the benefits of annealing but does not require cooling at a controlled rate.

TEMPER OR DRAW - Heating uniformly to some predetermined temperature under the critical range, holding at that temperature a designated period of time and cooling in air or liquid. This treatment is used to produce one or more of the following end results: A) to soften material for subsequent machining or cold working, B) to improve ductility and relieve stresses resulting from prior treatment or cold working, and C) to produce desired mechanical properties or structure in the second step of "Quench and Temper" treatments.

Related Terms

CARBON - The most important alloying element in steel. Strength, hardness, hardenability and ductile-to-brittle transition temperature are increased with increasing carbon content up to approximately .60%. Toughness and ductility are decreased with increasing carbon content.

HARDNESS - Resistance of metal to plastic deformation, usually by indentation. However, this may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting.

MARTENSITIC HARDENING - Thermally induced hardening of metals that relies on crystal structure change. Martensite is the name of the hard room temperature crystal structure of steel. Transformation to this structure is achieved via quenching. Carbon and alloy steels, carburizing steels, tool steels and 400 series stainless steels all rely on this hardening mechanism.

PRECIPITATION HARDENING - Thermally induced hardening of metals that relies on precipitation of solute in a solid solution of 2 or more metals. Formation of precipitates hardens alloys such as 17-4 and 13-8 stainless, maraging steels, beryllium copper, titanium alloys, inconels and hastelloys.

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