Heat Treating Terms and Definitions

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Heat Treating Terms and Definitions

Heat Treating Terms and Definitions

HARDNESS - The ability of a metal to resist penetration. The principle methods of determining hardness of steel are the Rockwell, Brinell and Scleroscope Tests.

HEAT TREATMENT - An operation or combination of operations involving the heating and cooling ofa metal or an alloy in the solid state for the purpose of obtaining certain desirable conditions or properties.

MARTEMPERING OR MARQUENCHING - This is a method of hardening steel by quenching from the austenitizing temperature into some heat extracting medium, usually salt, which is maintained at some constant temperature level above the point at which martensite starts to form (usually about 450 F.), holding the steel in this medium until the temperature is uniform throughout, cooling in air for the formation of martensite and tempering by the conventional method. The advantages of this method of interrupted quenching are a minimum of distortion and residual strains. The size of the part can be considerably larger than for austempering. MARTENSITE - A microconstituent or structure in quenched steel which has the maximum hardness of ally of the other steel structures resulting from the transformation of austenite.

NITRIDING - See case hardening.

NORMALIZING - Heating steels to approximately 100 F above the critical temperature range followed by cooling to below that range in still air at ordinary temperatures. This heat treat operation is used to erase previous heat treating results in carbon steels to .40% carbon, low alloy steels, and to produce a uniform grain structure in forged and cold worked steel parts.

OIL HARDENING - A process of hardening a ferrous alloy of suitable composition (generally alloys) by heating within or above the transformation range and quenching in oil.

PEARLITE - Another microscopic structure of steel which is produced by slow cooling or air cooling low to medium carbon and low alloy steels from the austenitic state.

QUENCHING AND TEMPERING - In this operation the procedure consists of heating the material to the proper austenitizing temperature, holding at that temperature for a sufficient time to effect the desired change in crystalline structure, and quenching in a suitable medium - water, oil or air depending on the chemical composition. After quenching, the material is reheated to a predetermined temperature below the critical range and then cooled under suitable temperatures (tempering).

ROCKWELL HARDNESS - A hardness test performed on a Rockwell hardness testing machine. Hardness is determined by a dial reading which indicates the depth of penetration of a steel ball for softer steels and a diamond cone for heat treated and harder steels when a load is applied.

SCLEROSCOPE OR SHORE HARDNESS - A hardness test performed on a Shore Scleroscope Hardness Tester. The hardness is determined by the rebound of a diamond pointed hammer (or tup) when it strikes the surface of a specimen. The hammer is enclosed in a glass tube and the height of the rebound is read either against a graduated scale inscribed on the tube, or on a dial, depending on the model used. This type of hardness testing is generally used on large parts which cannot be tested by either using a Rockwell or Brinell machine.

SUB-CRITICAL ANNEALING - Also Stress Relief Annealing. A heat treating operation used to relieve or dissipate stresses in weldaments, heavily machined parts, castings and forgings. The parts are heated to 1150 F., uniformly heated through, and are either air cooled from temperature or slow cooled from temperature depending on the type of part and subsequent finishing or heat treating operations.

TEMPERING - Also termed drawing. Reheating hardened, usually quenched, steel to some temperature below the lower critical temperature followed by any desired rate of cooling after the steel has been thoroughly soaked at temperature. Usual tempering temperatures are 300 to 1100 F.

WATER HARDENING - High carbon grades of tool steel, straight carbon steels and low alloy steels that are hardened by quenching in water during the heat treating operation.

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