Spheroidizing

Normalizing. Steel is normalized by heating into the austenite phase field at temperatures somewhat higher than those used by annealing followed by air cooling. Many steels are normalized to establish a uniform ferrite plus pearlite microstructure and a uniform grain size.

Spheroidizing. To produce a steel in its softest possible condition, it is usually spheroidized by heating just above or just below the eutectoid temperature of 727 C and holding at that temperature for an extended time. This process breaks down lamellar pearlite into small spheroids of cementite in a continuous matrix of ferrite, as seen in Fig. 14. To obtain a very uniform dispersion of cementite spheroids, the starting microstructure is usually martensite. This is because carbon is more uniformly distributed in martensite than in lamellar pearlite.The cementite lamella must first dissolve and then redistribute the carbon as spheroids whereas the cementite spheroids can form directly from martensite.

Process Annealing (Recrystallization Annealing). Process annealing takes place at temperatures just below the eutectoid temperature of 727 C. This treatment is applied to low-carbon, cold-rolled sheet steels to restore ductility. In aluminum-killed steels, the recrystallized ferrite will have an ideal crystallographic texture (preferred orientation) for deep drawing into complex shapes such as oil filter cans and compressor housings. Crystallographic texture is produced by developing a preferred orientation of the ferrite grains, i.e., the crystal axes of the ferrite grains are oriented in a preferred rather than random orientation.

Stress Relieving. Steel products with residual stresses can be heated to temperatures approaching the eutectoid transformation temperature of 727 C to relieve the stress.

Quenching. To produce the higher strength constituents of bainite and martensite, the steel must be heated into the austenite phase field and rapidly cooled by quenching in oil or water. High-strength, low-alloy steels are produced by this process followed by tempering. It must be noted that employing microalloying additions such as Nb, V, and Ti can also produce HSLA steels. These microalloyed steels obtain their strength by thermomechanical treatment rather than heat treatment.

Tempering. When quenched steels (martensitic steel) are tempered by heating to temperatures approaching the eutectoid temperature of 727 C, the dissolved carbon in the martensite forms cementite particles, and the steels become more ductile. Quenching and tempering are used in a variety of steel products to obtain desired combinations of strength and toughness.

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Bruce L. Bramfitt

International Steel Group, Inc.

Research Laboratories

Bethlehem, Pennsylvania

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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