Heat Treatment of Steel.

HEAT TREATMENT OF STEEL

INTRODUCE : One of the very important characteristics of steel is the ability to alter the microstructure through heat treatment. As seen in the previous sections, many different microstructural constituents can be produced. Each constituent imparts a particular set of properties to the final product. For example, by quenching a steel in water, the steel becomes very hard but brittle through the formation of martensite. By tempering the quenched steel, some ductility can be restored with some sacrifice in hardness and strength. Also, superior wear properties can be obtained in fully pearlitic microstructures, particularly if an accelerated cooling process is employed to develop a fine interlamellar spacing. Complex parts can be designed by taking advantage of the formability and ductility of ferritic sheet steel through cold rolling and annealing. The amount of pearlite in ferritic steel can be adjusted by carbon content and cooling rate to produce a wide range of hardness and strength. In quenched and tempered steels, a bainitic microstructure has a unique combination of high strength and toughness. Thus steel, more than any other metallic material, can be manipulated through heat treatment to provide a multiplicity of microstructures and final properties. The common types of heat treatment are listed below,

ANNEALING (FULL ANNEALING) : One of the most common heat treatments for steel is annealing. It is used to soften steel and to improve ductility. In this process, the steel is heated into the lower regions of the austenite-phase field and slow cooled to room temperature. The resulting microstructure consists of coarse ferrite or coarse ferrite plus pearlite, depending upon carbon and alloy content of the steel.

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 Steel 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, 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 fer-rite grains, 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.

SK NAJMUL (munna)

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