Types of Heat Treatment
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| Before going into the effect of carbon additions to iron on the above described transformations, brief general descriptions of the principal. Types of heat treatment given to steel will be reviewed at this point. Following this brief survey of the types of heat treatment, the effect of carbon additions to iron on crystal structures will be described, to rates to obtain desired results by various heat treatment.
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| Five operations are detailed in this lesson as the basis of heat treatment. Explanations of these operation follow.
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Full Annealing is the process of softening steel by heating and cooling cycle, so that it may be bent or cut easily. In anneling steel is heated above a transformation temperature and cooled very slowly after it has reached a suitable temperature. The distinguishing characteristics of annealing are; a. temperature above critical temperature and b. very slow cooling, usually in the furnace.
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| Normalizing is identical with annealing, except that the steel is air cooled; this is much faster than cooling in a furnace. Steel is normalized to refine grain size, make its structure more uniform, or to improve machinability.
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| Hardening is carried out by quenching a steel, that is cooling rapidly from a temperature above the transformation temperature. Still is quench in a water or brine for the most rapid cooling, in oil for some alloy steels, an in air for certain higher alloy steels. After steel is quenched, it is usually very hard and brittle; it may even crack if dropped. To make the steel more ductile, it must be tempered.
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| Tempering formerly called drawing, consist of reheating a quenched steel to a suitable temperature below the transformation temperature. How this process makes steel tough will be discussed later.
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| Stress relieving is the heating of steel to a temperature below the transformation temperature, as in tempering, but is done primarily to relieve internal stress and thus prevent distortion or cracking during machining. This is sometimes called process annealing. The transformation temperature was referred to varies with the carbon alloy content of steel. Until recently the transformation temperature was referred to as critical temperature.
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Reasons for Heat Treating
Heat treatment of steel is usually intended to accomplish anu one of the following objectives:
- · Remove stresses induced by cold working or to remove stresses set up by nonuniform cooling of hot metal objects
- · Refine the grain structure of hot worked steel which may have developed coarse grain size
- · Secure the proper grain structure
- · Decrease the hardness and increase the ductility
- · Increase the hardness so as to increase resistance to wear or to enable the steel to withstand more service conditions
- · increase the toughness; that is, to produce a steel having both a high tensile strength and good ductility, enabling it to withstand high impact
- · improve the machinability
- · improve the cutting properties of tool steels
- · improve the electrical properties
- · change or modify the magnetic properties of steel
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