TEMPERING
Steel that has been hardened by rapid quenching is brittle and not suitable for most uses. By tempering or drawing, the hardness and brittleness may be reduced to the desired point for service conditions. As these properties are reduced there is also a decrease in tensile strength and an increase in the ductility and toughness of the steel. The operation consists of reheating quench hardened steel to some temperature below the critical range followed by any rate the process lends itself to close control of the physical properties and in most cases does not soften the steel to the extent that annealing would. The final structure obtained from tempering a fully hardened steel is called tempered martensite.
Tempering is possible because of the instability of the martensite, the principal constituent of hardened steel. Low temperature draws, from 300′ F to 400’F (150′ – 205′ C), do not cause much decrease in hardness and are used principally to relieve internal strains. As the tempering temperatures are increased, the breakdown of the martensite takes place at a faster rate, and at about 600’F (315’C) the change to a structure called tempered martensite is very rapid. The temperiing operation maybe described as one fo precipitation and agglomeration or coalescence of cementite. A substantial precipitation of cementite begins at 600’F (315’C), which produces a decrease in hardness, Increasing the temperature causes coalescence of the carbides with continued decrease in hardness.
In the process of tempering, some consideration should be given to time as well as to temperature. Although most of the softening action occurs in the first few minutes after the temperature is reached, there is some addittional reduction in hardness if the first few minutes after the temperature is reached, there is some addittional reduction in hardness if the temperature is maintained for a prolonged time. Usual practice is to heat the steel to the desired temperature and hold it there only long enough to have it uniformly heated.
Two special processes using interrupted quenching are a form of tempering. In both, the hardened steel is quenched in a salt bath held at a selected lower temperature before being allowed to cool. These processes, known as austempering and martempering, result in products having certain desirable physical properties.
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