If it is desired to reline the grain structure and produce a lamellar pearlite, a full annealing cycle should be used. This consists of heating the steel to a temperature above the transformation range, holding for one or two hours, and then cooling at a predetermined rate to obtain the desired microstructure. Grain refinement is accomplished in this instance by the re-crystallisation of the steel in passing through the critical range both in heating and in cooling. The microstructure obtained in cooling any steel from above the critical temperature range is dependent both upon the temperature range in that ranges. Thus, it is obvious that the rate at which any steel is cooled determines the final microstructure, since the degree of transformation will depend on the amount of time allowed for it to occur. Therefore, the slower the rate of cooling and the higher the temperature at which complete transformation occurs during full annealing, the coarse the pearlite will be with correspondingly lower hardness.
Such treatment is performed usually on steel of 0.3 to 0.6 % carbon content which is to be machined.
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