The lower temperature limit of hot working is the lowest temperature at which the rate of re-crystallization eliminates strain hardening. This lower limit depends upon
(i) amount of deformation
(ii) time for which the metal is at that temperature
Thus lower temperature limit for hot working is decreased, for large deformation.
Most hot working operations are done in a number of steps. The working temperature of intermediate phases is kept much above the lower temperature limit. At this temperature, the amount of deformation is more, as well as the cost of working is lower. But at this temperature grain growth will be there. To have, finally the fine grain size structure, the working temperature in the last pass (or step) is lowered. This finishing temperature is usually above the minimum re-crystallization temperature.
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