Recent advances in hot working: Fundamental dynamic softening mechanisms

Abstract

The fundamental mechanisms responsible for the low rate of strain hardening during high temperature deformation and for a steady state of flow at high strains have been confirmed to be: I. Dynamic recovery, which limits the accumulation of dislocations through annihilation and which operates at all strains in all metals; and II. Dynamic recrystallization, which eliminates dislocations through the migration of grain boundaries and which only operates beyond a critical strain when the dislocation density becomes high enough to give rise to the nucleation and growth of new grains. These softening processes are retarded by the presence of solute atoms and second phase particles which reduce the mobility of both dislocations and high angle boundaries. These effects have some similarities to those observed under cold working and annealing but there is a strong dynamic element introduced by straining at the elevated temperature. As a result of the high strains imposed there is much more microstructural change than during creep loading. Industrial hot forming processes generally involve several stages of deformation separated by intervals during which static recovery or recrystallization take place. The interaction between dynamic and static softening processes under industrial conditions will be the subject of a sequel paper. This work will also consider the trends in hot ductility; the latter depends on the retardation of grain boundary cracking by dynamic recovery and recrystallization. Finally, since the effects of alloying in hot working have been treated only in a general way, the behavior of specific materials and their thermomechanical processing will be reviewed in the third and fourth papers of this-series.