Thermal recovery in 304 stainless steel following explosive shock loading and explosive forming

Abstract

The annealing behavior of type 304 stainless steel sheet samples shock-loaded at pressures between 750 and 1200 kbar, and cylindrical samples explosively expanded between 32 and 62 pct is characterized by rapid (catastrophic) hardness recovery between 750° and 850°C. Apparent aging effects, the occurrence of precipitates and dislocation loops of varying sizes, and a preferential etching at grain boundaries in thin electropolished foils is observed at 750 and 1200 kbar and 32 and 62 pct. Electron transmission microscope observations revealed that the catastrophic recovery and subsequent pseudorecrystallization were essentially synonymous, with no apparent nucleation and growth of new grains. Recovery is characterized mainly by the collapse of deformation twins and the accelerated annihilation of dislocations. Considerably less recovery is observed for lower shock pressures (<750 kbar), and for less explosive expansion (<25 pct) as evidenced by the presence of numerous dislocations at temperatures where complete recovery has occurred for higher pressure samples. It is suggested that a nonequilibrium vacancy concentration exists at the higher shock-pressures and larger degrees of explosive expansion, and that this feature is largely strain-rate dependent.