A. Najafizadeh - Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran
J. Jonas - Department of Metallurgical Engineering McGill University, Montréal, Canada

Experiments were carried out in which the dependence of the fractional softening on temperature, time and strain rate was determined in a ۳۰۴H stainless steel. Three prestrain ranges were identified pertaining to three different post-deformation softening behaviors: ۱) prestraining to below the DRX critical strain: strongly strain dependent softening by SRX alone with softening kinetics controlled by growth rate of the nuclei; ۲) prestraining to above the DRX critical strain: SRX + MDRX softening with weaker strain dependence of the kinetics but still controlled by grain growth; ۳) at a prestrain of e* and beyond: nucleation-controlled MDRX softening with the full inhibition of SRX. The transition prestrain e* can exceed the peak strain if the DRX grain refinement  ratio g= D۰/DDRX > ۴. The transition to MDRX-dominated softening can be attributed to a constant value of the normalized strain hardening rate independent of the preloading temperature and strain rate. The softening data from the compression tests show that at e*, the time for half softening t۵۰ exhibits a minimum. These data differ somewhat from observations obtained in the torsion testing of solid bars, in which no strain dependence of t۵۰ was detected at e* and beyond. Whether or not the strain dependence of t۵۰ vanishes in the MDRX range is sensitive to the test method employed to study the post-deformation softening.

Hot deformation, ۳۰۴H stainless steel, Metadynamic Recrystallization, Softening kinetics