The Strain Dependence of Post-Deformation Softening during the Hot Deformation of 304H Stainless Steel

Document Type: Research Paper

Authors

1 Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran

2 Department of Metallurgical Engineering McGill University, Montréal, Canada

Abstract

Experiments were carried out in which the dependence of the fractional softening on temperature, time and strain rate was determined in a 304H stainless steel. Three prestrain ranges were identified pertaining to three different post-deformation softening behaviors: 1) prestraining to below the DRX critical strain: strongly strain dependent softening by SRX alone with softening kinetics controlled by growth rate of the nuclei; 2) prestraining to above the DRX critical strain: SRX + MDRX softening with weaker strain dependence of the kinetics but still controlled by grain growth; 3) 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= D0/DDRX > 4. 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 t50 exhibits a minimum. These data differ somewhat from observations obtained in the torsion testing of solid bars, in which no strain dependence of t50 was detected at e* and beyond. Whether or not the strain dependence of t50 vanishes in the MDRX range is sensitive to the test method employed to study the post-deformation softening.

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