On Kinetics of Grain Refinement and Strengthening by Dynamic Recrystallization

The grain refinement and strengthening of an austenitic stainless steel are studied under multiple multidirectional forging at 1073 K up to strains of 4. The structural changes are characterized by the development of discontinuous and continuous dynamic recrystallization (DRX) leading to the grain refinement down to submicrometer level. The new ultrafine grains develop primarily along original grain boundaries and deformation microbands, leading to heterogeneous necklace-like microstructures at intermediate strains followed by rapid expansion of the DRX grains occupying the whole worked sample at large strains. The change in the fraction of ultrafine grains is commonly characterized by a sigmoid-type dependence on strain and can be expressed by a modified Jonson-Mehl-Avrami-Kolmogorov equation. The DRX development is accompanied by a stepwise decrease in the flow stress at reloading in each subsequent forging pass especially in intermediate strains. This stepwise softening results from rapid growth of freshly nucleated grains. In contrast, after forging, both the yield strength and ultimate tensile strength at room temperature increase progressively with an increase in the number of forging passes. The strengthening during DRX development can be attributed to concurrent contribution of work-hardening and grain refinement and can be expressed by a summation of recovered and recrystallized fractions.