Effect of hot working on structure and strength of type 304L austenitic stainless steel

Abstract

The development of microstructure and strength during forging in a single-phase austenitic stainless steel, 304L, was investigated by means of forward extrusion of cylindrical specimens. The temperature, strain, and strain rate of deformation were varied. A low strain rate was imparted by press forging (PF), and a high strain rate by high-energy-rate forging (HERF). Low forging temperatures produced dynamically recovered microstructures and monotonic increases in strength with increasing strain for low and high strain rates. At higher forging temperatures, the high-energy-rate-forged material exhibited softening, after the application of a critical amount of strain, as a result of static recrystallization which occurred within a few seconds after cessation of deformation. Analysis of isothermal compression test data, specifically the strain-to-peak stress associated with the onset of dynamic recrystallization, confirmed that dynamic recrystallization would not be expected for the deformation conditions imposed during forward extrusion in this study. Recrystallized grain size was found to vary uniquely with strain, initial grain size, and the Zener-Hollomon parameter. Recrystallization was much less prevalent in press-forged material and may have been affected by die chilling as well as the predominance of dynamic recovery. The variation of strength, recrystallized grain size, and extent of recrystallization with the deformation parameters, temperature and strain, are presented as a set of processing-property maps for each forging technique (έ). The findings are discussed in the context of developing process design criteria for forging alloy 304L.