Flow Behavior and Microstructure of Hot-Worked Fe-30.9Mn-4.9Al-4.5Cr-0.4C and Fe-21.3Mn-7.6Al-4.3Cr-1C Low-Density Stainless Steels

Abstract

Two as-cast low-density steels grades (austenite-based duplex Fe-30.9Mn-4.9Al-4.5Cr-0.4C and austenitic Fe-21.3Mn-7.6Al-4.3Cr-1C) with an initial dendritic microstructure were subjected to hot working conditions to understand the influence of deformation parameters on flow behavior and microstructural evolution. The alloys were produced using electric arc melting, and their phase constituents were determined using optical microscopy and X-ray diffraction analysis. This was then corroborated with the phase predicted from Thermo-Calc simulation. The as-cast alloys were machined to 10 × 10 × 7 mm specimen configurations for rectangular axial testing on the Gleeble 3500 thermomechanical simulator. The samples were deformed to a total strain of 0.5 at different deformation temperatures (800, 900, and 1000 °C) and strain rates (0.1 and 5 s−1). Thereafter, a hardness test was conducted on the deformed samples, and post-deformed microstructures were analyzed using optical and scanning electron microscopes. The results showed that the alloys’ dendritic structures were effectively transformed at temperatures below 1000 °C regardless of the strain rate. At all deformation conditions, the peak flow stress of Fe-21.3Mn-7.6Al-4.3Cr-1C alloy was at least 50% higher than that of Fe-30.9Mn-4.9Al-4.5Cr-0.4C alloy owing to the higher carbon content in the austenitic low-density stainless steel. The hardness of all the deformed samples was superior to that of the as-cast samples, which indicates microstructural reconstitution and grain refinement in the alloys. Dynamic recrystallization, dynamic globularization, and dynamic recovery influenced the softening process and the microstructural changes observed in the alloys under different deformation conditions.