Optimization of processing parameters based on high temperature flow behavior and microstructural evolution of a nitrogen enhanced 316L(N) stainless steel

The flow behavior of a high nitrogen grade 316L(0.14N) has been studied in a range of temperatures (1073–1423 K) and strain rates (0.001–10 s−1). The material exhibits two-slope behavior in the stress–strain rate relationship at ≤1223 K. It exhibits local negative strain rate sensitivity (LNSRS) in the regime bounded by 1073–1123 K and 0.1–1 s−1. In the LNSRS domain, material shows inhomogeneous flow, reduced ductility and cracks under tensile loading. The experimental data has been used to develop processing map and activation energy map. The material exhibits a wide stable domain below 0.1 s−1 spanning over 1073–1423 K, whereas the domain above 0.1 s−1 is demarcated as unsafe for processing. The stable domain consists of four peaks with efficiency of 35%, 45%, 50% and 45%, respectively. The activation energy for these peaks has been found to vary between 100 and 200 kJ/mol. The localization of heavily deformed grains and random bimodal distribution of grains have been identified as the signatures of instability. Based on the processing map, activation energy map and microstructural evolution, the optimum window for hot deformation has been identified as 1350–1423 K and 0.001–0.05 s−1 with peak efficiency of 50% and activation energy of 150 kJ/mol.