Effect of strain inhomogeneity on flow behavior, dynamic recrystallization and substructure evolution of a 904 L super austenitic stainless steel

In this work, two typical deformation processes were conducted on a 904 L super austenitic stainless steel to investigate its hot deformation behavior. Two regions with distinct effective strains were selected on identical cross section to examine the microstructural features, including substructures in the deformed matrix and dynamically recrystallized grains. The flow stress curves generally reflect the physical events that occurred at low strain range. When inhomogeneous microstructure and external factors such as friction become more remarkable at high strain level, some metallurgical events (e.g., partial dynamic recrystallization) are hard to be deduced from the flow curves. Meanwhile, strain dependent friction correction on the flow curves does not completely eliminate an obvious abnormal increase in flow stress, especially for the process with the deformation condition of 900 celcius -1 s-1. Based on the crystallographic analysis on the barreling zone of the deformed samples, it can be concluded that the strain-induced sub-boundaries that formed in the deformed matrix play a vital role in restricting the material flow behavior because these dense boundaries have a Hall-Petch strengthening effect. This is an intrinsic microstructure mechanism to explain the reason that the flow stress increases abnormally at large strains in the absence of dynamic recrystallization. In addition, the formation of twin boundaries at different deformation conditions are also discussed.