A Systematic Assessment of Various Stability/Instability Criteria in Predicting the Hot Deformation-Related Instabilities in Super-304H Stainless Steels

Abstract

In this study, different existing stability/instability criteria (viz Prasad, Murty, Gegel, Alexander, Jonas, and Semiatin) are used to characterize the hot deformation instabilities of super-304H stainless steel. The instability maps are developed based on flow stress data acquired from uniaxial hot compression tests performed at various temperatures (1173-1423 K) and strain rates (0.001-10 s−1). The deformation instabilities are further validated through detailed microstructural observations. Flow localization and cracking have been identified as the signature of deformation instabilities in this alloy. The microcracks have been found to be associated with the grain boundary precipitation of coarse (~ 1- to 5-µm-sized) Nb-rich particles (mainly NbC). The instability criteria developed based on dynamic material modeling (i.e., Prasad’s and Murty’s criteria) could accurately predict the formation of microcracks in the entire hot deformation range employed in this study. Albeit the Gegel’s and Alexander’s stability criteria could correctly predict the flow localization, it has overestimated the deformation instabilities at high temperatures (> 1250 K) and low strain rates (≤ 0.1 s−1). In contrast, Jonas’ and Semiatin’s instability criteria have under-predicted the unstable domains in the studied alloy.