Effect of initial grain size on the mechanical behaviour of austenite during deformation under hot-working conditions

Abstract

The present investigation has been conducted to study the effect of the austenitic grain size (AGS) on the flow stress behaviour of austenite, deformed in a wide range of temperatures and strain rates, employing samples of a commercial ER7 steel grade. To accomplish this objective, a novel general methodology for the analysis of the austenitic grain growth kinetics under combined arbitrary isothermal and non-isothermal austenitizing treatments has been developed. In this way, it has been possible to characterize the AGS effect when this parameter varies between approximately 135 to 260 μm. For this purpose, axisymmetric compression tests have been conducted in the temperature range of 850 °C–1250 °C, at strain rates in the range of 0.005–10 s−1. The experimental results show that the constant associated with the strength of grain boundaries in the Hall-Petch relationship, is approximately 57 MPa μm1/2. Accordingly, the athermal stress values are in the range of 3–5 MPa. Also, the exponent which characterizes the effect of the AGS on the time required to achieve 50% DRX, was found to be approximately 0.1, which also reflects a rather weak effect of this parameter on the evolution of the volume fraction recrystallized dynamically. The relevance of analyzing the DRX kinetics of the alloy based on the recrystallization time rather than on the applied strain is also discussed. The prediction of the change in the volume fraction recrystallized dynamically during deformation under variable strain rate conditions is analyzed and the verification of the conditions which promote DRX is highlighted. A set of sequential equations encompassing both a physically-based constitutive description, as well as a simple parametric relationship for the computation of the flow stress of the alloy, are proposed.