Strain hardening analysis of an austenitic stainless steel at high temperatures based on the one-parameter model

An austenitic stainless steel AISI 316L is deformed at temperatures from 700 to 1000 °C in the strain rate range 10−5–10−2 s−1. The strain hardening of this alloy is investigated by means of the one-parameter model. The mechanistic interpretation of this model has predicted that the best linear fits of the normalised differential data (normalised strain hardening rate Θ/μ vs. normalised flow stress σ/μ, with μ the elastic shear modulus) give normalised back-extrapolated strain hardening rates to zero stress (namely Θo/μ) that depend on the thermal activation parameter of flow stress s. At low and intermediate temperatures s is almost constant and, consistently, Θo/μ is found constant. However, at high temperatures s changes significantly with temperature and strain rate, so the strain hardening analysis of AISI 316L takes account for the predicted temperature and strain rate dependence of Θo/μ through s. From this analysis, Θo, steady state stresses σV, and parameters s′ (proportional to s) are determined from the fits of the differential data. A new constitutive equation for plastic flow in steady state conditions is proposed, in which Θo, σV and s′ are used as inputs. In the conventional Kocks–Mecking (KM) plot (σV/μ against the normalised activation enthalpy of plastic flow) the AISI 316L data show a significant scattering at temperatures higher than 800 °C. Conversely, in the modified KM plot based on the new constitutive equation, the high temperature data do not show any scattering at all. The parameters determined from the fits result to be consistent to their physical meanings. It is concluded that in this austenitic stainless steel at high temperatures the strain hardening behaviour is mainly affected by dynamic recovery driven by glide, rather than diffusion.