Predicting the critical stress for initiation of dynamic recrystallization

The critical stress for initiation of dynamic recrystallization (DRX) can be identified from the inflection point on the strain hardening rate (theta=d sigma/d epsilon) versus flow stress (a) curve. This kind of curve can be described by an equation that fits the experimental theta-sigma data from zero to the peak stress. Such a curve must have an inflection point and the simplest relation that has such properties is a third order equation. Hot compression tests were carried out on a 304 H stainless steel over the temperature range 900-1 100 degrees C and strain rate range 0.01-1s(-1) to a strain of 1. An appropriate third order equation was fitted to the strain hardening data. The results show that the critical stress at initiation sigma(c)=-B/3A where A and B are coefficients of the third order equation. It is evident that this value depends on the deformation conditions. The stress-strain curve was then normalized with respect to the peak stress, leading to a normalized value of the critical stress (u(c)) equal to u(c)=sigma(c)/sigma(p)=-B'/3A'. Here A' and B' are coefficients of the normalized third order equation. This value is constant and independent of the deformation conditions.