Therefore, according to Equation 2, the onsets of DT and DRX are associated with inflection points in a θ − σ plot and with minima in a −(∂θ/∂σ) versus σ curve. In most cases, there are two inflection points, the first of which indicates the onset of DT and the second is the initiation of DRX.
The −(∂θ/∂σ) versus σ curves for the first hit and second hit for medium-carbon low-alloy steel at various experimental temperatures and strain rates are shown in Figs. 3 a and 3 b, respectively. The two minima displayed in Figs. 3 represent the critical stresses at which DT were initiated during deformation. The graph in Fig. 4 show the dependence of DT critical stress versus temperature and strain rate. It can be seen that with the increase in temperature the critical stresses for DT decreases in both first and second hit deformation, whereas, the critical stress for DT increases with strain rate for both first and second hit deformation.