The soften mechanisms of martensitic heat resistant steels during hot deformation

Hot deformations for the experimental steels were carried out in the temperature range of 900-1200 degrees C and the strain rate range 10-3-100 s(-1). In this study, the critical condition for dynamic recrystallization (DRX) was determined by a cooperation of the minimum value of -(partial derivative theta/partial derivative sigma) and the deviation point of the linear slope of theta - sigma curve. From the analysis, it was found that only one slope ratio of critical strain, epsilon (c) versus critical stress, sigma (c) appeared in the 9Cr and 10Cr steels, whilst two different slopes in P92 and NS steels due to the augmentation of auxiliary softening effect of the dynamic strain-induced transformation (DSIT). The results also exhibited that ln(Z) kept an approximate positive linear relationship with ln(epsilon (c)) below the value of 40, but barely changed beyond it. However, the ln(sigma (c)) value increased continuously with the rise of the ln(Z) value at a smaller growth rate after the value of ln(Z) reached 40 than that of the initial part.