COMPARISON BETWEEN THE METHODS OF DETERMINING THE CRITICAL STRESS FOR INITIATION OF DYNAMIC RECRYSTALLIZATION IN 316 STAINLESS STEEL

Abstract

The critical stress for initiation of dynamic recrystallization ( σc) could be identified by the inflection point on the strain hardening rate (θ=dσ/de) versus flow stress ( σ ) curve. Several methods have been proposed to calculate this value on the basis of mathematical methods. One of them was proposed by Stewart, Jonas and Montheillet in which this critical point appears as a distinct minimum in the (dθ/dσ versusσ ) through differentiating from θ versus σ . Another one was presented by Najafizadeh and Jonas which obtain by modifying the Poliak and Jonas method. According to this method, the strain hardening rate was plotted against flow stress, and the value of σ c was obtained numerically from the coefficients of the thirdorder equation that was the best fit from the experimental θ σ data. In this study, hot compression tests were used in the range of 1000 to 1100 °C and strain rates of 0.011s 1strain of 1 for the above mentioned methods to compare the value of the σc for 316 stainless steel. The result shows that the method presented by Najafizadeh and Jonas not only simpler than the previous one but also, it has a good agreement with microstructures. Furthermore, the normalized critical stress for this steel was obtained uc = σc/σ p = 0.92.