Abstract: Several methods have been proposed to calculate the critical stress for initiation of dynamic recrystallization (σc) on the basis of mathematical methods. One of them is proposed by Stewart et al. in which this critical point appears as a distinct minimum in the (-dθ/dσ vs σ) through differentiating from θ vs σ. Another one is presented by Najafizadeh and Jonas by modifying the Poliak and Jonas method. According to this method, the strain hardening rate was plotted against flow stress, and the value of σc was attained numerically from the coefficients of the third-order equation that was the best fit from the experimental θ-σ data. Hot compression tests were used in the range of 1000 to 1100℃ with strain rates of 0.01–1 s-1 and strain of 1 on 316 stainless steel. The result shows that Najafizadeh and Jonas method is simpler than the previous one, and has a good agreement with microstructures. Furthermore, the value of normalized critical stress for this steel was obtained μc=σc/σp=0.92.

Key words: Dynamic recrystallization, Critical stress, Strain hardening rate