Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part I. Magnetic Measurements and Mechanical Behavior

The γ→α transformation in 304 stainless steel can be induced by plastic deformation at room temperature. The kinetics of strain-induced transformations have been modeled recently by Olson and Cohen. We used magnetic techniques to monitor the progress of the γ→α transformation in 304 stainless steel sheet loaded in uniaxial and biaxial tension at both low (10-3 per second) and high (103 per second) strain rates. We found that using the von Mises effective strain criterion gives a reasonable correlation of transformation kinetics under general strain states. The principal effect of increased strain rate was observed at strains greater than 0.25. The temperature increase resulting from adiabatic heating was sufficient to suppress the γ→α transformation substantially at high rates. The consequences of the γ→α transformation on mechanical behavior were noted in uniaxial and biaxial tension. Uniaxial tension tests were conducted at temperatures ranging from 50 to -80°C. We found that both the strain hardening and transformation rates increased with decreasing temperature. However, the martensite transformation saturates at ≈85 pct volume fraction α. This can occur at strains less than 0.3 for conditions where the transformation is rapid. Once saturation occurs, the work hardening rate decreases rapidly and premature local plastic instability results. In biaxial tension, the same tendency toward plastic instability associated with high transformation rates provides a rationale for the low biaxial ductility of 304 stainless steel.