Computer Simulation Analysis of the Deformation Behavior of 304L and 316L Steels at Cryogenic Temperatures

It is generally known that the yield stresses of 304 series steels at cryogenic temperatures are remarkably low compared to those of other more stable stainless steels such as 316L. In addition, load drops in the serration at cryogenic temperatures are larger for 304 series steels than in the latter steels. Martensitic transformation induced during deformation is thought to contribute to such behavior, but the details have not been clarified. Therefore, the effects of the transformation on the deformation behavior of 304L and 316L steels at cryogenic temperatures were examined using computer simulation. The simulation was carried out based on procedures previously developed by Shibata et al. As for the effects of the transformation, strain, heat and strengthening generated by the transformation were considered. Results showed that heat generated by the transformation increases the load drops in the serration. The low-yield stress was revealed by the transformation strain induced by an applied elastic stress. But Ogata et al. reported that X-ray diffraction detected no martensitic transformation at ambient temperature in 304L steel deformed at 4K by a very small plastic strain. On the other hand, Nyilas et al. reported that the transformation induced by a very small strain at cryogenic temperatures reversed to austenitic phase during heating to ambient temperature. Hence, it is expected that, in 304L steel, the martensitic transformation to ε induced by an elastic stress suppresses the yield stress at cryogenic temperature and the ε reverses to austenitic phase in the process of heating to ambient temperature.