Generation of isochronous stress-strain curves with a nonlinear least square fitting method for modified 9Cr-1Mo steel

This study deals with the generation of isochronous stress-strain curves for modified 9Cr-1Mo steel. Material property data (sy, D, and m) from tensile tests and strain-time curves from creep tests were obtained at 550 °C. On the basis of the experimental data, the creep curves were characterized by Garofalo’s creep model. The three parameters of P1, P2 and P3 in Garofalo’s model could be optimized properly by a nonlinear least square fitting analysis. The three parameters showed a good stress dependency with a linear relation. However, the P3 parameter, which represents the steady state creep rate, exhibited two-slope behavior with different stress exponents at a transient stress of about 250 MPa. The results verify that the long-term creep curves for G91 steel can be modeled by Garofalo’s model with only several short-term creep data. When the modeled creep curves are used, long-term isochronous curves of up to 105 h can be successfully constructed.