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

Abstract

This study deals with the generation of isochronous stress-strain curves for modified 9Cr-1Mo steel. Material property data (s_y, 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 P ₁, P ₂ and P ₃ 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 P ₃ 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.