Creep strength of Mod.9Cr1Mo steel under biaxial stress states

Abstract

This paper describes the development of tension, torsion, and combined tension-torsion creep testing machine with hydro static thrust bearing, criterion on creep rupture strength under biaxial stress states was examined. Tension, torsion, and combined tension-torsion creep tests were carried out at 873 K in air employing the developed creep testing machine and hollow cylindrical specimen of Mod.9Cr1Mo steel. Ratio of rupture time t_R/t_R^* increased with the decreasing principal stress ratio λ at constant Mises' equivalent stress, where t_R, and t_R* denote rupture times for biaxial and tension creep tests, and λ expresses ratio of minimum principal stress to maximum principal stress. Hayhurst's stress successfully correlated with tension, torsion, and combined tension-torsion loading creep lives but Mises' type equivalent stress and maximum principal stress were inadequate to predict creep lives of Mod.9Cr1Mo steel. Oxide film and micro cracks on specimen surfaces were observed using SEM. Micro cracks and oxide film cracks initiated to the perpendicular direction to maximum principal stress axis for -1≦λ≦0. Main crack grew in direction perpendicular to specimen axis independent on the maximum stress ratio λ. The residual lives under biaxial stress states were estimated using Omega method. The Omega method was able to estimate the residual lives within a factor of 4 under biaxial stress states.