ORIGIN AND EFFECT OF BACK STRESS ON CYCLIC CREEP DEFORMATION OF 316H STAINLESS STEEL

Components in power generation plants operate at high temperature and often go through complex cyclic loading sequence during its operations. Back stress' is generated during such cyclic loading due to inhomogeneity in deformation at micro scale and significantly affects the overall creep lifetime of the materials of these components. Using a time of flight neutron diffraction facility, we studied the origin of back stress and its effects on creep deformation rate of AISI type 316H austenitic steel during cyclic creep at 650 degrees C. The result shows, during high temperature cyclic loading of this material for any given level of stress, the magnitude of back stress vary significantly depending on the point in the cycle being observed. The effective back stress levels associated with dwells introduced at different points in the cycle shows good co-relation with the measured macroscopic secondary creep deformation rate. Moreover, a simplified approach using modified power law creep equation is proposed to quantify back stress from a known creep deformation rate.