Structural Integrity Mechanics and Creep Life Prediction of 304HCu Austenitic Stainless Steel Under Multiaxial State of Stress

Abstract

For increasing the efficiency of fossil power plants, the boiler tube material has to withstand higher temperature and pressure which calls for extensive research for identifying materials having high-temperature strength, good corrosion resistance and adequate mechanical properties. The 304HCu stainless steel 9304HCu SS is one of the candidate materials for boiler tubes employed in advanced ultra-super critical power plants. However, under above circumstances, material will subject to multiaxial state of stress that arises from internal pressure, weld joint, inhomogeneous structure, sudden change in dimension and change in cross section of the tube. Present study aims to introduce multiaxial state of stress through notches of different root radius on creep samples. Notch of different root radii, e.g. 0.25, 0.5, 2.5 was creep tested by keeping notch throat diameter 5 mm. Both plain and notch specimen had creep tested at same stress level at a particular temperature. Notch specimen possesses higher rupture life as compared to plain specimen. Based on these observations, the material is found to be ‘notch strengthening’. Then both SEM and optical micrograph were carried out on the unfailed notch which revealed that cavity density was decreased from notch root towards centre for relatively sharper notch, while random distribution of cavity for shallow notch. FE analysis has been carried out to understand the contribution of different components of stresses, i.e. von-Mises, maximum principal and hydrostatic stresses. Different models given by Cane, Hayhurst and Nix were examined for creep life prediction under multiaxial state of stress. The average hardness near to notch root was more in comparison with regions away from the notch root and shallow notch was found exhibit more hardness in comparison with sharper notch.