Grain Size Effect on Creep Properties of 304HCu SS and Modelling of Creep Curves Using Modified Theta Projection Approach

dvanced ultra-supercritical (AUSC) fossil-fired power plants are envisaged to operate at a higher steam temperature (983–1023 K) and pressure (32–35 MPa) compared to existing supercritical and ultra-supercritical thermal power plants. One of the candidate materials currently considered for application in boiler tube circuit for India’s National Mission on AUSC is 304HCu stainless steel (SS). In the present study, the influence of grain size on the creep properties of 304HCu SS was evaluated at a temperature of 923 K in the stress range of 180–240 MPa. The grain sizes investigated were 10, 20, 30 and 60 µm. At all the stress levels, the grain size of 30 micron exhibited the lowest value of minimum creep rate and highest rupture life. The variation of minimum creep rate with applied stress for all the grain sizes followed Norton’s power law relation, ε̇_min = Aσ^n . The values of ‘ A ’ and ‘ n ’ were strongly dependent on the grain size. The creep curves were modelled using an improved theta projection approach wherein the mathematical formulations for primary and tertiary creep regimes were modified for better correlation with experimental creep curves. Further, the variation of theta parameters with respect to stress and grain size was established through an empirical correlation. The rupture lives predicted from modified theta projection approach are in close agreement with the experimental values. The formulation of improved model is generalized to predict creep curves/total strain curves irrespective of duration in the three stages of creep, and the predictions were also verified with creep data of 316 SS and P91 steel.