Assessment of creep deformation and rupture behaviour of 304HCu austenitic stainless steel

Abstract

Reduction in CO2 gas emission and decrease in fuel consumption can result in increase in efficiency of the thermal power plants. Increase in efficiency is directly linked to the steam temperature and pressure which requires materials having high creep strength. This resulted in development of Advanced Ultra Super Critical (AUSC) power plant which aims to increase efficiency to more than 45 % and significant decrease in CO2 emission. The 304HCu stainless steel is one of the candidate materials to be used in AUSC power plant. It contains around 3 wt. % of copper, certain amounts of niobium and nitrogen and increased carbon content for enhancing creep strength. Creep tests are conducted for 304HCu stainless steel at 923K, 973K and 1023K over a stress range of 100-240MPa. The creep curve exhibited shorter primary regimes followed by marginal secondary regimes and extended tertiary regime. The variation of steady state creep rate with applied stress exhibited Norton’s power law relationship (εs = Aσn). The value of n (stress exponent) decreased with increase in temperature but decrease was more pronounced at 923K. The product of steady state creep rate and rupture life obeyed Monkman-Grant relation. The contribution of tertiary creep was found to increase with temperature. Microstructural degradation in the form of coarsened precipitates, dislocation cell formation and deformation bands was the primary reason for increase in tertiary regime of creep curve and increase in the value of damage tolerance factor (λ). A mathematical model based on finite element analysis coupled with continuum damage mechanics has been used to predict the creep deformation and rupture life of 304HCu SS. The prediction of creep curve based on this model was found to be in good agreement with experimental results.