Effect of cold forming strain on creep rupture properties of 47Ni-23Cr-23Fe-7W alloy

Abstract 47Ni-23Cr-23Fe-7W (HR6W) is a candidate material for high temperature areas in the Advanced Ultra-Supercritical (A-USC) boilers. In this study, creep rupture properties of cold-formed materials were investigated in comparison to the solution annealed materials in order to clarify the creep strengthening mechanism of the cold-formed HR6W. It was found that the creep strength and rupture life of the cold formed HR6W increased substantially as compared with solution annealed material and creep ductility decreased. According to investigation results of microstructure and hardness changes in the crept specimens, grain boundaries in cold-formed HR6W were covered with M 23 C 6 precipitates from the early stages of creep and the fine M 23 C 6 particles in the grains were very stable for long period of time as compared to the lower grain boundary coverage in the solution annealed materials. These results suggested that a high density of dislocation is maintained during the test and hence it exceeded 60,000 h. This was postulated to be reason why creep rupture strength of cold formed HR6W enhanced so remarkably in comparison to the solution annealed material.