Abstract
Segregation of S and precipitation of BN on creep cavity surfaces during creep and their effects on creep rupture properties are reported. Auger electron spectroscopy of creep cavitated specimens provided direct evidence of impurity segregation to creep cavity surfaces. Residual S segregated most strongly, and was observed on creep cavity surfaces in crept specimens of a coventional type 304 stainless steel. The segregated S lowers the surface energy of creep cavities, and promotes nucleation of creep cavities. It was thought that the extensive segregation of S brings on extensive creep cavitation and premature failure with low ductility. In the modified 304 stainless steel added with B, N, Ce and Ti, the segregation of S and the creep cavitation were greatly inhibited. Addition of Ce and Ti lowered the amount of S available to segregate to creep cavity surfaces, thereby replacing the segregated S by precipitation of BN. BN is very stable at high temperatures and expected to reduce the surface diffusion rate considerably due to precipitating on creep cavity surfaces. It was suggested that this precipitation of BN on creep cavity surfaces is closely related to the suppression of creep cavitation and high rupture ductility of the modified steel.
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