Self-Healing Effedt of B Segregation on Creep Cavitation in Type 347 Austenitic Stainless Steel
Norio SHINYA Junro KYONO and Kinker LAHA
Abstract:The previous work showed that the precipitation of boron nitride at creep cavity surface suppressed the cavity growth and provided a type 304 austenitic stainless steel with a self-healing effect for the creep cavitation. The self-healing effect for the cavitation improved the long time creep rupture strength and ductility remarkably. In order to provide a type 347 austenitic stainless steel with the similar self-healing effect, the composition of the steel was modified with the addition of boron and cerium. An improvement of rupture strength coupled with rupture ductility of the steel was observed with the boron and cerium additions. Mobile sulfur in the steel was removed by formation of Ce2O2S, Ce2S3 and TiS. Auger electron spectroscopic analyses of the chemistry of creep cavity surface, exposed by breaking the crept specimen at liquid nitrogen temperature under impact loading, revealed the segregation of elemental boron on the creep cavity surface. The boron segregation at the creep cavity surface in the absence of sulfur contamination suppressed the cavity growth and improved the creep rupture properties by the self-healing effect of the boron segregation. The self-healing effect of the elemental boron segregation was compared with that of the boron nitride precipitation. Key Words:347 stainless steel, Rupture strength, Rupture ductility, Creep cavity, Self-healing, Boron segregation, Surface analysis