Effects of notch geometry and impurity on creep fracture of low alloy steel

Abstract

he effects of stress state and impurity on the high temperature intergranular fracture of a commercial grade of 2·25Cr–1Mo steel have been investigated. Fracture surfaces and creep cavities were examined using scanning electron microscopy. Elemental segregation to the grain boundaries in the blunt and chevron notched specimens was analysed using Auger electron spectroscopy, which showed P segregation on the grain boundary facets and high contents of S on transgranular facets and in cavities. Whereas an increase of P content with time at 550°C was observed for the chevron notched samples up to 50 h (the time range used), a steady level had been found during creep aging for the blunt notch. However, in both cases, step cooling showed no effect on the segregation content of P after about 10 h at 550°C. The damage studies indicate that wedge cracking controls the damage, while a very few isolated cavities were found ahead of cracks, suggesting that cavities link up rapidly with the growing cracks. Energy dispersive analysis revealed the presence of Mn, Cr, and S in the cavities. It is suggested that P segregation, which is promoted strongly by the presence of Mn, could alter the grain boundary cavitation as could the cosegregation of Cr and P.MST/1489