Impurities segregation to grain boundary carbide interfaces and grain boundaries and the mechanism of elevated temperature intergranular cracking in heat-resistant steel

Abstract

It is shown that the time to elevated temperature intergranular failure in a heat-resistant steel is expressed by t=t0σ-nexp(Q/ RT) where n is the stress exponent, Q the activation enthalpy, and t0 the proportional constant. It is also shown that the segregation concentration of impurities is markedly higher in the dimples of grain boundary area than at the smooth grain boundaries and so the dimples observed usually at reheat or stress relief cracked surfaces are not the micro-ductile fracture areas but the carbide/matrix interfaces at the grain boundaries. Finally, it is shown that the elevated temperature intergranular cracking results from the smooth grain boundary cracking following the cracking of the carbide/matrix interfaces at the grain boundaries. © 2010 ISIJ.