Prediction of the Generation of Intergranular Cracking in Stainless Steels Under Creep Loading at Elevated Temperatures

Abstract

Initiation mechanism of intergranular cracking in SUS316L under creep loading at elevated temperatures was investigated quantitatively by applying electron back-scatter diffraction (EBSD) analysis. It was found that intergranular cracking started to appear by 40% of the normalized lifetime of the steel and the cracked grain boundaries were close to perpendicular to the direction of the applied load and the Schmid factor of one of the grains which consisted of the cracked grain boundaries was larger than 0.43. In addition, the difference of the Schmid factor between the nearby grains was large. Not only dislocations but also vacancies were found to accumulate faster around the cracked grain boundaries. The reason for the accelerated accumulation was attributed to the local mechanical stress concentration around the grain boundaries, which was caused by the large lattice mismatch between the nearby grains. The degradation process of the crystallinity of the grain boundaries was continuously monitored by using the image quality (IQ) value obtained from the EBSD analysis. The degradation process was analyzed by using the modified Arrhenius equation, in which the effective activation energy of the damage accumulation decreased locally with the large strain around the cracked grain boundaries. The local activation energy was confirmed to be more than 30% lower than that obtained under the thermodynamically stable condition.