Damage mechanism of the acceleration of intergranular cracking of stainless steel SUS316LN under creep loading at elevated temperatures

In this study, the simple creep test and intermittent creep test of SUS316LN, which has become a preferred materials for the structural components used in Boiling Water Reactors and Sodium Cooled-fast Reactors, were carried out to investigate the damage evolution. The effect of doping nitrogen into conventionally used SUS316L on the creep and fatigue strength has been proved in the comparison experiment between SUS316L and SUS316LN. At elevated temperatures, however, intergranular cracking still appeared under the application of low tensile stress. The mechanism of intergranular cracking at elevated temperatures, however, has not been clarified quantitatively yet. Therefore, in this research, EBSD method was applied to investigate the degradation process of the crystallinity around grain boundaries in this alloy from the viewpoint of the change of micro texture and atom arrangement. IQ (Image Quality) values, which indicates the average sharpness of the obtained diffraction pattern, were used for the evaluation of the local total density of defects. KAM (Kernel Average Misorientation) value was used for the evaluation of local plastic deformation in this study. In the creep test, the crystallinity decreased monotonically with the increase of creep damage. Combined with ΔKAM value, it was concluded that the accumulation of dislocations along specific grain boundaries and the difference of the magnitude of plastic deformation between two nearby grains were the dominant factors of intergranular cracking. Large difference of the magnitude of plastic deformation between two grains accelerated the accumulation of dislocations around the grain boundary. Therefore, the large difference of Schmid Factor between nearby grain is the dominant factor which determines the place where intergranular cracking starts to occur.