    Due to the growth of global population and severe environmental problems, the energy issues have become one of the most serious issues today. Therefore, new clean energies, such as solar energy, wind energy, ocean energy and nuclear energy, have been developed for minimizing the emission of CO~2~ and solving the energy crisis. Among them, about 10% of the world’s electricity is generated by about 440 nuclear power reactors, which is also the world’s second largest source of low-carbon power. Pressurized Water Reactors and Boiling Water Reactors, as the most common type of light water reactor, have generated a total power of 38042 MWe by the end of 2019 in Japan. In order to ensure their safe and stable operation of the reactors at the average age of 27.84 years old, annual inspections are being carefully performed periodically.

    In recent years, numbers of incidents caused by stress corrosion cracking, which degrades the reliability of alloys under the synergistic action of corrosive medium and tensile stress, have occurred in core shrouds and primary loop recirculation piping made of the low carbon stainless steel SUS316LN that had been recognized to be an SCC-resistant material. The crack propagation rate was found to accelerate under low frequency fatigue or creep loadings. The main reason for this acceleration is currently attributed to the change of the crack propagation path from transgranular to intergranular. Thus, grain boundary cracking is one of the most dominant factors which determine the effective lifetime. 

    In order to explicate the criteria of grain boundary cracking, it was found that the degradation of the crystallinity around grain boundaries decreased the effective strength of the damaged grain boundaries and accelerated intergranular cracking. Therefore, in this study, the change of the crystallinity of grains and grain boundaries under creep loading was continuously monitored and the initial intergranular cracking process was monitored quantitatively by using the crystallinity. The variation of the crystallinity of SUS316LN under creep loading was monitored at elevated temperature by applying an intermittent creep test and EBSD (Electron Back-scatter Diffraction) method. The IQ (Image Quality) value, which is obtained from the analysis of the sharpness of the diffraction pattern (Kikuchi pattern), was used for the evaluation of the total concentration of various defects such as vacancies, dislocations, local strain, and so on. In addition, KAM (Kernel Average Misorientation) value was also confirmed to be applicable to the analysis of local plastic strain. The continuous change of micro texture in grains and around grain boundaries were observed in detail by applying an intermittent creep test. As a result, it was confirmed that the IQ value around damaged grain boundaries was drastically degraded drastically. It was also validated that the fracture strength of the grain boundaries decreased monotonically with the decrease in the IQ value. Thus, intergranular cracks always occurred when the IQ value decreased to a certain critical value. Therefore, there is a correlation between the IQ value and the strength of a grain boundary. The decrease of the IQ value was attributed to the accumulation of both dislocations and vacancies around the specific grain boundaries under creep loadings. Meanwhile, the ΔKAM value obtained from cracked grain boundaries was much larger than that from uncracked grain boundaries. This large difference was caused by the large mismatch of Schmid factor of nearby grains, in other words, resolved shearing stress in each grain. Combined with analysis of IQ value, it was concluded that intergranular cracking was caused the accumulation of vacancies and dislocations around grain boundaries which consisted of grains with large mismatch of Schmid factor. 


Damage Mechanism of the Acceleration of Intergranular Cracking of Stainless Steel SUS316LN Under Creep Loading at Elevated Temperatures

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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