Effect of grain refinement on cracks occurring in SUS304L stainless steel under nuclear reactor operating conditions

• ・This study examined the impact of grain refinement on SCC in SUS304L, finding that larger grains reduce elongation under nuclear reactor operating conditions. • ・SUS304L with smaller grains had no cracks, while larger grains developed thicker oxide films and cracks. • ・Cr 2 O 3 films formed on smaller grains, but larger grains developed Cr 2 O 3 and Fe-based oxide layers. • ・Fine-grained SUS304L showed uniform oxygen diffusion and thin Cr 2 O 3 layers, suppressing cracks effectively. Using SUS304L stainless steel, which is employed in reactor structural components, the effects of grain refinement on stress corrosion cracking occurring under nuclear reactor operating conditions were investigated. As a result, after conducting slow strain rate testing (SSRT) in air and nuclear reactor operating environments, a comparison of the tensile properties of SUS304L with the same grain size revealed that elongation significantly decreased with increasing grain size under nuclear reactor operating conditions. In SSRT conducted in air, the k -value obtained from the Hall–Petch relationship was lower than the conventional values. Observations showed the absence of cracks on SUS304L with 0.59 and 1.52 µm grains; however, SUS304L with larger grains exhibited rougher fracture surfaces and side cracks. Thin oxide films were formed on SUS304L with 0.59 µm and 1.52 µm grains, while SUS304L with coarse grains of 28.4 µm or larger enabled the formation of oxide films with over 2 µm thickness. Cr 2 O 3 films were formed on SUS304L with 0.59 µm, 1.52 µm, and 28.4 µm, while Cr 2 O 3 and Fe-based oxides were formed on SUS304L with 39.5 µm and 68.6 µm. Crystal orientation analysis revealed linear surface layers without cracks in the γ-phase for SUS304L with 0.59 µm and 1.52 µm. In materials with larger grain sizes, surface irregularities and cracks were observed in the γ-phase. In fine-grained SUS304L, lattice diffusion caused uniform O diffusion in the γ-phase, resulting in the formation of a thin Cr 2 O 3 layer that suppressed cracks. In coarse-grained SUS304L, grain boundary diffusion caused Fe oxide formation at the grain boundaries, weakening them, and supersaturated O led to the formation of thick films comprising Cr 2 O 3 and Fe-based oxides, resulting in peeling and cracking.