Investigation of static and fatigue crack growth mechanism of borated stainless steel using in situ observation method

The borated stainless steel B-SUS304P-1 is used for storage and transport metal casks in the nuclear industry. According to our previous research, the boron addition reduces the fracture toughness but has a minimal effect on the fatigue crack growth. In this study, in situ laser microscopy observation during tensile, fracture toughness and fatigue crack growth tests of B-SUS304P-1 was performed to investigate the mechanism responsible for the effects of the boron addition on static and fatigue crack growth. During the tensile test, the borides were broken, but the base material remained intact. Results obtained using the digital image correlation method revealed that the strain of boride at fracture was very low compared with the macroscopic elongation of B-SUS304P-1. For the fracture toughness test, the borides in front of the crack were broken before static crack growth. The crack propagated along a zigzag path as it grew along the broken borides. Because of this crack growth mechanism, the fracture toughness was reduced by the boron addition. In contrast, for the fatigue crack growth test, only a few broken borides were observed in front of the crack because the stress intensity factor of the fatigue test was smaller than that of the fracture toughness test. The crack thus grew linearly, and the boron addition had a minimal effect on the fatigue crack growth of the stainless steel.