Fatigue behavior and mechanism of FV520B-I welding seams in a very high cycle regime

• FV520B-I smooth welding seams have obvious characteristics of VHCF.
• The VHCF behaviors of FV520B-I welding seams are affected by the surface roughness.
• The GBF formation was analyzed by the “effective short fatigue crack criterion”.
• The relationships of the VHCF life with characteristic area sizes were researched.

The fatigue properties of FV520B-I welding seams with different surface roughnesses up to 109 cycles were tested, and the fatigue behavior and mechanism in a very high cycle regime were studied. The test results showed that FV520B-I smooth welding seams had obvious characteristics of very high cycle fatigue, and cracks mainly initiated from the subsurface when the fatigue life was higher than 107 cycles. As the surface roughness increased to Ra ≈ 0.6, the S–N curve moved downward and cracks usually initiated from the surface. The surface roughness had no effects on the initiation mechanism of subsurface cracks in the welding seam. The initiation modes of cracks in FV520B-I welding seams were more than those of cracks in FV520B-I. The GBF formation mechanism of “dispersive decohesion of spherical carbide” was supported, and the possible crack growth in the GBF was analyzed according to the “effective short fatigue crack criterion”. The relationships of a very high cycle fatigue life and sizes of characteristic areas of fractures were further analyzed. The fatigue life increased with a decrease of inclusion diameter and an increase of the ratio of the GBF diameter to the inclusion (pore) diameter.