Fatigue properties of thick section high strength steel welded joint by hybrid fiber laser-arc welding

The fatigue properties of 12 mm Q890D high strength steel welded joint by hybrid laser-arc welding and their influencing factors were investigated in detail. The results showed that the fatigue limit of the welded joint was 406 MPa at two hundred thousand cycles load conditions, which was 45% of the yield strength of the base metal (BM). The welded joint could be divided into three layers, i.e., the backing layer, the filling layer, and the covering layer from the bottom to the top of the welded joint. The fatigue crack was initiated at the weld metal (WM) of the covering layer, then extended down to the WM and the heat affected zone of the filling layer, and finally fractured at the BM along the 45° direction. This was because that the BM consisted of massive polygonal ferrites and small granular carbides, while the WM was composed of martensite with a high density of dislocations. Additionally, the average hardness of the WM was 415 HV, which was 20% higher than that of the BM. Compared with the soft BM, the hardness and the strength of the WM were relatively high, resulting in higher resistance of the WM to the fatigue crack propagation.