Fatigue Crack Propagation Behavior and Fatigue Life Prediction of L360MS Pipeline Steel Welded Joint in Bimetallic Composite Pipe

Multi-layer and multi-pass butt welding of L360MS pipeline steel/N08825 nickel-based alloy bimetallic composite pipe was carried out by using NiCrMo-3 alloy as welding material. The hardness and tensile properties of welded joint of L360MS pipeline steel after stripping N08825 nickel-based alloy coating were measured. The fatigue crack propagation test according to the actual operating conditions was carried out on welded joints of L360MS pipeline steel. The crack length-cycle number curves in different regions were obtained by using the flexibility method. The fatigue crack propagation behavior in different regions was studied on the basis of caculation of the Paris formula, and the fatigue life was predicted. The results show that after tensile test, the joint specimen was fractured at base metal, and the weld was in a high-strength matching state. The hardness of the weld was in the range of 220–250 HV, which was significantly higher than that of the base metal (170–185 HV). The highest hardness appeared in the weld area adjacent to the fusion line, and the lowest hardness appeared in the heat affected zone. The fatigue cracks in the base metal, heat affected zone and weld mainly propagated in intergranular and transgranular modes. However, the fatigue crack propagation path in the weld was more tortuous, accompanied by bifurcation and even annular cracks. The Paris material constant C of the heat affected zone was the largest, followed by the base metal, and that of the weld was the smallest. Under the stress ratio of 0.1, the fatigue crack propagation rates of the base metal and the heat affected zone were faster, and that of the weld was the slowest. After modifying the material constant C, the life prediction formula of fatigue crack propagation along the depth direction was obtained. It was predicted that the fatigue life of the heat-affected zone of the pipe with semi-elliptical cracks along the circumference of the pipe body was the shortest, and the heat-affected zone was the weakest area of the fatigue performance of the welded joint.