Failure analysis of L415/316L composite pipe welded joint

Bimetallic composite pipe is composed of the outer layer (carbon steel or low alloy steel pipe) and the inner layer (high alloy corrosion-resistant steel pipe). Generally, when composite pipes can only be welded on one side due to their small diameter, high-alloy welding materials are usually adopted for the whole weld. In this case, the butt welding of L415/316L bimetal mechanical lined pipe adopts the welding sequence of sealing welding-root welding (inner layer)-transition welding-filler and cover welding (outer layer). In order to reduce welding material costs, lowalloy welding material E71T8-Ni1J is used for the outer layer. As a result, cracks at the mechanical joint between the inner layer and the outer layer after welding were found for phi 355 x (11 + 2)mm L415/316L bimetal mechanical lined pipe. Therefore, in this paper, optical microscope (OM) and electron probe micro-analyzer (EPMA) were used to analyze the weld microstructure and the element distribution near the crack, and the hardness of the welded joint was determined by Vickers hardness tester. Meanwhile, the finite element numerical simulation was used to analyze the residual stress near the mechanical joint between the inner and outer layers of composite pipes with different diameters. The results show that the pipe diameter affects the welding residual stress. The welding residual stress of phi 355 x (11 + 2)mm and phi 508 x (16 + 2) mm composite pipes is 301 MPa. There are two factors for the occurrence of cracks at the mechanical joint of phi 355 x (11 + 2)mm composite pipe. The mechanical factor is that the existence of stress concentration. The microstructure factor is the formation of lath martensite and upper bainite. The root cause of weld failure: the welding process was formulated or operated irregularly. The sealing welding layer should be originally austenite. Due to remelting, lath martensite and upper bainite are formed at the sealing welding layer.