In situ observation on fracture behavior of the three-phase zone of the L415/N08825 bimetallic composite pipe welded joint

Abstract

In this study, the fracture behavior of the three-phase zone of the L415/N08825 bimetallic composite pipe welded joint was investigated by in-situ tensile test and EBSD observation. According to the results of the tensile tests, the fracture occurs in the heat-affected zone (HAZ). There, the interface between the cladding and the base layer first separated, the N08825 layer fractures, and then the L415 layer finally fractures. The results of the in-situ tensile test show that the crack initiated in the unmixed zone of the interface between the L415 and N08825 layers, and then multiple crack initiation points appeared on the N08825 side, eventually merging and rapidly propagating. The crack propagation path is approximately straight, and the interface has little effect on the propagation path. The dominant fracture mechanism is microporous aggregated ductile fracture. The results of EBSD observation revealed that the high frequency of low angle grain boundaries (LAGBs) concentrated in the unmixed zone originated from the substructure. The CSL grain boundaries of the BCC structure crystal are mainly Ʃ3, Ʃ11, and Ʃ17b, and the CSL grain boundaries of the FCC structure crystal are mainly Ʃ3, Ʃ5, and Ʃ17b, which increase fracture resistance and improve toughness. In addition, the unmixed zone and its interface with L415 have a high level of stress concentration.