Molecular dynamics study on the microscopic mechanism of in-service welding damage and failure

Q Wu and AF Yu and ZT Kang and C Kuang and H Liu, ENGINEERING FAILURE ANALYSIS, 137, 106402 (2022).

DOI: 10.1016/j.engfailanal.2022.106402

The risk of burn-through is a major concern when conducting in-service welding repair for oil-gas pipelines. The governing mechanism of burn- through has not hitherto been studied systematically. During in-service welding, different micro-zones of the pipe wall metal below the molten pool have different crack initiation mechanisms due to different temperature and stress conditions. This paper uses molecular dynamics to study the initiation and evolution of micro-defects in different micro- zones of the pipe wall under the in-service welding pool and reveals the microscopic mechanism of in-service welding damage and failure. The research found that the pipe wall under the in-service welding pool is divided into a fusion zone and a coarse grain zone. For the coarse grain zone, the grain boundary region is subjected to large deformations and causes high strain. The stress concentration at the grain boundary (especially the triple junctions) is caused by the arrest of grain boundary slip, which leads to the initiation and propagation of microcracks in the grain boundary region. The displacement vector of atoms at the grain boundaries is generally larger than that of the atoms in the crystal, further indicating that grain boundary slip is the primary mechanism of microcrack initiation in the coarse grain zone. Compared with the coarse grain zone, the grain boundary width of the fusion zone increases, and the grain boundary atoms show apparent disorder. The peak value of the radial distribution function decreases, and the peak becomes wider, indicating that the grain boundary pre- melting phenomenon occurs in the fusion zone. In addition, as the tensile strain increases, the instability of the pre-melting zone expands rapidly and advances into the crystal grain. During in-service welding, the hightemperature deformation of the coarse grain zone and the fusion zone of the tube wall are mainly controlled by the intergranular mechanism. Through molecular dynamics, the mechanism of grain boundary slip in coarse grain zone and grain boundary pre-melting in fusion zone were studied from the microscopic scale. Thereby the microscopic mechanism of in-service welding damage failure was further clarified.

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