In-situ observation of high-temperature failure behavior of pipeline steel and investigation on burn-through mechanism during in-service welding

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. Here we perform a comprehensive analysis of the dynamic process of burn-through and the high-temperature failure behaviors of pipeline steel. Firstly, the in-service welding experiments were conducted to analyze the characteristics of burn-through. Influenced by the welding stress and especially the internal medium pressure, radial deformation of the pipe wall happens under the molten pool, thereby causing tensile stress. The pipeline metal under the molten pool contains the fusion zone and coarse grain zone. In order to study the crack initiation and propagation mechanism of the coarse grain zone and the fusion zone, the in-situ high-temperature tensile tests and in-situ high-temperature metallographic tests were carried out, respectively. The results indicate that during in-service welding process, the crack initiation mechanism of different microzones is different. For the fusion zone, the grain boundary melting leads to intergranular brittle fracture. While for the coarse grain zone, the stress concentration caused by grain boundary sliding makes the cracks appear easily at grain boundaries and triple junctions. Therefore, burn-through has intergranular cracking morphology. Microcracks originate from the fusion zone and penetrate along the weakened grain boundaries in the direction that perpendicular to the tensile stress and merge to form macroscopic cracks. Once the cracks penetrate to the inner wall, burn-through happens. We expect that these results would be valuable for clarifying the mechanism of burn-through and enriching the welding theories under severe conditions.