Analysis of the dynamic evolution behavior of radial deformation and cracks during in-service welding instability based on quasi-in-situ and in-situ experiments

Avoiding the risk of in-service welding instability is the primary problem to be solved to ensure pipeline repair safety. And it is of great importance to develop a model to predict burn-through instability. However, few literatures have investigated the bulge instability behavior based on in-situ or quasi-in-situ experiments, which restricts the development of a unified prediction model. Therefore, in the present paper, the dynamic evolution behavior of both radiation deformation and cracks during in-service welding instability was investigated. The inservice welding instability process was divided into two stages, namely, the bulge instability (i.e. molten pool bulge) stage and the burn-through instability (i.e. bubble-induced molten metal overflow and meltage-effect burn-through) stage. The former revealed the initiation and propagation of cracks under the combined action of temperature effect and strain effect. The latter uncovered that the raised remaining pipe wall metal was melted and the cracks were transformed to melting-corrode pinholes due to the meltage effect. Noted, the radial deformation was the root cause of burn-through instability, which resulted in cracks through the pipe wall and the raised molten pool attracted to the welding arc. Besides, the in-service welding instability zone was divided into four zones, i.e. crack-propagate radial deformation zone, bulge-assisted burn-through hole zone, bulgeassisted opening fracture zone as well as direct burn-through hole zone. Furthermore, the crack failure mode was a mixture of transgranular and intergranular fractures. And the transgranular cracks presented an obvious deflection inside the pro-austenite grains.