Failure analysis of fillet weld crack in linepipe

An in-service X80 gas pipeline was repaired with type-B sleeve. Cracks were not detected immediately after welding, but appeared on the surface of fillet weld after 2 days. Chemical composition analysis, welding pass inspection, optical microscopy, energy spectrum analysis, scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) were conducted to analyse the cause of weld cracking, as well as finite element simulation of residual stress. The results showed excessive hydrogen in weld seam resulted from humidity of construction environment. Meanwhile, the thickness and width of weld bead exceeded the specification because of high heat input welding. The above reasons brought microstructure deterioration and high residual stress of fillet weld. The crack initiated from the high stress position of the fillet weld surface and fractured in transgranular quasi-cleavage mode.