Crack analysis of induction heating bent pipe

There were 20 of 103 length of pipe found cracks on the extrados surface of the bent pipe made of Φ406.4×10mm L360 Longitudinal Submerged Arc Welding (LSAW) pipe, the cracks were mostly distributed in the extrados surface and the arc initial point of the bent pipe, the cracks were spindle-shaped and narrow at both ends with the longitudinal direction vertical to the axis of bent pipe. The mechanism and the causes of the cracks were studied by macro inspection, mechanical property test, microscope, scanning electron microscopy and energy spectrum analysis, which shown that the cracks were result from the contamination of copper or its alloy during or before the bent pipe production. The contaminated material like copper infiltrated and diffused into the base metal at high temperature, and could hardly stead to and transfer the normal stress from the base metal where the inertial crack was shaped. Meanwhile, the new crack was formed at the tip of inertial crack after the new infiltration and diffusion under the condition of both high temperature and tensile stress, so that the crack progressed and extended in this way.