LAMINATION DEFECTS AND DIAGONAL TUBE FAILURE NEAR BOILER SUPERHEATER

At the coal-powered Orot-Rabin power station (Hadera, Israel) an ASTM A210 low carbon steel pipe had burst, causing steam to escape through a hole during initial unit activation after maintenance repairs. The pipe is a diagonal ribbed steam-generating tube leading to the drum, mainly used to physically support a superheater coil assembly. During internal visual inspection, it was seen that a whole layer of metal had partially peeled off from the area around the hole, causing the tube wall to thin considerably prior to failure. Moreover, it was seen that the same area had undergone corrosion at an advanced stage. Optical metallography revealed that the metal layer had peeled off the pipe`s internal surface due to a wide two-dimensional lamination defect present beneath the surface, and that a large amount of copper was present near the peeling crack`s edge. SEM/EDS verified the existence of an abnormal concentration of copper-zinc alloy at the same location. This led to the final conclusion that the hole in the pipe had formed due to a combination of a wide lamination defect left over from production, and sever internal corrosion enabling the defect`s exposure and delamination, eventually causing wall thinning and failure.