ANALYSIS ON THE FAILURE OF BUTT WELD JOINT BETWEEN 2 one quarter CR-1Mo STEEL TUBE AND STAINLESS STEEL TUBE.

The joints studied were welded with the D309 electrode; the failures had occurred in superheater tubes of power boilers after 9 to 12 years' service. Arising from the outside of the tube and propagating inward, the cracking occurred intergranularly in the heat affected zone of the 2 one quarter Cr-1Mo steel along the fusion line. Neither tube bulging nor decarburization of 2 one quarter Cr-1Mo steel was detected. The loads to be weighted on a joint in service comprise a stress compound with internal pressure, thermal stress due to the difference between linear expansion coefficients, and bending stress. It was postulated that cracking is more attributable to creep damage than to fatigue damage, and that stress relief cracking may occur prior to the time of stress relief or at start up. Moreover, it was found that the absorbed energy at a high temperature correlated with the occurrence of stress relief cracking of 2 one-half Cr-1Mo steel. The chemical composition of the decarburized 2 one quarter Cr-1Mo steel and carburized deposited metal were also investigated.