1 1/4Cr1/2Mo鋼とHK40の異材突合せ溶接継手の使用中の割れ

抄録

After 15 to 22 months' operation of a steam reformer furnace, cracks were detected at the butt weld between reformer tube (HK 40) and top flange (11/4 Cr 1/2 Mo steel) with Inconel 82 filler metal. The tube being jointed was 18mm thick and 144mm outside diameter. The cracks occured circumferentially along the fusion line between the weld deposit and CrMo steel. The metal temperature during the operation was approximately 500°C.
The residual welding stress measurement at room temperature indicated that the circumferential stress is considerably higher than the longitudinal stress. Other stresses considered in the present study are the radial residual stress, which is estimated to be as high as the circumferential stress, and the thermal stress caused by the difference in the thermal expansion coefficient between the dissimilar metals. The stress due to the dead weight of materials and the one due to internal pressure can be neglected because of their low magnitudes.
Laboratory creep rupture tests using specimens with the dissimilar metal weldment showed that the specimens ruptured at HK 40 base metal in the short time creep duration tests, while the specimens subjected to the longer rupture life tests always ruptured at the fusion line between weld deposit and CrMo steel as observed on the reformer tubes.
The failure of butt weld joint can be explained in terms of the progressive change in stresses involved and rupture strength of the joint at various loads, as follows. Though the thermal stress and the residual stress are gradually relaxing during the service exposure, the rate of the stress relaxation is considered to be slower than the damage accumulation and thus the retained rupture strength for the weldment is decreased.