TYPE-III CREEP CRACKING AT MAIN STEAM LINE WELDS

After 204,000 h of operation, some welds in the main steam line of a t hermal generating station were examined ultrasonically. This examinati on found a major reflector close to a butt weld in the line. That refl ector proved to be cracking that was 90% through-wall. The cracking wa s not visible on the outside surface of the pipe. The cracking was typ e III creep cracking, and had initiated in the mid-wall region within bands of coarse-grained material. From that mid-wall position, crackin g had propagated outwards towards the OD surface, and inwards to reach the ID surface. There was no evidence of the involvement of any weld flaws in the initiation or the growth of the cracking. Stress analysis of the piping system, using commercially available software, indicate d that the stress in the major area of cracking exceeded that allowed by the ASME B31.1 design code. (C) 1997 Elsevier Science Ltd.