For reason of economy, the hot reheat pipework in many US power plants is fabricated from seam-welded low chrome-moly carbon steel spools. Unlike girth butt welds, where the critical weldment microstructures can off-load stress to the stronger parent material, seam welds are subject to the full pressure hoop stress. A number of failures have occurred, some of which have been evidenced by fast fracture over extended lengths of the seam weld with a massive and violent steam release.
Seam-welded pipework is thus widely recognized to be the most safety critical item in fossil-fired power plants.
Seam welding is generally performed using the automatic submerged arc process, although some examples of manually welded seams have been encountered. Following welding, the spools are either fully renormalized and tempered or sub-critically annealed. Failures have occurred in both renormalized and sub-critically annealed pipework. Investigations of the latter have identified that cracking develops in the heat effected zone (HAZ) in the band of inter and sub-critically transformed microstructures present adjacent to the parent material. Renormalizing eliminates the HAZ and thereby eliminates this source of potential weakness. Nevertheless, a number of failures of renormalized pipework have occurred as a consequence of cracking along the weld interface. These have generally been attributed to poor welding practices leading to the presence of slag and other inclusions.