A trigger of type IV damage and a new heat treatment procedure to suppress it. Microstructural investigations of long-term ex-service Cr-Mo steel pipe elbows

Two seam welded Cr-Mo steel pipe elbows that had been used as main steam piping were investigated. Type IV damage was observed at heat-affected zones of seam welds. Microstructures were investigated by scanning electron microscopy and it was found that micro-cracks and polygonal hollows were distributed as if they were on the former austenite grain boundaries. Fine bainite grains surrounded by coarse carbides and creep voids were also distributed in the same manner. Dense precipitations of carbides (DPCs) were found on extraction replicas by transmission electron microscopy. Because of the similarities in shapes, sizes and distributions, DPCs were regarded as root causes of the hollows and the cracks. Heterogeneity of carbon content at former austenite grain boundaries was suggested to be the origin of DPCs. Based on the results of the investigation, a new heat treatment procedure to suppress Type IV damage was proposed. (C) 2004 Elsevier Ltd. All rights reserved.