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

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.