Creep Damage at Fine Grained Heat Affected Zone of Seam Welded Cr-Mo Steel Pipes and a Proposed Mechanism of Its Origination and Growth

摘要

A seam welded Cr-Mo steel pipe elbow that had been used as main steam piping at a thermal power plant was investigated. Creep damages at fine grained heat affected zone of the seam weld, called as type IV damages, were observed. Hook-shaped microcracks surrounded by many polygonal hollows were observed by optical microscopy and scanning electron microscopy. Those polygonal hollows were seemed to form the same hook-shaped lines as microcracks. Fine bainite grains surrounded by coarse carbides and creep cavities were also lined in the same manner as the hollows. From the appearances of the hook, it was inferred that these damages were originated locally at the former austenite grain boundaries. Dense precipitations of carbides (DPCs) were found on extraction replicas by transmission electron microscopy. At the beginning of the usage, there might be high-carbon-content fine bainite grains and DPCs were created in these grains by aging. Because of the similarities in shapes, sizes and distributions, DPCs were regarded as root causes of the hollows and the cracks. Many creep cavities would appear at DPCs and fine bainite grains would easily be isolated. As DPCs line on the former austenite grain boundaries, exfoliated fine bainite grain boundaries would link each other to form a microcrack. In order to suppress type IV damage, it would be effective to avoid the origination of fine bainite with high-carbon-content grains on the former austenite grain boundaries. Adding normalizing twice or more to the heat treatment process for the plate materials before welding is proposed.