Overlaid weld metals of austenitic stainless steel in a pressure vessel of power reactor are usually postheated for a long period of time after welding. The heat treatment is considered a kind of sensitizing and it is important to check the soundness of the weld metal after heat treatment, especially about the precipitation of carbides.
Investigation was made about the distribution of delta ferrite and its change by reheating, precipitation of carbides and their identifications, and sigma phase. The main conclusions reached from this study are as follows:
(1) Precipitation of delta ferrite from molten weld metal depends on solidification phenomenon. There was a small amount of ferrite near the bond or toe in which the local solidification time was short, comparing with other parts of weld metal. At no specific location ferrite was found due to macrosegregation or extremely rapid solidification.
(2) Shape and amount of ferrite were changed by reheating after solidification. Ferrite was extremely grown after heating at 1350°C for 15 sec. It was spheroidized by heating at 1200°C.
(3) There were five types of carbides developed after postheating: a) dot-shaped carbide, b) massive carbide, c) precipitated carbide mostly due to diffusion of carbon from base metal, d) lamellar carbide by eutectic reaction, and e) precipitated carbide due to tempering of martensite.
(4) The first two types of carbides were M₂₃C₆ type except for a relatively intense postheating of 700°C. A M₆C carbide was detected with M₂₃C₆ carbide after postheating at 700°C. It should be noticed that dot-shaped carbides were linked to each other on an austenite grain boundary when there was no ferrite, and massive carbides co-existed sometimes with sigma phase.
(5) The fourth one, lamellar carbide, which had not been pointed out in previous study, precipitated on an austenite grain boundary near base metal after a fairly large extent of carbon diffusion from base metal. The eutectic phase was very soft and it would be a cause of trouble in a side bend test which is required as part of the procedure test of power reactor.