S. Tsukamoto, T. Shirane, M. Tabuchi, F. Abe, National Institute for Materials Science, Tsukuba, Ibaraki, Japan
Advanced high Cr heat resistant steels have been developed worldwide to increase the operating temperature and thermal efficiency of the fossil power plant. One of the major problems of the steels is reduced creep strength of the welded joint especially in the low stress and long-term ranges. The joint is normally fractured at fine grained HAZ (type IV failure). In the present study, formation mechanism and prevention of the type IV failure has been investigated. Newly developed boron containing 9Cr-3W-3Co-V, Nb steels (B steels) showed excellent creep properties. The creep strength of the welded joint was almost same as that of the base metal and was superior to the conventional heat resistant steel base metal. Type IV failure was prevented even in the creep rupture time of more than 20,000 hours. This is attributed to improvement of the HAZ microstructure. Fine grained prior austenite was formed in the conventional heat resistant steel HAZ, where the peak temperature reached around AC3 during weld thermal cycle. In this region, few M23C6 precipitates were formed at the grain boundary of the fine prior austenite, whereas a lot of M23C6 were precipitated at the ghost prior austenite grain boundary, which was observed in the base metal. This causes lack of grain boundary strengthening, leading to occurrence of the type IV failure. In the B steel HAZ, on the other hand, the original prior austenite and martensite observed in the base metal were reconstituted during weld thermal cycle. This allows M23C6 to precipitate at the same grain and block boundaries of the base metal during PWHT, leading to prevention of the type IV failure.
Summary: Boron addition in the ferritic heat resistant steels significantly improved the creep strength of the welded joint due to prevention of the Type IV failure. It is attributed to improvement of the HAZ microstructure. Coarse-grained prior austenite was formed by reconstitution of the original austenite.
