The microstructure and creep properties of the Mod.9Cr-1Mo steel of repair welds after long-term service for about 80,000 hours at around 600°C were investigated. Two types of heat affected zone (HAZ) were formed after repair welding performed at the interface between base metal and original weld metal. The first type was a HAZ formed in an ex-service original weld metal and another type was formed in an ex-service base metal. Creep rupture life of HAZ formed in an ex-service original weld metal was extremely short and it was about one fourth of that of HAZ formed in an ex-service base metal. From microstructural observation, many fine ferrite grains surrounded by carbides were present on grain boundaries and, therefore, duplex structure of ferrite and martensite was observed in the fine grain HAZ region formed in an ex-service original weld metal. These fine ferrite grains were considered to be formed by diffusion transformation of gamma phase to alpha phase instead of martensitic transformation during cooling of thermal history due to repair welding. It has been concluded that the creep strength of an ex-service weld metal is remarkably reduced by the formation of many fine ferrite grains due to repair welding.

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