Abstract
A systematic study on the microstructure evolutions and their relationships with mechanical properties of thick-walled 9Cr3W3Co1CuVNbBN martensitic steel pipe joint welded by narrow-gap automatic argon arc welding was carried out. The results showed that a narrow equiaxed grain zone (EGZ) and a significantly wide inter-critical zone (ICZ) were formed in the heat affected zone (HAZ) due to the larger grain size and lower VN precipitates in base metal (BM). The refined grains formed in WM and the higher solid solubility of C, Cr and W in EGZ and ICZ were the main factors leading to their higher hardness than that of BM. The decrease of dislocation density in over-tempered zone (OTZ) immediately adjacent to ICZ not only significantly improved the impact toughness of HAZ, but also led to a significantly lower hardness than that of BM. Nevertheless, the room/high temperature tensile fracture of the welded joints occurred in the BM zone, which was attributed to the cross-sectional shrinkage of the OTZ restrained by the nearby high hardness region ICZ, and the preferential formation of cavities near the coarse Cr23C6 carbides in BM.
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