Abstract
The interfacial microstructures of ex-service dissimilar metal weld (DMW) between austenitic stainless steel TP347H and low alloy ferritic steel G102 were investigated. A band of ultra-fine grains were found at the nickel-based weld metal/G102 steel interface. The ultra-fine grain band was characterized in detail by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), electron backscatter diffraction (ESBD), focused ion beam (FIB), transmission electron backscatter diffraction (T-EBSD) and transmission electron microscopy (TEM). The formation mechanism of ultra-fine grain band was proposed and the effects of it on service performance were discussed. The ultra-fine grains were ferrite without carbides, which were transformed from pre-existing martensite layer by carbon migration and localized recrystallization. This type of ultra-fine ferrite band triggered interfacial oxide notch by preferential oxidation of grain boundaries and increased the risk of interfacial cracking by microstructure degradation, which was a potential factor contributing to interfacial failure of DMW. The present work provided a new insight into the premature failure of such kind of DMWs.
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