Oxidation behaviour of T91/347H welds

T91 HAZ and weld metal zones of the T91/347H weldment exposed to simulated boiler environment have been observed to oxidize at higher rate and develop higher scale thickness than other zones. Optical microscopy, SEM/EDS and X ray mapping analysis have indicated lower Cr content in T91 HAZ by virtue of development of Cr rich carbides and delta ferrites in as welded condition. Availability of chromium carbides, intermetallics and martensitic morphology as well as the delta ferrites in an un-oxidized T91 HAZ zone has been found to be the main reason for the weak oxidation resistance of this zone. The higher corrosion in T91 HAZ may be by virtue of the lower presence of protective phases and higher availability of non protective oxides of Fe3O4 and Fe2O3. Higher corrosion of weld metal (WM) zone may be because of Cr depletion and higher oxides of iron.