The effect of long term ageing on the autogenous welding of dissimilar austenitic stainless steels

Austenitic stainless steels are used extensively throughout power stations in high temperature applications such as superheater tubes and fuel rod guides. For these applications, welding is often required to join sections of components or pipes/tubes due to their large sizes and lengths. In this paper, samples of a cast niobium stabilised stainless steel welded to a wrought 321 stainless steel were investigated. The sections were joined together using an autogenous Tungsten Inert Gas (TIG) weld. The effects of long term ageing at 750°C for up to 4000 hours have been studied. The ageing treatments were conducted in an inert atmosphere. Compositional changes and precipitates have been investigated using SEM with EDX and EBSD analysis. Niobium dissolved completely into the weld melt however it is observed to precipitate back out during long term ageing. Titanium carbonitrides however remained intact during the welding process, creating agglomerated particles throughout the weld bead. Ageing above 100 hours causes further Nb rich MX precipitates to form, which coarsen with longer ageing times up to 4000 hours.