Environmental degradation of dissimilar metal welds

A programme has been undertaken to investigate the susceptibility of austenitic/ferritic dissimilar metal weldments to stress corrosion cracking (SCC) in simulated normal and faulted PWR primary water environments, in the unlikely event that the protective austenitic cladding is breached. For a weldment made with stainless steel buttering and weld metal, SCC under constant extension rate loading in 290 degrees C water occurred only at elevated potentials corresponding to the presence of high dissolved oxygen levels, a condition unlikely to arise in practice due to the presence of a hydrogen overpressure. Under constant displacement loading, crack initiation was only observed in the presence of sulphate contamination and after pre-pitting at lower temperatures. Some evidence of crack initiation was observed at 150 degrees C in constant extension rate tests under sulphate free and low potential conditions, but the cracks did not propagate within the timescales of the tests. Similar behaviour was observed for a weldment made with nickel alloy 182 filler at 290 degrees C, but some evidence of intercolumnar/intergranular cracking in the weld metal was observed in 350 degrees C simulated de-oxygenated PWR primary water in constant extension rate, but not constant displacement tests.