Stress corrosion cracking of a low alloy steel to stainless steel transition weld in PWR primary waters at 292°C

Abstract

The effects of electrode potential and sulphate ( SO 2− 4 ) content in the water on the stress corrosion cracking (SCC) of a low alloy steel — austenitic stainless steel transition weld, A508-309L/308L, in pressurised water reactor (PWR) primary side waters at 292°C have been studied using slow strain rate testing (SSRT). The weld was post-weld-heat-treated at 620°C for 20 h before testing. Results showed that the transition zone in the weld had a higher susceptibility to SCC than either the bulk stainless steel or the bulk low alloy steel. The SCC in the transition zone was mainly intergranular in the austenitic layer, but transgranular cracking occurred at the interface and in the low alloy steel. The minimum potential for SCC, E c , in each water used was higher than the free corrosion potential range of −880 to −660 mV (SHE) and the susceptibility to SCC increased with increasing electrode potential. In sulphate doped waters, crack growth rates >2 × 10 −6 mm/s occurred at high applied potentials in the low alloy steel and/or in the austenitic layer but some less severe cracking occurred at the interface. Contamination of the water with SO 2− 4 increased the SCC susceptibility by both decreasing the minimum potentials for cracking and increasing the crack growth rate. However, the data suggest that transition welds should be immune from SCC in typical PWR primary side coolant water at 292°C even in the unlikely event that a break in the stainless steel cladding allowed access of the cooling water to the transition joint area.