Dissimilar metal butt welds are used in important parts of pressurized water reactors. Alloy 600
(UNS N06600) is welded to cast stainless steel (UNS S31600) using alloy 82 (UNS W86082)
or 132 (UNS W86132). Near the weld boundary of UNS S31600, the weld characteristics differ
from those of the original weld because of the dilution effect of UNS S31600. This study
examined the PWSCC susceptibility of nickel-based alloy of dissimilar metal butt welds taking
into consideration the dilution effect caused by UNS S31600. Constant load tests for estimating
the crack growth rate during the initiation stage were conducted and also slow strain rate tests
for PWSCC susceptibility and activation energy were conducted using dissimilar metal butt
welds specimens under simulated PWR primary water conditions.
No remarkable difference of activation energy and crack growth rate in the initiation stage was
observed between UNS W86082 and W86132. Higher PWSCC susceptibility was observed in
UNS W86132 than in UNS W86082. Crack growth rates in the initiation stage of dissimilar
metal butt welds were about one order of magnitude lower than that of UNS N06600 matching
metal welds. The smaller crack growth rate in the initiation stage suggests that dissimilar metal
butt welds may be less susceptible to PWSCC.
Using the activation energy and crack growth rate in the initiation stage obtained in this study,
the crack depth of nickel-based alloy of dissimilar metal butt welds taking into consideration the
dilution effect of cast stainless steel was calculated at the temperature of 320°C. As a result, it
was considered that the boundary formed between UNS S31600 and UNS W86082 or UNS
W86132 welding at the temperature of 320°C still remains in the initiation stage of PWSCC
after 20 years of operation.
Keywords: PWR, PWSCC, corrosion, nickel-based alloy, stress corrosion cracking, dissimilar
metal butt weld