Abstract
The sensitization behavior of types 308 and 316 stainless steel weld metals as internal overlays for reactor pressure vessels (RPVs) was studied with respect to the effects of postweld heat treatment (PWHT) at about 600 °C during RPV fabrication and low-temperature aging during operation. For the study, a criterion for the rate of intergranular corrosion (IGC) for detecting the susceptibility to intergranular stress corrosion cracking (IGSCC) in high-temperature oxygenated pure water was established by quantitatively evaluating the results from a modified ASTM A262E test. A criterion for expecting satisfactory resistance to IGSCC was found to be an IGC rate of about 1 µm/h. Type 308 weld metal can be sensitized as indicated by an IGC rate >1 µm/h, and can be healed, as indicated by an IGC rate <1 µm/h, depending on the length of PWHT. However, this healed weld metal can be resensitized by exposing it to 500 °C×24 h; in this condition, it shows a relatively high susceptibility to IGSCC. On the contrary, type 316 weld metal was almost immune to sensitization under the same heat treatments. By transmission electron microscopy (TEM), its excellent resistance was attributable to carbon fixation by molybdenum carbide precipitation within ferrite phases during PWHT. Reheat embrittlement of type 316 weld metal was also examined.
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