Niobium-Added Type 308 Stainless Steel Weld Metal Resistant to Sensitization by Postweld Heat Treatment and Low-Temperature Aging

Type 308 (UNS S30800) stainless steel weld metal, applied as an internal cladding of reactor pressure vessels (RPV) for light water reactors, is subjected to postweld heat treatment (PWHT) at around 600°C during RPV fabrication. This PWHT can sensitize or thermally stabilize (heal) the weld metal, depending mainly on the length of PWHT. Moreover, low-temperature aging (LTA) at 500°C for 24 h after PWHT, simulating the so-called low-temperature sensitization (LTS), can significantly influence the sensitization behavior. In the present study, the effect of niobium addition on LTS after PWHT was examined in detail. Optimization of the niobium to carbon ratio (Nb/C) and ferrite content was made, focusing on one-layer weld overlay metals by the electroslag welding process using wide-strip electrodes (typically 150 mm), for full resistance to sensitization and reheat embrittlement by PWHT + LTA (the developed metal was designated as Type 308NbL). A sufficient amount of niobium and a sufficiently fine distribution of ferrite phase were the key combination for excellent intergranular corrosion resistance in a modified ASTM A262E test. By electron microscopy, niobium carbides were found to precipitate, prior to chromium carbides, preferably along ferrite-austenite grain boundaries, not only by PWHT but also during the welding heat cycle.