Effect of thermal ageing on Alloys 690 and 52 in pressurized water reactor applications

In modern pressurized water reactors (PWR), the substitution of nickel-base Alloy 600 and its associated filler metals Alloy 82 and 182 by Alloys 690, 52 and 152 with higher chromium content has improved the integrity of the main components in the primary circuit. However, metallurgical changes during thermal ageing of Ni-base alloys, notably intergranular (IG) carbide precipitation and short-range ordering (SRO), can affect the long-term primary water stress corrosion cracking (PWSCC) resistance of PWR components, such as the steam generator for Alloy 690 and the reactor pressure vessel (RPV) safe-end dissimilar metal weld (DMW) for Alloy 52. In addition, it is known that post-weld heat treatment (PWHT) affects narrow transition zones within Alloy 52 weld metal at the ferrite/austenite interface of DMW, but no data is available on the effect of thermal ageing. Four conditions of Alloy 690 (solution annealed, cold-rolled and/or heat-treated) were aged between 350 and 550 °C for 10 000 h and characterized. In addition, two sets of PWR RPV narrow-gap (NG) DMW mock-ups were studied. The first weld was characterized before and after PWHT and the other after PWHT and ageing at 400 °C for 5000 and 10 000 h. No direct observation of ordering was made, but hardness and lattice parameter measurements indicated the formation SRO in Alloy 690, with a peak level at 420 °C. Heat treatment induced SRO before ageing, while stress relaxation, recrystallization and α-Cr precipitation were observed in cold-worked (CW) samples at higher temperatures. A disordering reaction was inferred in all conditions at higher temperatures. IG M23C6 carbide precipitation increased with increasing ageing temperature, as well as diffusion-induced grain boundary migration (DIGM). After comparison with recent studies, SRO was concluded to be locally as detrimental as long-range ordering (LRO) or 20% CW, but not a SCC driving force by itself. A combination of SRO and IG carbide precipitation was seen as detrimental to the SCC resistance. Results for Alloy 52 showed that PWHT increased carbon depletion in the low-alloy steel (LAS) side, and promoted a hard layer close to the fusion line inside the Alloy 52 weld metal. Thermal ageing did not affect the microstructure or hardness levels in neither LAS base metal nor Ni-base weld metal, but significantly reduced the hardness peak close the fusion line in Alloy 52 weld metal side. This strength mismatch reduction at the weld fusion line was seen as beneficial from both mechanical and SCC point of view