Fracture Toughness in the Ductile-Brittle Transition and Thermal Ageing Behavior of Decarburized Heat Affected Zone of Alloy 52 Dissimilar Metal Welds of Nuclear Components

Abstract

Dissimilar metal welds (DMW) are used in nuclear power plants between the nozzles of main components in low alloy steel and stainless steel pipes, or safe-ends connected to the main coolant line pipes. AREVA proposes for EPR™ an improved design of DMW involving narrow gap welding without buttering between the low alloy steel nozzles and the stainless steel safe-ends, and the use of a corrosion resistant weld filler metal (Alloy 52). AREVA performed a thorough characterization of this type of welds, which shows a particular microstructure close to the fusion line between the low alloy steel and the nickel base alloy, where the heat affected zone of the low alloy steel is decarburized. This paper presents results of fracture toughness tests performed with the crack tip located in this area, in the ductile to brittle transition in the as post-welded heat treated condition and after thermal ageing. The results show an excellent fracture toughness behavior of this particular area, compared to that of low alloy steel parent metal.

Dependence between $$\varvec{\upeta }$$ η -factor and crack location relative to a fusion boundary between hard and soft materials in a SE(B) specimen

“…The residual stresses can also affect the crack path by increasing the crack driving force. [55] Similar kind of behavior for HAZ cracks is not observed in [27] for a similar DMW. They did not observe a correlation between fracture toughness and distance to fusion boundary, Figure 37, and brittle fracture did not initiate in all cases next to the fusion boundary.…”

“…In Joly et al [27], the characterization was done with 25 mm thick C(T) specimens, whereas in this study the characterization was done with 5 mm thick SE(B) specimens. The larger specimen size yields a larger plastic deformation zone, compared to the smaller specimen, and proportionally, a larger part of the plastic deformation zone is in the weld metal, which can alter the stress response ahead of the crack [56,57].…”

“…In contrast to the ductile material properties of DMWs, the brittle fracture properties and crack path effects have not been investigated as extensively as in the ductile region [27,28]. Testing of the brittle fracture toughness properties of the HAZ can lead to a scatter larger than predicted by the normal Master Curve analysis developed for macroscopically homogeneous materials.…”

“…Ageing of the NIWEL DMW mock-up for 10 h at 400 °C corresponds to 60 years of operation in nuclear power plant conditions, when intergranular phosphorous segregation in the HAZ of a low-alloy pressure vessel steel is considered to be the main ageing mechanism [27].…”

Effect of Thermal Ageing at 400 °C on the Microstructure of Ferrite-Austenite Interface of Nickel-Base Alloy Narrow-Gap Dissimilar Metal Weld

“…Both mock-ups are representative of the RPV safe-end of a modern PWR design, using the narrow-gap welding technique without a buttering layer between the LAS and the weld metal (see Figure 1). Ageing at 400 • C for 10,000 h was estimated to correspond to 60 years of operation at PWR in-service temperatures in terms of grain boundary (GB) segregation of phosphorous in the RPV LAS [34]. The weld mock-ups consisted of SA 508 and AISI 316L base materials, SS 308L/309L cladding on the RPV side, and Alloy 52 weld metal in both cases (see Table 1 for chemical compositions).…”