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

Matias Ahonen,Zaiqing Que,Teemu Sarikka,Roman Mouginot,Iikka Virkkunen,Sebastian Lindqvist,Ulla Ehrnstén,Hannu Hänninen

doi:10.3390/met10030421

Matias Ahonen, Zaiqing Que + Show 6 more

Open Access

https://doi.org/10.3390/met10030421

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Abstract

Dissimilar metal welds (DMWs) are a key design feature in nuclear power systems, typically involving ferritic low-alloy steels (LAS), stainless steels (SS), and nickel-base alloys. They are, however, a potential concern regarding the structural integrity of nuclear power systems. In particular, the LAS/nickel-base alloy weld metal interface is known to develop a local strength mismatch upon post-weld heat treatment (PWHT). Very limited data is available on the effect of thermal ageing on the DMW interface. The aim of this study was to investigate the effects of thermal ageing at 400 °C for up to 10,000 h on a narrow-gap DMW mock-up representative of the weld between the reactor pressure vessel nozzle and its safe-end after PWHT, with a special focus on the LAS SA 508/nickel-base Alloy 52 weld metal interface. No significant effect of thermal ageing on the appearing microstructure was observed in either LAS base material, LAS heat-affected zone or Alloy 52 weld metal. However, thermal ageing reduced the local strength mismatch at the LAS/nickel-base weld metal interface formed during PWHT. The reduction of the strength mismatch was detected using nanoindentation measurements and was concluded to be associated with a decrease in the carbon pile-up in the weld metal caused by PWHT. Based on the obtained results, thermal ageing promotes carbon diffusion from the weld metal side of the fusion boundary further away into the weld metal and thus slightly decreases the local strength mismatch.

Highlights

With the exception of fuel cladding materials, the main materials used in the components and piping of primary circuits of pressurized water reactors (PWRs) are ferritic low-alloy steels (LAS), stainless steels (SS), and nickel-base alloys [1]
From the design and performance point of view, Dissimilar metal welds (DMWs) are critical and they have been related to a number of incidents involving intergranular stress corrosion cracking (IGSCC) [2,3,4], with sensitized microstructures found in SS and nickel-base alloy safe-ends and within weld metals [5,6]
Adjacent to the fusion boundary (FB), a layer of coarse grains is present resulting from the higher heat, where carbon has diffused to the low-carbon/high-chromium nickel-base weld metal side of the interface (Figure 2d)

Summary

Introduction

With the exception of fuel cladding materials, the main materials used in the components and piping of primary circuits of pressurized water reactors (PWRs) are ferritic low-alloy steels (LAS), stainless steels (SS), and nickel-base alloys [1]. LAS alloyed with low contents of chromium, nickel, molybdenum, and vanadium can lead to a good combination of high strength and ductility, which are used in structural components like a reactor pressure vessel (RPV) [2]. Austenitic SS are utilized for structural materials (types 304, 304L, 316, and 316L) and as corrosion-resistant cladding of the RPV and pressurizer (types 308 and 309). Nickel-base alloys are applied for high-strength components, some critical applications, and welds. From the design and performance point of view, DMWs are critical and they have been related to a number of incidents involving intergranular stress corrosion cracking (IGSCC) [2,3,4], with sensitized microstructures found in SS and nickel-base alloy safe-ends and within weld metals [5,6]

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