Roman Mouginot

Microstructural Characterization of Alloy 52 Narrow-Gap Dissimilar Metal Weld After Aging

The safe-end dissimilar metal weld (DMW) joining the reactor pressure vessel to the main coolant piping is one of the most critical DMWs in a nuclear power plant (NPP). DMWs have varying microstructures at a short distance across the ferritic-austenitic fusion boundary (FB) region. This microstructural variation affects the mechanical properties and fracture behavior and may evolve as a result of thermal aging during long-term operation of an NPP. This paper presents microstructural characterization performed for as-manufactured and 5000 h and 10,000 h thermally aged narrow-gap DMW representing a safe-end DMW of a modern pressurized water reactor (PWR) NPP. The most significant result of the study is that the thermal aging leads to a significant decrease in a hardness gradient observed across the ferritic-austenitic FB of the as-manufactured DMW.

Development of Short-Range Order and Intergranular Carbide Precipitation in Alloy 690 TT upon Thermal Ageing

Thermal ageing promotes intergranular carbide precipitation and atomic ordering reaction in most commercial nickel-base alloys, and it affects the long-term primary water stress corrosion cracking (PWSCC) resistance of pressurized water reactor components. Alloy 690 with 9.8 wt% Fe was solution annealed and heat-treated at low temperature, then aged between 350 and 550 °C for 10,000 h. No direct observation of ordering was possible, but variations in hardness and lattice parameter suggested the formation of short-range ordering (SRO) with a peak level upon ageing at 420 °C, while a disordering reaction occurred at higher temperatures. Heat treatment induced ordering before thermal ageing was compared to the solution-annealed state. Thermal ageing resulted in the precipitation of Cr-rich M23C6 carbides at grain boundaries and twin boundaries. Although no link between SRO and an increase in strain localization was observed, the combination of intergranular carbide formation and SRO over longer ageing times was deemed detrimental to the PWSCC resistance of Alloy 690 TT.

Effect of thermal aging on fracture mechanical properties and crack propagation behavior of alloy 52 narrow-gap dissimilar metal weld

Determination of the fracture toughness properties and thermal aging behavior of dissimilar metal weld (DMW) joints is of utmost importance for successful structural integrity and lifetime analyses. This paper presents results from fracture resistance (J-R), fracture toughness (T0) and Charpy-V impact toughness tests as well as fractography performed for an industrially manufactured narrow-gap DMW mock-up (SA508-Alloy 52-AISI 316L). Tests were performed on post-weld heat treated, 5000 h aged and 10,000 h aged material. The results show that this DMW is tough at the SA 508-Alloy 52 interface, which typically is the weakest zone of a DMW. The DMW joint maintains its high fracture resistance also after thermal aging. Crack propagates for a large part in the carbon-depleted zone (CDZ) of SA 508 but deflects occasionally to the Alloy 52 side due to small weld defects in µm scale. Ductile-to-brittle transition temperature determined from Charpy-V impact toughness tests increases due to thermal aging, but only to a minor extent. No significant change is observed for the T0 transition temperature due to aging.

Investigations of a Type 316L Steam Dryer Plate Material Suffering from IGSCC After a Few Years in BWR Conditions.

Detailed microstructural characterization has been performed on a sample made of Type 316L stainless steel, removed from a boiling water reactor (BWR) steam dryer inner roof plate, which had suffered from intergranular stress corrosion cracking (IGSCC) after only a few years of operation. The aim of the characterisation was to increase the understanding of possible material related characteristics that could have affected the observed cracking.