The shells of the PWR heavy components are made of quenched and tempered low alloy steel (type A508 cl.3). Slow thermal ageing of the steel may occur by inter-granular segregation of impurities, depending on the service temperature and the concentration of residual elements (mainly phosphorus) in the steel. This phenomenon may generate embrittlement (i.e. a shift of ductile to brittle transition temperature). The pressurizer service temperature is the highest of the primary loop (345°C), and this component may be affected by more important ageing effects than others. In order to support rules for embrittlement predictions and to anticipate the potential embrittlement of the pressure boundary of this component, AREVA launched a research program, on different materials (base materials, weld metals, and heat affected zones of welds) based on accelerated ageing (400°C up to 30,000 hours and 450°C up to 20,000 hours), and Charpy impact toughness measurements. The aim of this paper is to present available results and to analyze them in the light of the literature and the available models.
Filler material used for welding operations can lead to the occlusion of hydrogen gas in the arc atmosphere into the solidifying weld metal. This amount of hydrogen as well as the one originally present in the parent metal rapidly diffuses into the various regions of the weldment due to the high temperature depending also on the microstructure evolution and trapping effects. As the welded component cools down, depending on the metal’s microstructure in the heat-affected zone, the concentration of hydrogen in weld and the level of residual stresses, the risk of hydrogen assisted cold cracking in ferritic steel can arise. One of the most effective precautions against weld hydrogen cracking is to use of preheating and post-heating in order to reduce the hydrogen content, by diffusion in the structure and degassing, when residual stresses reach higher values at the end of cooling. The implant test is a stress controlled test applied on small specimen during welding to assess the susceptibility to heat affected zone hydrogen cracking. It may be used to define preheating temperature and postheating duration in order to prevent nuclear component assemblies from cold cracking risk. This paper will first present how to couple hydrogen diffusion, thermo-metallurgical and mechanical modeling in order to simulate the implant test. Finally, a Weibull type probabilistic criterion based on numerical approaches will be proposed to improve the implant test predictive capability in the case of multi-pass welding processes involving dissimilar materials.
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