Effect of Thermal Ageing on Properties of Pressure Vessel Low Alloy Steel

Abstract

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.

Effect of the bainitic and martensitic microstructures on the hardening and embrittlement under neutron irradiation of a reactor pressure vessel steel

“…This result shows the importance of the metallurgical microstructure on sensitivity to irradiation enhanced BIF. The microstructure appears to play a similar role as in the cases of thermal ageing presented in the introduction [5][6][7].…”

“…It has been identified since many years that simulated coarse grained heat affected zones present a larger susceptibility to thermal ageing than other microstructuctures. However, this tendency to embrittlement appears to be smaller for real HAZ [5][6][7]. In a previous study, the response of different coarse grained microstructures of a A533 type steel to step cooling heat treatment have shown a larger susceptibility of martensite to temper embrittlement and the enhancement of intergranular fracture for the microstructures containing martensite.…”

“…It was also observed for a DIN 22NiMoCr37 RPV steel thermally aged à 520°C, that coarse grained martensite presents mixed transgranular and intergranular fracture for notch specimens as the fracture for the bainite remains, after ageing, transgranular cleavage [9]. Recently, the sensitivity of martensite bearing microstructures to thermal ageing of A508 type steels without prior coarsening of the austenite grain has been demonstrated [7], [10]. Neutron irradiation, in general, increases only slightly the P segregation at grain boundaries and nonhardening embrittlement of RPV base metals is low [11][12][13].…”