The influence of prolonged exposure to operating temperatures and strain aging on the material properties of nuclear power plant reactor vessels and pipings

Abstract

This report presents information regarding the variation under the effect of operating temperatures of the mechanical properties of steels and their welded joints, in the production of NPP apparatus and pipings used in the types VVER and RBMK reactors. Together with the properties characteristic of the types 20, 22 K, 08X18H12T, 15X2MFA steels after exposure to the temperature range of 250–400°C for 10 4 h, the values of the mechanical properties of these materials and their weldments (cut from the real components of operating NPP) after the exposure for 10 5 h at material operating temperatures are given. This work has shown that variation in properties after such prolonged operation periods is negligible and moreover the values of UTS, YS, A and Z are not inferior to those required by the USSR ‘Norms of strength calculation’. The estimation of structural strength (low cycle fatigue resistance, static and cyclic crack resistance) has been carried out as related to the pipings manufactured from the 08X18H12T steel after their operation period of 10 5 h. No visible variations of these characteristics have been observed. The influence of strain aging in the operating temperature range of 150–400°C on low cycle fatigue resistance of the types 15X2MFA, 15X2HMFA, 22 K steels and their welded joints, produced by various welding processes (manual, automatic under flux, electroslag weldings) has also been studied. It has been found that with the application of specific temperatures and strain rates, the aging effect may predetermine the material long-term lifetime related to a decrease at the stage of fatigue crack initiation. The strain aging effect is most evident with low carbon weld metal after manual arc welding without preheating which is the welding process that may be applied during assembly.