STUDY ON THE PRECIPITATION OF Cu--RICH CLUSTERS IN THE RPV MODEL STEEL BY APT

Abstract

Reactor pressure vessel(RPV) is nonreplaceable component for the pressurized water reactor(PWR) in the nuclear power plants.RPVs are usually made of low alloy ferritic steels and A508-III steel is one type of these materials.After long-term service under the neutron irradiation,the ductile-to-brittle transition temperature(DBTT) of the RPV steel,which is the main parameter used to measure the degree of the embrittlement,will shift towards higher temperature.This phenomenon is termed irradiation-induced embrittlement,and it is a main factor to affect the operation safety and the lifetime of nuclear power plants.It is realized that the irradiation-induced embrittlement is mainly attributed to the precipitation of Cu-rich nanophases with a high number density.The precipitation process of Cu-rich nanophases can be well characterized by an atom probe tomography(APT) analysis for their size,composition and number density,and the Cu-rich nanophases obtained by the APT analysis are usually termed Cu-rich clusters.It is worthwhile to investigate the precipitation process of Cu-rich clusters by thermal aging for better understanding the mechanism of embrittlement.In order to accelerate the precipitation of Cu-rich clusters,experiment was performed by a RPV model steel containing higher Cu content than commercially available A508-III steel.RPV model steel was prepared by vacuum induction melting with higher content of Cu(0.6%,mass fraction).The specimens of the RPV model steel were tempered at 660℃for 10 h followed by air cooling after water quenching from 880℃,and then they were isothermally aged at 370℃for different time.The precipitation process of Cu-rich clusters is investigated by APT analysis.The results show that the Curich clusters are on the stage of the nucleation when the specimens were aged at 370℃for 1150 h.After specimens were aged for 3000 and 13200 h,the average equivalent diameter of the Cu-rich clusters increases from 1.5 nm to 2.4 nm,and the average Cu content in the Cu-rich clusters vary from 45%to 55%(atomic fraction).The number density of the Cu-rich clusters in both types of the specimens is at the order of 10~(22) m~(-3).The Cu concentration in the ferritic matrix is(0.15±0.02)% for the specimen aged at 370℃for 13200 h,which is still higher than the limitation of Cu solubility in the ferritic matrix at 370℃.It means that the precipitation process of Cu-rich clusters does not reach the equilibrium state.The analysis results also show that Ni,Si,P atoms,but not Cu atoms, segregate near the interface between the cementite and the ferritic matrix,and Mn,Mo,S atoms are enriched in the cementite.