EFFECT OF THE PRECIPITATION OF Cu-RICH CLUSTERS ON THE DBTT OF RPV SIMULATED STEEL

Abstract

Reactor pressure vessels(RPVs) are usually made of low alloy ferritic steels,among which A508-Ⅲsteel is a typical one.The long-term neutron irradiation can induce the embrittlement of RPV steels,and the embrittlement may lead to a reduction of the RPV service life.Generally, this behavior of the embrittlement is well established and is typically assessed by the increase in the ductile-to-brittle transition temperature(DBTT) of the RPV steels.For many years,extensive studies have revealed that irradiation-induced ultrafine Cu-rich clusters(CRCs) play an important role and CRCs with high number density cause hardening and embrittlement of the RPV steels.In order to investigate the effect of the precipitation of CRCs on DBTT of RPV steels by thermal aging,it is necessary to increase the Cu content in RPV steel.A 40 kg ingot of RPV simulated steel based on the composition of A508-Ⅲsteel with higher Cu content(0.6%in mass fraction) was prepared by vacuum induction melting,and it was forged and hot rolled to a plate with 7 mm in thickness.Specimens with a dimension of 7 mm×12 mm×60 mm were cut from the hot-rolled plate.The heat treatment routes of the specimens consists of a soaking at 880℃for 0.5 h,a water quenching,a tempering at 660℃for 10 h,and a final aging at 370℃for various times.The effect of the precipitation of CRCs on the DBTT of the RPV simulated steel was investigated by Charpy impact tests,as well as the microstructure analysis was carried out by TEM and atom probe tomography(APT).According to ASME 23 standard,Charpy-V specimens with a dimension of 5 mm×10 mm×55 mm were prepared and tested by TINIUS OLSEN 84 impact test machine.The TEM analysis shows that CRCs precipitate on dislocations in the specimen aged at 370℃for 3000 h,and the clusters become a little coarsened when the aging time is extended to 13200 h.For the specimens aged for 1150 h,CRCs were on the stage of the nucleation assessed by TEM as well as APT analysis,and they did not have an effect on the DBTT of the RPV simulated steel.For the specimens aged for 3000 h,CRCs precipitated with an average equivalent diameter of 1.5 nm and a number density of 4.2×10~(22) m~(-3),and it results in the increase of the DBTT from -100℃to -60℃.For the specimens aged for 13200 h,CRCs slightly coarsened to 2.4 nm of the average equivalent diameter,while the number density is similar to that of the specimens aged for 3000 h.In this case the DBTT rose to -45℃.Therefore,the present work shows the precipitation of CRCs induced by thermal aging reveals a smaller impact on the DBTT than that by neutron irradiation.From the thermal aging aspect,the much lower number density of CRCs and the absence of the defects induced by neutron irradiation in the matrix could account for this phenomenon.