Irradiation embrittlement of 2.25Cr-1Mo steel at 400degC

The present paper deals with irradiation embrittlement of normalized and tempered 2.25Cr-1Mo steel to be used for the pressure vessel material of the high temperature engineering test reactor (HTTR). Irradiation data were generated by 12 capsules using JMTR and JRR-2. The results of post-irradiation tensile and Charpy impact tests are reported and analyzed to assure the irradiation embrittlement sensitivity at the HTTR operation temperature of 400degC and provide implications on ferritic alloys for fusion applications. The irradiation hardening was small at irradiation temperature of 400degC with a fluence of ∼2 x 1019 n/cm2E>1MeV. No significant increase of the shifts in ductile to brittle temperature (DBTT) was observed at those higher temperatures. Additional neutron irradiation with changing the temperature from about 300degC to 400degC during irradiation induced substantial recovery of the irradiation hardening, irrespective of the prior hardening level at 300degC. Continued irradiation at 400degC to a fluence larger than 5 x 1019 n/cm2 altered the fracture appearance from the entirely transgranular to mixed transgranular plus intergranular which resulted in considerable shifts in DBTT. Thus, irradiation embrittlement of 2.25Cr-1Mo Steel at 400degC is mainly caused by irradiation-induced impurity segregation to grain boundaries. This was confirmed by electrochemical testing. (author)