Effect of cyclic heat treatment on the microstructure and mechanical properties of CLF-1 steel

Full text of publication follows: The two major blanket concepts for ITER TBM addressed by China are helium cooled lithium lead (HCLL) and helium cooled pebble bed (HCPB). Both concepts use reduced activation ferritic/martensitic (RAFM) steel as structural material. In Southwestern institute of physics (SWIP) of China, HCPB is the primary candidate blanket system and the RAFM steel of CLF-1 (Chinese low activation ferritic/martensitic steel) which is chosen as the primary candidate structural material is under research presently. In this paper, it is presented for the evolution of the microstructure, tensile and impact properties of RAFM steel CLF- 1 after cyclic heat treatment. The material investigated is a type of reduced activation ferritic/martensitic steel with higher tungsten and vanadium and lower manganese concentration compared with Eurofer97. The steel was supplied as bars in the normalized (1243 K/45 min/air-cooled) plus tempered (1006 K/90 min/air-cooled) condition. The cyclic heat treatment was performed at normalization temperature of 1243 K for 20 min then air cooled for 1, 2, 3, 4, and 5 times respectively. The effect of cyclic number on the microstructure and mechanical properties of CLF- 1 steel was observed and tested. The characterization of the microstructure was performed by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. The results show that it is fully martensitic microstructure in all experimental conditions. The strength and ductility were improved and ductile-brittle transition temperature (DBTT) was decreased due to its finer grain size after cyclic heat treatment. (authors)