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Dependence of fracture properties and hardening was examined as a function of helium production in tensile specimens of a martensitic steel F82H (Fe-8Cr-2W-0.1C-0.04Ta) irradiated at 300 C to 2.3 dpa by neutron irradiation in the JMTR (Japan Materials Testing Reactor). The specimens used in this study were F82H, F82H+60 ppm 11 B, F82H+30 ppm ( 11 B+ 10 B) and F82H+60 ppm 10 B. The helium range produced from 10 B (n,) 7 Li reaction was from 5 to 330 appm in the specimens. The tensile testing was performed at 25 C. The radiation hardening due to helium production was detected at 330 appmHe. The degradation of fracture stress due to helium production was approximately evaluated from the fracture strength and the reduction area. Effect of specimen size on tensile and Charpy impact properties in F82H doped with 60 ppm boron and 200 ppm nitrogen was also examined. The JIS 14A and SS-J3 (Small Size-Japanese-3 type) were used for the tensile specimens, and half size (55 mm in length, 10 mm in height and 5 mm in width) and 0.5-1/3CVN (18 mm in length, 3.3 mm in height and 1.65 mm in width) were used for the Charpy impact testing. The tensile properties were a similar to each other. However, the ductile-brittle transition temperature measured in smaller size specimen was somewhat lower than that in the standard size specimen.

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Effects of doping with 60 ppm B and/or 200 ppm N and heat treatments on the microstructures and the ductile-brittle transition temperature (DBTT) have been studied for ferritic/martensitic steel F82H. Prior austenitic grain size of standard F82H decreased from about 120 to 30 mm and also the DBTT decreased by about 50 C when the normalizing temperatures were changed from 1040 to 950 C. In case of F82H doped with B (F82H+B) normalized at 950 C the grain size increased from 30 to 40 mm and the DBTT increased by about 50 C as compared with that of the standard F82H. In case of F82H co-doped with B and N (F82H+B+N) the grain size and DBTT, however, were comparable to each one of the standard F82H. Localization of B and C with the size of a few mm at grain boundaries was observed in the F82H+B by using a time of flightsecondary ion mass spectroscope, but not in the F82H+B+N. The results indicated that the degradation of fracture toughness in F82H+B was caused mainly by the localization of B at the grain boundaries. The DBTT of F82H+B+N steels normalized at the temperatures from 950 to 1040 C was changed from À96 to À73 C, but the prior austenitic grain size remained nearly unchanged. The precipitate size, however, depended on the normalizing temperature. It was shown that the change of DBTT was related to the change in the mean size of the precipitates in the F82H+B+N steels. Keywords: heat treatment, microstructure, ductile-brittle transition temperature, ferritic/martensitic steel, boron and/or nitrogen doping, time of flight-secondary ion mass spectrometry