Heat Treatment Effects on Microstructures and DBTT of F82H Steel Doped with Boron and Nitrogen

Abstract

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 μm 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 μm 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 μm at grain boundaries was observed in the F82H+B by using a time of flight-secondary 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 −96 to −73°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.