APFIM CHARACTERIZATION OF 15KH2MFA CR-MO-V AND 15KH2NMFA NI-CR-MO-V TYPE STEELS

A microstructural characterization of 15Kh2MFA Cr-Mo-V and 15Kh2NMFA Ni-Cr-Mo-V type steels that are used in the pressure vessels of Russian VVER 440 and VVER 1000 nuclear reactors, respectively, has been performed with the use of the techniques of atom-probe field-ion microscopy (APFIM) and transmission electron microscopy. The microstructure of these materials was found to be tempered martensite and bainite. A high number density of coarse (approximately 50 to approximately 500 nm) blocky M7C3 carbides and some inclusions were observed. In addition to these coarse carbides, some finer (approximately 10 nm diameter) approximately spherical MC carbides were also observed in the VVER 440 steel. Field-ion microscopy has revealed that the lath boundaries in both unirradiated VVER 440 and VVER 1000 reactor steels are decorated with an ultrathin semicontinuous film of molybdenum-carbonitride precipitates. Atom-probe analysis has revealed a high enrichment of phosphorus at the lath boundaries.