Heterogeneity of structure and properties of 12Cr18Ni10Ti and 08Cr16Ni11Mo3 stainless steels irradiated up to high damaging doses in reactor Bn-350

Full text: Earlier, during investigation of post-operating properties and structure of responsible units of fast neutron reactors there was shown /1, 2/ that depending on character of preliminary treatment of austenite stainless steel (austenization, cold deformation, mechanical and thermal treatment) radiation effects could be different. In /2/ one could observe heterogeneity at swelling of cold- worked hexahedral ducts along perimeter, in particular, the swelling of corners was less than plates'. At the same time after mechanical-thermal treatment the corners swell in 3-5 times of magnitude higher than plates. By the present there are several assumptions about nature of this phenomenon. One of them is a difference of deformation degree of material in corners and plates of the duct. It is known that /3/ external effects (including deformation) induce martensitic γ→α transformation in austenitic steels, due to which the structure and properties of steel are changed. In particular, paramagnetic FCC matrix reveals sites with ferromagnetic BCC structure. Steel heating, containing martensitic α-phase higher than ∼ 450-800 deg C, results in reverse γ→α transformation in material, which in its turn leads to formation of phase phase-hardened austenite. We can expect that only peculiarities of processes of direct and reverse martensitic transformations, which took place during preliminary austenitic steel treatment, will predetermine its behavior under irradiation. Taking into account the above mentioned there have been carried out complex material-scientific investigations of 12Cr18Ni10Ti and 08Cr16Ni11Mo3 steel samples cut off from different sites (both adjacent to corners and far from them) of hexagonal ducts of spent fuel assemblies of BN-350 reactor. There were used samples in the form of plates of different sizes: 5x10x2 mm - for metallographic investigations (microscope Neophot-2) and determination of microhardness (PMT-3); 2x20x0,3 mm - for mechanical tests (testing machine Instron-1195), for determination of ferromagnetic α-phase content in sample (F-1053) and study of thin dislocation structure and porosity (JEM 100CX). As a result there was determined that, firstly, hydrostatic density of 12Cr18Ni10Ti steel samples cut out of duct corner of the fuel assembly CC19 is higher than the density of steel samples cut out of plate. Secondly, microhardness measured on perimeter of hexagonal duct appeared to be higher in corners then in plates. At that radiation strengthening of steel was maximal in sites of 12-15 mm in width, 7-8 mm being distant from a corner. According to metallographic investigations the microstructure in these sites considerably differed from the structure in corner and in the center of duct plate. The quantity of ferromagnetic phase is much more near corners than in the middle of the plate according to its determination and distribution on plate width. There was revealed increased content of magnetic phase on internal surface of a duct wall. The nature of magnetic phases has been discussing in both cases. Analysis of the obtained results shows considerable heterogeneity in structure and properties of irradiated stainless steels even within one distance from the center of active zone. It is also recommended to study separately corners and plates of hexagonal ducts at characterization of fuel assembly ducts. (author)