Kostenevich E. Mathematical modeling of the residual stresses in the anticorrosive cladding zones of the RPV WWER-1000.

The thesis is devoted to the solving of the actual problem of determining residual stresses in the anticorrosive cladding zones of reactor pressure vessel (RPV) WWER-1000, taking into account microstructural phase transformations, as well as variations of technological parameters of the arc cladding and heat treatment. Based on mathematical modeling, the microstructural composition and kinetics were determined during phase transformations of steel 15H2NMFA (2.5Cr-Mo-V) at characteristic thermal cladding cycles for different parts of the RPV - cylindrical, where automatic arc cladding under flux by strip electrodes is used, and the nozzle zone, which is cladded by a manual arc method with coated electrodes. The calculation results showed that a bainite-martensitic structure is formed in the heat affected zone (HAZ) of the base material of RPV during cladding. The results of calculating of the microstructure with the experimental data of dilatometric analysis of austenite decomposition during cooling and metallography of templates of 15H2NMFA (2.5Cr-Mo-V) steel were validated. For the first time, by the results of experimental studies, thermokinetic diagrams (CCT diagrams) of the austenite decomposition of 15H2NMFA (2.5Cr-Mo-V) steel for characteristic welding cooling rates were constructed, on the basis of which the formation of a bainite-martensitic structure in the HAZ of 15H2NMFA (2.5Cr-Mo-V) steel during cladding was experimentally confirmed. For the first time, the results of mathematical modeling of residual stresses taking into account microstructural phase transformations and variations of the cladding conditions and heat treatment parameters of the RPV WWER-1000 were obtained. In comparison with the existing data, the main differences of the calculated residual stresses after cladding and heat treatment are uneven distribution in the transverse direction of the cladding due to the applying beads periodicity, a deeper zone of tensile residual stresses, specially for the strip cladding, and the compressive residual stresses in the HAZ of the base material due to the formation of a bainite-martensitic microstructure. For two different parts of the RPV - the cylindrical part of the RPV and the inner surface of the DN850 nozzles simplified characteristic distributions of residual stresses after operations of anticorrosive cladding and heat treatment according to the high tempering with a duration 20 hours at a temperature of 650°C were proposed. It is established that the use of updated data on the distribution of residual stresses taking into account microstructural phase transformations during anticorrosive cladding of the RPV WWER-1000 allows to reduce the conservatism in evaluating of the resistance to brittle fracture in emergency situations “pressurized thermal shock” for subclad cracks, specially with small depth (up to 7 mm).