The brittle fracture surfaces features of the samples of WER-1000 reactor pressure vessel steel after fracture toughness tests

摘要

Mechanical properties of RPV materials continuously changes under the influence of neutron irradiation and longtime thermal exposures during the operation of nuclear reactor. For safety operation of nuclear power plant these changes are under control. One of the main parameters that define fracture toughness of the material is critical stress intensity factor Kjc that is determined by the value of J-integral. In this paper fractographic studies of the samples of 15Kh2NMFAA steel in the initial state and after irradiation fast neutrons fluence of ～35×10~(22)m~(-2). As it is known from the literature at fracture toughness tests the initiation of brittle fracture occur at a certain distance from the top of the stress concentrator - grown fatigue crack. It is defined both by the stress field at the crack tip and the location of different local concentrators (inclusions) in the material along the crack front. To search the correlation between the measured value of Kjc and the distance from the brittle static fracture initiator to the top of the fatigue crack (CID) is of scientific and practical interest. Also of special interest is to search the structural features of fractures in the area of CID, which determine the values of fracture toughness at the criterion level depending on the material state. In this paper a link between the CID on fractures and experimentally obtained values of Kjc of the samples in different states and tested at different temperatures was found. It was established that larger values of CID correspond to higher values Kjc. Neutron irradiation is well known to shift the temperature dependence of Kjc to higher temperatures, i.e. embrittlement of material occurs. This change in properties is taken into account in the validation of the possibility of safe operation of reactor pressure vessel. In this paper it was shown that there is a linear dependence between the value of Kjc and the distance between the initiator of brittle fracture and the concentrator (CID) in the zone of brittle fracture. This dependence is observed in a wide temperature range when a brittle fracture of the material is realized. It was also found that the irradiated material compared with non-irradiated samples at the same test temperature goes along with a decrease in the Kjc and the distance CID. Herewith the dependence between Kjc and CID for the irradiated material is the same as that for the unirradiated material. This obviously indicates that neutron irradiation reduces the critical stress of brittle fracture due to the appearance of new local stress concentrators or reduce the link with matrix that initiate brittle static fracture of the material.