Research of Creep Parameters of VVER Reactor Steel 15Kh2NMFA-A at Temperatures of 500 to 1200°C

Abstract—The paper studies the creep characteristics of 15Kh2NMFA-A vessel steel in the temperature range 500–1200°C, which can be achieved during emergency operating conditions of VVER-type nuclear reactors. The creep curves of the 15Kh2NMFA-A steel in the studied temperature range obtained during sample tensile tests were analyzed, with construction of the parametric Larson–Miller relationship. It is shown that in the temperature range 600–850°C, the generalized Larson–Miller temperature dependence has a changed slope, resulting in a high error in estimating the time to fracture of 15Kh2NMFA-A steel under creep conditions in this temperature range. To reveal the reasons for the change in creep resistance in the specified temperature range, studies of the metal microstructure using optical and scanning electron microscopy on specimens in the initial state and after creep testing were carried out. Studies of the microstructure of steel have shown that in the temperature range 600–700°C, coagulation of V(CN) type carbonitrides occurs, leading to a change in the slope of the Larson–Miller parametric relationship. It is shown that in the temperature range of Ac1 to Ac3 there is an abrupt change in creep characteristics associated with phase transformations. The feasibility of dividing the Larson–Miller parametric relationship into several sections depending on temperature, taking into account structural changes in the steel, is substantiated, which makes it possible to increase the accuracy of estimating the time to fracture of 15Kh2NMFA-A steel by at least an order of magnitude. Based on the results of the study, a calculated dependence of the long-term strength of steel on temperature was obtained on a basis of 6, 24, and 120 h.