Regularities of the creep deformation and failure of 15Kh2NMFA-A steel within the temperature range of 900–1200 °C

Abstract

This paper presents and summarizes main results of the research to define the high-temperature creep parameters and strength characteristics of the unirradiated steel 15Kh2NMFA-А tested in the temperature range of 900–1200 °C. This ferritic-pearlitic steel is one of the main types of steel used for the production of the VVER RPVs of the Russian design. The results of research of the strength properties and creep parameters for 15Kh2NMFA-А steel are meant to support, first and utmost, the estimation of the integrity and predicting the behavior of the VVER pressure vessel during the SA, when it may deform due to high-temperature creep and the overpressure in the RPV. One of the main goals of the investigation was to determine such strength properties and creep parameters of the investigated steel as, for instance, steady-state creep rate, creep tensile strength under the condition of isothermal creep at different temperatures and test stress. The creep tensile tests were conducted in vacuum and in the protective argon medium. A series of creep tests was conducted in air. The analysis and comparison of the test results demonstrated significant impact of the surficial oxidation process on the nature of their damage and the time to failure of the test samples. The absence of a protective medium during testing, as observed in the case of the test in air, causes decarburization and the damage of the surficial layer of the steel samples, so that the specimen fractures prematurely. Based on the statistic estimation the (B) and (n) parameters for the secondary creep rate equations were defined for the researched steel within the temperature range of 900–1200 °C. The agreement between the values of the steady-state creep rate of 15Kh2NMFA-A steel from the conducted creep tensile tests and similar calculation values from the obtained parameters (B) and (n) for the temperature range from 900 to 1200 °C was satisfactory. The paper also presents the definition of the LMP parametric dependence coefficients, completed on the basis of the creep test results for 15Kh2NMFA-А steel with a time span up to 100 h in the given temperature range. The comparative analysis of the defined LMP dependence demonstrated good agreement of the predicted values for the failure time with similar values defined during the experiment.The research of the steel specimens’ microstructure following the tests demonstrated a difference in the mechanisms of damage accumulation and fracture at the temperatures over 1000 °C in comparison to similar mechanisms at lower temperatures. It is important to apply more precise research methods to understand this phenomenon to a better extent; such research may take place later.