The regularities of creep deformation and failure of the VVER’s pressure vessel steel 15Kh2NMFA-A in air and argon at temperature range 500–900 °C”

The paper presents the results of the determination the creep strength properties and both the microstructure and regularities of high-temperature creep behavior of unirradiated ferritic-pearlitic steel 15Kh2NMFA-A within the temperature range from 500 to 900 °C and the creep process duration up to 100 h. It is extremely necessary to obtain the such experimental data characterizing the creep strength properties of this steel under the condition of high-temperature creep to predict the behavior of the VVER’s RPV structure in the severe accident when the pressure vessel structure undergoes both force and high-intensive thermal action from the melted core materials. This ferritic-pearlitic steel 15Kh2NMFA-A (2% Cr, 1% Ni, 0.5% Mn, 0.5% Mo) is one of basic steel types used in fabrication of reactor pressure vessels of the VVER type designed in Russia. This investigation included several series of creep tension tests of the specimens made of 15Kh2NMFA-A steel. In addition, an analysis of the microstructure of the steel under study was carried out for a number of samples after the creep tests. For the creep tests, several sets of specimens made of the fragment of unirradiated VVER-1000 RPV. The specimen’s working part was 8 mm in diameter and the gauge length was ~50 mm. Isothermal creep tests under uniaxial tension were carried out in air up to 650 °C and in shielding argon medium within the temperature range from 700 to 900 °C. Basing on mathematical processing of test results, the values of steady-state creep rate and the values of corresponding parameters (B) and (n) were determined. The analysis of obtained results showed that within the temperature range from 750 to 900 °C, the creep mechanism of 15Kh2NMFA-A steel is the same. A sharp variation of parameters (B) and (n) (describing the steady-state creep rate of this steel) is observed within the temperature range from 500 to 750 °C. At temperature of more than 750 °C, the value log(B) is varied within the range from −9.65 to −8.48 and the values of parameter (n) are within the range from 4.72 to 4.82. The results of creep tests show that both creep failure time and secondary creep rate values of the 15Kh2NMFA-A steel at 800 °C coincide very closely with the analogous ones for the specimen tests under analogous conditions at 850 °C. It is obvious that such singularity is due to the simultaneous presence of several phases in the 15Kh2NMFA-A steel during the phase transformations in temperature range from 770 to 820 °C. The formation of oxide layer on the specimen surface under the creep condition in air at temperature of 800 °C and more than the given one changes considerably the model of deformation and fracture of the investigated steel under the creep condition. We defined the parameters for the constitutive equation of the Larson-Miller type (LMP) for this steel by statistical processing the results of creep tests within the temperature range from 500 to 900 °C. The analysis of the obtained LMP equation for this steel revealed an important fact that within the range of temperature values Ac1-Ac3, where microstructure transformations of steel take place, we observe essential differences between experimental data and corresponding values defined by using the obtained LMP dependence. When estimating rupture time of this steel under the condition of high-temperature creep, such a difference between the specified values results in a considerable error (by more than the order of magnitude)