Short-term strength properties and features of high-temperature deformation of VVER reactor pressure vessel steel 15Kh2NMFA-A within the temperature range 20–1200 °C

Abstract

The problem discussed in this work concerns the estimate of integrity of the VVER reactor pressure vessel in the severe accident. In particular, the main goal of discussed investigation is to determine short-term mechanical properties of ferritic-pearlitic vessel steel 15Kh2NMFA-A (2% Cr, 1% Ni, 0.5% Mn, 0.5% Mo) within the temperature range 20–1200 °C. This steel type is one of basic structure steel types used in fabrication of reactor pressure vessels of Russian design, particularly, the VVER vessels. To perform short-term tests of this steel, several lots of steel specimens 8 and 10 mm in diameter with gauge part of length 40–50 mm, correspondingly, were made. The test specimens were made of the fragment of unirradiated standard VVER-1000 reactor pressure vessel of Russian design fabricated by OMZ Group “Izhorskiye zavody” (Russia). The short-term tensile tests of steel specimens were performed both in air within the temperature range 20–1100 °C and in vacuum within the temperature range 900–1200 °C. Basing on the results of the tests, strength-and plastic/strain parameters of this steel (ultimate tensile strength, yield stress, tensile Young’s modulus, steel elongation at fracture, the coefficient of linear thermal expansion) were determined. The analysis of obtained results made it possible to reveal a series of singularities of 15Kh2NMFA-A steel strain. In particular, it was shown that in the temperature range 770–820 °C (Ac1-Ac3), the investigated steel undergoes inner phase/structure transformations, which have an essential influence on strength- and strain properties of investigated steel. The analysis of temperature effect on strength properties (ultimate tensile strength and yield stress) and Young’s modulus of this steel made it possible to reveal three temperature ranges in which these parameters have characteristic features of change. In the first of these ranges (the temperature values from 20 to ~650 °C), a moderate degradation of strength- and elastic properties of steel is observed. In the second range (from ~650 to 850 °C), a sharp decrease of values of steel strength and elasticity occurs, and at test temperature of more than 900 °C, a monotonous decrease of these mechanical properties is observed. The comparison of the results obtained in the present work with the earlier results of tests at temperature of more than 900 °C showed their satisfactory agreement.