Study of the effect of the deformation martensite on the corrosion resistance of NPP equipment and pipelines made of austenitic steels

Abstract

Austenitic stainless steels are widely used in nuclear and thermal power engineering due to their high mechanical, corrosion and technological properties. We present the results of studying the effect of deformation martensite on the corrosion resistance of chromium-nickel steels of the austenitic class. Samples of heat exchange tubes of steam generators, tube bends, plates (constituents of steam turbines), safety valves used in NPP equipment were studied. The tests were carried out using metallographic, X-ray diffraction, atomic emission and atomic absorption spectral analyses. Electron microscopy was used to determine the content of the ferrite phase. It is shown that irregular dark gray spots located along a line parallel to the sample axis contain iron oxides. The appearance of such defects observed only on the outer surface of the products is attributed to the technology of their manufacture. It is also shown that severe plastic deformation which occurs during production or operation leads to formation of the deformation martensite which is subject to corrosion at this the corrosion cracking is accompanied by stress. The absence of δ-ferrite in the metal of samples is also revealed. The deformation martensite formed during operation of the product at the point of contact with a harder material leads to appearance of a large number of microcracks, which develop according to the fatigue mechanism under cyclic loading. The results obtained can be used to assess the probability of the formation of deformation martensite in chromium-nickel austenitic steels.