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

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.

Thermal Stability of Athermal and Deformation Martensite in Ni27Ti2AlMoNb Steel

The study was devoted to the thermal stability of the phases present in chromium-less and practically carbon-less Ni27Ti2AlMoNb steel. Steel with an austenitic structure was subjected to martensitic transformations using a thermal treatment, namely cooling to below the temperature Ms (beginning of the martensitic transformation), or by plastic deformation using the TRIP effect. The cooling in liquid nitrogen gave a martensitic-austenitic structure with athermal martensite α’a. The martensite had a lenticular morphology, and its volumetric share was 51%, which is typical of duplex type steels. The 50% squeeze of the austenitic steel resulted in the formation of deformation martensite α’d with a lamellar structure (this transformation is only possible below a certain temperature Md). During the experiments, the thermal stability of the two phases, α’a and α’d, at temperatures of 450°C and 550°C was examined. The changes taking place in the steel structure when the athermal martensite and deformation martensite were annealed below and above the temperature As (beginning of the austenitic transformation) were observed using a vibrating sample magnetometer and an optical microscope.