The paper deals with the stress corrosion cracking of heterogeneous welded joints. The welded joints were made of austenitic base metal (AISI 321) and ferritic weld metal by arc welding. Two methods were used to analyze the stress corrosion cracking of samples of this type of welded joint. The first method was a slow strain rate test (SSRT) using a tabletop device at a temperature of 60°C with a graded strain rate for three specimens (10−5, 10−6, and 10−7 1/s). The second method was exposure in a corrosion autoclave at a temperature of 270°C and pressure of 12.26 N/mm2, while prestressing devices were used to achieve the prestressing at the level of yield strength and 3% plastic deformation. One specimen was bent to 60°. Boric acid solutions were used as the medium, which was supposed to simulate the environment of the primary circuit of nuclear power plants, a type of water-water energetic reactor. The surface and the presence of corrosion products, cracks, fractures, or pits on specimens were monitored using scanning electron microscopy, stereomicroscopy, confocal microscopy, and light microscopy. A gravimetric analysis was performed, as well, to determine the corrosion rate after exposure to the autoclave.

The microstructural evolution and mechanical properties of WWER 440 steam generator steel GOST 22K after long-term operation were thoroughly examined in this study. The samples were taken directly from a steam generator using the small punch test method. The uniqueness of these samples lies in the fact that they were real operating materials used in a nuclear power plant with different years of operation. The microstructure was characterized using optical microscopy and transmission electron microscopy supplemented by selective electron diffraction and semi-quantitative chemical microanalysis. It was found that with the prolongation of the operation time of the steam generator, the density of carbides increased slightly, which was reflected in a decrease in the mean distance between particles, but these differences were very small, which indicates the microstructural stability of GOST 22K steel. The stability of this steel was also confirmed by measuring its mechanical properties, which changed only minimally depending on the years of operation. The tensile strength values were in the range of 508 to 579 MPa. In the case of the ductile-to-brittle transition temperature (DBTT), a slight increase was found after 6 years of operation. The DBTT did not change significantly with subsequent operation.

Austenitic stainless steels (ASSs) are characteristic with a combination of good mechanical and corrosion properties. Therefore, they are used in the primary circuits of nuclear power plants. Under the influence of a corrosive environment containing chloride ions and mechanical loading, the phenomenon of stress corrosion cracking occurs in ASSs. SCC can also be initiated by the surface condition of ASSs. Machining is usually the last stage of production, during which a significant deformed zone with high residual tensile stresses can be created, which can accelerate the initiation of stress corrosion cracking. Research is focused on analyzing the influence of final turning on microstructural changes of the surface-machined layer caused by various turning parameters (e.g.: cutting speed, feed, depth of cut, cutting tool geometry). No significant microstructure changes were observed between the samples by light microscopy, so we focused on transmission electron microscopy (TEM) on thin lamellas prepared using the focus ion beam (FIB) technique. TEM observation confirmed the presence of a deformed zone and a passivation layer. In the case of the sample that was turned with a higher feed and cutting speed, the passivation layer was discontinuous. Such a microstructural change can significantly affect the corrosion resistance of ASS.

From the point of view of the safety of nuclear power plants, it is necessary to determine the properties of the manufactured material used. One of the possible risks is the use of inappropriate turning on the material, whose subsequent surface and thus corrosion properties in such an extreme environment are significantly stricken. In this work, high cutting speed (596 m⋅min ⁻¹ ) was used for turning austenitic stainless steel 08Ch18N10T using a CBN cutting tool. Such a high cutting speed significantly affected surface integrity and surface properties such as residual stresses, roughness, hardness and surface toughness (metallographically determined and analysed using X-ray tensometry and acoustic emission). The area that was turned contained deformed grains, which contained deformation twins and slip bands. All these factors can affect stress corrosion cracking susceptibility and corrosion resistance. For this reason, the sample was exposed in a MgCl2 solution according to the ASTM G36, while the surface crack density and the crack lengths or depths were determined. The effect of residual stresses on the direction and propagation of surface cracks has been proven. DL-EPR test recorded sensitization that did not occur at the grain boundaries but in the turning direction where pitting lines appeared. Exposure in a simulated primary circuit environment was performed in an autoclave, where no signs of stress corrosion cracking were confirmed.

Extending the service life of nuclear power plant components beyond their originally designed operational period requires a detailed understanding of the microstructural stability of the materials used. This study focuses on low-temperature precipitation in the austenitic stainless steel 08CH18N10T, which is employed in the main circulation piping of pressurized water reactors. During long-term operation in the temperature range of 100–320 °C, secondary phases such as M₂₃C₆ carbides and intermetallic phase sigma (σ) can precipitate, which can lead to local chromium depletion at grain boundaries, subsequent sensitization of the steel, and susceptibility to intergranular corrosion. The research includes the analysis of samples taken from the decommissioned V1 unit of the Jaslovské Bohunice Nuclear Power Plant, which has been in operation for 28 years. The samples were subjected to thermal aging under laboratory conditions, with an emphasis on evaluating microstructural changes and their impact on corrosion resistance. Based on the experimental results, it can be concluded that the thermal stability of all tested materials is suitable for the operation of the main circulation piping, as the service temperatures to which the main circulation piping is exposed during operation remain below the activation of precipitation that would lead to sensitization and, consequently, susceptibility to intergranular corrosion. Activation of low-temperature precipitation was observed only at 450 °C, while at temperatures up to 400 °C, the structural stability of the material was confirmed, demonstrating its suitability for operation within the specified temperature range of the nuclear power plants’ main circulation piping.