No 3 (2024)

The main direction in solving the problem of increasing the reliability of field equipment, is the creation of new steels with higher resistance to corrosion-mechanical destruction. Currently, to produce oil and gas pipeline systems, low-carbon, low-alloy steels are used, in which lath carbide-free bainite is formed when quenched in water. Such a structure provides a combination of high strength and resistance to brittle fracture. However, issues of increasing corrosion resistance are still open. The purpose of this work is to identify the structural condition of low-carbon, low-alloy, pipe steels, providing a combination of high mechanical properties with increased corrosion resistance in oilfield environments. The studies were carried out on the latest generation 08KhFA, 08KhFMA and 05KhGB steels, most popular when manufacturing oil and gas pipelines. Samples for the study were cut from the pipes and quenched from the austenite region in water, which formed the structure of lath carbide-free bainite. The quenched samples were tempered at temperatures of 200, 300, 400, 500, 600, and 700 °C. To identify the relationship between the morphology of bainite structures and their properties, the samples after quenching and tempering at each temperature, were subjected to metallographic analysis, X-ray diffraction analysis, mechanical tests, and corrosion resistance tests. The work shows the sequence of structure transformation, temperature ranges of phase and structural transformations, changes in mechanical properties, and corrosion resistance that occur during tempering of lath carbide-free low-carbon bainite. It is shown that tempering of lath carbide-free bainite (08KhFA, 08KhMFA and 05KhGB steels) does not affect the rate of carbon dioxide corrosion. It has been found that medium tempering forms the structural condition of carbide-free low-carbon lath bainite providing a combination of high mechanical properties and high corrosion resistance in oil field environments. For each of the steels under study, the authors give recommended heat treatment modes.

The problem of hydrogen embrittlement remains relevant in many areas, so the FeCrNiMnCo alloy (Cantor alloy) generates increased interest among researchers as one of the materials least exposed to the negative effect of hydrogen. Nevertheless, the issue of the influence of microstructure parameters on hydrogen embrittlement of the Cantor alloy and multicomponent alloys of the FeCrNiMnCo system in general remains understudied. This work studies the influence of grain size on the susceptibility of a nitrogen-doped high-entropy Cantor alloy to hydrogen embrittlement. For this purpose, states with different grain sizes (43±21, 120±57, and 221±97 μm) were formed in the (FeCrNiMnCo)₉₉N₁ alloy, using thermomechanical treatments. It is experimentally found that grain refinement leads to an increase in the strength properties of the alloy under study and promotes an increase in the resistance to the hydrogen embrittlement: in samples with the smallest grain size, the hydrogen-induced decrease in ductility is less than in samples with the largest one. A decrease in grain size causes as well a decrease in the length of the brittle zone detected on the fracture surfaces of samples after tension. This is caused by a decrease in hydrogen diffusion during the hydrogen-charging process and a decrease in the transport of hydrogen atoms with mobile dislocations during plastic deformation due to a decrease in grain size.

The authors carried out the study of the influence of 3D printing modes on the structure and chemical composition of 30HGSA steel (chromansil) samples produced by the method of additive electric arc surfacing. To study the influence of the electric arc surfacing mode on the chemical composition of the steel under study, an optical emission analysis of the samples was carried out. The influence of the surfacing mode on the resulting structure was assessed over the entire height of the deposited walls at magnifications of ×50, ×100, ×200 and ×500. Optical emission analysis identified a change in the material chemical composition associated with the loss of chemical elements. It was found that the degree of loss of C, Cr and Si increases almost linearly and is directly proportional to the surfacing heat input (Q, J/mm). The exact influence of an increase in the surfacing heat input on the Mn content was not found, but a relationship between the degree of its loss and the voltage (U, V) during surfacing of samples was identified. Microstructural studies of all samples did not reveal a large number of systemically formed structural defects characteristic of cast and welded products (pores, shrinkage cavities, etc.), which confirms the high quality of the metal in goods produced by electric arc surfacing. Analysis of micrographs taken in different areas of the samples allowed determining that the metal microstructure does not undergo significant changes under different surfacing modes; the main tendencies in changes in the structure along the height of the sample are preserved. All samples demonstrated the formation of a highly dispersed structure, regardless of the 3D printing parameters. The most favorable metal structure, suitable for subsequent use in the production of goods using additive manufacturing, was recognized as the structure of the sample deposited using mode No. 5 (I=160 A, U=24 V, Q=921.6 J/mm). This mode can be used for further study of the problems of additive electric arc surfacing of 30HGSA steel.