The use of traditional materials leads to failures and breakdowns of expensive equipment, so advanced materials are needed that can provide reliable and durable solutions. The ability to control the quality of duplex stainless steels (DSSs) can greatly help with the development of new compositions or choosing existing DSSs. In this case, it is necessary to consider the final consumer properties—corrosion resistance and mechanical properties, which depend on the phase composition, contamination with non-metallic inclusions (NMIs), and the presence of undesirable secondary phases. In this research, specimens of cast DSSs of different grades, produced at laboratory and industrial scales, were studied. A technique for quantifying the microstructure of DSSs was developed. A thermodynamic database was chosen that adequately describes the processes of phase formation in DSSs. The effects of heat treatment on the microstructure and corrosion properties of cast DSSs were studied. The effects of the structural state on the changes in consumer properties of the final product are shown. It is shown that using various deoxidation technologies, it is possible to obtain both NMIs that are dangerous in terms of corrosive activity and ones that are relatively safe.

This work studied the effect of the amount of additives and the composition of refining slags on the metallurgical of steel grades of the S355G10, S420G2, and S460G2 types. The objects of study were samples of industrial melts after out-of furnace refining of steel with different amounts and nature of additives. Based on the results obtained, the recommended amounts of additives and the composition of the refining slag have been developed, which makes it possible to reduce the metal contamination with non-metallic inclusions.

One of the most popular methods for ranking duplex stainless steels (DSSs) and predicting their corrosion properties is the calculation of the pitting resistance equivalent number (PREN). However, since DSSs are two-phase materials with a significant fraction of secondary phases and precipitates, the application of the PREN can be highly limited. This article attempted to use a new approach to describe the corrosion resistance of these steels. The corrosion resistance of two DSSs of the same class was investigated. Under identical solution heat treatments in the temperature range of 1050–1200 °C, the crevice corrosion resistance of one steel increased, while that of the other decreased. It was demonstrated that the amounts of austenite and ferrite changed similarly in these steels, and the different corrosion resistances were associated with the behaviors of secondary phases: niobium carbonitride and chromium nitride. SEM-EDS analysis was conducted to analyze the redistribution of elements between phases in both cases, showing good agreement with the thermodynamic modeling results. The PREN was calculated for each phase depending on the treatment temperature, and a method for calculating the effective PREN (PREN_eff), accounting for phase balance and secondary phases, was proposed. It was shown that this indicator described corrosion properties better than the classical PREN calculated for the average steel composition. This study demonstrated how the calculation of critical temperatures (the temperature of equal amounts of ferrite and austenite, the temperature of the beginning of chromium nitride formation, and the temperature of the beginning of σ-phase formation) could describe the corrosion resistance of DSSs. Maximum possible deviations from these temperatures were defined, allowing the attainment of the required corrosion properties for the steels. Based on the conducted research, an approach for selecting new compositions of DSSs was proposed.

The present work is devoted to the study of the influence of the parameters of the structural anisotropy of rolled products on the low-temperature impact strength of alloyed steels. A quantitative metallographic analysis of the microstructure of rolled steel samples obtained after testing for low-temperature toughness was carried out. It was established that the main reason for the decrease in the low-temperature impact strength of rolled steel samples is a highly developed segregation band enriched with carbon films formed at the stage of steelmaking conversion in violation of the technology of continuous casting of steel. The microstructural analysis of rolled stock samples was used in the work, and studies of the fracture surface of rolled stock samples were carried out with a scanning electron microscope using X-ray microanalysis methods. The studies carried out showed that the metallurgical quality of sheets of one heat, as well as individual samples within one sheet, varied over a wide range, from satisfactory to unacceptably low. It was established that the main reason for the decreasing low-temperature impact strength of rolled products was a highly developed segregation band enriched with carbon films, formed at the stage of steelmaking in case of violation of the continuous casting of steel technology. The multivariate statistical analysis carried out showed that only the size of the segregation band has an effect on the low-temperature impact strength of 10 mm thick rolled coil samples.

Currently, duplex stainless steels are increasingly used in industry. Austenite and ferrite in these steels are in approximately equal proportions. During manufacture of cast products from these steels, a chemical and structural heterogeneity is formed in the castings, for the elimination of which heat treatment is carried out. In practice, within the framework of one class or even one steel grade, the chemical composition and, as a consequence, the phase ratio can vary over a wide range without reaching their optimal values. In this paper, the authors investigated the influence of chemical composition and solidification conditions on the structure and properties of cast duplex stainless steels and developed thermodynamic criteria for the selection of casting alloys, taking into account the temperature of beginning of the polymorphic transformation of δ-ferrite into austenite and the average equilibrium rate of this transformation. It was found that in the studied steels with 21 – 26 % of chromium, crystallization proceeds with the formation of δ-ferrite dendrites, and austenite is formed in the solid metal at the places of the former interdendrite spaces. It is shown that at cooling rates, which are realized in practice when obtaining, for example, casings of centrifugal pumps or other products of a similar size, the transformation of δ-ferrite into austenite is practically suppressed when the temperature reaches 1180 – 1200 °C. On the basis of this, a criterion for the development of compositions with the required phase ratio without heat treatment was proposed. The evolution of the structure during heat treatment at temperatures of 1050 – 1250 °C was studied and it is shown how by choosing the optimal temperature of annealing and quenching, depending on the actual chemical composition of steel, it is possible to achieve an acceptable level of pitting potential in steel with a lower alloying, and vice versa, non-optimal heat treatment of a high-doped alloy leads to a catastrophic decrease in corrosion resistance. It is shown that in the steels under consideration optimal properties are achieved at 70 % of δ-ferrite.

Introdution. Austenitic steel (e.g., AISI 304, AISI 321, AISI 316, AISI 403, 12-Cr18-Ni10-Ti, etc.) is widespread, which is caused by high corrosion resistance and the corresponding possibility of use in aggressive media. The following most common types of 12-Cr18-Ni10-Ti steel defects can be distinguished: integranular corrosion, martensitic orientation of the α-phase and ferrite δ-phase. The purpose of work: to analyze the defects formation reasons of the 12-Cr18-Ni10-Ti steel grade billets and to develop recommendations for their elimination. The methods of investigation. Tests of 12-Cr18-Ni10-Ti steel samples for resistance to integranular corrosion, metallographic analysis of defects were carried out in this work. Hardness measurements were carried out for various degrees of billets reduction. Thermodynamic calculations of phase equilibrium in multicomponent steel for different temperatures were performed by the Thermo-Calc software. Results and Discussion. It is determined that in order to prevent integranular corrosion, it is necessary to reduce the nitrogen and carbon content in steel at the stage of ladle refining to 0.05%, and also to ensure the concentration of titanium in steel is not less than the permissible value — 0.3%. These measures contribute to the reduction of Cr23C6 chromium carbides responsible for integranular corrosion. It is necessary to reduce the degree of compression of the billets to a level of no more than 50% to prevent the appearance of a ferromagnetic martensitic α-phase, since the formation of this defect is associated with a high degree of compression during drawing. The high-temperature phase of δ-ferrite exists in the metal structure in a wide temperature range. Reducing this range to 100 degrees or less by optimizing the composition of the carbon and chromium alloy in accordance with GOST 5632-2014 leads to a significant reduction of the amount of ferrite. However, it is not possible to completely eliminate it from the structure of steel. For all cases, it is necessary to assign austenization of billets in the temperature range of 1,050…1,100 °C.