The properties of duplex stainless steels (DSSs) depend on the ferrite–austenite ratio, on the content of secondary phases and on the contamination with non-metallic inclusions. To assess the quality of DSSs, it is necessary to use an integrated approach which includes controlling for the volume fraction, the morphology and the distribution of all phases and non-metallic inclusions. Samples of several grades of DSSs were obtained using various heat treatments, such as solution annealing and quenching from 1050 to 1250 °C to obtain different amounts of ferrite and to provoke annealing at 850 °C to precipitate σ-phase. As a result, a metallographic technique of phase analysis in DSSs based on selective etching and subsequent structure parameters estimation according to ASTM E1245 was developed. We demonstrated that the developed method of quantitative analysis based on selective etching and metallographic analysis according to ASTM E1245 allows us to obtaining much more accurate results, compared to the point count method described in ASTM E562 and to the XRD method.

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.

Thermodynamic criteria have been developed for a reasonable prediction of compositions of advanced duplex stainless steels. The success of these criteria was studied under the influence of the primary and secondary alloying elements for duplex stainless steels, and the results of these studies have been generalized using multiple regression equations. It has been revealed that the chemical composition optimization of well-known commercial DSS grades, as well as the development of new grades with specified properties can significantly benefit from the use of these equations. Hot physical experiments have shown that thermodynamic modeling well describes the ferrite content found in experimental steels of different compositions at different quenching temperatures.

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 properties of duplex stainless steels (DSSs) depend on the ferrite–austenite ratio and on the contents of secondary phases. Therefore, it is necessary to control the volume fractions, morphologies, and distribution patterns of all phases. The phases in the samples were identified using thermodynamic modeling and scanning electron microscopy. Investigated specimens were obtained after different heat treatments, such as solution annealing and quenching from 1050 to 1250 °C to obtain different amounts of ferrite and annealing at 850 °C to precipitate the σ-phase. Therefore, a metallographic technique for assessing the phases in DSSs based on selective etching and subsequent analysis according to ASTM E 1245 was developed. It was shown that the developed method of quantitative analysis based on selective etching and metallographic assessment according to ASTM E 1245 allows obtaining much more accurate results compared to the proposed ASTM E 562 method, which correlates well with the XRD quantitative phase analysis.

Duplex stainless steel is a unique material for cast products, the use of which is possible in various fields. With the same chemical composition, melting, casting and heat treatment technology, pitting and crevice corrosion were observed at the interphase boundaries of non-metallic inclusions and the steel matrix. To increase the cleanliness of steel, it is necessary to carefully select the technology for deoxidizing with titanium or aluminum, as the most common deoxidizers, and the technology for modifying with rare earth metals. In this work, a comprehensive analysis of the thermodynamic data in the literature on the behavior of oxides and sulfides in this highly alloyed system under consideration was performed. Based on this analysis, a thermodynamic model was developed to describe their behavior in liquid and solidified duplex stainless steels. The critical concentrations at which the existence of certain phases is possible during the deoxidation of DSS with titanium, aluminum and modification by rare earth metals, including the simultaneous contribution of lanthanum and cerium, was determined. Experimental ingots were produced, the cleanliness of experimental steels was assessed, and the key metric parameters of non-metallic inclusions were described. In steels deoxidized using titanium, clusters of inclusions with a diameter of 84 microns with a volume fraction of 0.066% were formed, the volume fraction of which was decreased to 0.01% with the subsequent addition of aluminum. The clusters completely disappeared when REMs were added. The reason for this behavior of inclusions was interpreted using thermodynamic modeling and explained by the difference in temperature at which specific types of NMIs begin to form. A comparison of experimental and calculated results showed that the proposed model adequately describes the process of formation of non-metallic inclusions in the steel under consideration and can be used for the development of industrial technology.