Duplex stainless steels are widely used in many fields due to their excellent corrosion resistance and mechanical properties. However, it is a challenge to achieve duplex microstructure and excellent properties through additive manufacturing. In this work, a 0.09% N 25Cr-type duplex stainless steel was prepared by additive manufacturing (AM) and heat treatment, and its corrosion resistance was investigated. The results show that, compared with S32750 duplex stainless steel prepared by a conventional process, the combination value of film resistance and charge transfer resistance of AM duplex stainless steel was increased by 3.2-5.5 times and the pitting potential was increased by more than 100 mV. The disappearance of residual thermal stress and the reasonable distribution of Cr and N elements in the two phases are the reasons for the improvement of the corrosion resistance of AM duplex stainless steel after heat treatment. In addition, the extremely high purity of AM duplex stainless steel with no visible inclusions resulted in a higher corrosion resistance exhibited at lower pitting-resistance-equivalent number values.

Prompted by the unexpected observation of the pitting of the weldments of a highly corrosion- and pitting-resistant duplex stainless steel, SAF2507, in chloride solutions with nitride addition, the pitting and corrosion resistance of SAF2507 and its weldments were investigated in chloride solutions with and without different levels of nitrite. The Incoloy 825 and 316L austenitic stainless steels were included for the purpose of developing a comparative appreciation. The microstructures of the weldments were characterised, and 316L showed a profound influence of nitrite addition in inhibiting pitting, while \'meta-stable\' pitting transients that were clearly visible in the chloride solution without nitrite were absent when nitrite was added. Both the parent metal and the weldment of SAF2507 had similar pitting potential (Ep) in 0.1 M NaCl without nitrite, which was the highest Ep among the three alloys tested. Additions of nitrite at low concentrations had an inhibitive effect on pitting, whereas higher nitrite contents had a deleterious effect on pitting resistance. On the other hand, Incoloy 825 showed a trend of Ep ennoblement with an increasing nitrite content of 0.1 M NaCl, and the weldment underwent greater ennoblement. Moreover, 316L showed a trend similar to Incoloy 825; however, the Ep ennoblements were significantly more pronounced for both the weldment and the base metal of 316L.

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 (PRENeff), 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.

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.

Duplex stainless steel (DSS) exhibits good mechanical properties and corrosion resistance, and has attracted more and more attention within the fields of both science and technology. However, the increasing levels of N and of Cr, Mo, etc., as alloying elements in DSS increase production difficulty. In particular, the N element increases the risk of Cr2N precipitation, which can seriously deteriorate the thermal plasticity of DSS, while increasing its strength. For this reason, a low-N-content 25Cr-type DSS was designed in order to adapt additive manufacturing processes. With regard to the nano-inclusions of oxide precipitation and effective grain refinement, and considering the benefits of selective laser melting fabrication, a low-N 25Cr-type duplex stainless steel with a 0.09 wt.% N content achieved high mechanical properties, with a yield strength of 712 MPa and an elongation of 27.5%, while the V-notch impact toughness was 160 J/cm2. The microstructure evolution and the reasons behind the improvement in mechanical properties will be discussed in detail.

Additive manufacturing is an important and promising process of manufacturing due to its increasing demand in all industrial sectors, with special relevance in those related to metallic components since it permits the lightening of structures, producing complex geometries with a minimum waste of material. There are different techniques involved in additive manufacturing that must be carefully selected according to the chemical composition of the material and the final requirements. There is a large amount of research devoted to the technical development and the mechanical properties of the final components; however, not much attention has been paid yet to the corrosion behaviour in different service conditions. The aim of this paper is to deeply analyze the interaction between the chemical composition of different metallic alloys, the additive manufacturing processing, and their corrosion behaviour, determining the effects of the main microstructural features and defects associated with these specific processes, such as grain size, segregation, and porosity, among others. The corrosion resistance of the most-used systems obtained by additive manufacturing (AM) such as aluminum alloys, titanium alloys, and duplex stainless steels is analyzed to provide knowledge that can be a platform to create new ideas for materials manufacturing. Some conclusions and future guidelines for establishing good practices related to corrosion tests are proposed.

The hydrocarbon industry constantly requires a better understanding of stainless-steel welding metallurgy. Despite the fact that gas metal arc welding (GMAW) is one of the most commonly employed welding processes in the petrochemical industry, the process is characterized by the presence of a high number of variables to control in order to obtain components that are dimensionally repeatable and satisfy the functional requirements. In particular, corrosion is still a phenomenon that highly affects the performance of the exposed materials, and special attention must be paid when welding is applied. In this study, the real operating conditions of petrochemical industry were reproduced through an accelerated test in a corrosion reactor at 70 °C for 600 h, exposing robotic GMAW samples free of defects with suitable geometry. The results show that, even if duplex stainless steels are characterized for being more corrosion resistant than other stainless steels, under these conditions it was possible to identify microstructural damage. In detail was found that the corrosion properties were strongly related to the heat input during welding and that the best corrosion properties can be obtained with the higher heat input.

Duplex stainless steels exhibit an excellent combination of corrosion resistance and strength and are increasingly being manufactured through powder metallurgy (PM) to produce large, near-net-shaped components, such as those used for offshore applications. Hot isostatic pressing (HIP) is often used for PM production, in which pre-alloyed powders are compacted under high pressures and temperatures. Recent developments in HIP technology enable fast cooling as part of the process cycle, reaching cooling rates comparable to oil quenching or even faster. This enables the integrated solution annealing of duplex stainless steels directly after compaction. In contrast to the conventional HIP route, which requires another separate solution annealing step after compaction, the integrated heat treatment within the HIP process saves both energy and time. Due to this potential gain, HIP compaction at a high pressure of 170 MPa and 1150 °C with integrated solution annealing for the production of duplex stainless steels was investigated in this work. Firstly, the focus was to investigate the influence of pressure on the phase stability during the integrated solution annealing of the steel X2CrNiMoN22-5-3. Secondly, the steel X2CrNiMoCuWN25-7-4, which is highly susceptible to sigma phase embrittlement, was used to investigate whether the cooling rates used in the HIP are sufficient for preventing the formation of this brittle microstructural constituent. This work shows that the high pressure used during the solution heat treatment stabilizes the austenite. In addition, it was verified that the cooling rates during quenching stage in HIP are sufficient for preventing the formation of the sigma phase in the X2CrNiMoCuWN25-7-4 duplex stainless steel.

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.

Quantitative metallography to understand the morphology of different crystallographic phases in a material often rests on the segmentation and classification of electron backscatter diffraction (EBSD) maps. Image analysis offers rich toolboxes to perform such tasks based on \'scalar\' images. Embracing the entire wealth of information provided by crystallography, operations such as erosion, dilation, interpolation, smoothing and segmentation require generalizations to do justice to the very nature of crystal orientations (e.g. preserving properties like frame indifference). The present study gives such extensions based on quaternion representation of crystal orientations. A dual-phase stainless steel specimen is used to illustrate the different steps of such a procedure.