The scarcity of fresh water in some areas of the world has increased interest in desalination plants, designed to extract fresh water from seawater to be used for human purposes. The result of this process is not only fresh water, but also some brines very corrosive for the materials used in the components of the desalination plants due to their high chloride content and the presence of CO2 and H2S, which can reduce the corrosion resistance of the components even if present in modest quantities. The materials used in the desalination plants are usually subject to stress-cracking corrosion (SCC), generalized corrosion, localized corrosion (intergranular, pitting, crevice, etc.). These environments make it necessary to use materials that guarantee an excellent mix of high mechanical properties and great resistance to corrosion. The material most used in this field is Duplex Stainless Steel (DSS), a biphasic steel, composed of a ferritic matrix and austenitic islands, typically distributed equally to obtain the maximum mechanical properties. One of the most used DSS is SAF 2507 (X2CrNiMo25-7-4) composed of 25% Cr, 7% Ni, 4% Mo and 0.3% N. This chemical composition is able to guarantee a PREN higher than 40. However, Duplex is a material affected by embrittlement problems due to the formation of deleterious phases when kept at a high temperature for a sufficient time. In particular, above 600°C carbides and nitrides precipitate favouring the sensitization. In addition, deleterious phases such as Chi and Sigma are formed, which severely limit the toughness and corrosion resistance of the material. On the other hand, between 300°C and 550°C the spinodal decomposition of the ferritic phase causes a severe embrittlement. Casting processes are required for the production of large components used in desalination plants. However, sand casting does not allow to reach sufficiently rapid cooling rates to prevent the formation of deleterious phases, especially in the case of thick sections. The only possibility to remove them is to carry out a solubilization annealing heat treatment.

The aim of this research is to optimize the solubilization heat treatment of DSS obtained from thick castings. By analysing the samples solubilized at different temperatures and for different holding times, it was possible to obtain the relationship between the microstructure and the mechanical and corrosion resistance properties of the material. In this way it possible to define the specific heat treatment in order to obtain the best performance of the material.

The analysed material was extracted from a SAF2507 casting of about 250x250x500 mm cut into bars with a band saw. The heat treatments were performed with a heating rate of 200°C/h, maintenance from 0 minutes to 2 hours at temperatures from 950°C to 1150°C and cooling in water. The microstructural characterization was performed by observing the samples polished and etched with the Murakami reagent under an optical microscope. The images obtained were then processed with a graphic software and the phases were quantified through an image analysis software. The mechanical characterization was performed by means of tensile and impact tests. The corrosion resistance was finally defined through immersion tests. All tests were carried out according to the reference ASTM standards. The analyses have shown that upon reaching the core temperature of 1100°C it is possible to completely solubilize the sigma phase present in the material. However, such a high temperature produces an imbalance between the ferritic and the austenitic phase which reduces the mechanical properties. To rebalance the microstructure it is therefore necessary to carry out a second step in the heat treatment at a lower temperature, equal to about 1000°C. This step heat treatment has allowed a considerable reduction in the energy consumption of traditional heat treatment.