## 1 Introduction  
Duplex stainless steels have a balanced microstructure of austenite and ferrite (approximately in equal proportions) in their equilibrium state. The most used ones are:  
* Fe 23 Cr 4 Ni 0.1N (AISI 2304, UNS S32304),
* Fe 22 Cr 5.5 Ni 3 Mo 0.15N (AISI 2205, UNS S32205), and
* 25 Cr 5 Ni 2.5 Mo 0.17N Cu (AISI 2505). The alloying elements in these steels promote either austenite or ferrite formation, expanding the temperature range in which one of the phases is stable. The main alloying elements in duplex stainless steels include chromium and molybdenum, which promote ferrite formation, as well as nickel, carbon, nitrogen, and copper, which promote austenite formation [1, 2, 3, 4, 5]. Applications of these steels include [6, 7, 8, 1]* Components exposed to cavitation phenomena: hydromechanical equipment (rotors and blades of pumps and hydraulic turbines, ship propellers for maritime and river vessels). * Offshore drilling platforms for oil and gas (drilling and processing equipment, etc.). * Seawater desalination plants. * Chemical and petrochemical industries. Duplex stainless steels have a higher yield strength and tensile strength compared to standard austenitic stainless steels, and they also exhibit greater toughness than ferritin stainless steels. This allows for the reduction in thickness and weight of components or equipment made from these types of steels. The lower nickel content (an expensive element) compared to austenitic stainless steels justifies their economic efficiency. Additionally, due to their high chromium content and the presence of other alloying elements such as nickel and molybdenum, duplex stainless steels exhibit superior resistance to pitting corrosion and stress corrosion cracking in chloride-containing environments compared to standard austenitic stainless steels (such as X5 CrNi18-10 and X2 CrNiMo17-12-2). Because of the reduced nickel content, the production cost of duplex stainless steels is lower than that of austenitic stainless steels. The presence of nitrogen in the chemical composition plays an important role in increasing mechanical strength characteristics, raising the temperature at which austenite begins to form from ferrite, and promoting the precipitation of chromium nitrides at the interfaces of ferrite grains--ferrite and ferrite-austenite. The proportion of the two phases, austenite (A) and ferrite (F), can be widely modified depending on:  
* The content of alloying elements. * The heating temperature for solution treatment of intermetallic phases. Moreover, the application of heat treatments triggers the following phenomena with consequences for microstructure changes [1]:  
* Precipitation of carbides such as M\({}_{23}\)C\({}_{6}\) and nitrides such as Cr\({}_{2}\)N at the grain boundary of ferrite-austenite, without consequences regarding intergranular corrosion. * Light precipitation of the \(\sigma\) phase (from ferrite) during heating. * Hardening through annealing treatments at temperatures ranging between 400 and 900 \({}^{\circ}\)C.  
The main disadvantage of duplex stainless steels is related to their hardness and low resistance to wear through friction and cavitation erosion, factors that shorten the lifespan of component parts [8; 9; 10; 11; 12]. When covered with hardening alloys, these steels not only suffer from sensitization to the precipitation of chromium carbides M\({}_{23}\)C\({}_{6}\), as is the case with austenitic stainless steels, but they are also susceptible to embrittlement due to the formation of secondary phases, i.e., intermetallics, carbides, and nitrides, as a result of prolonged exposure to high temperatures. Significant precipitation of secondary phases can lead to a loss of corrosion resistance and, sometimes, hardness loss.