Austenitic-ferritic stainless steels are alloys with controlled additions of Cr, Ni, Mo and N which, after a suitable thermomechanical treatment, results in a balanced microstructure with similar proportions of ferrite (δ) and austenite (γ). Thus, it is possible to obtain good combinations of high mechanical properties and corrosion resistance through microstructural refinement, which also leads to a relatively good Pitting Resistance Equivalent value (PRE) at both phases. However, inadequate heat treatment and/or welding processes might result in the precipitation of deleterious phases, leading to poor mechanical properties and/or corrosion resistance. In this sense, the use of non-destructive microstructural characterization techniques becomes a valuable resource to access such alterations. Therefore, this work evaluates the precipitation of deleterious phase’s in welded thermal aged joints by portable Double Loop Electrochemical Polarization Reactivation (DL-EPR) taking into account a preliminary metallographic replica characterization. The results show that the proposed portable method can detect even a small percentage of deleterious phases, in addition to having a strong potential to be a non-destructive microstructural characterization technique.

Superduplex stainless steels (SDSS) are frequently employed in the petrochemical industries where is required high mechanical strength, toughness, and corrosion resistance. However, these properties can be affected by deleterious phases formation due thermomechanical processes applied in the field during pipes and vessels construction. This work propose the nondestructive microstructural characterization of deleterious phases precipitated in SDSS isothermally treated in 800 and 850ºC using portable double loop electrochemical polarization reactivation tests (DL-EPR). The results obtained in this nondestructive test are quite close to those obtained by conventional test, and can be correlated with the amount of deleterious phases precipitated. It can be concluded that the microstructural degradation of superduplex stainless steel can be evaluated by portable DL-EPR test with slow sweep rates, using a special cell and a proper electrolyte at room temperature.

ABSTRACT AISI 430 is a non-stabilized ferritic stainless steel grade with carbon content lower than 0.12%.After hot and cold rolling this material is annealed. The slow cooling after soaking at temperatures between 900oC and 1000oC promotes the formation of a high quantity of carbides and nitrides, while the rapid cooling partially suppresses the formation of these precipitates, but introduces martensite in the microstructure. Intergranular martensite can also be produced in the weld metal and in the heat affected zone (HAZ) of welds of non-stabilized ferritic stainless steels. In this work, several heat treatments between 900oC and 1000oC, with different cooling rates, were performed in a commercial sheet of AISI 430 grade. Also, an autogenous welding was produced with GTAW process, and post weld heat treatment at 700°C was carried out. The different microstructures produced were analyzed by optical and scanning electron microscopy (SEM). The degree of sensitization was measured by double loop electrochemical potentiodynamic tests (DL-EPR). The pitting corrosion resistance was evaluated by cyclic polarization tests in 3.5%NaCl solution. Hardness and toughness tests were also performed in selected heat treatment conditions. The results indicate that the slow cooling results in a higher degree of sensitization than observed in the material rapid cooled from the annealing temperature. The ferritic martensitic structure produced by water cooling has higher pitting potential and lower degree of sensitization, but is brittle at room temperature. A subsequent tempering treatment between 600 and 800oC can increase the toughness, but the corrosion resistance may decrease due to carbides precipitation.The heat affected zone of AISI 430 welds contains intergranular martensite, which is brittle and susceptible to corrosion attack. Post weld heat treatment at 700oC decomposed the martensite into ferrite and carbides and improved the corrosion resistance.