Estudo dos efeitos dos tratamentos de solubilização e envelhecimento nas propriedades mecânicas e na resistência à corrosão de tubos de aço inoxidável superduplex UNS S39274

Abstract

Superduplex stainless steels (SDSSs) are austenitic-ferritic corrosion resistant altos (CRA) with pitting equivalent number (PREN) higher than 40. This class of steel combines excellent mechanical properties and corrosion resistance, for this reason these alloys have found widespread use in aggressive and hostile environments such as in the chemical and petrochemical industry and especially in the oil and gas production and refining. These characteristics are mainly attributed to the presence of two phases: ferrite (α) and austenite (γ) distributed nearly in equal proportion. However, SDSS are subject to suffer deleterious precipitation of alpha prime (α') phase, more pronounced at 475ºC. This precipitation causes embrittlement of material, promoting an increase in hardness, in yield strength, in the ultimate tensile and decrease in total elongation. This work aimed to to study the effects of solubilization and aging treatments on the mechanical properties and corrosion resistance in the SDSS UNS S39274 under the conditions As-Received, As-Received solution-treated at 1150ºC for 40 minutes and aged at 400ºC for 8 and 12 hours and at 475ºC for 4, 8, 12 and 200 hours. The microstructure was observed by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) showing two phases, ferrite (α) and austenite (γ). The effects of isothermal treatments on the mechanical properties of the material were studied using Vickers hardness and microhardness, uniaxial tensile and Charpy impact tests. The results show an increase in hardness, yield strength and ultimate tensile as a function of aging time of the samples at 400ºC and 475ºC. On the other hand, they point to a decrease in elongation and, above all, in Charpy impact energy. These changes in mechanical properties are more evident in SDSS UNS S39274 samples aged at 475ºC. Corrosion tests by Electrochemical Polarization Cyclic Reactivation (PERC or DL-EPR) indicate that there was not sensitization of the analyzed specimens. Which suggests that the material had high corrosion resistance.