Estudo da precipitação de fases secundárias em um aço inoxidável duplex UNS S32750 (ASTM 2507)

Abstract

The aim of this work is the study of secondary phases precipitation in duplex stainless steel UNS S32750 (ASTM 2507), submitted to different aging heat treatments. The duplex stainless steels are named this way because they have a microstructure composed of the ferrite phase (BCC) and austenite phase (FCC) in equivalent quantities. These alloys are used in applications that need mechanical resistance (yield limit in the order 450-550 MPa) and corrosion resistance (range PRE 25 to 43), in the saline environments, petrochemical industry, chemistry, paper factory, etc. Secondary phases, such as intermetallic, nitrides and carbides, are subject to be precipitated over a wide temperature range (500-1,000°C). First, the material was characterized in the state received by the manufacturer. The chemical composition was obtained by WDS and EDS. The partition coefficient indicates the preference of molybdenum to ferrite and nickel to austenite. Thermodynamic calculations simulation was done to obtain phase diagram and phase fraction, where it was found the temperature range for secondary phases precipitation. X-ray diffraction indicated the presence of the ferrite and austenite phases, in equivalent amounts. In the analysis by optical and scanning electron microscopy, lamellar morphology was found, where interspersed regions of ferrite and austenite are observed, a typical characteristic of hot rolled duplex stainless steels. Subsequently, aging heat treatments were onducted, in the temperature range of 750-950°C, with time ranging from 15 minutes to 10 hours. In the optical microscopy images, it can be observed that the sigma phase precipitates preferentially in ferrite-austenite boundaries and within the ferrite phase, with morphology resulting from the ferrite eutectoid reaction in sigma + secondary austenite and / or sigma with massive aspect. In scanning electron microscopy, it was observed that the sigma phase precipitates and grows in several morphologies, such as allotriomorphs, side plates, secondary side plates, from phase transformation in grain boundaries; while the interior of the ferrite phase was decomposed by the eutectoid reaction in sigma and secondary austenite. In Vickers microhardness, it was verified that the increase in heat treatment time, the microhardness increases until to reach 770HV, which indicates a large amount of precipitated sigma. In chemical microanalysis by EDS, it was observed the elements alloy molybdenum (6-10% in mass) and chromium (25-30% in mass), tend to diffuse to form sigma phase, however the molybdenum diffusion is slower, due to the size of atomic radius, that cause chemical composition gradients in the sigma phase. In the phases quantification by the systematic manual point count, it was verified that the secondary phases precipitation kinetics are at temperatures between 850°C and 900°C, while at 750°C was the lowest precipitation kinetics; it was verified that aging heat treatments with times greater than 5 hours, at temperature range 750-900°C, approximately 50% of the volumetric fraction of the material was transformed in secondary phases (sigma and secondary austenite).