ResumoNeste trabalho, amostras de aço Lean Duplex UNS S32304 foram submetidas à temperatura de 800°C em um forno acoplado a um difratômetro de raios-x durante intervalos de tempo de 30, 60 e 90 minutos. Com o objetivo de analisar em tempo real a transformação das fases constituintes, foram realizadas várias varreduras insitu durante os diferentes tempos de permanência em 800°C deste material. Assim, observou-se a formação da fase sigma neste aço, tanto nos contornos de grão da ferrita como nas interfaces ferrita/austenita. Os valores de fase sigma obtidos para cada tempo de exposição foram calculados através do método de Rietveld sobre cada difratograma obtido e variaram de 11% a 17%. Notou-se que a fase sigma exibiu crescimento contínuo até 60 minutos e variação após este período. Todavia, observou-se que a formação de fase sigma a 800°C ocorre de forma significativa neste material após 30 minutos de aquecimento, o que pode comprometer as propriedades do mesmo. Palavras-chave: Aço lean duplex; Difração de raios-x in-situ; Microestrutura; Fase sigma.
LEAN DUPLEX STEEL CHARACTERIZATION BY ANALYTICAL METHODS
AbstractIn this work, samples from a Lean Duplex Steel UNS S32304 were subjected to a temperature of 800°C for different intervals of time, being 30min, 60min and 90 min. All samples were allocated inside a small high temperature furnace attached to an xray diffractor. The goal of the work was to analyze the phase transformations occurring in those samples in real time. Formation of sigma phase was observed both in the ferrite grain boundaries as in the ferrite/austenite interface. The amount of sigma phase formed was measured by quantitative phase analysis using the Rietveld method and result values ranged from 11% to 17 % in volume. It was also noticed that the sigma phase exhibit a continuous growth up to 60 minutes, become stable for a few minutes to then grow again. In any case, it was observed that sigma phase formed significantly in the material after 30 minutes soaking, which can lead to a decrease in the material properties.
Duplex stainless steels (commonly known as DSS) are a class of stainless steels with a microstructure formed by two main phases: ferrite and austenite. They are used in a wide range of applications, such as chemical processing, in maritime environments and in the oil and gas industries. In most cases, DSS are chosen based on their strength and corrosion resistance for various environments. When exposed to temperatures above 600°C though, the balance of alloying elements can be modified due to precipitation of various secondary phases, such as sigma (σ) and chi (χ). The sigma phase is typically enriched with Cr and Mo, so its formation can lead to a drastic deterioration in toughness, corrosion resistance, and weldability of duplex stainless steels. To prevent damages to these steels due to the formation of sigma phase, the understanding of such transformation becomes mandatory, not only during the development of these steels but also during their processing. In this research, samples from a lean duplex steel UNS S32304 are subjected to a temperature of 800°C and analyzed in situ by X-ray diffraction. Thus, the kinetics of phase transformations occurring in duplex stainless steels are observed in real time.
Aiming to reducing the gas emissions in the steel mill, in the present study the kinetic behavior of self-reducing pellets containing elephant grass coal and iron ore was investigated through kinetic isotherms for different temperatures. The pellets were isothermally treated in different time intervals for X-Ray diffraction and SEM analyzes. The results obtained by XRD showed that the higher mass of iron in the sample was achieved until the first 24 min. The SEM analysis allowed visualizing the presence of a metallic porous structure closer to edge and whiskers in the center of sample.
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