Further development of high chromium cast irons (HCCI) is based on tailoring the microstructure, necessitating an accurate control over the phase transformation and carbide precipitation temperatures and can be achieved by thermal treatments (TT). To understand the underlying mechanisms controlling the transformation kinetics during the different stages of the TT, it is imperative to adjust the TT parameters to have information of the transformations occurring during non-thermal and isothermal heating cycles, since proper selection of the TT parameters ensures the optimum use of the alloying elements. In this work, the boundaries of the phase transformations for a HCCI containing 26 wt pct Cr for different cooling rates (continuous cooling transformation, CCT, diagram) were established by applying dilatometric measurements. Based on the CCT diagram, a temperature-time-transformation (TTT) diagram was constructed by isothermally holding the samples until complete phase transformation. For determining the initiation and finishing of the transformation, the lever rule assisted by derivatives was applied. The phases present after transformation were determined by combining X-ray diffraction (XRD) and metallographic characterization using optical microscopy (OM) and scanning electron microscopy (SEM). Finally, the data obtained from the dilatometer was experimentally verified by isothermally heat treating some samples using laboratory furnaces. The transformed phase fraction from OM and SEM images was then correlated to the fraction obtained from the TTT diagram.
In previous works the possibilities and limitations of the application of calculations in the Al-Fe-N system to describe the precipitation of AlN in steel, both in the solid state and during the solidification were discussed and some difficulties related to the extension of these calculations to more complex steel systems, due to limitations in the thermodynamic data were also presented. Presently, the precipitation kinetics of AlN in ferrite (BCC) and austenite (FCC) is discussed. The correct description of the precipitation of AlN in both phases is relevant to: (a) the precipitation at higher temperatures, in the austenite field, that occurs in some steels, (b) the concurrent precipitation of this nitride with the annealing treatment, when the steel is mostly ferritic, used in the processing of some types of deep drawing steels (c) the precipitation of this nitride in some silicon alloyed electric steels at relatively high temperatures, when these steels can have significant fractions of BCC and FCC in their microstructure. The precise knowledge of the precipitation-dissolution behavior of AlN in special in these two latter classes of steels is of great importance to their correct processing. In this work, a computational tool for simulating multiparticle precipitation kinetics of diffusion-controlled processes in multi-component and multi-phase alloy systems is employed in an attempt to describe these precipitation processes. The results are compared with experimental data on precipitation. The assumptions necessary for the application of the multi-particle modeling tool are discussed, agreements and discrepancies are identified and some possible reasons for these are indicated. Furthermore, the impact of the use of different sources of data on steel processing development is discussed and the need for further studies highlighted
ResumoOs Aços Inoxidáveis Duplex possuem uma estrutura constituída basicamente por ferrita e austenita na proporção aproximada de 50%. Este trabalho estudou a influência da temperatura de um tratamento térmico na morfologia e fração de ferrita e austenita no aço UNS S32304. Os tratamentos térmicos realizados consistiram no aquecimento das amostras, desde a temperatura ambiente até a temperatura de 1250°C, seguido de um resfriamento lento em um segundo forno estável em uma determinada temperatura. As amostras permaneceram neste segundo forno por um intervalo de tempo necessário para que suas temperaturas atingissem exatamente um valor desejado, sendo então imediatamente retiradas e resfriadas em água a 20°C para preservar a estrutura resultante do ciclo térmico aplicado. As amostras foram metalograficamente preparadas, atacadas e analisadas com o auxílio de um microscópio ótico. As frações de fases medidas por MO se aproximaram dos resultados obtidos por simulação utilizando o software Thermo-Calc. A morfologia das fases variou significativamente com a diminuição da temperatura de tratamento, uma vez que a austenita se mostrou mais fragmentada nas temperaturas mais baixas, porém o bandeamento não deixou de existir na estrutura.A transformação da ferrita em austenita se mostrou fortemente dependente do tempo. Palavras-chave: Transformação de fases; Morfologia; Aço inoxidável duplex UNS S32304. INFLUENCE OF HEAT TREATMENT TEMPERATURE ON MORPHOLOGY AND PHASE FRACTIONS IN A DUPLEX STAINLESS STEEL TYPE UNS S32304 AbstractThe Duplex Stainless Steels have a structure composed mainly of ferrite and austenite in the approximate ratio of 50%. This work studied the influence of heat treatment temperatureon themorphologies and fractions of ferrite and austenite in the UNS S32304 steel. The applied heat treatment consisted of heating the sample from roomtemperature until 1250°C followed by slow cooling in a second stable furnace at a certain temperature. The samples remained in thisfurnaceuntil their temperatures reacheda desired value, and then they were removed and immediately cooled in water at 20°C aiming to preserve the structuresobtained from the applied treatments. The samples were metallographically prepared and analyzed with the aid of an optical microscope (OM). The phase fraction measurements, done inOM,were close tothe results obtained by computational simulation, using the Thermo-Calc software. The phase morphologies varied significantly as the temperature decreasewith the heat treatment temperature decreasing.At lower temperature, there are more fragmented austenite grains. However, for all studied conditions, the banded structure still exists.The ferrite to austenite transformation were strongly time dependent.
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