Influência dos elementos de liga na estabilidade da fase g presente em aços hp fundidos por centrifugação: avaliação por termodinâmica computacional

Abstract

HP-class steels are used in tubes used in steam-powered reform furnace. These tubes are submitted to wall temperatures around 950ºC. Burners provide the external heating and internal cooling due endothermic reform reaction. In service microstructural modifications provide less fluency resistance. The present work analyses the influence of the alloy elements in the value of the phase G obtained and develops a methodology of utilization of the software Thermo-Calc® that simulates the phases present in HP steel tubes even when occurs the segregation inherent to their fabrication process. Through thermal treatments and analysis in scanning electron microscope analysis, and image processing techniques, the present phases were quantified, and the dispersive energy spectroscopy ratified the phases observed in the simulations. Scheil diagrams and fraction phase diagrams were plotted. Phases Quantifications presented a plateau near 1000h of heat treatment where the amount of phase G is stabilized. Deep etching allowed quantifying the percentage of area of the matrix and the interdendritic region. Dispersive energy spectroscopy characterized the phases Cr23C6, NbC and (Nb,Ti)C, G phase and austenitic matrix. These tests validated the data obtained by the Scheil diagrams methodology to generate an enriched composition that mimics the composition of the interdendritic region and then use this alloy composition to generate phase stability diagrams. Through the factorial analysis it was possible to verify a great influence of Si in the formation of the G phase, other elements such as C and Nb also have shown influence on its stability.