Castro-Fernández, F. (Francisco)

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    Análisis termodinámico de la descarburación de aceros
    (Sociedad Española de Cerámica y Vidrio, 2000) Castro-Fernández, F. (Francisco); Gómez-Acebo, T. (Tomás)
    El control del contenido en carbono en aceros pulvimetalúrgicos durante el tratamiento de sinterización requiere plantear el equilibrio y la cinética de la reacción del metal con la fase gaseosa. En este artículo se aborda el estudio termodinámico. En la práctica industrial se suelen emplear diferentes atmósferas, que pueden contener CO, CO 2 , CH 4 ó H 2 . Con ayuda de pro - gramas de cálculos termodinámicos como Thermo-Calc, es posible realizar estimaciones complejas de equilibrio. Se ha com - probado que la representación del diagrama de fases de un acero en el espacio de coordenadas potenciales actividad de carbono frente a 1/T facilita la selección de las atmósferas gaseosas de reacción. Se presenta también el modo de calcular analítica y gráficamente la actividad de carbono en función de la relación de presiones parciales p (CO)/ p (CO 2 ) y p (CH 4 )/ p (H 2 ). Como ejemplo, se ha estudiado el control de la descarburación en el hierro puro y en las aleaciones Fe-Cu y Fe-Cr.
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    Steam oxidation of ferritic steels : kinetics and microstructure
    (Sociedad Española de Cerámica y Vidrio, 2000) Aríztegui, A. (A.); Castro-Fernández, F. (Francisco); Gómez-Acebo, T. (Tomás)
    The ferritic 2.25Cr–1Mo steel has been subjected to isothermal and non-isothermal oxidation treatments in water steam at several temperatures ranging from 550 to 700 °C for up to 56 days. Under isothermal conditions this steel follows a parabolic oxidation kinetics, with an activation energy of 324 kJ mol –1 . This value corresponds to an apparent activation energy for the global process, which includes both outward diffusion of Fe cations and inward diffusion of oxygen. The oxidation products present in the oxide scales, which were characterised by X-ray diffraction and SEM, are in total agre - ement with the Fe-O phase diagram. Thus, magnetite is the most stable oxide at low temperatures and wustite starts to form above 570 °C. Further studies of the effect of cooling rate have shown that wustite formed at 700 °C transforms into magne - tite during a slow cooling, whereas a rapid cooling inhibits this transformation to a certain extent. For non-isothermal oxidation treatments consisting of a holding period at 550 °C followed by a single or double 4 hours exposure at 700 °C, the oxidation process seems to occur in sequence, thus presenting an additive effect of the oxidation treatments carried out at each temperature. This effect was observed both, for the type of oxide grown, and for the kinetics of the process. Microscopic observations of the oxide scales formed after the various oxidation treatments revealed that the oxide scales are constituted by sublayers of distinct microstructure and chemical composition changing from their surface to the substrate interface.