SOLUBILITY OF NITROGEN IN LIQUID STEEL IN PLASMA INDUCTION REACTOR

A dc graphite plasma torch and refractory lined induction furnace equi pped with a porous plug were used to study the kinetics of nitrogen ab sorption and desorption in molten steel. The experiments were conducte d on undeoxidised melts and melts deoxidised with aluminium. The addit ion of a BaO-TiO2 refining slag was shown to either reduce the initial nitrogen contents by up to 80% or lead to an increase in the nitrogen depending on the melt composition. The effectiveness of the slag was found to be dependent on the amount of aluminium in the bath. Insuffic ient amounts of aluminium resulted in nitrogen reversion to the bath. Argon and nitrogen were used as both plasma and bubbling gases. Both s traight, i.e. electrode cathode and melt anode, and reversed polarity arcs were tested. Operating the arc in the reversed polarity mode may result in aluminium reversion from the slag to the melt. The results o btained in this work were used to clarify the mechanism of the nitroge n reaction with liquid steel in the presence of a plasma arc. Nitrogen absorption during nitrogen bubbling and nitrogen plasma gas treatment was found to follow a second order relationship with nitrogen dissolv ed in the undeoxidised melt, implying that the slow chemical reaction at the interface controlled the reaction rate. Melts processed in the same conditions but deoxidised with aluminium followed a first order r elationship, indicating that mass transfer in the melt was the rate li miting step. (C) 1993 The Institute of Materials.