Nitrogen solubility in liquid manganese and ferromanganese alloys

Manganese and ferromanganese alloys are widely used as additives in steel making processes. The increased demand for ultra-low-carbon and -nitrogen steels requires the production of high-purity ferromanganese alloys. The nitrogen solubilities in liquid manganese, manganese-iron, manganese-carbon, and manganese-iron-carbon alloys have been measured by the gas-liquid metal equilibration technique in the temperature range of 1623 to 1823 K. The equilibrium nitrogen content in pure liquid manganese at an atmospheric nitrogen pressure is high, and it does not follow Sievert's law, i.e., f sub N is not unity. The reduced nitrogen partial pressures by dilution with argon enabled us to obtain more reliable information on the thermodynamics of nitrogen in liquid manganese. The nitrogen dissolution follows Sievert's law at nitrogen contents below 1 wt pct. The standard free-energy change for the dissolution of nitrogen in pure liquid manganese has been determined as -67.222 + 30.32T J/g atom, with the standard state of nitrogen taken as a 1 wt pct solution. Carbon and iron in manganese-rich melts decrease the nitrogen solubility significantly. The first- and second-order interaction parameters between nitrogen and other elements in manganese alloy melts have been determined. The activity coefficient of nitrogen in a ferromanganese alloy melt can be expressed as log f sub N = 0.005 (pct N) + 0.029 (pct N) exp 2 + 0.015 (pct Fe) + 0.09 (pct C) + 0.013 (pct C) exp 2 where the interaction parameters are independent of temperature in the temperature range of 1623 to 1823 K.