Structure and Properties of Nitrided Ferroalloys

The structure and the physicochemical properties of nitrided chromium- and manganese-based ferroalloys have been studied experimentally. Nitrogen in the alloys is shown to be mainly in the form of nitrides Mn2N, CrN, and Cr2N, the fractions of which are dependent on the nitrogen content and the methods of alloy manufacture. The liquidus temperature of the nitrided chromium-bearing ferroalloys is lower than that of analogous nitrogen-free ferroalloys, which positively influences the rate and degree of assimilation of elements by steel. The introduction of 8–16% nitrogen in the ferroalloy compositions decreases their real densities, which enables one to produce alloys with the optimal density. The apparent densities of the nitrided chromium and manganese ferroalloys are very low (3000–4200 kg/m3) due to their high porosity; as a result, they will be melted at the steel melt surface and will have a low degree of nitrogen assimilation. Higher degrees of nitrogen assimilation can be achieved as powdered alloys are injected in a gaseous nitrogen jet. The degree of transition of nitrogen to steel from the ferroalloys is 47–84%; it is mainly dependent on the holding time of a nitrided steel in liquid state, the physicochemical characteristics of nitrided ferroalloys, the composition of an alloyed metal, and the types and content of nitride-forming elements (Cr, Mn, Ti, etc.) in it. The highest degree of transition of nitrogen to steel (84%) was reached using an as-cast nitrided chromium sample with 8% N in the first minute of holding. Further holding up to 30 min of the liquid sample at 1500°C leads to a decrease in the degree of nitrogen assimilation to 52%.