Effect of Surface Active Elements on Nitrogen Content of Iron under Arc Melting

Abstract

The nitrogen content of arc-melted iron containing surface active elements, such as oxygen, sulpher and selenium, has been measured at various nitrogen partial pressures (N₂-Ar mixtured gas).
The surface active elements markedly increase the nitrogen content in arc-melted iron, and this nitrogen content increases semi-regularly with an increasing nitrogen partial pressure. For instance, in Fe-O system, the nitrogen content in arc-melted iron with under 200 ppm oxygen content in 5% N₂-Ar atmosphere is a relatively low value such as about 300 ppm nitrogen. It is seen that the nitrogen content markedly increases with an increasing oxygen content in the range from 200 to 600 ppm oxygen in iron. These nitrogen contests are 3-10 times greater than the solubilities of non-arc melted iron. This anomallous nitrogen absorption has only been observed in arc melting and could not be explained by using the thermodynamical data on steel making reactions.
On the other hand, it is known that FeO on the melt is formed in.the range of 200-400 ppm oxygen contcenration in the bulk.
This oxygen concentration range also agrees with the range in which the nitrogen content in arc-melted iron markedly increases. This fact suggests that gas absorption mechanism would change as FeO is formed on the sufrace.
A model of gas absorption mechanism in molten iron in arc melting was set up with the idea that the solubility of nitrogen in arc melting is not a true equilibrium value, but a steady state concentration between dissolution into and evolution from the melt. That is, the nitrogen is mainly absorbed through the atmosphere in arc which is in a highly activated state with a high chemical potential of nitrogen, and transported by fluid flow to the outer, cooler, region of the pool. Evolution of gas in the supersaturated outer regions is much affected by the state of its surface which is closely associated with the amount of surface active elements in the melt. When the surface of melt is covered with the surface active elements such as FeO, or FeS, the rate of evolution of gas will be extremely suppressed by FeO or FeS on the melt.
Therefore, it is considered that the nitrogen content in the melt will become, higher than in non-arc melting.