Nitrogen Absorption in Arc Melting of Iron

Abstract

The nitrogen content of arc-melted liquid iron has been measured in the atmosphere of argon containing nitrogen with various partial pressures. The arc melting was carried out in a small vessel, in which the controlled gas mixture flowed, under the condition of an arc-current of 180 A and straight polarity for the non-consumable tungsten electrode. By a preliminary experiment, the temperature of the molten pool was measured as about 1, 800°C by using W-W.Re thermocuple.
The results obtained were as follows:
(1) The nitrogen content of the arc-melted pure iron is proportional to the square root of the partial pressure of nitrogen up to about 0.09 atm P_N2, and above 0.09 atm the nitrogen content becomes a nearly constant value.
(2) In the range of the nitrogen partial pressure lower than 0.09 atm, the nitrogen content of the arc-melted iron is about 5.2 times higher than the value expected from the solubility of nitrogen in the ordinary, non-arc melting. Such anomalous high dissolution of nitrogen may be caused by the activation of nitrogen in the atmosphere by arc.
(3) From the difference of the nitrogen content in iron between the arc melting and the non-arc melting, the free energy difference 4F₄⁰ between the activated and the ordinary nitrogen is obtained to be 6.82 Kcal/g·atm at 1, 800C. If it is assumed that the activation of nitrogen is essentially the dissociation of the nitrogen molecule to the atomic nitrogen, the above energy difference corresponds to the dissociation constant of 0.15.
(4) Calculating the dissociation equilibrium of nitrogen from its dissociation energy and the specific heats of each species, it is shown that the dissociation constant reaches 0.15 at 5, 100C. This is the apparent temperature which the arc-atmosphere would reach in the arc-column, if it is assumed that the composition of the nitrogen gas in the vicinity of the metal surface is the same as that in the arc-column.