RATE OF NITROGEN DESORPTION FROM MOLTEN FE-CR-S ALLOYS

The rate of nitrogen desorption from liquid Fe-10Cr-S alloys is a seco nd order process with respect to the nitrogen concentration in the mel t; this is true irrespective of the content of the surface active elem ent, sulphur, in the melt. The rate controlling step is deduced as the association of adsorbed nitrogen atoms on the melt surface. The rate of nitrogen desorption is progressively decreased with increasing sulp hur content of the Fe-Cr-S melt at 1700 degrees C, However, at sulphur contents above 0.25%, the desorption rate levels off to a constant re sidual value, The addition of chromium to the iron melt is observed to have an adverse effect on the rate of nitrogen desorption; this reduc ed rate is due to the strong interaction between chromium and nitrogen . A mathematical model is developed to interpret the effect of sulphur on the rate of desorption of nitrogen. In the vacuum refining of stai nless steel, there is evidence that the extent of nitrogen desorption is proportional to the amount of carbon removed during decarburisation , and that argon stirring which gives a lower end carbon content appea rs to have much less influence on the nitrogen level. The presence of surface active sulphur in the stainless steel melt has a strong advers e effect on the rate of desorption of nitrogen, and strong nitrogen bo nding with chromium further inhibits the desorption kinetics.