Effect of dissolved oxygen and sulfur on nitrogen absorption rate in high Cr stainless steel melt

Abstract

The rate of nitrogen dissolution in liquid iron and stainless steel melt was investigated using a crucible melting induction technique. When the surface-active elements such as sulfur and oxygen were low in the melt, the nitrogen dissolution rate was controlled by the liquid phase mass transfer and the rate was first order with respect to the nitrogen content in the melt. When sulfur and oxygen content was high, the rate was second order indicating that the nitrogen dissolution rate was controlled by the dissociation of nitrogen gas at the interface. At an intermediate concentration of surface-active elements, the rate was controlled by both mechanisms (mixed control). Chromium in liquid stainless steel increases the nitrogen solubility significantly. However, the nitrogen dissolution rate was first order at relatively high sulfur or oxygen level in Fe-Cr melts. Chromium is known to decrease the activity coefficients of sulfur and oxygen in liquid iron and consequently reduces the surface-active effects of sulfur and oxygen at high Cr content. Therefore, in the present study, the nitrogen absorption experiments were carried out for Fe-S, Fe-Cr-S, Fe-O and Fe-Cr-O melt at 1600°C. under different nitrogen pressures. The effect of sulfur and oxygen contents on the nitrogen dissolution mechanism was determined as a function of Cr content in the melt. The absorption coefficients of sulfur and oxygen in Fe and Fe-Cr melts were determined as a function of Cr content. A mixed control model [1] was adopted to describe the nitrogen dissolution behavior at an intermediate oxygen content in Fe-Cr-O melt.