Partition of nitrogen in solidifying iron-carbon-silicon alloys

Abstract

The partition of nitrogen between austenite and liquid iron was examined from the measurement of solubilities of nitrogen in these phases. On primary austenite crystallization, nitrogen was rejected into liquid iron at high temperatures and the partition behaviour was reversed at low temperatures. Silicon lowered the critical temperature where the partition behaviour of nitrogen changed. The partition of nitrogen between cementite and austenite was investigated by analysis of nitrogen in iron-carbon-silicon alloys, which were quenched in ice-water from an equilibrium state at subeutectic temperatures, and in the cementite extracted from them. Nitrogen was enriched in cementite for Fe-C-0.5 wt% Si alloy, similar to the case of pure Fe-C alloy. However, silicon reduced the degree of enrichment of nitrogen in cementite. By the use of the partition coefficients, the variations of nitrogen concentrations in the coexisting phases were evaluated during the solidification of cast irons. In the irons with lower carbon concentrations, the supersaturation of nitrogen in liquid iron attained during solidification increased with increasing silicon content, and silicon had a detrimental effect to promote the formation of nitrogen gas blowholes in low-carbon cast irons.