The Effect of Surface Movement on the Evaporation Rates of Alloying Elements from Liquid Iron under Vacuum

Abstract

The rates of evaporation of alloying elements from Fe-Cu, Fe-Sn and Fe-Cr melts under vacuum have been investigated by using an experimental apparatus which was made up of a LANGMUIR'S evaporation cell and of target plates for collecting evaporated material. These melts were so heated as to change stirring conditions by resistance heater or high frequency induction.
The evaporation rates are of first order with respect to all the alloying elements. The evaporation from Fe-Sn systems is mainly controlled by the evaporation step on the surface. However, in the cases of Fe-Cu and Fe-Cr systems, both the evaporation step and the diffusion step through the liquid boundary layer are rate controlling, i.e. in these systems, the total evaporation process is of mixed. controll.
The surface concentration of these alloying elements was obtained by analyzing the condensed material on the target plates. The observed values are in good agreement with the theoretical values based on the model proposed by R. G. WARD. This agreement implys that the vaporization coefficients of these alloying elements are unity.
When tin which is surface active element is evaporated from the specimen, surface movement due to surface pressure (Marangoni effect) is observed, and the mass transfer coefficient is increased.