Evaporation of Fe and Cr from induction-stirred austenitic stainless steel. Influence of the inert gas pressure

In Vacuum metallurgy one of the purposes is the reduction (or at least the accurate prediction) of the evaporation losses. It is well known that the a ddition of an inert gas in a vacuum furnace increases the recondensation of the volatile elements and then reduces the evaporation losses. We may defi ne the pressure P-1/2 required to halve the evaporation rate. The objective of this study is a theoretical and experimental evaluation of P-1/2 in the case of an austenitic stainless steel, and the analysis of the parameters which influence this value. The experimental programme was carried out on an austenitic stainless steel to determine the net flux of evaporation from a well-mixed liquid in an am bient pressure of argon ranging from 0.03 to 133 Pa. P-1/2=30 Pa is estimat ed from the experimental curve. The mechanisms of volatilization have been modeled using both a system base d and a mechanistic approach, and the calculation of the pressure P-1/2 giv es respectively 45 and 90 Pa. The numerical simulations (mechanistic approa ch) emphasize the strong expansion of the vapor from the high density regio ns close to the liquid surface. The macroscopic velocity of the vapor decre ases as the argon pressure in the chamber increases since the average frequ ency of collision with the argon atoms increases. We have set up a sensitivity study in order to analyse the effects of the g eometry and scale of the furnace and of the liquid temperature on the facto r P-1/2. Since geometry and temperature vary in large scales for the indust rial applications, the use of the experimental value P-1/2=30 Pa obtained i s discussed.