Abstract
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 addition of an inert gas in a vacuum furnace increases the recondensation of the volatile elements and then reduces the evaporation losses. We may define the pressure P1/2 required to halve the evaporation rate. The objective of this study is a theoretical and experimental evaluation of P1/2 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 ambient pressure of argon ranging from 0.03 to 133 Pa. P1/2=30 Pa is estimated from the experimental curve.
The mechanisms of volatilization have been modeled using both a system based and a mechanistic approach, and the calculation of the pressure P1/2 gives respectively 45 and 90 Pa. The numerical simulations (mechanistic approach) emphasize the strong expansion of the vapor from the high density regions close to the liquid surface. The macroscopic velocity of the vapor decreases as the argon pressure in the chamber increases since the average frequency of collision with the argon atoms increases.
We have set up a sensitivity study in order to analyse the effects of the geometry and scale of the furnace and of the liquid temperature on the factor P1/2 . Since geometry and temperature vary in large scales for the industrial applications, the use of the experimental value P1/2=30 Pa obtained is discussed.
