Magnesium and Rare Earth Elements Behavior During Vacuum Induction Melting of Ni-base Alloys. Dr. Ph. Thesis
Abstract
Introduction.
Magnesium is among the most active oxygen and sulfur reducing elements and it is extensively used in superalloy melting practices for hot workability improvement. Magnesium is often added to the melt in combination with rare earth elements. (REM). In this case additional important effect of magnesium reveals in preventing of interaction between REM and magnesia crucible lining.
There are some optimum magnesium and REM concentrations in alloys, depending on residual sulfur concentration. Being an element with extremely high vapor pressure at steelmaking temperatures, magnesium intensively evaporates from the melts.
Therefore, it is necessary to know and account must be taken of main features of magnesium evaporation at vacuum induction melting.
Following stages may control magnesium evaporation from the melt: mass transfer in molten metal; evaporation (desorption) reaction; mass transfer in gaseous phase through the boundary diffuse layer. Literature data on magnesium evaporation vary considerably.
An application of REM at Ni-base alloy production is described in many publications. This addition reduces free sulfur content in metal and prevents atomic sulfur segregation at grain boundaries during solidification. As a result the technological and mechanical properties are improved.
However, extra concentration of REM in metal causes reverse effect - deterioration of properties. It is owing to the REM intermetallic phases, formed during crystallization as a result of low solubility in the base of alloy.
It is necessary to get all sulfur and REM to the end of ingot solidification in a bound state to avoid inferior properties of metal. This task is rather complex, because REM interacts not only with sulfur, but also with oxygen. Moreover, the affinity REM for oxygen is larger, than for sulfur, and low activity of oxygen is the necessary condition of sulfides and oxysulfides formation.
Besides that, in the system metal-lining-slag the oxygen activity not always may be brought down to the level, necessary for sulfide formation. The reason is that oxygen input in metal is possible from oxide phases: nonmetallic inclusions, slag, scull, and lining. Therefore, an addition of REM to the melt in one would think sufficient quantity may not ensure in the end the association with sulfur. Consequently, it is important to have available information on interaction between REM, lining, slag, and scull.
In this thesis is discussed our own results on magnesium evaporation process and peculiarities of REM oxidation at VIM of Ni-base alloys.
The main conclusions:
Magnesium evaporation constants from Ni during melt holding in 25-kg crucible vacuum induction furnace were experimentally found (in vacuum and in argon, PAr = 13,3 kPa; T = 1773 ± 15 K). The evaporation constant value in vacuum was 1.4·10-4 m/s and the one in argon was 0.87·10-4 m/s.
Mass transfer constant values in a Ni melt Km and in a gas Kg (argon, PAr=13.3 kPa), and evaporation constant Ke value for 1773 K were computed: Km=2·10-4 m/s, Kg=1.5·10-4 m/s, Ke=1.6·10-1 m/s.
On the base of comparison of these values it was concluded that in vacuum the evaporation of magnesium from nickel melt is limited by mass transfer in the melt, and in argon by mass transfer in the melt and in the gas simultaneously.
Experimentally there was found the influence of stirring activity (velocity of melt flows and their turbulence) on magnesium evaporation rate constant.
Experimentally it was defined that aluminum decreases and chromium, molybdenum, and iron increase the magnesium evaporation rate constant from Ni-base alloys.
Phase diagrams of Ni-Al-La-Mg-O system were computed and concentration regions of lanthanum, magnesium and aluminum in nickel melt, in which the most stable phases are La2O3, MgO, La2O3·Al2O3, MgO·Al2O3, were found.
The new method of oxygen activity computation in alloys which contain elements with high affinity to oxygen was suggested. By this method it was defined soluble oxygen value and oxidation product phases during melting N-base alloy, wt. pct.: 15 Cr, 3 Al, 2 Ti, 0.01 La, and 0.01 Mg, in MgO crucible.
Experimentally there was found the ability of prevention of lanthanum oxidation by MgO lining if magnesium is present in the melt. Lanthanum does not react with MgO lining if magnesium and lanthanum concentration in nickel are 0.01 – 0.02 wt. pct. (typical concentrations in Ni-base alloys) because magnesium and lanthanum oxides have equal stability in these conditions.
It was found that the most stable oxide in system Ni-Al-La-Mg-O ([La] = 0.01-0.02 wt. pct., [Mg] = 0.01-0.02 wt. pct.,) is La2О3·Al2O3. This phase coats the crucible wall and prevents interaction of lanthanum with MgO lining despite of lower stability of MgO and MgO·Al2O3.
On the oxidation rate of rare earth elements during vacuum induction melting of Ni-base alloys affects the slag and the crucible scull composition. Chromium, silicon and iron containing oxide phases in the slag and crucible scull are main sources of oxygen for oxidation of rare earth elements.
The algorithm for computation of liquid melt temperature during VIM and the general plan of VIM control system were developed. The main goal of the system is to decrease deviations of magnesium and rare earth elements from optimal values in VIM ingots. [Rus.]
The Dr. Ph. Thesis content was published at following articles:
1. A.A. Alekseenko, F.I. Shved, Yu. H. Shwartsman, D.A. Soskov/ Magnesium Evaporation During VIM./ Perspectives of special electrometallurgy development. Branch proceedings .– Moscow.: Bardin CNIIChM, 1989. Part. III. pp. 40-41. (Rus.)
2. A.A. Alekseenko, F.I. Shved, A.B. Sergeev, B.M. Starostin/ Magnesium Evaporation from Ni-base alloy CrNi62BMoCoTiAl in 25-ton Vacuum Induction Furnace ISV-25// 1-st National symposium "New heat proof materials".– Moscow.: Academy of Sciences USSR, 1989. Part. I. pp. 118-119. (Rus.)
3. A.A. Alekseenko, F.I. Shved, A.B. Sergeev, R.F. Pershina/ Magnesium Evaporation During VIM // Stal. 1989. No12. pp. 31-33. (Rus.)
4. A.A. Alekseenko, F.I. Shved/ Magnesium Evaporation from Ni-base Alloys During Vacuum Induction Melting// Modern concerns of electrosteel making. XI International conference thesis proceedings. – Chelyabinsk: SUSU. 2001. p. 111. (Rus.)
5. A.A. Alekseenko, F.I. Shved/ Rare Earth Elements Interaction with Crucible Material, Scale, and Slag During Vacuum Induction Melting of Ni-base Alloys// Modern concerns of electrosteel making. XI International conference thesis proceedings. – Chelyabinsk: SUSU. 2001. p. 112. (Rus.)
6. Alexeenko A., Shved F. Magnesium evaporation from Ni-base alloys during vacuum induction melting// Proceedings of the 2001 international symposium on liquid metal processing and casting. – Santa Fe, New Mexico: American vacuum society, 2001. P. 18-30.
7. Alexeenko A., Shved F. The interaction of rare earth additions with refractory lining material and slag during vacuum induction melting of Ni-base alloys// Proceedings of the 2001 international symposium on liquid metal processing and casting. – Santa Fe, New Mexico: American vacuum society, 2001. P. 61-71.
