The Interaction of Rare Earth Elements with Oxygen and Sulfur during Vacuum Induction Melting of Ni-base Alloys
Abstract
Service and technological properties of Ni-base superalloys significantly depend on the balance between residual concentrations of sulfur and sum of rare- and alkaline-earth elements. Practically it is difficult to achieve optimal ratio of the above elements, even if vacuum induction melting is applied. Control of the alkaline-earth elements contents is difficult because of its intense evaporation from the melt, and at same time control of the rare-earth elements is difficult because of its interaction with lining, crucible scull and slag. Thermodynamical calculations and laboratory experiments were carried out. The aim of the investigation is better understanding of these processes and improving of Ni-base superalloys micro-compositions control.
Experimental procedures. All the laboratory heats were melted in a 25 kg VIM furnace. The crucibles filled with pure magnesia were used for experimental heats. After kilning of the lining one preliminary rinsing heat was melted, therewith nickel was used as a charge before experiments with pure nickel and Ni-Al-alloy before experiments with alloy Ni-Al(2 wt. pct). After tapping of the rinsing heat the crucible was allowed to cool in vacuum.
During the experimental heats after the charge was molten and the metal was heated to the specified temperature argon was let in to pressure 13.3 kPa. The heats of pure Ni were deoxidized by aluminum. Ni-Mo-Mg masteralloy was added thereafter and next to its solution - lanthanum. The isothermal hold during 40-60 min followed with periodical sampling and new additions of Mg-bearing masteralloy to maintain its amount in in the melt at the definite level. One heat was melted for comparison without Mg addition. Lanthanum behavior also investigated during the hold of CrNi62NbCoMoTiAl alloy in crucible with the scull, containing chromium, silicon and iron oxides. Lanthanum was added to calculated amount 0.6 wt. pct at the melts of pure nickel, and to 0.2 wt. pct - at the melts of Ni-Al(2 wt. pct) alloy. Melt temperature was measured by immersion thermocouple Pt/Pt-Rh 6/30.
We founded that if rare-earth elements and magnesium concentration are 0.01 – 0.02 wt. pct. (typical concentration in Ni-base superalloys), rare-earth elements don’t interact with MgO lining because of two factors. First: the rare-earth oxides and MgO have equal thermodynamical stability in Ni melt. Second: MgO lining is passivated by spinel layer, in case of aluminum addition in Ni melt. It was found that rare earth oxidation rate during Ni-base alloys vacuum induction melting is controlled by composition of crucible scull and slag. The crucible scull and the slag can contain some components with lower thermodynamically stability then rare earth oxides. Due to that crucible scull and the slag are often main oxygen sources for rear-earth additions oxidation during Ni-base alloys vacuum induction melting. [Rus.]
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