Use of Tribocor 532N for a carbon-free induction furnace for vacuum-induction melting and casting of uranium and its alloys

Abstract

Vacuum-induction melting (VIM) of uranium and its alloys commonly utilizes graphite crucibles and components that are typically coated with crucible/mould washes [1, 2] of yttria, zirconia or erbia. Carbon/ carbide contamination of the uranium and/or alloys arises from several mechanisms [3-5] that are enumerated in Table I along with the level of pick-up observed in large castings produced from feedstock containing 33 + 15w.p.p.m. carbon. The carbon/ carbide contamination has very undesirable effects on metallurgical properties, and many efforts have been made to reduce or eliminate this contamination. Because the carbon contamination all results from the use of graphite components in the VIM operation, the most obvious method of eliminating carbon/ carbide contamination in uranium systems is to replace the graphite components with carbon-free (or noncarbon) components. This letter summarizes the results of our effort to develop a carbon-free induction furnace at the Oak Ridge Y-12 Plant. Initially, the carbon-free furnace utilized oxidebased ceramics (alumina-silica or alumina-zirconiasilica) along with a plasma-sprayed tungsten susceptor [6]. Recently, however, a new material (Tribocor 532N, an alloy produced by Fansteel, Inc., of 50 wt % niobium, 30 wt % titanium, 20 wt % tungsten nitrided at > 1800°C [7-13]) has been shown to be relatively inert with uranium and its alloys [14] and, thus, to be an ideal candidate material for a noncarbon crucible/ susceptor. The Tribocor 532N material is a heavily nitrided metal (1.6ram (0.063in.) nitride-affected depth, with 0.25 mm (0.010 in.) of mostly nitride) and thus has a very hard outer layer (microhardness ~2300kgmm -2 at 100g load) consisting mostly of titanium nitride. The hard, abrasion-resistant (Ti, Nb)N surface of Tribocor 532N prevents gouging and puncturing upon loading heavy uranium parts, a main source of the uranium graphite reaction as a contribution to carbon pick-up (see Table I). The uranium-carbon monoxide reaction [3] in standard graphite-component VIM furnaces occurs primarily from in-leakage of air followed by an oxygen reaction with the graphite crucible to generate the carbon monoxide which is readily gettered by the molten uranium. This carbon monoxide source is virtually eliminated with Tribocor 532N: although some oxidation of Tribocor 532N can occur at production-level vacuum levels (6.7 to 33.4 Pa or 50 to 250#m), the surface appears to remain inert and intact. Conventional crucible/mould coatings [1, 2] do not react with Tribocor 532N as they do with graphite; thus the coating-graphite reation is eliminated. Coating outgassing could contribute a low level carbon pick-up (estimated below l w.p.p.m.), which could be eliminated by using an inorganic-binderbased coating. Zirconia coatings are quite effective with Tribocor 532N and can be used instead of yttria. Yttria is preferable to zirconia for coating graphite only because of its greater stability with graphite. Zirconia and Yttria are equally good with uranium [15, 161. In addition to eliminating sources of carbon con-