I tried to cast some niobium in place.... Not a pretty picture!
I formed a curved piece to fit at the top of the ring, a sort of
"spiral galaxy" shape, and then waxed the rest of the piece
around it, encasing the bottom, and all around the edges. There
was a 2mm hole through the middle for a tube to solder into
place, underneath to the gold. The piece of Niobium was 20 ga.
I invested normally, with Kerr Satin Cast 20, and used an 8 hour
burnout, stepping up 300, 750, 1000, 1350 and then back down to
casting temp of 1000. I melted the 14K yellow gold with
Oxy/Acet., and poured it into the flask on the vacuum table. We
let it cool for about 10 minutes, no visible red on the button,
and quenched it in water.
The piece of Niobium was all crusty, and some of the gold had
flowed over it in spots and created a partial skin over the
niobium. I peeled the gold skin off and under it was a white
layer of some weird oxide that was flaky and hard, about .2mm
thick. I scraped and wire wheeled the area and there was some
niobium underneath, but it is very weathered looking - like old
rusty iron looks after it is wirebrushed. It is the silvery color
of niobium. The melting temp of Niobium is 4474 degrees F.
There seems to have been some chemical / oxide action going on
here, but I have no idea what happened. I've cast Titanium in
place with no trouble. Does anyone know what may have happened,
or any suggestions? (Don't try this at home!)