[Zeke]

Carbide-mild steel-cutting oil? That should have worked.

But slow for this is really slow. And lifting out of the cut every few seconds for just a second or 2.

Believe it or not, fewer teeth would have been better. Carbide likes a good bite, not just skimming along. You may have done better by really bearing down on the DP if it's got the beans.

Another thing that might have happened is that the work got hardened. Try your cutter in a fresh cut. If it works, don't let the process get hot. Keep jumping in and out of the cut.

Worst case is that you can make a good pattern for a jigsaw to follow. And if you have a metal lathe and a way to mount the work, rough it out with a disc grinder and finish with the lathe. You don't want it too jagged or you'll have trouble.

If you can find some thick wall pipe you may be able to cut sections and use cross plates. You weld, right? I mean fundamental stick or something.

Edit: I see while I was typing that you could have bought a kit for 50. I'll bet the bit was all of that. And I wouldn't worry about it being from china. The bit probably was.