Broken Drill Bit in Inconel Tube 2

[moltenmetal]

We've got a small piece of high speed steel drill bit twisted off inside an Inconel 600 thermowell (a horizontally oriented blind hole about 1/4" diameter and 8" deep, with the drill bit between about 4" and 4.5" from the open end). The geometry, location and size of the well and bit etc. are such that removing the bit by EDM or drilling out the bit are pretty much infeasible. The well is welded into a vessel and removal/replacement of the well is a major deal.

I was thinking of corroding the broken piece of bit out of the well over a period of several weeks, which is feasible. I was thinking that nitric acid, say 7% concentration, would be a good choice. Is my choice of corrodent a good one? Any other suggestions for speedier corrodents with no additional risk to the Inconel? Anything else to watch out for?

I'll be doing a beaker test before I attempt the real thing.

 

[moltenmetal]

tomwalz:

Turns out that etching in the beaker was far more effective than in the thermowell itself- three weeks of exposure didn't even loosen it perceptibly. It seems we DID get an adherent scale after a time which inhibited further corrosion, and I was too chicken to go the nitrating acid route. So we picked away at it with carbide tooling until the pieces were small enough to blow out.