Gordon England Surface Engineering Forum



Re: mystery material removal


Posted by J Weinbrecht on July 16, 2003 at 23:04:22:

In Reply to: Re: mystery material removal posted by Jennifer Weinbrecht on July 16, 2003 at 22:23:04:

: : : Has anyone ever had a situation where multiple passes (unintentional - apparently due to turntable failing) resulted in a band of missing material of the substrate, discovered after stripping? The band of missing material is about 0.002 inch lower than surrounding substrate where multiple passes did not occur. There was only one pre-heat pass. Not sure when turntable stopped during 12 passes. Tungsten Carbide on Mirage 250, air plasma spray. Very strange.

: : Hi Jennifer

: : Yes, very strange. I would be inclined first to check that the original dimensions were in fact correct. Check that the grit blasting prior to spraying was ok ( area being over blasted ). Check that the material was not removed during coating stripping. I can't see the plasma spraying having anything to do with this, particularly as the area is not associated with the excessive coating build-up. Could component distortion from localised overheating be confused with material loss?

: : Hope this helps

: :
: : Regards Gordon

: Hi Gordon,
: Thanks for prompt attention to my question. Sorry this next message is so long, but here is more information:

: OD is on a turbine rotor shaft, about 5 inch diameter. Two pilot diameters about an inch away from each other, one is about 3/4 inch wide, one about 1/2 inch wide. First, the diameters were ground for coating and were "0" runout, then inspected, shot peened, and plasma sprayed. At finish grind, the coating was cracked, so the shaft went to chem strip for a plasma rework. After chem strip, it was inspected and was a consistent surface. It went to plasma dept for mask, grit blast prep, and spray. Again went to the grinder for finish grind. Grinder had "lack of fill" in one inch band across both diameters, but, strangely, also had heavy overspray adjacent to lack of fill. Again went to chem strip for rework. As coating stripped, "bull's-eye" discoloration on parent material and band of missing material appeared. Band is up to 0.013 lower than "0" surface on larger pilot diameter and registers spike on eddy current. We are simulating pattern on a scrap shaft today, by stopping the turntable during the passes, the coating built up and fell off, and we are creating blue discoloration on the shaft, but will need to strip it to see if we have the "low spot" we're talking about. The band is distinctly demarcated, with a little ridge along the edge of each side of it, and we have some "ripples" along the diameter next to the low spots.

: Yeah, we looked at blasting, etc, but it really seems to have happened during the spray operation.

: However, in all these many years, none of us have seen this pattern. Would you like to see a picture?

: Thanks,
: Jennifer

P.S. I should say none of us have ever seen material missing like this. Oh yeah, and, the area is most definitely associated with very heavy build-up in overspray, as mentioned. Also, the width of the band of missing material matches perfectly with the pattern of our test run today.

The shaft is hollow, of course, but fairly thick here, didn't measure it yet but I'm thinking maybe 1/2 inch or more. When we ran the gun without the powder, we just didn't get the shaft hot, but when we ran it with the powder, we started cooking it.

I know you can remove material during plasma spray - we had a guy burn a hole through a part about ten years ago. However, this is a new and fascinating anomaly. How hot do you think we got the surface of the part? It's only blue in the "bullseyes" and not all over the low area. It just looks like something eroded it off, but very evenly - not a curved surface like you'd expect from blasting.


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