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Bright Annealing of 21-6-9 Small Tubular parts and discoloration on the ID

Q. Hi All,

I am dealing with a strange issue that I am trying to solve. We manufacture small tubular fittings from 21-6-9 steel that we perform a 1 hour anneal on at 1950 °F in our vacuum furnace. We do a full argon cool down to 150 °F before releasing vacuum. We are getting a rainbow discoloration on the inside diameter of the parts that we believe is an oxide layer, coming our purple/greenish. My issue is, I can't figure out how it is happening. It doesn't happen all the time, and it occurs most commonly on the smaller sizes like 1/8" - 1/4" diameter. However, I have seen it all the way up to 1/2" diameter.

We've run a bunch of tests thinking it was related to our wash process before anneal, however the cleanliness vs dirtiness of the part hasn't consistently shown to have an effect. I have had samples I put in the furnace soaked in machining coolant come out perfect and parts I scrubbed and dried by hand come out discolored. The last test I ran, I sectioned a part lengthwise and ran it side by side with a part that was still fully intact and the sectioned part did NOT discolor but the intact part did.

The furnace is leak tested every week. The release temp seems safe for atmosphere exposure. I am at a loss for what might be causing this. Could it just be air trapped in the ID of the small parts and the vacuum doesn't create enough flow through the ID to get the oxygen out? Or could it still be some foreign contaminant on the part?

Ethan McBride
Quality Director - Los Angeles, California
October 16, 2025

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