Removing gas nitride residue from stainless

[Comatose]

We have a family of new parts made of 400 series stainless, that have to be hardened and then gas nitride. They come back incredibly hard (1300 vickers!) but with a mottled yellow-grey residue all over them. The residue doesn't wash off with the usual things I have tried (simple green in ultrasonic, citrisurf) but it does come off if I bead blast it. As received versus after bead blast in the picture. Alconox in an ultrasonic makes it better, but not shippable.

Bead blasting each one is a lot of manual work, though, and I'll need to make about 50,000 of these.

Does anyone have any idea what exactly this residue is and how I can remove it in a batch or no-touch process? My heat treating shop doesn't have any insight but they're a pretty small operation.

Thanks!
John

 

[Comatose]

Oh, and it's not baked on coolant: I sent them one batch just rinsed in DI water, and one thoroughly degreased and then double rinsed in DI water and blown dry, and they both came back looking the same.

 

[Hightemp]

50,000!

I don't know what is causing it, but if bead blasting works, maybe vibratory finishing would? Those are a lot cheaper (purchase and operation) than a conveyor-style blaster.

 

[henrya]

How about a fairly strong citric acid bath as if you were passivating them?

 

[chipss]

Any surface finish requirements? Run them through a fine particle vibratory bowl polisher?

 

[FeinMecanix]

air leakage on cooling can lead to discoloration. try another heat treat shop and ask them if they can guarantee non blemish finish...

 

[Thunderjet]

air leakage on cooling can lead to discoloration. try another heat treat shop and ask them if they can guarantee non blemish finish...

We glass seal some larger (for us) parts and have had to rethink our oxygen purge operations. We were getting a similar but not identical reaction from our ovens.
You may try to ask about maybe a vacuum H.T. option from your heat treater.

Just an idea.

 

[BT Fabrication]

acid passivation. big shops will dunk it all in.

 

[Comatose]

Acid passivation does absolutely nothing to this residue, for what that's worth...