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discoloration after heat treat

discoloration after heat treat

discoloration after heat treat

I am a metal stamper producing a star washer out of 410 annealed stainless. The part was always bright after heat treating.
In trying to minimize distortion, different heat treat options were being tried. The heat treater asked if discoloration was a problem. I asked my customer, he said no.
The parts came back dull grey almost black appearance. I sent them to the cusotmer, he doesn't like the look. He says they are "corroded". I know that the cause was a vacuum issue, what I want to try and do is solve the problem with these parts.

My questions.
1: Is the black harmful in any way to the part? My heat treater says no, that the discoloration is visual and not mechanichal.
2: Is there a way to restore the brightness? somewhere else on the web I saw a solution to bluing.
"We took some Phosphoric Acid (10% by volume) and used reverse current gotten from a few flashlight batteries wired together. The part was made anodic and the cathode was a piece of Stainless wrapped in cotton shich was saturated with the Phosphoric acid solution. In seconds the blue color came off and the bright color was restored"
Is this a possibility?
3: What about plating?
Any suggestions are welcome

RE: discoloration after heat treat

You could try rotary finishing w/ an abrasive media and water. You might need to passivate after finishing.

RE: discoloration after heat treat

The heat treater is not correct. Any time stainless steel is oxidized, more chromium is pulled into the oxide than its percentage in the underlying surface. This depletes the underlying metal surface of chromium, which hurts its corrosion resistance. After you remove the oxide, you must also remove the chromium depleted surface zone by pickling with any of a number of acids and/or electrolytic methods to restore the normal composition on the surface.

Michael McGuire

RE: discoloration after heat treat

Listen to McG, this is serious.
If the parts were a light yellow or staw color then it wouldn't be an issue.  Dark and dull are bad.
We presume that you are concerned about corrosion resistance.  If so then pickling is your best option.  If you don't like the matt finish that results you can look into chemical brightening (these are usually phosphoric acid based).
Abrasive finishing and passivation will give you good looking parts and help the corrosion resistace some.  It won't be as good as pickling, but how good do you need it?

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

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