Electropolishing is warping my thin stainless parts

[antknee]

I have some stainless parts that I am electropolishing, the parts are discs that are 0.2mm thick and 15mm in diameter. What I'm finding is that they are warping and I can't figure out what will stop them warping. I can't change the thickness which is very thin and I've reduced the current so much that they seem to take forever to finish - 4 hours! I'm removing about 0.05mm thickness all in and losing maybe the same in diamter, that is ok and what I want, it is just that I want them to stay flat. What is happening do you think? Any pointers?

 

[btrueblood]

Residual stresses in the parts - you are cutting away and thus relieving the stresses from rolling the sheet on one face, and the residual stress left on the opposite face are causing the part to warp. Have you tried annealing or stress-relieving the parts prior to electropolishing? OR using annealled material to start with?

 

[antknee]

Your explanation makes sense although I did find that the quicker I polished the more warping I got, not sure why. I haven't annealed and don't think I could, what is stress-relieving?

I'm using very hard 17-7Ph that came in condiditon C and has been preciptation hardened.

Thanks, Ant.

 

[MikeHalloran]

Are you electropolishing one face at a time?
If so, can you terminate the periphery of the disc and electropolish both faces at once?



Separately from that:
The increased warping with higher current density suggests localized heating, possibly from random concentration gradients in the electrolyte.
Are you agitating or circulating the electrolyte?
Can you/ do you use a shaped cathode? E.g., a faced rod of the same diameter as the disc, located in close proximity to the disc?



Mike Halloran
Pembroke Pines, FL, USA

 

[antknee]

I'm electropolishing just one face and the other face is to be left unpolished. I'm not agitating at all, i don't have any means of doing that, the cathode is a rectangular stainless sheet. I've uploaded a picture of the test rig I'm using, it works, just not so well - they're warping. The cathode is tucked under the part and out of sight but i've pulled it out a little so you can see it, the anode is the part, the power supply is a 12V 300mA adapter and it's wire is attached to the anode with some conducting tape, the electrolyte is weak hydrochloric acid.

 

[EdStainless]

Find a tank where you can do this on edge, not laying flat.
This will at least give you some natural convection.
A simple agitator would help, Just something to gently stir the solution.
Make sure that your connection to the part covers a large area, preferably all of the back.
Your cathode should match your part shape.
Uniformity is your friend.

I presume that this material is in the CH900 condition.
How much machining was done after aging?
It might be worth re-aging (at 885) part way through machining.
You may have way too much internal stress.

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Plymouth Tube

 

[antknee]

It is in the CH900 condition. The stainless was bought in the C conditon, laser processed and then aged. It hasn't had any re-aging, I didn't know that was possible? How would it help?

I will use a disc instead of a rectangular sheet and ensure that the connection is much bigger, I think just the wire in one spot will be causing a problem.

The bath/tank i'm using is shaped to prevent bubbles sticking under the part and that does seem to work.

Thanks,

Ant.

 

[EdStainless]

If you are doing much machining or forming a re-age would serve as a stress relief.
As long as you don't exceed the original temp you shouldn't change the strength, but staying just below it is a good precaution.

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Plymouth Tube