52478

Electropolishing 304L and 316 Stainless  

June 23, 2009

We weld 304L to 316 (no filler), and need to clean the weld to a mirror finish.

We're getting all of the heat tint off with HTR-30 (nitric acid & ammonium bifluoride) and electropolishing, but end up with a haze that we still have to buff off.

We're using:

* Proprietary citric acid solution,
* 12VDC/10A power connected,
* Positive connected to the part (anode),
* Negative to my stainless steel cathodes,
* using a special fixture which keeps the parts about 1/8" away from the cathodes,
* the solution boils after about 1 minutes, consuming 15A,
* we've run it 30 secs, 1, 2, 3, 4 & 5 minutes with similar results.

Ross Johnson
Manufacturer - Charleston, South Carolina

A couple things you can try:
1.) Immediately after welding try removing heat tint with a stainless steel wire brush then electropolish.
2.) Same as above, passivate then electropolish.
3.) Make sure you wipe the discoloration off after it comes out of the passivating so it is a uniform gray color. The electropolish.
If any of those don't work it may have something to do with the temperature/cooling cycle at the heat zone near the weld.

I copied and pasted part of letter #49519 ==> I am going to try some different weld/electropolishing samples to try and replicate/resolve your issues:

It's likely that the details of the heat treatment are inappropriate rather than the act of heat treating. If it's to have it's normal corrosion resistance and mechanical properties, all 304 material has to be heat treated, and that applies to wrought pipe whether it's formed into pipe hot or cold, elbows that are cold worked into shape, components that are cast to shape, and so on. You may not see the straight pipe being heat treated, but it has been. The only appropriate heat treatment for this material is heating into the range 1000-1100 degrees Celsius and fast cooling (components of the section thickness that yours look to be would probably air-cool fast enough, but quenching into water is the normal cooling method). And 1100 is better than 1000! If you use a lower temperature, although it may well remove the ability to attract a magnet, it will also cause undesirable and harmful metallurgical phases to form instead of having a microstructure of 100% austenite. Those other phases, dispersed in small quantity through the austenite, likely are what is causing your polishing problem. Bill Reynolds consultant metallurgist Ballarat, Victoria, Australia The heat treatment is performed at 1000 degrees Celsius and air cooled. We will try heating to 1100 degrees Celsius and water quenching. I will post results. Mr. Reynolds, thanks for your reply. Jay Antonio - Kunshan, Jiangsu, China Success!! Followed Bill Reynolds' advice: heat treating at 1100 degrees Celsius and water quenched.The elbows are being electropolished perfectly. Thanks for the advice Mr. Reynolds and thanks to finishing.com Jay Antonio - Kunshan, Jiangsu, China

Cliff Kusch
electropolishing shop - North Vancouver, BC, Canada