NITROCARBURIZING 1

[grayseal]

We have a supplier who applies a wear/corrosion resistance surface finish to plain carbon steel bars. The problem is that when we receive the part, it has an objectionable black smut on the surface. The part is used in a medical environmenet. We believe that the process used is some variation of liquid salt bath nitrocarburizing. What is the black smut, and more importantly, how can it easily be removed?
grayseal

 

[unclesyd]

The easiest thing to do is tell you vendor that you want clean parts, no smut. If he can't do it look for another vendor.

What type smut are you seeing, loose or tightly adhering?

If it is loose the best way to remove ii is tumbling, vibratory finishing, or some other mechanical means like buffing.

If it is very tight one need to know the specifics of the applied process. One process I know leaves a variegated surface and the black areas are permanent.

 

[grayseal]

unclesyd,

Thanks for the reply. We have tried to have the supplier clean the parts with unacceptable (varied) results. Since I am not in Purchasing, I can't change supplier. We have tried tumbling, vibratory finishing, hot water wash, etc., again with unacceptable results. The smut is removed by simply wiping with a rag, but this is labor intensive.
I would like to know the specifics of the process, but this is proprietary information.
Thanks agian,
grayseal

 

[unclesyd]

I'm doing all these recommendations and have not asked for the size of the part.

One thing to try is tumbling with soft media like dry oak sawdust, corn cobs, walnut shells, or a mixture of one or the other with a plastic media.

http://www.black-walnuts.com/nutshell.asp

http://www.greenproducts.com/specifications.htm

If you could contact Mr Victor Briosi a Victor Metal Finishing he would put you on track and possibly do some samples.

http://www.vicmetals.com/

 

[swall]

There are some salt bath nitriding processes, such as QPQ, that quench out of the 1000F nitride salt into a 500F oxidizing salt bath that imparts a black oxide finish on top of the nitride. Perhaps your vendor supplied this process and the oxidzing step was out of control, leading to a mottled black finish?

 

[grayseal]

unclesyd,
The parts are cold rolled bar 3/8" x 1" x 11". When we have tried tumbling, the bars tend to collide with each other removing all coating on the sharp edges.

swall,
You are exactly correct on the QPQ process. This is the process the supplier is supposed to be providing. I guess unclesyd has the best suggestion, find another supplier.

Thanks all,
grayseal

 

[TVP]

grayseal,

If the parts are going through a QPQ process, then they are going to be black. Changing suppliers won't change this fact. The oxidizing step after nitriding turns the surface layer black-- it is not a conventional smut, but an inherent part of the coating. Specify that the parts be nitrided only (no oxidizing step, no QPQ) if you do not want them to be black.

 

[lgearhart]

TVP, grayseal said the black smut is removed with a rag. That doesn't happen with any QPQ processed parts I've come across. Well, any correctly processed ones, at least.

grayseal, it's certainly worth chatting with another vendor to see if they'll claim they can solve your problem. Realize that the surface finish of your original cold rolled may have something to do with the vendor's difficulties. If they're trying to nitrocarburize bars with scale still on them, maybe that's the source of the problem.

Best of luck!

 

[unclesyd]

QPQ is Quench Polish Quench with a final polish. I have never seen smut of any sort on a received product.

All the parts I've seen, with the exception of some SS, with the QPQ process are very shinny and slick black.

 

[metman]

"...Since I am not in Purchasing, I can't change supplier..." True and yet you should be able to influence them.

Apparently you are currently handling each part individually to remove the smut. If you quantify the cost of smut removal, maybe purchasing will be given impetus to find another vendor even if the cost is greater. It certainly appears to be a processing problem which you should not have to solve in-house if it truely is a vendor problem.

 

[TVP]

Thanks for the clarification Lee. I forgot about that comment by the time I got around to replying. The P in QPQ is polish, which is usually effected by vibratory tumbling in some type of media. Is it possible that the parts are being polished again after the oxidizing bath, and that this media is leaving the residue?

 

[Tmoose]

What does your drawing say? Many of ours have a standard note about parts being deburred and cleaned of all debris.