Nitriding vs Corrosion resistance 3

[Chiwahua]

We have a custom-design bearing race that needs to be surface hardened, but also needs to be corrosion resistant.

We believed that nitriding could achieve those two features.

The part is made out of 4140. On the draft, it is specified nitriding (but no specific type or process for the nitriding).

The nitriding was assumed to give corrosion resistance. This was a mistake because at first, the parts came back from 1st supplier and it had that light gray finish (see image). We were pretty surprised that it was not black because all the other parts that we have been "nitriding" at a different supplier came back black.

We dropped some water on the part (light gray) and it rusted within a few minutes..

We then sent the parts to our "usual supplier" to have him "re-nitriding" his way and it was black like expected. I believe that it's the oxydation that gives this black and protective finish (right?).

Now, we have a problem:
- We have parts that are manufactured in China and we are looking to have them surface treated over there, but I don't know what process or type of process to specify in order to get corrosion resistance parts along with the hard surface. Is it "plasma nitriding? salt bath nitriding? is it the post treatments that give the black and protective finish?.

Thank you for your replies, it is much much appreciated!

 

[Tmoose]

A moderately hard-working ball or roller bearing race (or gear tooth) needs the hardness to extend pretty deep below the surface to withstand the Hertzian stresses down there.
I think most nitriding is relatively shallow, even after lots of time in the tanning booth.

Are the other components in the bearing at risk of corrosion too?

 

[mp87]

is it the post treatments that give the black and protective finish?

The black finish is likely due to post-oxidation, which is expressly aimed at improving corrosion resistance.

This presentation may also be of some help.

 

[JackMats]

The basic steps are nitrocarburising followed by a post oxidising stage. Your usual supplier seems to be doing things correctly. The post oxidation stage, as the name suggests, helps form a protective oxide layer on the surface. It is likely the other supplier didn't do this, maybe check the PO or give them a quick call to ask what they did.

Salt bath will be the common method used and will be fine for this application.

 

[tbuelna]

As noted, a two step liquid salt bath nitrocarburize process will both case harden and provide good corrosion resistance. However, the case depth will only be .006" to .008" max. Which may not be adequate for the hertzian contact stress conditions of rolling element bearing race surfaces. One nice thing is that the process does not involve quenching, so it does not produce much distortion.

 

[Chiwahua]

Thank you all for your inputs!

Tmoose: Yes it's all at risk of corrosion. It's a component exposed to every type of weather.

It is understood that the case depth is very thin.

This component was initially designed using 50w steel with no treatments except zinc-plating!!
This component was largely under-designed (by shape, by material and treatments). I'm pretty confident that we considerably improved the component just by using 4140 Heat treat and nitriding.

I don't expect this component to last 30 years but it is a good improvement I think.

So from what I understand, it must be specified: "Nitrocarburizing with post oxidising"

 

[EdStainless]

While nitrided surfaces are slower to corrode in mild environments, such a treatment affords no protection in more severe situations.
This is why you see bearings made from 440C and 17-7PH (0r 17-4PH) for severe service.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Tmoose]

Cronidur

http://www.bardenbearings.com/content.barden.us/en/products/customerdesigns/cronidur/cronidur_1.jsp

http://www.progressivealloy.com/pdf/cronidur30.pdf

http://www.schaeffler.com/remotemed...chaeffler_2/tpi/downloads_8/tpi_226_de_en.pdf

 

[tbuelna]

"So from what I understand, it must be specified: "Nitrocarburizing with post oxidising"

It's likely not quite that simple. To get the best result from the liquid salt bath nitro-carburize of your 4140 alloy steel part, you'll want to quench and temper the part beforehand at a temperature just slightly above the max nitro-carburize process temp, which is something like 1075degF. This will give the best core strength after case hardening. You'll also want to specify a standard for performing the nitro-carburize process, such as AMS 2753.

 

[Coctyle]

There are also tradenames such as BlackNitride and NiTemper, among others. BlackNitride, as the name implies, includes the black oxide step inherently. It is a salt bath process. NiTemper is a gas process and does not automatically include black oxide.

Ask your "good" vendor about the process. If they will at least tell you what they call it, Google it. It is probably not proprietary to the vendor, but rather to the maker of the chemicals. Multiple vendors might offer the same thing and might be willing to give you more info to get your business than the vendor that already has your business.

In my experience with one vendor who does NiTemper, a polymer quench was offered, which was supposed to produce a black finish. Unfortunately, it was not as consistent or appealing in appearance as needed due to cosmetic requirements of the product. We then tried NiTemper followed by the hot black oxide (liquid) process that we use for other steel products (that do not have nitrided surfaces). That did not work well. A black surface appeared, but it could literally be washed off with alcohol or scratched off with a fingernail. It seems that the iron nitride compound layer did not react properly with the chemicals. My point is that you might have problems if you specify "Nitrocarburize with post-oxidation" or "Nitrocarburize followed by black oxide".

This has been mentioned briefly, but black oxide provides little if any corrosion resistance by itself. It holds onto oil, thereby enhancing the protection of oil compared to plain steel.

 

[MechyMarco]

I agree with EdStainless. If you want a corrosion resistant bearing you should opt for 440C. That's pretty much what it was made for. I've used it in other applications and absolutely love it. Check out this document for some solid technical specs on it:

https://www.nickelinstitute.org/Tec...esfortheSelectionandUseofStainlessSteels.aspx

Calgary, AB
Mechanical Design Engineer, P.Eng.

 

[tbuelna]

A lower cost alternative to 440C for rolling element bearing races is E52100 coated with thin dense chrome. This is offered by many bearing OEMs for corrosion resistant applications.

http://www.kaydonbearings.com/Endurakote_bearings.htm

 

[WKTaylor]

Fud-4-thot...

Suggest using only 4140 'special aircraft quality cleanliness, normalized' per AMS6529

Also, FYI, possible HT/Nitriding/Carburizing specs to consider working-to...
AMS2759/6 Gas Nitriding and Heat Treatment of Low-Alloy Steel Parts
AMS2759/7 Carburizing and Heat Treatment of Carburizing Grade Steel Parts
AMS2759/8 Ion Nitriding
AMS2759/10 Automated Gaseous Nitriding Controlled by Nitriding Potential
AMS2759/12 Gaseous Nitrocarburizing, Automatically Controlled by Potentials

NOTE.
Aerospace is beginning to use Corrosion Resistant Nitrogen Steel [CREN, CRENS] AMS5898 or AMS5925 for bearings [races, balls/rollers, etc]. This tends to 'kill a bunch of birds with one stone': attain great corrosion resistance, high hardness and good wear resistance. Where the CRES surfaces contact dissimilar metals [IE races in contact with shafts or housings such as aluminum, LA steel, etc], these surfaces usually receive flash cadmium, or [preferably] zinc-nickel alloy plating, with post plating chromate treatment.





Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Chiwahua]

Thank you everyone for the replies!! I will make sure to check into your propositions. I'll keep you posted