Nitriding over hard chrome plating

[ktgottfr]

Greetings,
I have a shaft of which a small area has been hard chrome plated. I now wish to liquid nitride the remaining surface of the shaft. I have been told by another engineer that nitriding will not affect the chrome plated layer, but I was hoping to get an opinion from someone with more experience with these processes.
Is it acceptable to nitride a piece that is already partially chromed? Can I assume that the chromed area will be unaffected? Thanks!
-Kristjan

 

[DVWE]

I would ask that question to the nitriding company. I know that chromium, at a certain percent, makes the nitriding process more difficult.

http://www.nitrex.com

 

[metengr]

Based on what I have seen in the literature, nickel, silver and chrome plating can be used to protect steel surfaces during liquid nitriding. Most use copper flash plate because of cost, and it can be easily removed.

 

[swall]

ktgottfr--what you are proposing is bassackwards. The normal procedure is to nitride first, using copper plating to mask the areas where nitride is not wanted, strip the copper, then chrome plate.

 

[ktgottfr]

Thanks for the responses. Based on metengr's comment, it sounds like I should be ok to nitride over top of the chrome plating. But I will ask the Nitriding company what their thoughts are...

However, swall's comments bring this into doubt, because if chrome does indeed protect the steel from the nitriding, then why would standard procedure be to use copper to mask the areas during nitriding, then strip the copper and add chrome?

 

[ktgottfr]

The nitriding company tells me they have never heard of anyone putting copper flash on a part before nitriding, then stripping and adding chrome. They say they have nitrided a few parts that had chrome already on the piece with no complaints - they say the chrome is unaffected by the nitriding.

So, I am going to have the parts nitrided over the chrome and see what will happen. I also have a couple parts that have a 10%nickel 90%Tungsten Carbide spray coating on them that I am going to get nitrided. I am hoping the Tungsten Carbide will not be too badly affected. I will update with my results when I get the parts back.
-ktgottfr

 

[mfgenggear]

I have parts masked with copper prior to gas nitride on a daily basis. so I suggest you discuss this with more heat treaters.

Yes you can nitride over the chrome but its finish will look poor and the plating depending on its application may
be in jeopardy. Is it a bearing surface?

is the requirement selective nitride or nitride all over?

if selective Nitride, it's best to mask off with copper, then Nitride, then strip copper, then chrome plate there after "if it's engineering chrome".

Ion Nitride is a whole diffrent issue.tooling for masking may be required.

if it's cosmetics it will be more pleasing to the eye if chrome plating is applied after nitride.
other wise it will need polishing.

 

[ktgottfr]

I am actually liquid nitriding these parts, not gas or Ion. Perhaps this is the difference?

 

[mfgenggear]

Liquid Nitride (AMS2753/AMS2755) is normally applied all over. be-aware the case is very light .0001-.0005 typically.

parts are usefully finished prior to Liquid nitride
unless the nitride will be machined off on some areas.

There will be a dimensional change, depending on the material type.

If my memory is correct it is possible to copper mask but most heat treaters do not recommend or like it.
copper will sometimes will start to expel or dissipate.