Can QPQ be applied to 13 Cr or Nickel Alloys? 2

[johnchrc]

Can QPQ be applied to 13 Cr or Nickel Alloys? Back in the 90's I was told it couldn't.

What will it do to the passivisation? Does it eliminate the need?

- CJ

 

[TVP]

QPQ is a nitriding (nitrocarburizing) process that is applicable to ferrous metals. So, 13 Cr martensitic stainless steels can be treated with this, but nickel alloys would not. Whether or not you would want to treat 13 Cr is a different topic.

 

[johnchrc]

I want to prevent pitting in stagnant areas of tool under high stress. You have to dip coat but could blast away of machine QPQ from FLow path.

Do you know why the material needs flow by it to keep surface passified or CO2 resistant?

- CJ

 

[TVP]

QPQ diffuses nitrogen into the surface of the steel. This creates a hardened region (diffusion zone) and a chemically-reacted surface layer (iron nitrides) that is called the compound layer or white layer. The surface is then oxidized to create a thin, black oxide layer. Diffusion zone is measured typically in 10's or 100's of micrometers, compound layer is 5-40 micrometers, oxide layer is 0.5-2 micrometers. There is no passive, chromium oxide layer that normally constitutes the surface of a stainless steel.

 

[johnchrc]

Quotes from Dr James Marrow,Manchester Materials Science Centre.

13 CR to a self-repairing oxide film. He refers to it as passivity.

A compact, continuous film requires ~ 11wt% chromium.
Passivity increases with chromium content up to ~17wt% chromium.
Most stainless steels contain 17-18wt% chromium.
Passivity is due to a self-repairing oxide film.
A compact, continuous film requires ~ 11wt% chromium.
Passivity increases with chromium content up to ~17wt% chromium.
Most stainless steels contain 17-18wt% chromium.

I have been told that and producers purchase 13Cr flowpath materials to prevent CO2 wt. loss, pitting and erosion/corrosion. I'm told that stagnant pockets will pit. As if you have to continually remove and replace the surface protection. I don't understand this concept but it is what I have been told for years. I have seen some serious pitting samples in stagnant surfaces.

Is that what you object to in statement no passive, chromium oxide layer that normally constitutes the surface of a stainless steel? I know if you increase MO from .02 to .2 and nickel from .15 to 5. you have Super 13Cr and the pitting stops everywhere.

I think vendors want QPQ because it looks cool and is hard to prevent erosion and galling. It isn't put on pipe threads, just straight connections with seal.

I was told in the 90s you could not even apply QPQ to SS and Nickel Alloys. Now they do?





- CJ

 

[TVP]

In my response, I meant that there is no passive layer when the part is nitrided (QPQ process). The passive layer is purposely removed and not allowed to reform-- the part is exposed to a C & N "atmosphere" instead of an O atmsophere. So, this means that it is possible to apply QPQ on some stainless steels, but it is not necessarily desirable. It is fundamentally impossible/impractical to apply QPQ to nickel alloys, as the nickel matrix does not have appreciable solubility for N.

I was in no way attempting to describe what happens to a normal stainless steel component that is subjected to stagnant or flowing conditions.

 

[CoryPad]

The reason stagnant fluid leads to pitting is not that the surface material is repassivated, but rather that the corrodent (e.g. Cl-) is allowed to stay in contact with the metal surface and participate in the electrochemical corrosion process.

Certainly molybdenum and nickel increase the corrosion resistance of ferrous alloys. Molybdenum in particular is helpful with pitting resistance.

You are correct, QPQ helps with wear. It is not so good for corrosion.

In general, stainless steels are not treated with QPQ. There are nitriding processes for stainless steels, e.g.:

http://www.nitrex.com/english/technologies_modern.htm

http://www.swagelok.com/downloads/webcatalogs/EN/ltcss article.pdf

 

[johnchrc]

CVP,

I get that. That is why I was indicating you would have to machine the QPQ from the flow path so the passive layer could form. However, I don't know how the QOQ nitriding compares to bare 13CR so just leave the part alone.

And you are saying Nickel alloys can't have QPQ applied nor does it make sense. This is what I was told back when the only place in south TX who was licensed to apply it was in Chapel Hill, TX. I come to new company and they tell me they QPQ Nickel Alloys. Don't know why and who is doing it.

Thanks. I think you confirmed what I remember from working with these materials. It has been around 13 years + and now I'm back working with completion equipment for multi-zone horizontals.

- CJ

 

[tbuelna]

As noted, QPQ is basically a liquid salt bath nitrocarburizing process. It is intended for case hardening alloy steels and some corrosion resistant steels without producing quench distortions, and also providing some added corrosion resistance. Melonite is one commercial name for this process. AMS 2753 is a spec covering the process. If you don't need a case hardened surface, then this process is probably not the best choice.

 

[johnchrc]

Yes, case hardening is needed if it provides the appropriate need. Many people love it and the "look".

We apply QPQ to reduce friction and prevent galling on the parts I'm reviewing. The alloy steel version is phosphated then they go back and xylan coat a sliding surface. I don't know why they don't just QPQ the alloy steel. On the 13Cr material, xylan coating the surface seems appropriate. My guess is they want the OPQ to help with anti-galling on the end threads of the part so they dip the lot and come back to finish coating. The multi-coated parts with QPQ and Xylan have got to be a logistical nightmare and quite expensive if a single coating is sufficient. A single coating with low friction and anti-galling properties would be ideal and pitting protection for 13Cr.


If QPQ prevents pitting in 13Cr then it would help 13 Cr assemblies other than reducing friction. But an extended xylan should also help but I think it's durability and potential for pin holes may cause issues.




- CJ

 

[tbuelna]

Here is a process similar to QPQ that works well with high chromium alloys:

http://www.hefusa.net/salt-bath-nitriding-arcor.html