QPQ (Nitriding) API threads 1

[johnchrc]

I have an assembly thta I want to coat with QPQ for erosion and galling protection. Nothing to do with corrosion protection. I am worried threads will have a tendency to crack such as during thread make-up at highly stressed last thread.

Should a part that is using QPQ with an API thread have the thread blanked, part QPQ'd and then cut have the API thread machined after which an appropriate coating can be applied? Or is this overkill and do I even need to worry?

Material is Low Alloy Steel , 110ksi 30-36 HRC P110 equiv.

 

[unclesyd]

We use a lot components where we use the QPQ process and if my memory serves we correctly we stayed out of the threaded area.

I would give Nitromet a call to discuss your concerns. I don't know haw their technical reps are but when it was Kolene the response was very good.

 

[swall]

Yes, nitrided threads are a no-no. If you pursue the QPQ or other nitriding process, you must mask the threads with copper plating prior to nitriding. If your configuration will allow, you could make up a threaded piece to screw over the threads during nitriding and remove it afterward.

 

[Cockroach]

As a rule, I liquid nitrate all threads, period. I have been doing this for about twenty years or so, not for corrosion protection which it does, but for wear and resistance to galling, metal-to-metal pickup, etc. Never had a problem with it, which is why I continue to use it.

I did have an issue with a local vendor gas nitrating, they had a water quench application shortly after the piece leaving the furnace which had to do more for handling purposes and moving volume through the furnace than any metallurgical reason. I found that every thread was cracking at the roots due to thermal shock. So the process does vary from vendor to vendor.

I enjoy liquid nitration more than the gas alternative because of the lower activation energies associated with heating. Also, the liquid application uses an oil quench which lends itself well to lubricity in the threading.

Therefore I greatly disagree with SWall, with all due respect, there is no real reason other than monitoring the vendor's process.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Cockroach]

One other thing, when SWall is referring to "masking off" threading, this can only be done in a gas nitration application. There is no equivalent process for liquid nitration practices, you either do the whole piece or nothing.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[redpicker]

What are you trying to accomplish?

Are these casing/tubing threads? RSC threads? Do the threads need to seal, or does the connection have a mechanical seal? Are you trying to prevent errosion/galling on the sealing surfaces, thread flanks, or some other location?

In general, nitriding of threads is a no-no. The nitrided layer is very brittle and the thread roots are highly stressed, which can lead to cracking in service. While I do not doubt Cockroach's observation, the consequence of a cracked thread root in service would make me stay away from nitriding.

Really, though, it would depend on the service.

rp

 

[Cockroach]

Interesting enough, I did a metallurgical analysis on threading at and surrounding area. Reported cracking are typical of spider marks, not indicative of a true crack. The other surprise find was that such markings were confined to the nitrided layer, 0.005/0.008 inch depth in the liquid application case. I was taken back a little to have learnt that mose of the spider marks were arrested at the transition layer to parent material.

Your comment on thread roots, RedPicker, triggered my memory on that.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[johnchrc]

Thanks for all the responses! The opinion appears mixed which is why I posed the question... for clarification. I think I will follow Coackroach's response regarding the process as well as considering the thread loading/application. He also has done research that shows the crack growth was confined to the nitrided area. Good stuff!

What am I trying to accomplish?
Improving the life of parts via improved wear resistance. Improved galling protection on threads. Pretty part. It will only be used on low alloy and carbon steels. Not SS or Nickel Alloys. As discussed, my primary concern was cracking from surface hardening and potential for growth from micro-fractures at last thread. Coackroach answered my question and I know of Engineers, like Coackroach, that have used QPQ on their parts for years without problems, but others who have indicated they have had failures. Bottom Line - this isn't a yes/no answer. Need to look into application and process.

Thread Type:
I will be using parts with straight threads and API tapered connections. Premium connections will be blanked and then machined and coated per vendor specifications.

Make-Up Issues
One problem with QPQ is torquing through the connection as the surface is hard to get a pipe or chain wrench to bite... and when they do many times the teeth bite through the surface creating other problems. I solve this with flats or with straight or course threaded knurls. ANYONE HAVE SOLUTIONS TO THIS?

As far as blanking goes, if I am going to coat the threads with a different coating, I will blank the part for the thread and cut after the QPQ process, then coat the thread.

Corrosion & QPQ
If I am concerned about corrosion and want to use low alloy steel, I have found Armoloy (depending on licensed vendor) works great and also performs all the advantages QPQ provides, only better. It is just expensive and has longer lead time. XADC is another trade name used by Armoloy.