Nitriding or Xylan on threads 1

[JHWC]

Hi All,

I would like to clarify the doubts I have.

1. Threads with xylan coating is usually preferred. However, some threads are nitrided. Why is this so?

My colleague mentioned that nitriding on thread may cause galling. But in one written specification, it is said to prevent galling. I am confused, what is what?

Therefore, I would like to ask when do we do nitriding or xylan on threads.

Thanks for all the help given. Truely appreciated.

Jeffrey

 

[tbuelna]

As far as I know screw threads are not nitrided. The reason is that the thread tips will thru harden and become very brittle, and then they will fracture off when loaded at installation. A case hardened surface on screw threads is not the best approach to inhibit galling. Instead, things like different metal/hardness combinations, more accurate threads, smoother thread surfaces, and DFL coatings (like Xylan) or metal plating that provide anti-seize are used.

Xylan is a polymer coating that works great as an anti-seize and dry film lubricant, while also providing some corrosion protection. But one issue with relying on film coatings for corrosion protection of screw threads or surfaces subject to high contact stress/sliding, such as bolt/nut flanges or wrenching surfaces, is that the film coatings sometimes tend to get abraded off when the fastener is installed.

You did not mention details of your application, but I can't think of any situation where Xylan would not be a better choice than nitriding in terms of preventing galling.

 

[JHWC]

Hi tbuelna,

Thanks. I have the same reasoning as you.

In this case, the application here is the stem used in the gate valve.

The stem itself consists of threads and main body. I understand the rational behind for doing nitriding for the body.

In this case, can we do nitriding and follow by xylan coating on the thread itself?

Thanks,
Jeffrey

 

[tbuelna]

As I noted above, I would not recommend nitriding screw threads. If you require case hardening of journal or sealing surfaces on the stem, you should mask the screw threads when nitriding these surfaces or machine the threads after nitriding. After final machining you can apply Xylan coating where required. Just be sure to compensate for the coating thickness when you finish machine the surfaces that will be coated. I believe the typical Xylan process uses a plated metal base followed by the polymer topcoat, and the final coating thickness is around .001".

 

[JHWC]

Thanks tbuelna!

 

[CoryPad]

I assume this is for carbon steel parts. Nitriding increases the allowable contact stress on the part, and provides a small increase in corrosion protection. The Xylan coating reduces friction and provides moderate to high corrosion protection. Nitriding usually produces a compound layer on top of all surfaces, with a thickness around 20 micrometers. It also diffuses below the surface, up to a depth of approximately 300 micrometers. Xylan usually is around 10 micrometers, and typically is applied over zinc phosphate conversion coating, but it can be applied over zinc electroplating or even zinc-rich non-electrolytic coatings, which themselves are 10 micrometers in thickness. Unless the screw threads are very small, nitriding can be used without through hardening. I have specified nitriding for the internal Acme thread on a power screw system.

 

[JHWC]

Thanks CoryPad...

 

[samsam1973]

Nitriding is beneficial on the threads provided the case depth is narrowed down. We nitride to a depth of .002" to .003" on threads for better results.