Simultaneous Aging and Ion Nitriding

[tsb15]

I am looking to age and ion (plasma) nitride one of the three following materials to be used for a shaft in a downhole mud motor application. The possible materials are: maraging C-250, Custom 465, or 13-8 stainless. The requirements are: 200 ksi min. yield, Rc 48 max, and uniform shrinkage from heat treating. The core can be no harder than Rc48 because portions of the shaft get HVOF coated with carbide for rotational mechanical seals. Typical aging for most steels is just a few hours whereas the ion nitriding process requires 40 hrs. The shafts currently in use are 17-4 PH at H900 condition. The application requires two shafts to be screwed together and this connection is galliong severely and several shafts have broken. Thus the shaft material yield must be increased and hopefully hardening the threads will reduce/eliminate the galling. Any recommendations?

 

[swall]

What is section size? A 40 hour ion nitriding cycle is awfully long unless you have a massive cross section.

 

[tsb15]

It is about 1" od on average. I was given this time frame from a heat treating vendor.

 

[israelkk]

From my experience I do not think that raising the hardness will solve the galling problem completely, it may reduce it and may not. It is better to silver plate one of the two threads. As far as I know there is a limit to the minimum temperature that the ion nitriding is done. Meaning that I am not sure you can can have H900 and ion nitriding you may need to reduce core hardness to H-1000 but I may be mistaken.

One more issue to take into consideration is that the nitiding reduces the corrosion resistance of the substance metal. By the way Maraging c-250 is not a stainless steel and will corrode without a proper surface treatment.

 

[tsb15]

You are correct about the ion nitriding temperature...it is 975F. I would age the stainless steels to H1000 or H1050 but the maraging to only 800F and not nitride. I'm not sure which option would offer the best resistance to galling.

 

[EdStainless]

Your real problem in the thread. It needs to go away. Splines would be my first choice.
At 200ksi yield you will have almost no fatigue life. Every little nick and machining mark has a chance of initiating cracking.
Custom 465 will give you the best mechanicals at this strength level.

If you are looking to use the nitriding to reduce galling then you will only nitride one part, and it only needs to be superficial. If a little doesn't work, more won't help.

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