How much is this part going to move during Nitriding?

Below drawing is for a 4140 pre-hard crankshaft, which will get Nitrided to a surface hardness of RC58 and a depth of .015"

The nitriding is required for surface hardness on two needle bearing journals which require RC58+ according to SKF.

My question is, how much is the shaft likely to bend/grow during the Nitriding process? It is 8" long and the first 2" are hollow, with just a thin outer wall. The OD of the thin part is the surface the bearing runs on.
Of course we need to leave some amount of stock on there for finish grinding after Nitride, but if too much is left, the hardness will drop off as the material is removed.
If not enough is left, the risk is the part bending to the point where it doesn't clean up.

I was going to guess to leave .005" per side on that diameter, although that's a guess, I don't have much experience with Nitriding large parts. I would prefer to leave more like .001".

Any other suggestions for how to make the part are also welcome. We are open to changing the material and process, but not dimensions. I have had bad luck in the past with through hardened crankshafts being too brittle, so I'd prefer to go with a case hardening or hardening + coating?

Thanks a lot,
Mark

A lot depends on how good your heat treater is. You might consider 4340 (harden blank then machine) or ETD 150 instead. Leave .003 or .004 to finish grind. YMMV.

When the Moore Special Tool Company, had to nitride their lead screw threads, they cut them slightly wrong, so after the heat treat movement, the thread grinding could leave a symmetrical amount of hardened material on each flank.

Very cool...

Regards,

Stan-

Nitriding is done well below critical temperature. Most nitriding is done between 950 and 1100 degrees. This is in the stress relieving range for 4140. It will also draw the prehard to somewhere between Rc26 and Rc34. If you wanted to reduce any movement run a stress relief on the material before you make the part. My understanding is that nitrided items will not grow an appreciable amount.

Thanks for the suggestion on the 4340. I read that it has higher tensile strength than 4140, but conflicting info on if Nitriding produces a harder surface or not. I'll keep looking into it. Stress relief before-hand is something we'll do for sure.

Mark Winsor said:

Of course we need to leave some amount of stock on there for finish grinding after Nitride

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Ideally, you'd rough machine, stress relieve, then finish machine and grind, then nitride last. Second best would be stress relieve your blank, do all machining and grinding, then nitride last.

Mark Winsor said:

Thanks for the suggestion on the 4340. I read that it has higher tensile strength than 4140, but conflicting info on if Nitriding produces a harder surface or not. I'll keep looking into it. Stress relief before-hand is something we'll do for sure.

Click to expand...

It _can_. Depends on how it's called out. We make a Gear Shaft for Toyota that calls out 4140 HRC 48 - 52, Nitrided to HRC 65 for .06" deep. And yes, straightness is at the mercy of who's doing the Heat Treating. Specifically, the Quench. Go ahead... ask me how I know...

Do you finish grind after the hardening and nitriding? How much stock do you leave ?

Mark Winsor said:

Do you finish grind after the hardening and nitriding? How much stock do you leave ?

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Yes, of course. Honestly? Depends on who's doing the Heat Treating/Nitriding. If it's me*, then I leave .010" for finish grind. If it's someone else, I leave .020" for finish grind. This is because I take enough care to prevent as much movement as possible and someone else will not.

* - I don't do Nitriding, but will do the initial Heat Treat. Some times I will do it this way. Other times ( if I'm too busy or there's an extreme rush because of someone else's lack of planning ) I will let the place that does the Nitiriding do the initial HT as well.