Elevated temperature restraining fixture during heat treat (nitriding)

[gringopapo]

Hi all,

We heat treat gears here and in dealing with the multitude of distortion problems that come about because of our complex geometry one idea that has been floated is placing a restraining fixture on a problem area during heat treat. Specifically I'm talking about sub-critical heat treating (ie nitriding in this case). I've looked around for literature on the practice of restraining fixturing and haven't seen much except in the brazing industry. I'm wondering how feasible this is. Problems I can see are thermal contractions/expansions negating the baseline datum of the restraining fixture, distortion of the fixture causing additional unwanted distortion in the part, by adding a heat sink around the part I will change the nitriding process.

The magnitude of taper we are seeing is in the range of .008-.010". The taper is occurring on the feature that is being nitrided, however my gut feel is that nitriding would not cause that much taper (of course, I'm happy to open up a new discussion if I'm out on left field on my gut feel).

We have tried stress relieving the part and removing very minimal stock but keep getting this distortion. We've also measured how much thermal sag we may be getting due to fixturing at the nitriding temperature but have found thermal sag to be minimal and irrelevant at these temperatures.

So my question is: Is there viability in using a restraining fixture at elevated temperature? If so, is there a material recommendation, or just use the same material as the part?

 

[CoryPad]

If you can perform stress relief prior to nitriding, then that is your best option. What alloy are you using? What did you use for stress relief and nitriding process parameters?

 

[MagBen]

Agreed with stress relief which should be done prior to nitriding, not after. residual stress present from steel making or machining could cause later distortion during nitriding. stess relief after nitriding has been proven to be not effective.

Due to no phase transformation and no quenching, the distortion for nitriding is supposed to be less a problem than carburization. There is no diemnsional change but only slight growth due to volumetric change from N diffusion.

For heavy parts, fixturing should help. How to place and support parts is important. warped grids, unflat trays should be avoided. i ti salso important to make sure gears are freely exposed to heat transfer medium for a unform heating rate.

 

[arunmrao]

I agree with both of the comments. Stress relief is important . In my over 20 years of making fixtures, I do not remember selling one for nitrided parts. Essentially temperatures being far lower, and lack of any phase transformation, there are no changes in dimension .

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