Cryogenic Processing ???

[dfly9891]

Hi All,

What do you know of Cryogenic Processing? I came across this link...

http://www.johnsonpackings.com/cryo/cryo.htm

I called them up and asked about the process. They soak the parts at LN2 temps for a number of hours, then temper the parts in the oven and repeat a few times.

I have some 304 SS and TI Grade 5 parts that will be soaked in 25K temperatures with some cycling. Furthermore, I have a 17-4 shaft that will rotate at 200+ [krpm]. So, I'm wondering if I should give this process a try in order to minimize warpage of the 304 bearing housing and the 17-4 shaft. Shaft diameter is 3/8", length 2". Bearing housing ID is .430 and OD is about 3.5"

If I go with it, I would0 turn the parts down to rough dimensions, cryo-process them, then finish machine. Thoughts?

 

[iainuts]

I design cryogenic pumps and other equipment and have never used this, though I know some folks have found crogenic processing of parts as you describe to be useful in a couple of unique instances. Basically, if you're consistantly having problems with parts holding tolerance after exposure to cryogenic temperature, then you may want to try it, but my experience is that there's no significant benefit as far as holding tolerance is concerned. Parts don't grow, shrink, or otherwise warp at low temperature regardless of machining and/or welding. Parts can be cold shocked from ambient to liquid helium temperature and there's still no obvious change other than shrinkage, and that includes parts with tolerances of +/- 0.001" on a 3" or even 6" dimension. Once the part warms back up, it comes back to the original dimension.

 

[EdStainless]

There are a number of reasons why this treatment might help, but many of the claims have zero basis.
Regardless of the service temp cycling parts prior to final machining can help minimize future distortion. You are either increasing or decreasing residual stresses, but either way you are reaching a steady state condition.

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Plymouth Tube

 

[Burdy]

What grade of 17.4ph are you using? I've used HH1150 for a number of years for shafts with high torque requirements within a cryogenic environment.
We soaked every one of our components prior to installation. There always appeared to be less failure issues and better performance when doing this.
Initial wear rates on surfaces in contact with each other was also reduced.

 

[Frederick]

Deep cryogenic processing has been shown to relieve stresses in many metals. I am not sure why it does, only that research shows that it occurs. For instance, it is used extensively for tightly toleranced components that are to be used in spacecraft to keep the parts from warping out of tolerance. An associate of mine has been treating such parts for over a decade.

Do your rough machining first, cryo treat and then finish machine. The added benefit is that the parts will be easier to machine and it will be easier to achieve a proper surface finish, especially when grinding. By the way, this is the sequence being used on some of the finer racing engines now days.

 

[dfly9891]

Frederick, Thanks for your post. You've convinced me to cryo-treat this rotating assembly.

Now I'm wondering how much stock I should leave on the "roughed out" part. I'm thinking about .020 - .030 on diameter (I don't want leave too much stock on it causing me to loose some of the benefits of Cryogenic processing, and too little so that the net shape cannot be recovered after the part has "moved" in the cryo-treatment). I couldn't imagine that it would move more than a few thou or so... or?


Application Details:
Shaft material will either be 17-4PH-H900 or Inconel 718.
Shaft length and diameter are 3" x 1/2"
Rotational speed is just north of 200,000 rpm.
Delta T across the long axis of shaft is about 325K.


An off topic but related question if I may: In the case where one end of the shaft is exposed to 25K and the other end 340K at the speeds stated above, which material would you recommend I make the shaft from? 718 or 17-4PH?

In the case of using 17-4, which Heat Treat would you use? From my initial investigations, H900 seems like a good fit.

As for Inconel 718... I know that Nickel, having an FCC crystal structure, has a low ductile-brittle transition temp... but then again, 718 is only about 50% nickel. So I'm not sure if I can characterize it as such. It's definately rigid enough and has a 3x lower thermal conductivity than 17-4. I do need a RHC of around 58-62... and I gather that coating 718 is tricky at best?

These are the issues I'm parsing out at the moment... and I'm rather new at this... need some experienced input here.

Anyone out there feel like shedding some light on these topics?


Thanks.