Cryogenics with Liquid Nitrogen

I would like to get some feed back on using liquid nitrogen for shrink fitting parts. I want to know what is the actual physical change in properties to the steel are when using liquid nitrogen. Are there any change in strength or any other effects after warming to its natural state? The material I'm using is 440C and it doesn't get hardened after. I have .0006 to .0007 press fit. The part I want to cool is a washer .187 thick X .600 diameter with a .12 hole in the center.

If I use a coefficient of linear expansion of 5.7. I might be able to use Dry ice with Acetone bath. However, it that don't work I would like to have the option of using liquid nitrogen, but I don't know what the physical change in properties might be.

I would appreciate any input

Thanks'

You should expect the changes to be subtle and (generally) beneficial. I don't know about stainless (440C), but cryogenic quenching (after the usual quench, NOT from hardening temperature) is reputed to have good effects on carbon steels. Basically, any retained martensite that can possibly be converted will be converted. This is beneficial to strength, hardness and long-term stability. But the impact is more like getting the last few drops out of a bottle than a whole new drink.

There's been a fair quantity of snake oil claims on behalf of cryogenic quenching, too.

With any process that has very low temperatures, you will have to contend with condensation. This causes two problems. First is the water itself - you may entrap the moisture during the fit. Second, it will warm the part very quickly. Otherwise, I see no problems, as long as the washer is an internal fit. If it is going over a shaft - you have contraction where you want expansion.

Thank you both for you input.

Sfriedberg I have read the same things about having a benefits after quenching but I really thought that was just for after heat treatment process.

Bruce, the washer is an internal fit. I understand about the condensation factor. However, I didn't realize about the water warming the part quickly. With that being said, I think I'm a little skeptical about using the Dry ice & Acetone bath because its pushing the limits of shrinking the part only .0006.

Thanks again.

Used to install bushings in wheel loader buckets, 6" to 9" bores and 1/2" wall thickness and soaked them in liquid nitrogen untill it would stop boiling and then install the bushing into the boss. .005 to .006" press and most of the time the bushing would slide in by hand. Use caution while handling both the liquid nitrogen and the parts. We did have a hyraulic cylinder to pull the bushings in if they would not slide in but they would not go in with out being cooled.

And ALWAYS remember. Liquid nitrogen is !comparatively! harmless. If you get a few drops on your bare skin, no problem, they just slide off. Nitrogen is slow at drawing out heat for a number of reasons (low densitiy, vapor barrier etc) and it takes a while to do damage to skin.That does not mean you can handle it or the parts like it was water. It just means that you need no ridiculous safety measures to handle the liquid, common sense should be enough.

But the PARTS are NOT harmless. The metal is at -200 C° and it is a good conductor of heat and will not form a protective vapor barrier agaginst a warm surface either. The metal will give burns almost instantaneusly.

P.S. LN2 in single compartment cars and elevators is generally a big nono.......If the dewar vessel shatters due to an accident......

To get 200 degrees difference heating is generally far cheaper!

thermite said:

Do we presume this is a 440C washer being pre-seated into a 440C hole/recess/counterbore to be retained by the shrink fit...
.. or is the 'host' (or the washer) some other material?

Click to expand...

Yes it is going into another 440C hole to do the retaining . Thank you for you input.

Zonko said:

And ALWAYS remember. Liquid nitrogen is !comparatively! harmless. If you get a few drops on your bare skin, no problem, they just slide off.

Click to expand...

The worst thing is the fine spray from boiling LN2, the small drops tend to stick to the skin making small blisters. Not dangerous or painful, just a bit annoying. But watch your eyes. The liquid itself is pretty harmless, you can even stick your finger into it for at least half a second without any chilling.

If you go the liquid nitrogen (LN2) route for shrink fitting a hardened grade of steel, you need to LN2 treat the steel first, check your
dimensions, then do the actual LN2 shrink. Reason is that the LN2 treatment may cause some additional martensite to form and this
can change your dimensions

All the precautions for handling LN2 listed so far are good but I will add a few. Zoncko's point about handling of parts that have been chilled is especially important.

Wear gloves that can be "thrown off" quickly and easily. If some splashes inside a glove you want to get it off quickly.
Wear, at the very minimum, safety glasses. Even better goggles.
Wear loose fitting clothing and trousers that go over the top of your boots or shoes. You don't want any splashing inside your shoes. If your clothes get soaked (for whatever reason) you want some distance between the cloth and your skin.

It is common practice to use an open top container made of styrofoam (small beach cooler) because they are cheap, relatively rugged, and easy to handle. However, if you notice that they LN2 inside is no longer "water white" clear and starts to get a bluish tinge to it it is becoming liquid oxygen rather than plain liquid nitrogen. This is VERY dangerous. The boiling point for LOX is about 15K higher than it is for LN2. This means gaseous oxygen will condense out of the air and go in to solution with the LN2 "enriching" it. LOX + styrofoam will burn explosively. LOX plus a whole lot of things one normally doesn't think of as flammable will burn.
Fortunately most uses of LN2 are too short for much LOX to to accumulate so it is very rare. If it does happen the safest thing is to dilute it with LN2 (if you have any left) and dump it on bare ground (sand, gravel, concrete NOT asphalt.) Or even a steel bucket.

-DU-

Nice read. Yeah , LOX and sawdust were used as a mining explosive because all you needed was a big Linde style cryo cooler, which is essentially a several hundred KGF per square centimeter compressor and some fancy pipework.

IIRC the practice was abandoned as modern ammonium nitrate explosives became cheap and trucks fast with good roads into distant corners of the world because the damn LOX mix was pretty unsafe.....

Wingshot said:

I would like to get some feed back on using liquid nitrogen for shrink fitting parts. I want to know what is the actual physical change in properties to the steel are when using liquid nitrogen. Are there any change in strength or any other effects after warming to its natural state? The material I'm using is 440C and it doesn't get hardened after. I have .0006 to .0007 press fit. The part I want to cool is a washer .187 thick X .600 diameter with a .12 hole in the center.

If I use a coefficient of linear expansion of 5.7. I might be able to use Dry ice with Acetone bath. However, it that don't work I would like to have the option of using liquid nitrogen, but I don't know what the physical change in properties might be.

I would appreciate any input

Thanks'

Click to expand...

If there are any changes worth noting, they are small enough that Boeing does not bother to raise them in their docs WRT cold shrinking bushings and bearings into aircraft parts. Part of my daily work life over the last few years.

I would feel pretty comfortable saying that your main concerns are the fit of the part (calculate the actual shrinkage based on the shrinkage per inch, per degree temperature change, and remember to calculate THAT by the part size), and the safety around the cryro temperature liquids. Other than that, getr done.

Cheers
Trev

Updated With Prints



Wow, I never thought I would get this much response and feed back. Thank you all for you feed back.

I have uploaded my prints with actual dimensions to put everything in prospective. The main reason I started this thread was I wanted to change our process of assembly of the two parts. We currently are heating the block with the female feature to 800 degree's prior to assembly. We have to hold a locational tolerance of .002 with the washer from the bottom of the part when assembled. We don't machine anything after assembly.

We have had some issue's with scraping the part after the assembly of the two parts due to the close locational tolerance of the washer. As-well as galling of the I.D. and the 32 micro finish etc. When we heat the block we really have to watch for particles or dust getting into I.D. It's a pretty slow process and I want to increase production.

We do use a fixture to hold down the block after heating to 800 degree's. The fixture is a plate with a male stud sticking up (which is locating the .002 tolerance). Then we made a male rod with a magnet in the shape of male stud which locates off of the I.D. of the washer. Then we stick the washer on magnet and push it into the I.D. and located it off the top of the male stud in the fixture. It does work, but just want to increase production.

Thanks again for everyone's help.
Kevin

Hi Bill. I'm not sure why we aren't machining it out of one piece, but I'm sure its a good reason. I just don't what it is. I'll get back to you.

Thanks again.

Well Bill. I asked about why we just don't make it out of one block of 440C complete. I was told we had talked to the engineer at the company we were making it for and he plain and simple said you have to make it like a assembly. Well you know the answer to that don't you? The customer is always right.

Thanks again for everyone's input.

This was a very interesting read, thank you.

A LN2 story. True story. The shop driver was told to check the LN2 flask as he was going to the welder supply house anyway and see if it need topped off. The driver thought he would grab a piece of steel off the rack and use it as a dipstick. With his great wisdom, he grabbed a piece of 9/16 diameter tubing about 10' long and dunks it into the LN2. After a couple of seconds the tubing turns into an LN2 fountain, raining bits of LN2 all over. The driver danced, screamed, and fled the area.

One of the machinists had to go over and pull the pipe from the dewar. It took a while, as they were all laughing so hard they could hardly breathe.