Cryogenic treating and plastic- what would happen?

[greasemonkey]

the reason i ask is that i will be getting some tools cryo'd and am wondering if it will affect the plastic and rubber?

im thinkin i should also take out springs as it may make them too stiff?

any help would be great!

 

[Q_Goest]

Hey there Greasemonkey. It sounds like what you're doing is getting some tools dunked in liquid nitrogen or at least cooled to roughly that temperature somehow. Sounds like the tools have rubber and plastic parts on them in addtion to springs, and you want to know if these parts also get cold if anything will change with them.

Freezing parts to that low a temperature generally doesn't do much to them. There are plenty of examples of plastic and rubber parts along with springs regularly exposed to cryogenic temperatures that don't crack or have a change to their properties (such as in cryogenic pumps which is what I design for a living). The parts, once warmed up, will come back to their original condition with no affect on them.

The only problem I see is potential shrinkage of the plastic or rubber parts. The plastic and rubber parts are going to shrink much more than the metal, and this can put stress on the rubber and plastic parts which might make them crack. Having said that, I doubt that will happen, even where the rubber is wrapped around a piece of metal such as on the grip. I've seen many cases of plastic and rubber parts being clamped or prevented from contracting and still don't crack. If you have any doubts, I'd say experience is a good reference here so if the person who's doing it for you has seen similar things in the past and not seen any problems with it, I'd accept that. Certainly I haven't seen any significant problems with plastic or rubber cracking under those circumstances. Alternatively, you might test one piece first, say something with a rubber hand grip or a plastic handled screwdriver. Something with a big block of plastic is the most likely thing to crack or break. Plastic doesn't conduct heat very well and the conduction coefficient drops dramatically as temperature drops which means large blocks of plastic can have significant thermal gradients in them which can lead to stress and cracks. Cooling slowly is a way of preventing cracking in this case.

As for springs, don't worry at all about them. Even carbon steel springs will come back to their original shape/position/properties when warmed back up.

 

[Danger]

May I ask just why on Earth you'd want to do that?

 

[greasemonkey]

May I ask just why on Earth you'd want to do that?

ah, well i use these tools a LOT at work so making them harder and tougher is ideal, and its damn cheap too, at around $25(Aus) for 3kg's, it is excellent value.
Its also like an experiment just to see first hand how much tougher the tools will get.

should be interesting, will post up my findinds in a few weeks time.

thanks for that Q_Goest, very helpful. ill prob leave the plastic bits on then. he did say he put in a complete go-kart motor (less ignition) and it came out pretty good, i assume it would have had rubber seals and gaskets.

im still doubtful of the springs, as i would have though the treatment would make them tougher and less resistant to movement?

 

[Danger]

Ahhh... I've never heard of toughening metal by cooling it. Sort of reverse annealing, huh? What does it do to the crystal structure?

 

[greasemonkey]

taken from a magazine

"all metals are formed using heat. When a metal cools a substance known as austenite forms within the metals structure. The grain structure of austenite is very irregular and unstable.
Frezzing the metal at very low temperatures causes some of the austenite to disperse out of the metal while the remainder is transformed to martensite (four times harder than austenite).
This results in in a very dense uniform grain strucure with improved molecular alignment theat relieves stress in the metals core stucture and increases overall strength."

-conventional heat treating such as nitriding only harden the first few microns of the surface, where as the cryogen treatment acts on the whole metal, from the surface to the core.

-components are frozen to minus 196 celsius for at least 8 hours. it is then slowly brought back to ambient room temp.

-the metal becomes stronger by up to 300%

-resists micro cracking, the first stage of fatigue failure but more importantly it releives residual stresses within the component structure

used mainly in motor racing from go karts to top fuelers and also in tooling for mining industrys

 

[GENIERE]

Ahhh... I've never heard of toughening metal by cooling it. Sort of reverse annealing, huh? What does it do to the crystal structure?

Some golf club manufacturers also use the process. Supposedly it removes internal stresses by altering the chrystalline structure.

 

[PerennialII]

Keeping the polymer glass transition in mind is still probably a good idea, at least if toughness etc. issues have any relevance:

http://plc.cwru.edu/tutorial/enhanced/files/polymers/therm/therm.htm

... when thinking about "cooling metal" and improving it's properties better be sure what properties and what failure mechanism considering, otherwise can lead to some unexpected surprises.

 

[Danger]

Thanks for the info. Sounds like something that might be useful in some of my hobby projects.

 

[greasemonkey]

Keeping the polymer glass transition in mind is still probably a good idea, at least if toughness etc. issues have any relevance:
http://plc.cwru.edu/tutorial/enhanced/files/polymers/therm/therm.htm
... when thinking about "cooling metal" and improving it's properties better be sure what properties and what failure mechanism considering, otherwise can lead to some unexpected surprises.

can you please explain that last sentance? failure mechanism considering?

 

[PerennialII]

can you please explain that last sentance? failure mechanism considering?

... martensitic transformation can result in great increase of strength, but martensite is typically a very brittle phase, which has poor resistance to for example brittle failure (cleavage). The dilemma of increasing strength and losing toughness - usually need to make sure both retain an acceptable level for the considered application. Naturally the type of microstructure & the martensite you attain will depend heavily on the steels alloying & C-content, can be a problem or not one at all.