Mike and all,
Cryo is a good process to do, especially if you have a new part
without any service life. Cryo sort of "completes the heat treatment"
by helping to insure that all transformations have been completed.
You will probably see an improvement in properties especially with
steel parts because some steels require a zub-zero treatment in order
to complete transformation. This is always a good thing.
Service life also does the same thing. Many may recognize that it is
always good to use a "seasoned" crankshaft for rebuilding. That is
because when parts are in service, the stresses seen often complete
transformations and, along with long exposures to sevice temperatures,
provide stress relieve as well.
So to optimize core properties, use a seasoned crankshaft and cryo
treat it. It is true that the core properties could be improved
leading to higher tensile strength (and therefore fatigue strength).
But this gain would be slight. Would I bother with cryo treating a
crankshaft? No. I don't see much benefit for our applications with
the materials used for our parts. I don't see a huge benefit from the
amount of money I would have to spend. Would I do it if it were free?
Maybe, if I had the time. Would I do it if I were building formula 1
crankshasfts? Maybe. But I can tell you, Formula 1 crankshafts are
not cryotreated now, but they are nitrided! I currently do
development work now for a manufacturer of crankshafts for Indycar,
NASCAR, and F1 and we don't cryotreat their parts.
It is true that nitriding only affects the surface of the parts?
Absolutely! But that is what you want. You want a strong (hard)
surface layer that resists fatigue crack initiation. If parts were
through nitrided, they would be very brittle and may even crack if you
dropped them. You need the "flexible" core to help support the
nitrided layer in order to have a system that works. Remember,
crankshaft life is all about fatigue. If you have adequate core
strength coupled with a crack resistant nitride layer, you have a part
that will probably outlast you.
So why "billet" cranks? Because they provide a better base for
nitiding. Why 4340? Because you can maintain the same core hardness
through thicker sections. Same with EN30B (for our European friends)
and other more expensive derivatives.
So, what would I do (not that it is really important what I do :))?
If your reconditioning a stock crankshaft, have it stress relieved at
around 1000 F before grinding, have it ground, and have it nitrided,
preferably ion nitrided. Ion nitriding produces a much better surface
finish over gas and makes cleanup easier.
If you feel that you want to hedge your bets, go ahead and have the
crank cryo treated before nitriding. But don't skip the nitride
process. It is very important! A few FOT members have experience
with non-nitrided cranks, none of it has been positive.
Hope this helps,
Chuck
Spitfire Racer
Hope to see you all at Hallet
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