Another reason to re-gear

[SteveO]

Wow, that sucks.

Before I put gears into my front diff (during the SAS), I sent my gears off Bobby to have my cryoed as well. They have been doing great thus far.

[MTL_4runner]

I asked my friend (a Machinist) about cryo and he told me that unless your going to do serious wheeling its not worth it but if you do hard core then getting all the air pockets out(cryo rearranges the moliques forcing the air out of the metal there for strengthening the metal as much as 4 times what it when you get them from suppler drop the part into a vat to like -320 and then slowly brings it back to normal temp thus forcing all air out of metal and hardens metal) is very worth it.

Just to be clear, there aren't really any air spaces in the metal instead there are micro voids (on an atomic level.....so small one molecule of air could not even fit in) which when too large, may weaken the steel (this is why iron is brittle and steel is tough.....the carbon atoms present in the steel structure lock each other in place). What cryogenic treating does is to transform the steel structure itself by nearly eliminating the atomic voids thereby eliminating the weaker points in the structure. These voids actually cause areas of high stress (ie failure points) so by removing most of the voids you lower the internal stresses of the part and the likelyhood it will fail in use. The removal of the voids essentially locks the crystaline struture of the steel together and makes it more uniform. The cooling of the parts down to near absolute zero actually slows down the molecules themselves and allows them to move closer together thereby having the effect of organizing the crystal structure (locking mechanism) of the steel. The harder the steel is on the surface, the more critical it is to retain toughness internally. In the case of something like gears where you need both (surface hardness for wear longevity and part toughness for resiliency and resistance to torque loading), this can make a dramatic difference in when failures might occur.

Here's some good articles on the subject:
http://lennon.csufresno.edu/~rlk16/cryo.html
http://www.gocryo.fr/technicals.php?page=3