Cryogenic treatment can be
used to improve the hardness of certain grades of steel.
Many years ago it was used as the "magic bullet" to raise
the hardness of steel that has a high nickel content, such
as 9310 or 4820, so that the gears don't wear out
prematurely.
However, raising the hardness
on the factory gears can lead to embrittlement and make
the gears more prone to cracking. We generally find
that the hardness of the factory gears in the Mitsubishi
transmission are around 60 HRC and that they harden up to
62-64 HRC after they have been Cryogenically treated,
which is too high. The proper procedure to correct
this is to temper the gears to bring the hardness
back down to just under 60 HRC.
We
personally have mixed feelings about Cryogenic treatment
and most places don't temper the hardness down to a more
acceptable number. If a shop can't tell you what the hardness of the
gears were prior to treatment, what they hardened up to
and what they tempered them back down to then they are
probably doing your gears more harm than good. Also, if
there is any gain in durability to be had from Cryogenic
treatment it's going to be quite low, less than 8% when
treating the factory gears. On top of that, when you
take into consideration the way that the gear teeth wind
and unwind from this process it makes one question the
validity of doing Cryogenic treatment to the factory gears
in the first place. If you want to have us Cryogenically
treat your gears we ship them to the most respected shop
in the business and let them work their magic.
What
is the nickel
content of the gears in your Mitsubishi transmission?
Depending on the transmission model and the gear, it can range
between 3-10% which is no where near enough to withstand
the high
impactive shock loads for very long before failure
happens. Ideally you would want to see a bare minimum of 10%
nickel content for durability with the critical highly
stressed gears around 14% nickel content if the gear
is to last long enough to wear out from use before
breaking teeth
off.
