OK guys (and gals) here it goes:
Nitriding will increase the fatigue strength of a crankshaft by
setting up compressive stress at the surface. In other words, it
helps keep a crack from starting at the areas that see bending
stresses like the journal radii. In addition to this, it also
improves wear resistance at the bearing surfaces by increasing the
surface hardness.
There are a few types of surface hardening methods that use nitrogen
to increase the hardness. If you have Kas's first new book, I wrote
an article in there describing the various types and my
recommendations on which is the best. My company does all types at
various locations throughout the world, so my recommendations are not
based on getting business, but rather my engineering background and
experiences.
Most processes are done in the 900 F range. Some are done at lower
temperatures, but they are really only good for wear improvement.
Nitriding is a diffusion process, which means that it is time and
temperature dependant. The higher the temperature and the longer the
time, the deeper the resultant nitride case. The limitation is the
final tempering temperature of the part to develop the core
properties. If you exceed that temperature (usually around 1000 F),
you will soften/weaken the part.
On my crankshafts I do perform a stress relieve prior to taking it to
the machine shop. Usually around 950 F. I then have the crankshaft
ground to final size, plasma nitrided, and polished. I recommend
plasma (or ion) nitriding because it leaves the surface smooth so that
further grinding is not required.
With respect to cryo treatments, Susan is right. It helps insure the
austenite to martensite transformation is complete. However, the
martensitic crystal structure has more volume than the austenitic
structure, so the parts actually expand (microscopically), not shrink
as Susan indicated. On some steels, the martensite transformation
finish temperature is below room temperature, so cooling subzero
insures complete transformation. This results in better core
properties. Cryo treatments should always be followed by a final
temper to transform the brittle martensite to the strong and ductle
tempered martensite. Cryo treatments will not give you a marked
improvement in fatigue life like nitriding will, but it does help. I
don't know why cryo treatments work on materials without such a
transformation, but it does seem to.
Keep in mind that all Formula 1 crankshafts are ion nitrided to very
deep case depths and ground to size after nitriding. This is to
increase fatigue life on their spindly cross sections. Also keep in
mind that this process takes over four days to complete, which
translates into large $. For us, the typical case depth
recommendations I made in Kas's book work just fine. I've been
running the same crankshaft in my 1500 Spitfire for over four years
now processed to my recommendations. I've roasted many bearings (as
other 1500 racers are probably painfully aware), simply polished the
bearing material off of the journals, crack checked, and reused it.
Works great.
If you have any other questions just ask. This is probably more info
than you wanted anyway, but I'm always happy to share info that will
help my fellow Triumph racers.
Thanks,
Chuck
Spitfire Racer
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