So Chuck, would it be possible to do both treatments? If you cyrogenically
treat, grind, plasma nitride and polish, would the nitride process also
temper the crank?
Mark Weathers
>From: chasgee@aol.com
>Reply-To: chasgee@aol.com
>To: fot@autox.team.net
>Subject: Re: Nitriding and Crankshaft Durability
>Date: Fri, 14 Oct 2005 17:32:27 -0400
>
>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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