nitriding a crank

[grumpyvette]

FROM THE SCAT CRANK SITE

http://www.scatcrankshafts.com/

So . . . What the heck is NITRIDING?

The nitriding process, unlike induction hardening, is done in
an oven. The cranks are suspended in a closed chamber
which is lowered into the furnace for heating. At a
determined temperature, amonia and nitrogen gas is
introduced into the chamber and circulated all around the
cranks and chamber. This heated gas reacts with the
carbon on the surface of the crank at a depth of
approximately .010, making the surface hard.
Nitriding is done at a temperature that is less than
the critical temperature which, unlike induction
hardening retains maximum strength of the core of
the crank.
Nitriding treats the crank evenly from top to bottom
and side to side. It sets up a surface tension that
stiffens the crank and increases the fatigue life by 18%
to 20%. Induction hardening sets up stress risers that
lowers the fatigue life.
The process is expensive. The equipment is very high tech
and is computer controlled. It has high energy and labor
cost. Typical cycle time is 24 or more hours in the furnace.
It uses expensive ammonia and nitrogen gas. The process is
designed for each specific alloy steel. If the steel is not to
spec, the crank will come out of the oven bent, broken or
swollen. In reality, the nitride process is SCAT's 100%
check of the steel to make sure that each crank a customer
receives is exactly what we say it is.
Are there any down sides to nitriding? And the
answer is yes, there are two.
1) If you have a failure and the crank requires regrinding to
restore surface hardness, you must re-nitride the crank.
But then the crank is new again. Some say you should
have more confidence in yourself than planning to rebuild
before you have even run the engine for the first
time.
2) Cost . . . You know the saying . . . You get what you pay
for. There is no question a nitrided process is more
costly. SCAT is committed to excellence and therefore
will not compensate the quality of our crankshafts by
using an inferior heat treating process to save money.
By using Hi-tech equipment and processing we are able
to furnish our customers the finest performance cranks
at an affordable cost.
Ion

READ THIS
http://www.circletrack.com/drivetrainte ... index.html

 

[grumpyvette]

cryogenic treatment of metals, increased wear resistance

http://www.cryogen.com.au/

http://www.finishing.com/340/78.shtml

http://www.metalscience.com/techinfo_ASM.php

http://www.cdpautomachine.com/ecatalog/cryo.html

"There are three mechanisms related to cryogenic treatment of steels. The conversion of retained austenite (RA)to martensite is one. This mechanism is important and brings several benefits, including a contribution to increased wear resistance. Additionally, it provides for a more homogeneous grain structure, free of (grain) imperfections and voids, which contributes to enhanced thermal properties, (e.g. better heat dissipation). This is because the imperfections act as points of diffusion, effectively "blocking" or de-grading the thermal properties of the metal at those points.
A second mechanism, even more important to increased wear resistance, is the precipitation of eta-carbides in carbon steels. This has been documented by a team of Japanese researchers in a technical paper presented at ISIJ.
In order to understand its significance, I think that it is important to realize that the introduction of carbon to iron is what fundamentally makes steel. Carbon,(C) a non-metal, is chemically dissolved into iron (Fe). Chemically, the largest amount of carbon that can be dissolved into iron is somewhere around 7%. When people talk about "high carbon" steels -- those that are recognized for their high wear resistance properties -- they are often thinking about Tool Steels that may have somewhere between 0.7% and 1.2% Carbon content. So the point is that a little bit of carbon goes a long way in enhancing the wear resistance of steels.
Remember that Carbon -- AKA diamond --is the hardest element. By chemically blending it with iron (Fe), it effectively protects the iron molecules by providing a tough, highly wear resistant molecularly bonded partner.
On the down side, the more carbon that you add, the less ductile that the metal becomes. You could also say that it becomes more brittle or that it loses toughness (in a machine tool sense). So it is always a balancing act of having high carbon for high wear resistance versus not too much whereas the steel fails due its reduced ductility/ increased brittleness.
The whole point of this discussion is that CARBON is critical to wear resistance in steels. When carbon steels (and cast irons, etc.) undergo a cryogenic treatment, free carbon atoms are able to locate themselves within the chemical lattice of the iron / carbon (Fe-C) matrix in a place where they are more atomically attracted. This modification to the carbon microstructure (technically called "the precipitation of eta-carbides") can vastly improve wear resistance of carbon steels, cast irons, etc. In general terms, the more carbon, the better the effect.
Now, why does this occur? Again, it is all the result of TTT (Time Temperature Transformation)process. When steels are brought to a very low temperature (e.g. -300 F) for extended periods, heat is removed. As a result, molecular activity is reduced -- or molecular movement is minimized. (Remember at theoretical absolute zero, which is about -460 F, there is NO molecular movement.) So as heat comes back into the steel, e.g. as it gradually warms up, kinetic activity (molecular motion) increases and carbon atoms actually "tweak" themselves into a more ideal position within the chemical matrix. In a simply stated version, free carbon atoms are attracted to open spots within the iron matrix. This mechanism, ever so slight, can have big implications on increased wear resistance. It is the mechanism that the Japanese team documented and in my view is the one that is most critical to improving wear resistance in carbon steels.
As a final note, the third mechanism is residual stress relief. Einstein observed that matter is at its most relaxed state when it has the least amount of kinetic energy (or molecular activity). With a proper cryogenic treatment, any metal will be relaxed and residual stresses relieved. It is perhaps the least recognized benefit of cryogenic treatment. Parts that "walk" or "creep" during machining are the result of residual stresses in the metal that have been machined away that were keeping the part in a certain plane. So more and more people are cryogenically stress relieving metal parts to reduce the creep and walk factors that causes parts to go out of round or flat and fail critical tolerances. This is most successfully done after rough cut and before final machining. Again, this can benefit any metal and is unrelated to the other mechanisms cited above."