While cryo treating has been around for many years, and is often used by racers with heavily abused hardware, new advances have been made in this material treatment process. We had our parts shipped off to NW Cryogenics' new facility in Palmdale, California for their new dry, deep cryo treatment, which brings a material below -300 degrees, far lower than older style cryo. The NW system is also dry so that we don't have to worry about dunking a part into a tank of liquid nitrogen that might shock the part or yield inconsistent temperature gradients in the part. Bearings and differentials can also be thrown in since the process is dry, saving the time to repack parts with grease.
What cryogenics essentially does to a part is to relieve internal stresses left from forging, machining, or an incomplete quenching and annealing process as a part is manufactured. Like most ferrous metals used in cars, the steel, whether cast, forged, rolled, or stamped, is made of iron and varying levels of carbon imbedded within. The grade of steel depends on what additional metals are alloyed in it, its carbon content and very importantly, how the steel is manufactured. Carbon's role in steel is to increase its overall strength when compared to iron. But depending on the alloy, intended use and how a part is made, these properties can vary greatly.
During manufacturing, the temperature the steel is heated to and how it's cooled down or quenched makes a big difference in how the final material behaves. As steel changes from a molten liquid state to a cooler malleable solid state, the iron atoms begin to form up and link in a cubic crystalline structure. As these crystal lattices form, carbon atoms are captured within each cubic crystal structure, made up of eight iron atoms at each corner. While still very hot, the smaller carbon atoms are locked in by the iron atoms in a Face-Centered Cubic Crystal (FCC) structure with a carbon atom in each face of the cube. This phase is called austenite. As steel cools to room temperature, the smaller carbon atoms diffuse out of the crystal structure as steel assumes its stable room temperature form, called ferrite.
In order to keep the carbon atom inside the crystal structure, the steel is quenched to rapidly cool and contract the metal and trap a single carbon atom in the center of the cube in a Body Center Cubic (BCC) structure. By trapping the carbon within eight iron atoms, in a phase called martensite, the steel becomes stronger while being denser than conventional ferrite.
Most hardened steels are quenched to promote the formation of martensitic steel. The problem is that quenching will only transform up to 85-percent of the steel into martensite. Deep cryogenic freezing can be looked at as a sub-ambient quenching and annealing process that further increases martiensite formation by another 8 to 15-percent making a part stronger and more uniform. This works for anything from gears and tooling steel to guitar strings.
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TRD has discontinued all but the 2-way clutch type LSD for later model AE86 Corollas. This
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NW Cryogenic's deep cryo process starts by gradually cooling a part with nitrogen gas to -300 degrees Fahrenheit. That temperature is maintained for at least eight hours. After the cooling cycle is complete, the item is slowly warmed back to room temperature, then heat-treated, with temperatures of 100 to 400 degrees Fahrenheit, depending on the composition of the item. Finally, the item is gradually returned to room temperature. The complete process takes a minimum of 24 hours and is a one time deal that does not require additional service as a part wears.
WPC Surface Treatment
Now that the transmission and gear components are all uniformly strengthened, we wanted to increase the surface hardness of the gears themselves to minimize wear. The WPC process is fairly unique, because it not only hardens a part's surface, it also increases durability by reducing friction, which is why many top motorsports teams use this process.