Freezing the Performance
One way most NASCAR teams have found to stay in the race is by cryogenically treating their parts. Let’s take a look at how cryogenics improves the performance of race-car parts.
In thermal processing our goal is to create a structure that results in the optimum properties for the material. Cryogenics is a thermal process that affects the material at the crystal-structure level by reducing the number of point defects, thereby creating a more consistent crystal structure. It is believed that this structural modification results in improved heat dissipation in many metals and improved electrical conductivity in nonferrous materials. It may also improve the corrosion resistance of some materials.
The cryogenic process is able to accomplish this by bringing the temperature of a part down to -300°F over an 8-hour period. It is then held for 8-20 hours and brought back to room temperature slowly over a 15-hour period. Other properties found to be positively affected by the cryogenic process are abrasion resistance, fatigue life, surface finish, strength, toughness and dimensional stability.
Several studies show that in some alloys fine eta-carbides are apparently initiated during the slow cooldown and are precipitated during the ramp-up to room temperature. In addition to a martensitic transformation obtained by this thermal cycling, these carbides result in a marked improvement in abrasion resistance. This is one of the reasons a high percentage of NASCAR race teams cryogenically treat parts such as gears, camshafts, connecting rods, cylinder heads, pistons, valves, rings and bearings. The company manufacturing most of the transmissions used in Nextel Cup cars processes all of its components cryogenically.
Carbide precipitation and martensitic transformation can’t explain why brake-rotor life is improved up to 300% with cryogenics. NASCAR has led this movement, but the USPS and many police departments now require cryogenics for all of their rotors because of the dramatic life improvement. This improvement, in cast iron, has been related to the reduction in pearlite spacing and interfacial platelets with cryogenic treatment. Heat dissipation is improved, resulting in less distortion and the initiation of fewer microcracks. In fact, tests show untreated rotors have four to five times the distortion and three times the microcracking seen in treated rotors. Using performance brake pads, the untreated rotors ran an average of 105°F hotter.
For the reasons already discussed, NASCAR has found that strength is optimized for a given alloy and part configuration. It is key to attain the maximum possible strength-to-weight ratio so that the cars can do their job, survive the torture of the race and not be weighed down by parts that are unnecessarily heavy. Strength optimization has been found to improve the performance of various types of springs used on racecars as well. Cryogenically treated valve springs show significant improvement in fatigue life, lose less preload and show a smaller loss of spring constant. Applying this benefit to coil springs has also been found to increase suspension life and improve the handling of Nextel Cup cars.
While it may be easier to understand why this process works for steels, it has also been found to improve the properties of various nonferrous materials, including some plastics. The concept of the creation of a more consistent crystal structure makes mechanical-strength improvements understandable in many materials. It is this same concept that increases the electrical conductivity of copper alloys used for electrical applications and improves the tonal qualities of musical instruments that use copper. This same crystal-structure improvement is also apparently responsible for the noted corrosion resistance of magnesium alloys, some of which are used in automotive castings.
So the next time one car or team seems to have the edge, it just might be the cryogenic advantage. IH