TPI 355 Machine Work - Something Old, Something New Part II

Ray T. Bohacz Apr 1, 2006 0 Comment(s)
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Holding The Horsepower
Power is useless if it isn't reliable, and that is where 300 Below Cryogenic Tempering Services become useful. The bane of all metal is the residual stress that remains from the manufacturing and machining processes. These are broadly grouped as thermal and mechanical stress. The first situation occurs when metal of any type is drilled, cut, machined, or welded, and is the by-product of the uncontrolled cool-down rate in the affected zone. Bending, forming, die-casting, or the rigors of machining impart mechanical stress as the molecules are forced or torn into a new shape. Engine components usually suffer from the two of them. Though both are catastrophic, thermal stress is dominant and more destructive. It makes bores go out of round, shifts line-bore paths, causes premature valvespring failure, and initiates other nice stuff like that. By removing the residual stress, the component becomes stronger and more stable.

Cryogenic processing incorporates a controlled deep-freezing process with a very defined warm-up rate. This process accelerates the molecules until they align themselves properly, which returns the intended dimensional stability to the component. Stability through stress relief adds power by keeping the bores concentric longer, thus maintaining ring seal; it increases the life of an aluminum connecting rod on the order of 300 percent to 500 percent and triples the usefulness of something as mundane as a brake rotor. Unlike heat treating that tends to warp or distort the piece, cryogenics has no impact and requires no additional machining. Dimensional tolerances should always be checked after each process; as the molecules return to their natural position, measurements may change. The molecules return to the position they maintained prior to manufacturing and machining. After the cryogenic process they stay this way.

The original deep computer-controlled cryogenic process was developed by Peter Paulin of 300 Below Inc. In the early '60s, NASA and the military experimented with freezing to -120°F for stress relief and dimensional stability. As with many technologies, the microprocessor allowed the full benefit of a process to unfold, and enabled 300 Below Inc. to develop this integration. Its documented cryogenic process has the ability to neutralize both mechanical and thermal stress with no negative effects. The secret is the way it is able to control the temperature drop from a computer cycle. The temperature of the part is lowered to -100°F through normal refrigeration (in a processor that resembles a chest freezer). Nitrogen gas then plummets the Fahrenheit to -300°, and the part remains in the processor for 24 to 36 hours, depending on its mass and composition.

Then it's time for the warm-up cycle to begin. The temperature is raised very slowly to 375°F and then slowly returned to room temperature. During the deep freeze, the molecular structure draws closer, and stops just short of absolute zero (-459.69°F), where no molecular movement exists. During warm-up, the molecules accelerate, and the grain structure of the metal takes its natural form, thus limiting distortion and adding strength and dimensional stability.

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