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            Gear Technology, The Journal of Gear Manufacturing
,                                                                                                March/April, 1993
Completing the Heat Treating Process 

Durability is the most important criterion used to define the quality of a gear. The freezing of metals has been acknowledged for almost thirty years as an effective method for increasing durability, or "wear life," and decreasing residual stress in tool steels. The recent field of deep cryogenics (below -300°.F) has brought us high temperature superconductors, the superconducting super collider, cryo-biology, and magneto-hydrodynamic drive systems. It has also brought many additional durability benefits to metals.

The deep cryogenic tempering process for gears is an inexpensive, one-time, permanent treatment, affecting the entire part, not just the surface. Gears may be new or used, sharp or dull, and reshaping will not destroy the treatment.The process has a number of obvious benefits, including increases in tensile strength, toughness, and stability through the release of internal stresses. The exceptional increase in wear resistivity, generally exceeding 200%, is the greatest benefit.

Steel surfaces receiving wear, such as gears, sharper cutters, drill bits, end mills, taps, dies, surgical scissors, bearings, racing engines, slicers, and granulator knives, all benefit from this inexpensive treatment. New applications are being discovered regularly.

Martensitic Transformation

A research metallurgist at the National Bureau of Standards states, "When carbon precipitates form, the internal stress in the martensite is reduced, which minimizes the susceptibility to microcracking. The wide distribution of very hard, fine carbides from deep cryogenic treatment also increases wear resistance." The study concludes: "...fine carbon carbides and resultant tight lattice structures are precipitated from cryogenic treatment. These particles are responsible for the exceptional wear characteristics imparted by the process, due to a denser molecular structure and resulting larger surface area of contact, reducing friction, heat, and wear. "Metallurgists have been skeptical of the cryogenic process for some time, because it imparts no apparent visible changes to the metal. The thinking is that since proper heat treating changes 85% of the retained austenite to martensite, and the deep cryogenic process only transforms an additional 8 - 15%, deep cryogenic treatment is an inefficient process. These are correct premises, but an inaccurate conclusion . Deep cryogenically cooled metals also develop a more uniform, refined microstructure with greater density. Microfine carbide "fillers" are formed, which take up the remaining space in the micro-voids, resulting in a much denser, coherent structure of the tool steel. The end result is increased wear resistance.

These particles are the same ones identified and count in the accompanying study using a scanning electron microscope with a field particle quantification. (An automatic particle counter.) It is now believed that these particles are largely responsible for the great gains in wear resistivity. Unlike the case of coatings, the change created is uniform throughout and will last the life of the tool, regardless of any subsequent finishing operations or regrinds. It is a permanent, irreversible molecular change.

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