Two main changes in the microstructure of the steel occur as a result of cryogenic treatment. These changes are the principal reasons for the dramatic improvement in wear resistance.

First, retained austenite (a softer grain structure always present after heat treatment) is transformed into the harder, more durable grain structure - martensite. The range of retained austenite in a material after heat treating may be as high as 50% or as low as 3%. The amount depends on the heat treating operator and the accuracy of the heat treating equipment. Cryogenic treatment simply continues the conversion initiated by heat treatment, whereby almost 100% of the retained austenite is converted to martensite. As greater amounts of retained austenite are transformed, and wear resistant martensite is increased, the material obtains a more uniform hardness.

Second, fine eta(n) carbide particles (precipitates) are formed during the long soak (chromium carbides, tungsten carbide, etc., depending upon the alloying elements in the steel). These are in addition to the larger carbide particles present before cryogenic treatment. These fine particles or "fillers", along with the larger particles, form a denser, more coherent and much tougher matrix in the material.

The surface energy of martensite is higher than the surface energy of austenite due to the differences in their atomic structures. In potential adhesive wear situations, the martensite is less likely to tear out than is austenite. The probability of wear particles forming in a steel in which the austenite has been transformed to martensite is less than for the steel containing some retained austenite. The adhesive wear coefficient is decreased, and the wear rate is decreased. For cryogenically treated tool steel, some of the junctions that would break off and form a wear particle if the steel were untreated, simply shear at the junction interface. In abrasive wear situations, both the martensite formation and the fine carbide formation work together to reduce wear. The additional fine carbide particles help support the martensite matrix. This makes it more difficult to dig out lumps of the material.

When a hard asperity or foreign particle is squeezed onto the surface, the carbide matrix resists plowing and wear is reduced. This is analogous to the fact that concrete made with cement, gravel (large particles) and sand (fine particles) is more resistant to wear than concrete made with cement and gravel alone.

Almost any kind of tool steel or dynamic part, for whatever application, will exhibit some kind of life increase. As less tools, or parts are needed, there is substantial savings in dollars. Additional savings include less downtime and short runs, less maintenance and change-over, which allows for lower production costs.

In fact, our Cryo-Processing treatment exceeds other heating and freezing methods by 200 to 400 percent.
See the chart below for some wear improvement data after conventional heat treatment - sub zero - versus cryogenic treatment:

RF Barron study results, Louisiana Polytechnic Institute

The following statistics shows increase in tooling performance after using Cryotron's Vari-Cold process:

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See Also:

• Research and Development
• Industrial Applications
• Material Analysis