CryoTune In Detail

The computer controlled temperature cycle from room temperature to -300°F and back occurs at roughly one degree per minute so as to minimize new stresses in the parts. Rapid temperature change, particularly when cooling, is often the cause of harmful residual stresses. By using this type of curve, thick parts and those with a wide variety of shapes can be processed easily. The “soak” at -300°F for over 20 hours causes some interesting things to happen, metallurgically, which we will attempt to describe.

Improving Heat Treated Steels

    The most dramatic results of this process is with hardened steels. This applies to through or case hardened alloy, tool, carbon and heat treatable stainless steels.
    Besides the welcome elimination of harmful residual stress, there are two primary reasons that steels hardened conventionally can be greatly improved with deep cryogenic processing.
    1. Without going into the technical aspects of hardening steels, it is important to note that the transformation of steel heated to solution, which is called austenite, to the hard structure known as martensite, occurs by decreasing its temperature at a controlled rate. Unfortunately at the normal room temperature floor there is often an incomplete transformation to the desirable martensite. Much of the matrix may be unconverted, residual austenite. The deep cryogenic process allows this transformation to continue, to the extent where hardness increases of over 5 points on the Rockwell C scale may be noticed. A follow-on tempering step at 300-800° is used to eliminate brittleness, much as in the original hardening operation.
    2. Perhaps more importantly is the type of new structures that are formed. New, small carbides, named n-carbides, are formed at extremely low temperature without an increase in volume. Although the explanation for this molecular movement is beyond the scope of this article, it is thought to occur because electrons are slowed down when chilled. Electrical energy can keep atoms apart. When this energy is reduced they are free to move and combine into new molecules. Over time the new n-carbides become evenly distributed in the matrix, filling in the gaps and making it less porous. This densification is the primary reason why cryogenically treated steels become so resistant to abrasion.

Abrasion and Wear - What Is It?

    Imagine that steel is porous, with “peaks and valleys” at the wear surface. Under sliding friction, for example, metal-to-metal contact will break off the “peaks.” This particle may then gouge one or the other surface before it goes away, one of the reasons we use lubricants or coolants.
    A “CryoTuned” surface will exhibit a more dense matrix with much smaller “peaks and valleys.” This supportive structure resists the degradation of the wear surface and lasts much longer.
    Similarly a ground cutting edge on a cryogenically treated tool will be sharper and less jagged than an untreated tool. The microscopic “teeth,” being smaller, cause a finer wear pattern as they break off. For this reason, cutting tools that are re-sharpened require up to 30% less stock removal to restore edge geometry! This factor alone will contribute to increased tool life.

Micro-Cracking - A Source of Catastrophic Failure

The “densification” of hardened steel after the deep cryogenic process minimizes the formation of micro-cracks. These of course are the source of larger cracks when parts are under tension. This phenomenon is demonstrated when observing fewer failures of cryogenically treated parts even if abrasion is not a factor. For example an automobile drag racer who used to break drive axles regularly experienced nearly no failures after treating his axles. This had little to do with abrasive wear or hardness and everything to do with resisting torque and tension. Improvement occurs throughout the part and is a permanent change to the metal.

Stabilization of Ferrous, Non-Ferrous and Plastic Materials

    Along with the molecular movement described above, stress between molecules is eliminated as a matter of course. This benefit is realized in a wide variety of materials beyond hardened steels.
You can't afford to have a "splitting headache" here...     Residual Stress typically occurs when materials are formed, cut or after heating. The level varies according to how much force is put into the work and the condition of the work to start with. Often a part needs to be stress relieved more than once during its manufacturing process. Considerable residual stress occurs after work is cooled from an elevated temperature. This is especially true if cooling rates vary through the work, as in the case of welding or if castings or forgings are varied in thickness.
    Residual stress “rears its ugly head” two ways. When the work is subsequently machined, the relieved stress can cause the work to move, making it difficult to hold close finish tolerances. Even a finished part containing residual stress can move over time in service, especially if temperatures fluctuate as in the case of engine blocks and heads. Parts that must remain dimensionally stable for long periods must be stress free when placed into service.
    Any material containing residual stress is a candidate for CryoTune®. Hardened or unhardened steel bars, sheet and plate, forgings, iron castings and weldments all contain harmful residual stress. Aluminums in all shapes - hardened or not, copper, brass, other non-ferrous products to include exotic alloys can all be stabilized with cryogenics. Nylon and other plastics also respond well.

Double Benefit?

Hardened metals that are treated for additional wear-resistance also become stabilized. Unhardened metals and non-ferrous alloys can be stabilized but may not show increased resistance to wear.

CryoTune® - A Gentle Process!

    The most exciting thing about CryoTune® is that the process is very gentle. In the case of stabilization, it is the only real option for parts that cannot take a conventional thermal stress relieving treatment. This can include assemblies, parts that can move undesirably at elevated temperatures, packaged or coated parts and those that must maintain a certain cosmetic appearance. Light bulbs, nylon panty hose, printed circuit boards and electric motors have all been treated successfully.

    No changes in dimensions have been noted after CryoTune® treatment. This makes is excellent for aftermarket applications.

    For additional details about pricing, lead times, and how it can complete a total metallurgical solution, contact
Edwards Heat Treating.

Sexy curves!

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