Relieving built up and residual stresses with Deep Cryogenic Treatment

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Most engineers in the new millennium will agree that enough cannot be said and done about stress relief. However with the usage of strictly heat concepts, companies may be falling short on a complete relief of residual stresses. The fact that relieving stresses built up from heat treatment, manufacturing and machining, can be done so with a cryogenic stress relief is often overlooked. Thoroughly removing residual stresses from aluminum, copper, brass, and plastics is most often mistaken as: not an option. Removing stress with cold instead of heat is a thorough and complete relief of stress and not just a surface treatment.

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With all the exhibited types of residual stresses built up in a material there is a great need to alleviate that stress without redistribution or build up of more stress.

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Stresses and stress relief

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Brief description of stress:

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What takes place during the manufacturing, production, and machining phases is this: on a molecular level materials are subject to strenuous events which cause molecular displacement or gaps in the matrix during solidification stages. These gaps are out of place molecules which are not working in uniform of one another, but are displaced and can even be working against each other. This causes a lack of strength and toughness, and places a great amount of stress on the newly formed object.

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For a complete scientific definition on stress, refer to this engineering page.
 

Conventional heat treatment and tempering:

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Heat treated parts go through extreme amounts of stress inducing phases. Consider taking a bar of steel that’s been heated to 800°C for an extended period - so the temperature is even throughout the metal - then plunging that steel into water that is 20°C. The rapid cooling reduces the surface temperature almost instantaneously while the interior of the structure is still red hot. Although the heat does dissipate, it does not dissipate uniformly. Therefore the radical cooling of the outer and inner core in a non uniform way causes compressive stresses, which need to be stabilized.

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The part now needs to be tempered to stabilize the newly formed martensite. If this is not completed, the inherent stresses are so built up they can cause the part to crack or literally explode. Because such high temperatures are needed to relieve the stress, more austenite grain structure is transformed into martensite, creating yet another stress layer.

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The best and most complete way to remove built up stresses from heat treating is through the use of cryogenic temperatures. A properly done Deep Cryogenic Treatment will ensure the correct amount of stress relief. However, if not done properly (assisted warming after cryogenic soak), then excessive residual stresses will be reintroduced back into the material.

The use of a cryogenic stress relief will not redefine the hardness value or change the color of the metal in any way, as it will with heat tempering. It is completely non destructive to the carbon and free of oxides.

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The use of a cryogenic stress relief will not redefine the hardness value or change the color of the metal in any way, as it will with heat tempering. It is completely non destructive to the carbon and free of oxides.
 

Stress relief using cryogenic technology:

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A cryogenic process simply allows the molecules to redistribute in a uniform like fashion, albeit very slowly. This releases any stored up energy that may be trapped in the object. This in turn increases strength and toughness within the object as it relaxes back to its proper positioning. The molecules are now working in an orderly, uniform fashion with one another.  There are also changes happening on the surface of the material; the gaps along the surface are reduced and the peak and valley composition of the surface is reduced as well.

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Deep Cryogenic Treatment done in a controlled environment, such as a cryotron cryogenic processor, is a very viable stress relief and can be used where heat cannot. It can be used prior to, during or after machining of materials. It is a thorough stress relief and not just an exterior stress relief as is the case with shot and laser peening. The cryogenic process, as in the case with heat, must be strict and deliberate with slow changes in temperatures occurring. The warm up process must be just as absolute.
 

Aluminum, Brass, Copper, plastic and composite materials in castings, forgings, etc...

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can all be stress relieved by the above method of a cryogenic treatment providing it is done properly and slow and deliberately enough to allow for molecular realignment and not redistribution of, or creating of, more stresses by rapid temperature fluctuations.

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Facts about stress:

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There are two different types of stress: textural and body stress:

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Textural stress -  arises because of microscopic inhomogeneities in a metal

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Body stress – stress created or produced by an external load. This can now be broken into two categories:

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Contingent body stress  – dissipates when the load is removed
 

Residual stress – remains after load is removed, clearly seen as the retention of stress from inhomogeneous plastic deformation

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It is impossible to cold work a steel without creating stresses on the metal.

 

Most common major residual stress cause is machining or altering the parts by any form of metal removal.

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Grinding can produce temperatures in excess of 1650°C. This produces great amounts of surface stress.