In this column, we will discuss the application of cryogenically treating gears and the outcomes expected.
There has been much discussion regarding the application of cryogenics to improve the properties of steel. I will try to illustrate the different types of cryogenic treatments of steel and discuss the ramifications on steel properties.
Interest in cryogenics has grown tremendously in the past 30 years and has been applied to many different types of components, such as parts for motor racing  , gears and bearings  , oil drills , knives, surgical instruments  and even brass musical instruments . The focus of this article will be on steel.
There are two different basic types of cryogenic treatments . First, there is the Shallow Cryogenic Treatment (SCT) where parts are placed in a freezer at -80°C, and then warmed to room temperature. Secondly, there is Deep Cryogenic Treatment (DCT) where parts are slowly cooled to -196°C, held for many hours, then gradually warmed to room temperature. Either treatment is applied after quenching, but before tempering. Each material should be tested at different temperatures (-185°C, -130°C, and -80°C) to determine the optimal temperature. This can be done rapidly using calorimetry or acoustic emission.
The cooling time or rate is usually restricted to prevent thermal shock or cracking. Usually, a rate of 0.3°C/min to 1.2°C/min is adequate. Hold times of 24 hours are generally sufficient, and times over 36 hours do not yield significant improvements. The rate at which parts are warmed is usually not critical or controlled. A Design of Experiments (DOE)  on 18 percent martensitic stainless steel used for piston rings showed that the largest factor was the holding temperature, the time of holding at temperature, followed by the cooling rate. The heating rate showed little effect.
Effect on Properties – Ferrous Alloys
Much of the literature regarding cryogenically treated steels discusses the improvement in mechanical and wear properties in terms of three different phenomena:
- Transformation of retained austenite into martensite.
- Creation of finely dispersed eta-carbide precipitation.
- Removal of residual stresses.
The martensite start (Ms) and martensite finish temperatures are readily calculated. Remember, from previous columns, that the martensite start temperature can be estimated from the equation :
The martensite finish temperature can be estimated from the equation :
SCT was originally developed to completely transform any retained austenite to martensite. Rarely are temperatures much below -180 degrees Celsius necessary for most common alloy and tool steels to completely transform retained austenite to martensite. One additional benefit of SCT on steels is the dimensional stability during tempering, due to retained austenite being converted to martensite.
There have been many reports of DCT steels precipitating fine carbides    , and the complete conversion of retained austenite to martensite. Finely dispersed carbides have been observed in cryo-treated steels. The mechanism is presently unknown.
The reduction in residual stress is associated with the reduction in retained austenite. This is particularly true with carburized steels, as the surface compressive residual stress would increase (more compressive) as more retained austenite is converted to martensite. This increased compressive residual stress has the benefit of increasing the fatigue life of the carburized component.
Benefits to Mechanical Properties
An extensive review of the benefits of cryogenic processing on steel components is provided by Baldissera and Delprete . A significant increase in the hardness and wear resistance of tool steels was observed by treating at -85°C and -196°C. Stainless steels and carbon steels showed less than a 10 percent improvement. Both retained austenite conversion to martensite and carbide precipitation can improve the wear resistance by increasing hardness.
The increase in hardness is confounded with the formation of fine carbides. This results in improvements to wear resistance due to higher hardness, but it is difficult to determine if this was due to fine precipitates or increased hardness.
Cryogenic processing of steels is an important tool to improve wear and fatigue. For applications to improve wear of tool steels, the application of cryogenics is a significant improvement, reducing wear. This is attributed to both reduction in retained austenite, and the precipitation of finely dispersed carbides. For carburized gears, there is a large benefit in the creation of greater residual compressive stresses at the surface, improving the fatigue strength. Wear is also improved due to the conversion of retained austenite to martensite.
As always, should you have any comments regarding this column, or have suggestions for any columns in the future, please contact the editor or myself.
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