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Cryo Treatments are interesting and IMHO they should be considered with a great deal of cynicism.
There is no doubt that many tool steels, particularly those used for the manufacture of high precision parts benefit from this type of treatment as it generally assist with dimensional stability.
The main benefit of this process is for the treatment of steels that suffer with problems of retained Austenite following heat treatment.
Retained Austenite is a metastable phase and there is a thermodynamic driving force which wants to allow its transformation to a Martensitic type of structure. The activation energy to allow this transformation can be supplied from either a mechanical force or can be thermally activated either by low temperatures of increased temperatures.
This transformation involves a small expansion in the volume of the component and this can lead to distortion and subtle changes in shape.
When considering close fitting parts or transducer materials for the manufacture of load cells these very small variations in size can cause problems and it has been common practice to cryogenically treat parts prior to final grinding.
Once the steel has been cooled to typically Liquid N2 temperatures full transformation will occur and the parts will be stable.
The type of material used for the manufacture of con rod, which would typically be a 4340 type steel and normally these steels are fairly stable due to the influence of carbon and tempering effects which make the retained Austenite relatively stable and immune to the effects of mechanical stresses. The volume of retained Austenite in 4340 type steels is typically in the region of 2% and tempering above 600 degF will cause it fully transform.
In terms of metallurgy there is no benefit to be obtained in cryo treating this type of steel in terms of either mechanical properties or dimensional stability.
There has recently been some evidence to suggest that the cryo treatment of chrome-silicon wire springs can result in an improvement in fatigue performance compared to an untreated spring and this is due to the development of compressive residual stresses at the surface of the wire.
The basic measurements of the residual stresses suggest that this influence is less than would be expected in a shot-peened spring but no comparative data has been produced.
Frankly I wouldn't bother but it will do no harm and if you are going to cryo treat them do it first.
I carried out a significant amount of work on the fatigue behaviour of shot peened gears more years ago than I like to admit and presented a paper on this subject in Amsterdam and have followed this subject with interest for many years.
Some work more recently published in Sweden on behalf of Scania reported the following observations:
In order to find out how the fatigue limit is affected by the material
hardness, a number of gears were tempered whereas other remained untempered.
Similarly the retained austenite content was altered in some of the gears with a
cryotreatment.
The shot peening of these gears was performed by dual shot peening (200% Coverage)
The results show that the fatigue limit is enhanced when the shot peening was
performed with hard media.
The amount of retained austenite does not seem to affect the fatigue limit for samples shot peened with this hard media.
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