The purpose of cryogenic treatment is to decompose any residual austenite (retained austenite). Transformation of the retained austenite to martensite results in dimensional stability to the heat-treated piece.
But what creates retained austenite? Retained austenite remains untransformed when the quench procedure (from the austenite region) is either too slow or it is quenched from too high an austenitizing temperature. Alloying can also create a very low transformation temperature, and quenching stresses can prevent further transformation.
When quenching a piece of steel (alloy steel or tool steel), we are limited to what is pictured in Figure 1.
Oil can be modified slightly simply by the addition of blended accelerators to speed up or slow down the cooling rate of the quench medium.
The most accurate method of cooling (but the most expensive) is the use of blended gases using: nitrogen + helium or nitrogen + hydrogen. The mixed gases are blended based on specific quenching requirements.
If we now consider the Time-Temperature-Transformation diagram for a particular steel, we can see the effects. If quenched from too high an austenitizing temperature or too slowly, the steel will begin to cool into the regions that are to the right of the critical cooling curve. This will have the tendency to create the retained austenite or untransformed austenite.
Untransformed austenite will progressively (over time) become untempered martensite. If we now consider the molecular size of the austenite molecule in relation to the martensite molecule, it will be seen that they are of different sizes. This means that two things will occur: an increase in volumetric size an an increase in the hardness value. Both occurrences can have catastrophic effects on the hardened steel, including cracking or the steel could become too hard for its application.
The continuation of our discussion on cryogenic treatment will continue with part 2.