The basic reason for cryogenically treating steels by freezing methods is principally to ensure both dimensional stability and to decompose retained austenite. (That is, austenite that remains as a direct result of either cooling too slowly or cooling from too high an austenitizing temperature).
If we have done everything correctly in terms of the appropriate cooling rate and from the appropriate austenitizing temperature, we should accomplish an almost complete transformation from austenite to martensite. This means that we should have transformed most or almost all of the austenite into untempered, fresh martensite (providing we have the appropriate amount of carbon present in the steel and cool from the appropriate temperature at the appropriate cooling rate).
However, if we check the as-quenched hardness and it is somewhat lower than the expected, there is a strong probability that not all of the austenite transformed to martensite when the steel was quenched. Hardness can be anywhere from 3 Rockwell C scale points low to 6 or 7 points low.
So what now? We have two choices to consider. The first choice is to anneal the steel and redo the heat-treatment procedure. The second option is to cryogenically treat the steel.
In other words, we are accelerating the change that will take place if we did not treat the steel after quenching. You will recall that any retained (residual) austenite will transform into fresh untempered martensite progressively over time. This means that if the retained austenite has not been dealt with, the steel will change both hardness and size, which has the potential to lead to a catastrophic failure.
So, the cryogenic treatment speeds up the decomposition of austenite. One needs to remember that once the cryogenic treatment has been undertaken, fresh martensite has been created. Martensite is the most unstable condition that the steel can be in. It is absolutely necessary to temper the steel after cryogenically treating it so that the metallurgical condition is tempered martensite. After the cryogenic treatment, the hardness value will increase if retained austenite was present.
Note: Before the tempering procedure is performed, allow the steel to warm to ambient temperature from the cold-treatment temperature naturally. Do not force it up to ambient temperature as this could also put the steel at risk for cracking.