This is the final part nf the subject of cryogenic treatments (known also as deep freeze or cold treatment). The last method of treatment is concerned with the use of liquid nitrogen, which can reduce the temperature of the steel being treated to as low as -195°C (-320°F), depending on where the steel is placed. This means either in the gaseous nitrogen or the liquid nitrogen.
The liquid nitrogen can be stored in an external bulk storage tank, or it can even be stored in what are known as “dewars.” The receptacle can be a specially designed freezer box that is designed and insulated for such low-temperature applications or the simple insulated box, described in the previous articles on this subject.
Whichever freezing method is used, it is most important that the immediate area of the operation of the unit is well ventilated and that the appropriate safety clothing, which must be worn when handling either the liquid nitrogen or the frozen dry ice (CO2), is present.
Once again, the main purpose of the cold treatment is to decompose any retained austenite that may be present in the heat-treated steel. Remember that the volume of any retained austenite is much different than that of fresh martensite.
It is because of the decomposition of any retained austenite to fresh martensite that dimensional stability will be accomplished. One MUST also remember that because of the decomposition of the retained austenite to fresh martensite that the fresh martensite WILL need tempering.
The selected tempering temperature can be anywhere between 150°C (300°F) up to the appropriate tempering temperature that is required for the steel being treated.
How long do we soak at the low temperature in order to decompose the retained austenite? First, that will depend on the maximum cross section of the part and its mass and volume. Freezing times can be as long as 15-20 hours at low temperature for large cross sections using liquid nitrogen and about 25% longer for using the dry-ice technique.
Cryogenically treating steel is not a new subject. The writer has used the technique for more than 50 years. The technology has also found favor with gunsmiths and weapons manufacturers to produce a weapon that is dimensionally stable and free of retained austenite.
There are many uses for cryogenic technology, but its main use is that of the reduction/decomposition of retained austenite. Retained austenite is caused by quenching from too high an austenitizing temperature or quenching too slowly. What it really means is that one should study the Time-Temperature-Transformation diagram (or the Isothermal Transformation diagram) in order to establish what the critical cooling rate is for the steel being heat treated.