This is the final part on the subject of cryogenic treatments (known also as deep freeze or cold treatment).
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
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
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
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.
Cryogenic Treatment (part 4)
By David Pye
David Pye is the owner and operator of Pye Metallurgical International Consulting, Saint Anne's on Sea, Lancashire, U.K. He has 25 years of practical experience in captive and commercial heat treatment, metallurgical laboratory operation and industrial furnace sales. He also has teaching experience on a very wide range of heat-treatment and metallurgical subjects.
He can be reached at email@example.com.
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