The continuation of cryogenic
treatments is to ask why do we cryogenically treat steels, and how do we
identify those steels that will need to be cryogenically treated?
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.
Cryogenic Treatment (part 2)
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 pye_d@ymail.com.
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