Cold Hardening

You may have heard a lot of different opinions on cryo-treatments. With all various methods discussed on the topic I did a little research and after much checking about and reading lots of boring, long, and extremely technical (almost to the point of needing a doctorate in the subject) I had almost given up hope of finding a paper (with a simple cause and effect correlation).

Sub-zeroTreatment_final.pdf

This is what I found, it has clear concise information that is presented in table form that can be easily applied, by even the most inexperienced blade maker. With a clear knowledge of what Sub-zero treatment dose to different types of carbon (and alloy) steel, and how it effects wear resistance, carbide formation, and toughness.

Some cliff notes from the paper:

Heat treatment of metals has transformed over centuries from
black art to science. Through worldwide research, metallurgists
have developed phase diagrams, continuous transformation diagrams,
process/property correlations, and heat treatment cycles.
Metallurgists understand how and why an alloy responds to a
heat-treatment cycle, but they also understand that changing any
processing variable will influence the final properties.
Researchers have only recently begun to study sub-zero cooling
cycles. For many years, sub-zero treatment of metals had the
reputation of being a quick fix for poor heat treatment practice.
Sub-zero processing technology has not been widely adopted by
the metals industry due to a lack of understanding of the fundamental
metallurgical mechanisms and due to the wide variation
in reported research findings. Early unsubstantiated claims that
cryogenic treatments can solve every problem from bad golf
balls to holes in woman’s hosiery have further diminished the
interest in exploring cryogenics as a credible process. Recently
however, researchers have conducted significant research into
the fundamental understanding of the mechanisms governing
sub-zero processing.Dr. Randall Barron at the University of Louisiana
was one of the pioneers to conduct significant research
into sub-zero processing. His research shows that sub-zero
processing of steels can improve various mechanical properties
including:
Hardness and strength
Wear resistance
Dimensional stability
As an earlier proponent of sub-zero processing, the paper and
corrugated board industry has achieved significant improvements
in wear resistance for different types of cutting tools.
Cutting knives, chipper knives, trimmers, bracket trimmers,
guillotine blades, slitters, score cutters, and envelope dies have
increased their useful life by factors of 2 to 5 [3–8]. Table 1
shows the average useful life of particular tooling pieces with
and without the benefit of sub-zero treatment. A parameter
called Wear Ratio, defined as the ratio of life after sub-zero treatment/
average tool life without sub-zero treatment, gives a measure
of the amount of improvement this process can impart
when applied correctly. Differences in wear life, shown in Table
2, between parts cold treated at about –80°C (–110 °F), and parts
cryogenically treated at –190°C (–310 °F) using liquid nitrogen,
raised questions about the causes of the improved wear resistance.
However, the overall results from these studies could not
be disputed and further research has been conducted to gain a
better understanding of the underlying mechanisms.
...
Numerous factors impact how sub-zero treatments affect an alloy.
Processing factors like time, temperature profile, number of
repetitions and tempering practice, in conjunction with material
parameters such as prior heat treatment and alloy composition
will alter the final results.
Cold treatment of higher-alloy and carburized steels is an additional
step in the heat treatment hardening process. The cold
treatment process occurs in the temperature range of –70 to
–120 °C (–90 to –190 °F) and completes the transformation of the
steel microstructure from austenite to the stronger and harder
martensitic structure. The hardness of a steel increases with
increased percentage of martensite in the structure. Thereby
wear resistance increases as it correlates positively with
hardness (wear resistance in addition depends on the presence
of carbides as further discussed in section 2.3). A specific percentage
of retained austenite may be desired for applications
such as bearings or gears where the metal may require some
toughness to absorb impact or torsion loading.
...
“In addition to the well-known effect of transforming retained
austenite to martensite, with the consequent increase in
hardness, deep cryogenic treatment, or cryotreatment, has an
effect on martensite. It causes crystallographic and microstructural
changes which, on reheating, result in the precipitation of
a finer distribution of carbides in the tempered microstructure,
with consequent increases in both toughness and wear resistance.”