Cold Cuts
By Pete Paulin
By Pete Paulin
Although its
effects on metal composition are subtle, deep cryogenic tempering can yield
dramatic improvements in tool performance.
In the search for cutting tool engineering that can increase productivity,
prolong cutting life, and decrease costs, gains of 15% to 20% are considered
significant. One recently developed tool treatment is showing far greater
promise, in some cases improving tool life by 200% to 400%. The method, called deep cryogenic tempering,
subjects tools placed in a specially constructed tank to temperatures below
-300° F for a number of hours using liquid nitrogen as the refrigerant. The
process supplements standard heat/quench tempering, completing metallurgical
changes that heat-treating begins. Since 1965, when commercial deep cryogenic
treatments first became available in the United States, a handful of studies
and reports have been released noting the improved performance of treated lathe
tools and of tool steels used in the steel industry. According to the
literature, some machine elements, such as progressive dies used in
metalworking, have lasted six times longer after deep cryogenic treatment.
Drills, end-mills, and taps also have shown significant improvement. Another advantage of deep cryogenic
tempering revealed through research is its ability to change the entire
structure of the tool material, not just its surface. As a result, the
treatment is not negated by subsequent finishing operations or regrinds.
Cool and Controlled
Test Results: Percent Increase in Wear
Resistance After Cryogenic Tempering
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Materials that showed significant
improvement |
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AISI# |
Description |
At
-110° F |
At
-310° F |
|
|
D-2 |
High
carbon/chromium die steel |
316% |
817% |
|
|
S-7 |
Silicon
tool steel |
241% |
503% |
|
|
52100 |
Standard
steel |
195% |
420% |
|
|
O-1 |
Oil
hardened cold work die steel |
221% |
418% |
|
|
A-10 |
Graphite
tool steel |
230% |
264% |
|
|
M-1 |
Molybdenum
high speed steel |
145% |
225% |
|
|
H-13 |
Chromium/moly
hot die steel |
164% |
209% |
|
|
M-2 |
Tungsten/moly
high speed steel |
117% |
203% |
|
|
T-1 |
Tungsten
high-speed tool steel |
141% |
176% |
|
|
CPM-10V |
Alloy
steel |
94% |
131% |
|
|
P-20 |
Mold
steel |
123% |
130% |
|
|
440 |
Martensitic
stainless |
128% |
121% |
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|
Materials that did not show significant
improvement |
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|
430 |
Ferritic
stainless |
116% |
119% |
|
|
303 |
Austenitic
stainless |
105% |
110% |
|
|
8620 |
Nickel-chromium-moly
alloy steel |
112% |
104% |
|
|
C1020 |
Carbon
steel |
97% |
98% |
|
|
AQS |
Graphitic
cast iron |
96% |
97% |
|
|
T-2 |
Tungsten
high speed-steel |
72% |
92% |
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For all its
advantages, however, deep cryogenic tempering is no panacea. In some cases it
has produced sterling results. One manufacturer of titanium alloy parts reports
that, after treating the M-42 twist drills it uses, the company needed 63%
fewer of the tools to do the same work. In another instance, a 400% improvement
in tool durability was achieved using cryogenically treated C-2 carbide inserts
to mill 4340 stainless steel. However, other metals, such as T-2 tungsten HSS,
have been left with little or no change after treatment. Even where deep
cryogenic treatment has been shown to be effective, results have not been
consistent. But as researchers and commercial applicators have learned more
about the deep cryogenic process, they have discovered ways to produce more
repeatable and predictable effects. The trick is in the precise management of
the cooling sequence. The older tanks did not adequately control temperatures
as they were brought to deep cryogenic levels.
Today’s state-of-the-art deep cryogenic systems use a computer linked to
the tank that duplicates the optimal cooling curve. It carefully regulates the
temperature change and brings a measure of consistency to the process. Under a
computer’s control, temperatures inside a cryogenic tank are brought down
according to a prescribed timetable. The process unit manufactured by 300°
Below Inc., Decatur, IL, cools the material slowly to -317° F, holds the temperature at that point for 10 to 40 hours,
then raises it to +300° F before slowly returning it to room temperature. It is
a dry process that, unlike other deep cryogenic processes, does not bathe the
materials in liquid nitrogen, a practice that the designers of 300° Below’s
unit believe is more likely to cause damage from thermal shock.
Chilly Changes
Slowly cooling a tool steel to deep
cryogenic temperatures and soaking it at this low temperature for several hours
changes the material’s microstructure.
Almost all of the austenite (a soft form of iron) retained in the steel
after heat-treating is transformed into a harder form, martensite, by deep
cryogenic tempering. A second result of
a deep cryogenic “soak” is the formation of fine carbide particles, called
binders, to complement the larger carbide particles present before cryogenic
treatment. (Depending on the alloying elements in the steel, these particles
might be chromium carbide, tungsten carbide, etc.) One recent study by Randall
Ban-on, Department of Mechanical Engineering, Louisiana Polytechnic Institute,
Ruston, LA, looked at how the changes brought about by cryogenic treatment
affected steel’s ability to resist abrasive wear. This type of wear occurs when a body penetrates and gouges a
material’s surface. The gouging body
may be a surface asperity on a mating part, a free abrasive grit particle from
an external source, or an internally generated wear particle. The Barron study
found that the martensite and fine carbide formed by deep cryogenic treatment
work together to reduce abrasive wear. The fine carbide particles support the
martensite matrix, making it less likely that lumps will be dug out of the
cutting tool material during a cutting operation and cause abrasion.
When a hard asperity or foreign particle is pressed onto the tool’s surface,
the carbides further resist wear by preventing the particle from plowing into
the surface. Some of these benefits
may be achieved through standard tempering, which also transforms austenite
into martensite. But standard tempering may not bring about a complete
transformation in some tool steels. For example, 8.5% of an O-l steel remains
austenite after it is oil quenched to 68° F. If M-1 is quenched from 2228° F to
212° F, then tempered at 1049° F, the retained austenite is 11%. Additional
improvements in tool performance can be achieved if this retained austenite can
be transformed to martensite. As
Barron’s study has confirmed, adding a cryogenic step to the treatment process
does just that. In the chart accompanying this article, data drawn from another
study of treated metals by Barron indicates which samples exhibited improved
abrasive wear after cryogenic treatment. In addition to results obtained from
samples treated at liquid-nitrogen temperature (-310° F), the chart also lists
results of treatment at dry ice temperature (-110° F).
Predicting Effectiveness
Knowing how deep cryogenic
tempering works, we can predict which materials will benefit most from
treatment. Generally, if an alloy contains austenite, and this austenite
responds in some degree to heat treatment, further improvements will be seen
after deep cryogenic tempering. For instance, ferritic and austenitic (430
and 303) stainless steels generally cannot be hardened by heat treatment. Martensitic (440) stainless steels, on the
other hand, can be hardened by heat treatment. Therefore, the effect of deep
cryogenic treatment should be more pronounced for 440 stainless steel than for
the other stainless steels. C-1020
carbon steel and QS Meehanite iron also show no significant improvements in performance
after cryogenic treatment. Because these materials contain no austenite, sub
zero temperatures can cause no further metallurgical changes in them.
Critics
In addition to examining cryogenic
treatment’s effects on austenite transformation, Barron also looked at other, less
obvious metallurgical changes. His data helps answer those critics of cryogenic
tempering who doubt the effectiveness of a process that imparts so few visible
changes to the metal. These critics say heat-treating already changes 85% of
the retained austenite to martensite, leaving only 8% to 15% to be transformed
by deep cryogenic tempering. Using the process to produce such small results is
inefficient, according to the skeptics.
These observations are accurate as far as they go. But they fail to take
into account the fact that metals subjected to deep cold develop a more
uniform, refined microstructure with greater density. The particles that are formed through the precipitation of
additional micro fine carbide fillers take up the remaining space in the
microvoids, resulting in a tool steel with a much denser, coherent structure
that improves wear resistance.
Metallurgists have known that cryogenic tempering has this effect, but
they may not have realized the extent to which the
microstructure changes. Researchers in a study conducted at the Jassy
Polytechnical Institute in Rumania used a scanning electron microscope equipped
with an automatic particle counter to identify and quantify these smaller
particles. Through this examination
they found that cryogenic tempering creates a significant change in density
throughout the tool. Some people acknowledge the benefits of cold tempering,
but they question the need to use temperatures below -110° F. A review of the
data from the Barron study, however, shows that some tools, at least, perform
significantly better after processing at -310° F than they performed after
-110° F tempering. Among cutting tool steels, for instance, the wear resistance
of M-l HSS was almost one-and-a-half times the wear resistance of the untreated
material after a -110°F soak. After a -310° F soak, the steel exhibited
two-and-a-quarter times the wear resistance of the untreated material. T-l, the
traditional tungsten HSS, went from a little under one-and-half times the wear
resistance after a -110° F soak to one-and-three-quarters the wear resistance
after a -310° F treatment. It may be concluded from these tests that treatment
at -110° F does improve wear resistance, but deep cryogenic treatment at -310°
F improves wear resistance much more. To allow time for the very fine carbide
particles to form and the retained austenite to be transformed into martensite,
a long soak at deep cryogenic temperatures is necessary.
Summary
Deep
cryogenic treatment has been shown to result in significant increases in the
wear resistance of steels such as D-2, M-2, O-1, and 52100. The basic
mechanisms at work during the cryogenic process help control wear by producing
a tough surface, which helps to prevent particles from tearing out of the
material and resist penetration of the surface by other particles.