Abstract
Cryogenic temperatures
are defined by the Cryogenic Society of America as being temperatures below
1200K (-2440F, -1530C).
Durability is the most important criterion used to
define the quality of a tool steel. Cryogenic treatment and tempering of metals
has been ac- knowledge for almost thirty years as an effective method for
increasing durability, or "wear life" and decreasing residual stress
in tool steels. Deep cryogenics (below -300°F) is creating many new
applications in science. High temperature superconductors, the super-conducting
super collider, cryo-biology, magneto-hydrodynamic drive systems for ships, and
low temperature physics have all developed recently. The deep cryogenic
treatment and tempering process for metals is economical. It is a one time
permanent treatment, affecting the entire part, not just the surface. The
treatment may be applied to new or used tools, sharp or dull, and reshaping
will not destroy the imparted properties. Benefits achieved from subjecting
tools to this treatment include: increases in tensile strength, toughness, and
stability through the release of internal stresses.
Cryogenic Treatment for Improved Properties
A research
metallurgist at the National Bureau of Standards in Boulder Colorado,
states, "When carbon precipitates form, the internal stress in the
martensite is reduced, which minimizes the susceptibility to micro cracking.
The wide distribution of very hard, fine carbides from deep cryogenic treatment
also increases wear resistance." The study concludes: "...fine carbon
carbides and resultant tight lattice structures are precipitated from cryogenic
treatment. These particles are responsible for the exceptional wear
characteristics imparted by the process, due to a denser molecular structure
and resulting larger surface area of contact, reducing friction, heat and
wear." There have been skeptics of the cryogenic process for some time,
because it imparts no apparent visible changes to the metal. Since proper heat
treating can transform 85% of the retained austenite to martensite and the deep
cryogenic process only transforms an additional 8 to 15%, the deep cryogenic
treatment has been considered an inefficient process. While these percentages
are correct, the conclusion drawn from them is inaccurate. In addition to the trans-
formation to martensite, the subjected metals also develop a more uniform,
refined microstructure with greater density. Although known to exist, this type
of microstructure was only recently quantified scientifically. Particles known
as "binders" are coupled with the precipitation of the additional micro
fine carbide "fillers". The fillers take up the remaining space in
the micro-voids, resulting in a much denser, coherent structure of the tool
steel. These particles are identified and counted in the above study cited,
using a scanning electron microscope with field particle quanti- fiction (an
automatic particle counter). It is now believed that these particles are
largely responsible for the great gains in wear resistivity. The permanent
irreversible molecular change created is uniform throughout the tool, unlike
coatings, and will last the life of the tool, regardless of any subsequent
finishing operations or regrinds.
Fig. 1 shows two
photomicrographs (1000 x) representative of samples from the same S-7 bar
stock. The first is untreated S-7. The second was deep cryogenically treated.
Both samples initially were conventionally heat treated; that is, austenitized
and oil quenched. The deep cryogenic treatment consisted of varying ramp with
pause at -150°F for 1 hr, at -270°F for 2 hr and soaking for 8 hr at -310°F,
followed by tempering at 300°F for 1 hr, AC to room temperature and tempering
at 225°F, AC. In this micro- structure, note the considerably greater number of
fine particles coupled with fine carbides in comparison with the untreated
sample. The martensitic transformation is readily apparent.
Deep
Cryogenic Treatment Potential
The cryogenic cycle is
an extension of standard heat-treatment, and creates many outstanding increases
in durability. Some examples are as follows. A major aircraft manufacturer
testing deep cryogenic treatment found that with only six different tools
treated, the savings in tool purchases could exceed $5 million. An Arizona State study conducted by Laurel Hunt,
used deep treated C-2 debarring tools on INCONEL alloy 718, achieving a 400%
improvement based on weight, after five cats of .003 in. (.007 cm) on this
alloy. This deep cryogenic treatment of an 8% cobalt end mill has made dramatic
improvements in two important ways. The number of milling cats was increased
from three before deep cryogenic processing, to 78 cats after processing (26
times the wear life). Resharpening the end mills after deep cryogenic treatment
required only 1/3 the amount of stock removal to restore the tool geometry.
Rockwell, a major aircraft manufacturer, using C-2 carbide inserts to mill
epoxy graphite, doubles their output after deep cryogenic treatment of the
inserts. In a second test, a 400% improvement was achieved upon milling 4340
stainless steel with cryogenic treated tool. Other applications include:
Leading national stock car drivers who previously raced only 4-8 races between
equipment teardowns, drove in 40+ races before teardown after cryogenically
treating block, crank, cam, pistons and heads.
Confirmation
Of Lab Results For Field Tests (Shallow Cryogenic Cycles)
The latest research
data on cryogenic and tempering cycle confirms the long standing theory that
cryogenic treatment significantly enhances cutting tool life. Dr. Loan
Alexandra and Dr. Constantin Picos of the Polytechnic Institute of Jassy, Romania,
utilized the latest scientific equipment available, a JEOL IXA-5A Electron
Probe, a DRON-1 X-ray Diffractometer, a Qaantimet 720 Quantitative Microscope,
and a Chevenard Differential Dilatometer to supply the following results from
the extensive study. The study involved 7 samples (A- N, Fig. 2) each subjected
to a different heat/cool cycle as noted. Each sample was the equivalent of M2
steel. The carbide particles were physically counted, both before and after the
deep cryogenic treatment.
The team then measured the
samples with the equipment above, and with standard metallurgical evaluative
testing. The results confirm with tangible evidence the carbon participation in
cryogenic processing
Fig.
2 Standard heat treating, austenitizing, oil quenching and tempering, compared
to cycles with added cryogenic (-70°C) and tempering cycles. (Source :- Jassy
polytechnic institute / Alexandraue ).
Fig.
3 Cryogenic and tempering cycle doubles durability, decreasing austenite while
doubling micro fine carbides. (Source :- Jassy polytechnic institute /
Alexandraue )
The results of the testing,
Fig. 3, comparing standard heat treating to heat treating with the addition of
a shallow cryogenic soak (-70°C) are summarized as follows: austenite de-
creased from 42.6% to 0.9%; martensite increased from 66% to 81.7%; car- bides
increased from 6.9% to 17.4%; mean number of carbides counted @ 1mm sq
increased from 31,358.17 to 83,529.73; number of carbides less than 1 µm
increased from 23,410.24 to 69,646.09; Rockwell increased from 60.10 to 66.10;
tensile strength in- creased from 86.0 to 244.46; bending tensile rate
increased from 0.65 to 1.85; KCU (resiliency) increased from .0668 to 1.18; HRC
after 20 minutes hold at 675°C: 56.88 to 62.25.
Durability in terms of
length of cutting time increased from 20 minutes to 45 minutes with a
shallow cryogenic cycle. Fig. 2 illustrates the seven separate heat/cool
cycles used to temper the lathe cutting tools. The tools were then used to cut
0.5% structural carbon steel (see Table I). Durability was established by
measuring the radical component of wear.
Table :1 Parameters for
Lathe Cutting Tools in Wear Resistant Test
Intensive Speed
|
33.6 m/min
|
Depth
|
5 mm
|
Feed
|
0.62 mm per rev
|
Relief Angle
|
8 Deg
|
Hack Angle
|
5 Deg
|
Plan
|
45 Deg
|
Deep Cryogenic Cycle vs Shallow Cryogenic Cycle
Separate
laboratory testing has been accomplished by Dr. Randall F. Barron at Louisiana Tech University.
The results by Dr. Barron more than substantiated the Jassy study. In one
series of tests compared were five common steel alloys (see Fig. 4). First they
were wear tested as pro- cured, then as chilled to -120°F and finally tested
after treating at -317°F. In all cases the cold treatment improved wear
resistance; the colder the treatment, the more favorable the results. The
-120°F (dry ice) treatment improved ratios ranging from 1.2 to 2 times
depending on the alloy. This is consistent with the Jassy findings. However, the
deep cryogenic treatment in liquid nitrogen at -317°F soak improved wear
resistance by even greater ratios running from 2 to 6.6 times.
Process Advancement through New Equipment and
Computerization
The deep cryogenic
process has had an Achilles heel. It has been inconsistent. In the past,
improvements to cutting tools would vary from little improvement to over 1000%
increased in useful life. The key to effective improvements consistently is
proper processing. If a cutting tool is dropped in liquid nitrogen, without
tem- perature control, the tool could shatter. Metals require specific cooling
rates; temperature changes must be controlled exactly to obtain the optimal
cooling curve. The computer processor solves the problem, since it al- lows
exact duplication of the optimal cooling curve, repeatedly. The older cryogenic
tanks did not have adequate controls. A relatively new cryogenic system (model
2953, lead illustration) achieves consistent results. The new cryogenic
machines operate with controlled dry thermal treatment. "Controlled"
simply means that the process is performed according to a precise prescribed
time table. A process controller (Yokogawa UP 25) operates the descent; soak
and ascent modes (see Fig. 5). Generally, the material is cooled slowly to
-317°F, held for 20-60 hr then raised to +300°F, and slowly returned to room
temperature. The machine switches over to the electrical resistance mode for
the tempering operation. The "dry" process prevents the metals from
being subjected to liquid nitrogen, and eliminates the placing of an item in
the freezer and pushing a button. A breakthrough in system insulation has been
achieved as a result of the space program. The system is de- signed to
accomplish thermal transfer, and the more efficient the better. It is essential
to transfer themes from the liquid nitrogen to the metal parts being treated,
without losing the therms to the outside. Thus, the vacuum chamber is designed
for three- level insulation. Walls of the chamber are 3 in. stainless steel. On
the inside wall are 125 wrapped layers of aluminized polyester film. Inside the
chamber it self is 2 in. polyisocyanurate high density foam which is coated
with Ceramaseal amorphous vapor barrier comprised of micro spherical ceramic
globes. A 93% increase in processing ability is accomplished with the
"space shuttle insulation" in the vacuum walls, providing
considerable savings in processing and making the treatment economical
for a variety of items in addition to tooling.
Industrial
Practice and Advantages for Heat Treaters
Potentially every tool heat treated is a candidate for
the additional ser- vice of cryogenic treatment and tempering. It is economical
to provide the additional improvement of any perishable item. There are more
than a handful of large tooling manufacturers utilizing the process today for
manufacturing a premium line of cutting tools. More than 200 heat treaters
provide cold cryogenic services. However, 95% of these are only utilizing
-120°F (dry ice) treatments. Only a handful of companies provide cryogenic
treatment below -300°F, which results in much more impressive results and
accompanying benefits. A small Massachusetts
firm has been processing items for 12 years. The strings on a piano which was
previously tuned every six months were treated. The piano has not been tuned
for five years. Musicians who play guitar and violin firmly believe that the
strings are brighter in sound. Oscilloscopes confirm a shift after treatment. A
firm in Michigan
has been processing with the method for 27 years. They also specialize in
stress relief of the plastic material used in contact lenses, among other
items. A cryogenic treating company in Phoenix
treats many aerospace parts. Another processor in Ohio treats many carbide tools. The
treatment is gaining acceptance nationwide. The process is used in Europe and Australia under the trade name
CryoTough, a BOC treatment.
CONCLUSION:
While not a
"Magic-Wand" which will extend the life of everything, over 100 tools
such as reamers, taps, dies, broaches, drills, end mills, slicers and cutting
knives do respond consistently to this process. Cryogenic ser- vice can create
a "premium" more profitable tool line for a manufacturer. It is also
saving considerable tool expense for the end user. The process is effective
throughout the tool un- like a coating, so tools can be resharpened and retain
the benefits of the treatment until completely worn out. The process also works
with Tin coatings.
When a specific tool
receives wear extension, there is a 95%
certainty similar tools will respond consistently in the future to the same
exact cycle. Among the properties which define the cutting qualities of a tool
steel, durability is the highest importance. Results in this regard are
decisive in establishing the benefits of cryogenic treatment and also answer
the decades long question, "what happens when parts are tested in this
manner?"
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