Gears being treated cryogenically makes them more durable and greatly
increases the life of the gear.
Frozen Gears
Gear Technology, The Journal of Gear
Manufacturing, March/April, 1993
Durability is the most important criterion used to define the quality of a gear. The freezing of metals has been acknowledged for almost thirty years as an effective method for increasing durability, or "wear life," and decreasing residual stress in tool steels. The recent field of deep cryogenics (below -300°.F) has brought us high temperature superconductors, the superconducting super collider, cryo-biology, and magneto-hydrodynamic drive systems. It has also brought many additional durability benefits to metals.
The deep cryogenic tempering process for gears is an inexpensive, one-time, permanent treatment, affecting the entire part, not just the surface. Gears may be new or used, sharp or dull, and reshaping will not destroy the treatment.The process has a number of obvious benefits, including increases in tensile strength, toughness, and stability through the release of internal stresses. The exceptional increase in wear resistivity, generally exceeding 200%, is the greatest benefit.
Steel surfaces receiving wear, such as gears, sharper cutters, drill bits, end mills, taps, dies, surgical scissors, bearings, racing engines, slicers, and granulator knives, all benefit from this inexpensive treatment. New applications are being discovered regularly.
Completing the Heat Treating Process
Martensitic Transformation
A research metallurgist at the National Bureau of Standards states, "When carbon precipitates form, the internal stress in the martensite is reduced, which minimizes the susceptibility to microcracking. 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. "Metallurgists have been skeptical of the cryogenic process for some time, because it imparts no apparent visible changes to the metal. The thinking is that since proper heat treating changes 85% of the retained austenite to martensite, and the deep cryogenic process only transforms an additional 8 - 15%, deep cryogenic treatment is an inefficient process. These are correct premises, but an inaccurate conclusion . Deep cryogenically cooled metals also develop a more uniform, refined microstructure with greater density. Microfine carbide "fillers" are formed, which take up the remaining space in the micro-voids, resulting in a much denser, coherent structure of the tool steel. The end result is increased wear resistance.
These particles are the same ones identified and count in the accompanying study using a scanning electron microscope with a field particle quantification. (An automatic particle counter.) It is now believed that these particles are largely responsible for the great gains in wear resistivity. Unlike the case of coatings, the change created is uniform throughout and will last the life of the tool, regardless of any subsequent finishing operations or regrinds. It is a permanent, irreversible molecular change.
The two 100X micrographs shown in Fig. 1 represent samples from the same S-7. The second was deep cryogenically treated. The martenisitc transformation is readily apparent.
Field Testing Proves
Deep Cryogenic Potential
The cryogenic cycle is an extension of standard heat treatment, and creates many outstanding increases in durability. For example, a major aircraft manufacturer testing deep cryogenics found that with only six different tools treated, the savings in tool purchases could exceed $5 million.
The deep cryogenics treatment of an 8% cobalt end mill has demonstrated dramatic improvements in two important ways. The number of milling cuts was increased from three before deep cryogenics processing to 78 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 running C-2 carbide inserts used to mill epoxy graphite, doubled its output after deep cryogenic treatment. In a second test used to mill 4340 stainless steel, it achieved a 400% improvement.
Other applications include leading national stock car auto racers, who previously raced only 4-8 races between teardowns, went 40+ races after treating the block, crank, cam, pistons, and heads.
Lab Results Confirm the Field Testing
The latest research data on cryogenic tempering confirms the long standing theory that cryogenic treatment significantly enhances cutting tool life. Dr. Joan Alexzndru and Dr. Constantine
Picos of The Polytechnic Institute of Jassy, Romainia, utilized the latest scientific equipment available, a JEOL IXA-5A Electron Probe, a Diffractometer, a Quantimet 720 Quantitative Microscope, and a Chevenard Differential Dilatometer to supply the following results from the extensive study.
The study involved 7 samples (A-N in Fig. 2), each subjected to a different tempering cycle as noted. Each sample was the equivalent of M2 steel; each sample had carbide particles physically counted, both before and after the deep cryogenic treatment. The team then measured the samples with the equipment above, and with standard metallurgical evaluation testing. The results confirm with tangible evidence the carbide precipitation in cryogenic processing.
All the metal samples were taken from identical batch stock. The sample structure was comprised of .83%C, .38%Mn, .3%S, 4.1%Cr, 5.1%Mo, 1.9%V, and 6.3%W. Samples were all simultaneously standard heat treated at 1230° C, then oil-quenched. Four of the pieces were then subjected to the cryogenic cycle at -70°
C with varying tempers added after cold soaking.
Findings
The results of the testing conclude with the following findings and analysis comparing standard heat treating to heat treating with the addition of shallow cryogenic soak:
Austenite decreased from 42.6% to 0.9%.
Martensite increased from 66% to 81.7%.
Carbides 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 then 1um in size increased from 23,410.24 to 69,646.09.
Rockwell increased from 60.10 to 66.10.
Tensile strength increased from 86.0 to 244.46.
Bending strength increased from 86.0 to 244.46.
KCU (resiliency) increased from.668 to 1.18.
HRC 675° C after 20 minutes keeping: 56.88 to 62.25.
Durability of the 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 lathe cutting tools were then used to cut a .5% structural carbon steel. Durability was established by measuring the radial component of wear. Intensive Speed: @33.6m/min.; Depth: 5mm; Feed: 0.62mm per rev. Relief angle: 8°
Hake angle: 5°; Plan angle: 45°
Deep Cryogenic Cycle Doubles the
Results of the Shallow Cryogenic Cycle
Separate laboratory testing has been performed By Dr. Randall F. Barron at Louisiana Tech University, Ruston, LA. The results by Dr. Baron confirm the Jassy study even further. In one series if tests Dr. Barron compared five common steel alloys. First he tested them as procured. Then he chilled them to -120° F, tested them again, and then treated them at -317° F. In all cases the cold treatment improved wear resistance. The colder the treatment, the better. 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 at 317° F improved wear resistance by even greater ratios ranging from 20 to 6.6 times.
Process Developments
The deep cryogenic process has had an Achilles heel. It has been inconsistent. In the past, improvements to gears would vary from little improvements to over a 1,000% increase in useful life. The trick is in the processing. Temperature changes must be controlled exactly for consistent results. If a gear or tool is dropped in liquid nitrogen, it could shatter.
The computer processor solves this problem. The computer can duplicate the optimal cooling curve exactly time after time.
Older cryogenic tanks did not have adequate control. Using them was like trying to bake a cake in a wood-fired stove. The newest cryogenic tempering achieve consistent results. Furthermore, the price enables the gear manufacturer to improve his profit margin, improve his product, and increase market share with a superior product.
The Cryogenic Tempering Process
The new machines operate with controlled dry thermal treatment. "Controlled" simply means that the process is performed according to a precise prescribed time table. A 386 PC is utilized as the process controller operating the descent, soak, and ascent modes. The material is cooled slowly to -317° F, held for 20-60 hours, then raised to +375° F, and slowly and slowly returned to ambient temperature.
It is a “dry” process that, unlike other
deep cryogenic processes, it does not bathe the materials in liquid
nitrogen, which is more likely to cause damage from thermal shock.
How It Works
The
Barron study looked at how the changes brought about by cryogenic treatment
affected steel’s ability to resist abrasive wear.
It 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-1 steel remains austenite after it is
oil-quenched to 68° F.
If M-1 is quenched from 222° 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 Table 1, 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).
How Effective is it?
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.
Financial Potential
Liquid nitrogen is the largest
processing cost in cryogenically cooling gears.
The newest systems are designed to more efficiently transfer cold from
the liquid nitrogen to the metal parts being treated without losing the cold to
the outside. They have reduced the
processing costs by half, making it economical to process all types of steel
items, not just gears and tooling. Gears
now cost pennies instead of dollars to treat with this method.
Potentially every gear which is heat
treated is a candidate for the additional service of cryogenic tempering. Most customers are pleased to pay for the additional
improvement of any gear, especially at the nominal additional cost (less than $1
per pound!).
There are more than a handful of
large tooling manufacturers quietly utilizing the process today for
manufacturing a premium line of cutting tools.
They manufacture a “premium” tool which lasts 2-5 times longer for
pennies and charge dollars more for it – a great boost in a competitive market
place where profit margins have been squeezed.
Some heat treaters offer “cold
cryogenic services” utilizing –120° F (dry ice)
systems, but deep cryogenic treatment (below –300° F)
is where most of the benefits occur. Ultra low temperature treatments below –300°
F show much more impressive results.
Conclusion
While
not a magic wand which will extend the life of every product, over 100 items,
such as gears, reamers, taps, dies, broaches, drills, endmills, slicers, and
cutting knives do respond to the process. It
can create a “premium” more profitable tool gear for a manufacturer and save
a lot of tool expense dollars for end users.
The process is effective throughout the tool, unlike a coating, so tools
can be resharpened and receive the benefits until completely warn out.
The process also works with TiN coatings.
Among the properties which define the
qualities of a gear steel, durability is the highest importance.
These results are decisive in establishing the benefits of cryogenic
treatment in increasing durability.
