HEAT TREATMENTS

Mechanism and Applicability of Heat Treating at Cryogenic Temperatures

Deep cryogenic treatment and tempering machine with Squid nitrogen cylinder is newly designed for highly efficient thermal transfer in processing.

By PETE PAULIN 300'Below Inc. Decatur, IL

Durability is the most important criterion used to define the quality of a tool steel. Cryogenic treatment and tempering 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. Deep cryogenics (below -300F) 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 resharping 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. The exceptional increase in wear resistivity, generally exceeding 200% is the greatest benefit.

Steel surfaces requiring wear resistance, such as in the cases of drill bits, end mills, surgical scissors, bearings, racing engines, slicers and granulator knives, all benefit from the inexpensive treatment. New applications are being discovered continuously.

BASIS FOR IMPROVED PROPERTIESA research metallurgist at the Na

tional 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 are24

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 Bome 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 transformation 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 microfine 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 quantification (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 (lOOOx) 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 -150F for 1 hr, at -270F for 2 hr and soaking for 8 hr at -310F, followed by tempering at 300F for 1 hr, AC to room temperature and tempering at 225F, AC. In this microstructure, note the considerably greater number of fine particles coupled with fine carbides in comparison with the untreated sample. The mar- tensitic transformation is readily apparent.

Industrial Heating August. 1992

Fig. 1 Photomicrographs (1000X) of non-cryogenically treated and tempered S-7 steel (left) and deep cryogenically treated S-7 steel (right). Superior properties developed by the latter treatment are attributed to increased transformation of austenite to martensite and a more uniform, refined microstructure with greater density (precipitation of additional microfine carbides).

FIELD TESTING PROVES 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 deburring tools on INCONEL alloy 718, achieving a 400% improvement based on weight, after five cuts 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 cuts was increased from three before deep cryogenic processing, to 78 cuts 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.

Fig. 2 Standard heat treating, austenitizing, oil quenching and tempering, compared to cycles with added cryogenic ( -7 CPC) and

tempering cycles. (Source: Jassy Polytechnique InstitutelAlexandru).

Industrial Heating August, 1992

LAB RESULTS CONFIRM HELD TESTS

Shallow Cryogenic CyclesThe latest research data on cryo

genic 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 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, 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.

The results of the testing, Fig. 3, comparing standard heat treating to heat treating with the addition of a shallow cryogenic soak (-70C ) are summarized as follows: 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 @ lmm sq increased from 31,358.17 to 83,529.73; number of carbides less than 1 |un 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 tensile rate increased from 0.65 to 1.85; KCU (resiliency) increased from .0668 to 1.18; HRC after 20 minutes hold at 675C: 56.88 to 62.25.

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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 I Parameters for Lathe Cutting Tools in Wear Resistant Test

Intensive Speed @33.6m/min. Depth 5mm; Feed - 0.62mm per rev.

Relief angle - 8 Deg.; hake 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 procured, then as chilled to -120F and finally tested after treating at -317F. In all cases the cold treatment improved wear resistance; the colder the treatment, the more favorable the results. The -120F (dry ice) treatment improved ratios ranging from

Fig. 4 Wear improvement due to shallow cryogenic ( - 12CPF) treatment vs.

improvements due to deep cryogenic (-317 F) treatment

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 317F 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 temperature control, the tool could shatter. Metals require specific cooling rates; temperature changes must be controlled exactly to obtain the opti-

Fig. 3 Cryogenic and tempering cycle doubles durability, decreasing austenite while doubling microfine carbides.

mal cooling curve. The computer processor solves the problem, since it allows 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 -317F, held for 20-60 hr then raised to +300F, 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 designed to accomplish thermal transfer, and the more efficient the better. It is essential to transfer therms 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 itself is 2 in. polyisocyan urate high density foam which is coat-

26 Industrial Heating August, 1992

52100

Fig. 5 Instrumentation for control of the cryogenic treatment and tempering cycle.

ed with Ceramaseal amorphous vapor barrier 'comprised of microspherical 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.

ADVANTAGES FOR HEAT TREATERS AND IN INDUSTRIAL PRACTICE

Potentially every tool heat treated is a candidate for the additional service 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 -120F (dry ice) treatments. Only a handful of companies provide cryogenic treatment below -300F, 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.

CONCLUSIONWhile not a Magic-Wand which

will extend the life of everything, over 100 tools such as reamers, taps, dies, broaches, drills, endmills, slicers and cutting knives do respond consistently to this process. Cryogenic service 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 unlike 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.

Industrial Heating August. 1992

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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