CRYOGENICS: A TECHNOLOGY SEEKS LEGITIMACY

Plagued by controversy and populated by small mom-and-pop companies, cryogenic processing is a growing industry. It's not going away, but it exists in a Wild West environment. Slick operators selling money-making opportunities make things difficult for small-business people interested in serving their customers. Approximately 150 cryogenic processing companies are operating worldwide; most have annual sales of less than $10 million, and fewer than 10 employees.

The industry is fighting for respect and for recognition of cryogenic processing as a legitimate technology. Under-capitalization and limited research hamper the industry, but there's an abundance of anecdotal evidence that cryogenic processing works. Users often conduct their own tests and experiments during their cost-benefit analysis. Results, however, are not routinely shared outside those companies. Legitimate cryogenic processing companies are usually happy to provide customer references.

Proponents of cryogenic processing claim dramatically improved wear resistance for perishable tooling and dies. Longer tool or die life and reductions in machine downtime and maintenance, they assert, can significant-

ly cut operating costs. Confusion about the cryogenics process and terminology, however, coupled with inconsistent results, a wide variety of applications, and differing effects on various types of materials, make it difficult to determine the value and feasibility of using cryogenic processing.

Simply chilling metals to sub-zero temperatures for stress relief and stabilization is a very old technique. In fact, Swiss watchmakers have reportedly used cold mountain caves to condition and stabilize parts for more than 100 years. Extreme temperatures and computer-based controls are key elements that set cryogenic processing apart from this sort of traditional cold treatment.

Cryogenic processing to increase wear resistance is a fairly recent development that has spawned a new industry. Although cold conditioning reportedly increases the stability and wear resistance of some materials, questions remain as to which materials benefit from cryogenic treatment. It's also not clear whether the results of cryogenic processing offset its cost.

Cryogenic treatment is accomplished in various ways. The most popular approach in the United States is a deep, dry, controlled cryogenic process. Deep cryogenic processing takes place at around -320­F (196­C), near the temperature of liquid nitrogen. Shallow cryogenic processing takes place around -120­F (-84­C), near the temperature of dry ice. When material is immersed in liquid nitrogen, the process is considered "wet." A dry process is one during which the material is not immersed in a liquid. A computer (usually microprocessor-based) controls the process.

Tools sent to a commercial cryogenic processor are generally batched with items from other companies until enough are obtained to fill a specially constructed freezer that uses liquid nitrogen as the refrigerant. The temperature is gradually lowered or ramped down to -320­F. Items remain at that temperature for 20 to 60 hr (commonly referred to as the soak). Then, the temperature is gradually raised to room temperature or beyond. If the material requires additional tempering (to stabilize freshly transformed martensite, for example), the system slowly raises the temperature (to around 375­F or 191­C for most tool steels) before gradually reducing it to room temperature.

The effectiveness of cryogenic processing can be hampered by insufficient soak time, cooling or warm-

ing too quickly, and skipping the post-soak temper. Any one of these factors can cause inconsistent results-a problem that has dogged the cryogenic processing industry. Fortunately, today's cryogenic processors are able to provide more consistent results than older equipment.

Materials for myriad applications can reportedly benefit from cryogenic treatment. Property enhancements are claimed for steel, aluminum, brass, copper, nylon, plastics, and carbides. Applications include -but aren't limited to- aerospace, manufacturing, sports, music, firearms, motorsports, and tooling. New applications are being found all the time, and each seems to stir up its own share of controversy.

When it comes to tool steels, there's a little more acceptance of the technology. Metal tools and parts that may benefit from cryogenic processing include drill bits, end mills, cutters, dies, punches, bearings, cams, crankshafts, blocks, and pistons.

Cryogenic processing is commonly referred to as cryogenic tempering. One person who cringes at this term is Bill Bryson, Advisor In Metals (Union, NH), who says "there's no such thing as tempering with cold treatment." He explains that cryogenics is a continuation of the heat-treat process during which a ferrous steel's molecular structure continues its transformation from austenite to a more desirable, wear-resistant martensite. After cryogenic processing, however, this newly created martensite must be tempered with heat

to stabilize the freshly transformed, unstable microstructure. Driving his point home, Bryson says, "Make sure you are not misled by thinking 'temper' is anything less than grain refinement and structure stabilization accomplished by heat tempering. Cryogenic processing IS NOT a tempering process."

Ask Pete Paulin, CEO, 300 Below Inc. (Decatur, IL), about cryogenic tempering, and he cheerfully claims, "That's a term we coined about 14 years ago." When asked about some who take exception to that particular term, he responds, "well, those are people who don't understand that the term refers to more than just going down in temperature-it includes coming up in temperature. So, the process is cryogenic and tempering, and that's why we came up with the term cryogenic tempering." Improved wear resistance of tool steels after cryogenic process-ing, he says, is due to three factors: retained austenite (RA) conversion, carbide precipitation, and thermal-mechanical stabilization. Changes from the last two factors, however, can only be seen under a micro-scope. Paulin observes that many people schooled in metallurgy accept the notion that cryogenic treatment of steel converts retained austenite to martensitic structure. They maintain that if you heat-quenched properly in the first place, then you wouldn't have any retained austenite; so, cryogenic processing is unnecessary. "It's a true statement with an inaccurate conclusion," says Paulin, "some metallurgists conclude that RA conversion is the only thing going on because it's the only thing they can see."

That view, according to Paulin, ignores benefits of cryogenic processing that can be seen in plastics, aluminum, copper, brass, and other materials that don't have retained austenite to convert. Those materials may be enhanced by thermal-mechanical stabilization-expansion and contraction of the crystalline structure-which decreases latent stress in materials, and is one of the factors responsible for gains achieved with cryogenic processing. It's crucial, however, to transition material uniformly and maintain equilibrium to avoid thermal shock. In steel, a differential in the

rate of expansion from core to surface results in thermal shock, which makes the material brittle and very susceptible to cracking. "What we're doing," says Paulin, "is removing those stresses-not imparting them. That's diametrically opposed to the way that most heat treaters and metallurgists think about this process."

He also points out that "heat treating is really a misnomer because nothing happens when you heat steel to eutectoid temperatures-no changes take place there. All of the changes take place on the quench, which is just cooling from a higher reference temperature than the planet happens to be at." Martensitic start and finish temperatures (MS, Mf) are a function of the amount of carbon in the structure of steel. As carbon content approaches 1%, the MS Mf curve drops, which means at more than 0.4% carbon for most tool steels, and a sub-ambient quench is needed to fully convert to martensitic structure. "Metallurgists have long overlooked the fact that physical processes don't stop because the planet happens to be at 70°F. This is actually a continuum, it's a function of physics, and that's what we're doing-extending that quench to where it should be, optimally, as a function of heat treating."

Because there are no readily visible changes, skeptics abound, and a lot of metalworking professionals take a wait-and-see approach. A cryogenic processing company claiming they could improve the performance of

ceramic and carbide cutting tools contacted Bill Russell, director of research and development for Valenite (Troy, MI), about their treatment. He chose not to do business with them at that time. "It's somewhat of a fringe project for us. We don't know whether cryogenic processing is something we're going to get into seriously or not at this point, but I'm keeping a file on it." For Valenite, and many other companies, the decision on whether or not to investigate cryogenics involves such considerations as limited resources and higher priorities. "We have to decide what things we can focus on," says Russell. "So, I don't want to say cryogenics is or is not valid, because we just don't have the data. But I like some of the theory. We just need to prove to ourselves whether that theory translates to machining."

Some manufacturing practitioners enthusiastically embrace the technology. Steve Chapman, CNC machining department supervisor, Federal-Mogul Aviation Inc. (Liberty, SC) routinely uses cryogenic processing for drills. "It's one of those things you have to try because there are all kinds of stories." He inherited the practice from another engineer and continues it based on his own cost-benefit analysis. He's conducting additional trials and says he's looking at doing even more tool trials. Chapman believes the effort is worthwhile-especially since cryogenic processing is a one-time cost based on weight. He also comments that keeping an open mind and being willing to try new things is vital in today's competitive environment where, "we're constantly looking for ways to cut cost, improve processes, and improve our overall throughput."

Chapman explains they've been using cryogenic processing services for more than two years. He confirms that the life of a cryogenically treated 1/16" (1.59 mm) carbide drill increased from 120 to 416 holes when compared to an untreated drill. "We use the 1/16", 3/64" (1.19 mm), 1mm, and a small quantity of number 63 Bassett drills. As soon as they come in, I send them and have them treated. It has produced drastic life improve-

ments on all four drills." The cost for cryogenically processing these drills is roughly 33% of tool cost, says Chapman, but it's definitely worth it. "We machine the exotic, high-temperature nickel-alloy metals for turbine engine igniters. And a lot of times, you just don't get a lot of tool life out of standard, uncoated drills." He mentions that with some larger-diameter drills, you can keep re-sharpening and still see gains, because cryogenic treatment is a one-time process. Once you've identified the tool by etching or engraving, he says, you can use it over and over. "It's going to give you more tool life. It seems to stabilize the granular structure in the carbide."

Cryogenically processing carbides baffled the industry for about 25 years, says Paulin of 300 Below. "We've done some work in carbides. We process about a million pounds of steels per year, and have been able to find out why some carbides work and some carbides do not work at a cost of a couple hundred thousand dollars." Paulin claims the process is not grade-dependent- it's even more specific. It depends on the actual structures and target temperatures of carbides being treated, along with how they are processed.

"We've been experimenting a little over a year. And, there's no doubt about it; the process has increased the life of the product that we use," says Glenn Pisching, manufacturing engineer, Lincoln Electric Motor Division (Cleveland). He uses cryogenic processing on molds, broaches, and drills, and has pushed the process for about two years. "It's definitely put a rose on the top of my head, because it has demonstrated significant cost savings. Now, we have a group, consisting of three engineers, who will be working with this process and introducing it to more areas in our plant." Next, he says, they're going to try it on punches and dies.

Pisching began using the services… The very first mold he did was an $18,000 mold, and cryogenic processing nearly doubled its life. He's particularly impressed with what cryogenic processing

has done for his premium-quality H13 molds. Molten aluminum (1300-1500­°F or 704-816°­C) is poured into the molds to cast rotors. Exposed to very high temperatures, molds are usually susceptible to heat checks, the aluminum starts sticking, and it's harder to release the casting from the mold. "But right now, the mold that I have in this process hasn't even shown that type of wear-and it should have-it hasn't shown any type of heat checking, yet."

Broaches can only be used for so many pulls -or passes- before they must be ground and resharpened. Usually, Pisching says, he can broach 150-200 rotors. Now, he's finding that he can broach 500 or more rotors. As this article is written, he's still using the treated broaches, while he's sending untreated broaches out to be ground. "So, cryogenic treatment increases the life," he says, "even if it's 50-60%, it's definitely an increase. For what it cost to have the broaches treated, the process pays for itself."

Drills exhibited a significant improvement in life after cryogenic processing, says Glenn Pisching. The average drill costs $20-30 and his group would sometimes go through 5-7 drills per day, which is quite costly. Now, at Lincoln Electric Motor Division, they cryogenically treat 3" and 4" (4.76 and 6.35 mm) standard, TiN-coated Cleveland Twist Drills that are 12-18" (304.8-457.2 mm) long. The drills are used for plunge-drilling motor brackets made of cast iron and cast steel. Typically, he says, you can only drill to a depth 1½ -2 times the drill diameter before you need to pull it out to expel chips. After cryogenic processing, the Lincoln Electric team quadruples that depth. "The process has better than doubled the life of these drills. Before, we might get out 200-300 pieces, now we get 400-600 pieces."

Some companies have taken

processing is considered low tech. Another step intended to move the industry forward was the recent formation of an ASM International (formerly American Society for Metals) Materials Park/OM committee on cryogenics to address the need for more information, standardization, and training.

Like an unruly teenager, the cryogenic processing industry has lots of room for improvement, disagreement, and growth. Although the technology has come a long way, it's not part of the mainstream and should be scrutinized carefully. There's an undeniable credibility gap in the industry, but the promise of this technology is tantalizing. The challenge that users and service-providers face is learning when, where, and how cryogenic processing does or does not work or add value. Consider total cost and the nuances of each situation, ask probing questions, and experiment to determine whether it's practical and cost-effective for you.