The Cold, Hard Facts About Cryogenics
Cryogenic Processing an Exciting Frontier for Manufacturers
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By Mark Shortt
Editorial Director, Design-2-Part Magazine
Unlike coatings, cryogenic treating improves the entire tool or part, rather than just the surface. Longer part life, residual stress relief, and less downtime for replacing broken or worn parts are among the reported benefits.
The emerging technology of cryogenic processing (also known as cryogenic treating or tempering) is quickly earning respect as a technique for increasing the durability and dimensional stability of manufactured parts. Originally developed by NASA, and once used primarily to extend the life of industrial tooling, cryogenic processing now holds exciting possibilities for manufacturers of products in every corner of the commercial realm. Much of the current enthusiasm is a result of research showing that wear resistance of tool steels can be significantly improved by slowly cooling the tool steel to cryogenic temperatures (-310F to -320F), and cold soaking the steel at this low temperature for a minimum of 20 hours. However, the treatment process has also been found to enhance the abrasive wear resistance of various alloys and plastic materials.
Unlike various surface treatments, cryogenic processing is a one-time treatment that affects the material throughout its entire structure. Residual stress relief, thermal and dimensional stability, and greater machinability in addition to increased toughness are among the main benefits of the technique, which has long been known to extend the service life of end mills, drill bits, cutting blades, punches, and dies. Now, with the popularity of the process on the rise, engineers are becoming increasingly aware of its effectiveness on steel parts, such as springs, wheels, shafts, bearings, gears, sprockets, and valve components. Other uses include gun barrels, aluminum softball bats and golf clubs, aerospace castings and components, and automotive brake parts (rotors and pads).
"Each week, new products are found to benefit from cryogenics," says Lee Edwards, president of Edwards Heat Treating, San Leandro, California. "Sports equipment, cutting tools, stereo components, brass musical instruments, and racing equipment are just a few that are now being cryogenically treated [to achieve] greatly enhanced performance." Edwards adds that although cryogenics is not a substitute for conventional heat treatment, it can provide "tremendous improvement in carbon, alloy, and stainless steels." Besides increasing resistance to wear by "up to 300%," it can also diminish the propagation of cracks, he says.
Although there are many theories as to why cryogenic treatment is effective, actual measurements of results have remained relatively difficult to obtain. An interesting aspect of the process is that the treated tool or part shows no visible changes in color, size, or any other property that can readily be visually detected. There is also essentially no change to the hardness, or any other physical properties of the part, that can be readily checked or measured by tensile or impact tests, for example.
"A normal metallograph shows no changes," says Pete Paulin, CEO of 300 Below Cryogenic Tempering Services, Inc., Decatur, Illinois. "Nor do common tests like eddy current, ultrasonics, or Barkhausen effect belie the fact that the part has been cryo processed." For these reasons, Paulin says, the process is difficult to fit within the normal parameters of a quality control system.
As a result, the advantages of what began as Space Age technology many years ago are still largely unknown to many within the manufacturing industry. Although the benefits of cryogenic treatment can be supported by numerous anecdotal examples, they require precise measurement to prove their effectiveness: something that too few companies have been willing or able to undertake. The fact that many companies keep their processing techniques "close to the vest" to maintain a competitive advantage only adds to the mystery. To those who are unfamiliar with cryogenic treatment, the process can therefore seem more like magic than something measurable.
This is beginning to change, however. Intrigued by reports of as much as 300% improvement in wear resistance, manufacturing engineers are starting to see the benefits of using cryogenic processing to treat not just tooling, but also a broad range of parts, components, and products.
In the 36 years since its formation, 300 Below Cryogenic Tempering Services, Inc. has grown from its origins in a garage to its current position as a multi-million-dollar business. In 1966, the company's Cryo-Tech division was the first company to commercialize the process of deep cryogenic treatment. To scientifically document the benefits of cryogenic processing, Cryo-Tech commissioned Louisiana Tech University's Division of Engineering Research to conduct a series of studies to determine what happens to materials during the process. The research, led by Dr. Randall Barron, identified the fundamentals of wear, the factors controlling wear, and the mechanisms by which cryogenic treatment influences the wear resistance of materials. Five types of tests were run on common tool steel alloys: 52100, 0-2, A-2, M-2, and 0-1.
Photomicrographs taken during the research showed that cryogenic treatment changed the microstructure of tool steels that had been slowly cooled to cryogenic temperatures (-310F to -320F) and cold soaked at the low temperatures for a minimum of 20 hours. One of the findings was the transformation of retained austenite into harder martensite as a result of taking the steel to very low temperatures. According to Paulin, the study concluded that cryogenics, when applied properly, results in significant savings and can reduce the use of energy, natural resources, and raw materials. Today, those who use the process consistently report reduced costs, he says. If a $75,000 die receives a three-fold increase in service life as a result of a cryogenic treatment that costs $1000, the treatment process would save $150,000. "Generally, it is a 15% cost for a 200-500% gain in life," he says.
The Treatment Cycle
The controlled cryogenic process starts with the loading of a well-insulated treatment chamber with the materials to be processed. A microprocessor is programmed according to the weight of the parts being treated. Liquid nitrogen (LN2) is used as a cooling medium to lower temperature within the chamber to -315F. The temperature is lowered at a very slow rate, under precise control, because a rapid change in temperature can induce stresses caused by thermal shock. This descent phase can take anywhere from seven to 14 hours.
Once the temperature reaches 315F, the process enters the soak phase, which maintains this temperature for 24 hours. The long soak ensures that the entire cross section of the material in the chamber is completely treated. The final step in the process is the ascent phase, in which heaters raise the internal temperature of the chamber to ambient.
"There is not that much sophistication to cryogenic treatment when you get right down to it," says Bill DeFelice, president of Fountainhead Cryogenic Processing, York, Pennsylvania. "The whole process boils down to a slow rate of cooling followed by a long soak period, followed by a mild temper (300F) for some items."
Despite impressive reports of its benefits, cryogenic processing is not the answer to every situation, according to Charles Lenker, president of Leading Edge Cryogenics (LEC), Camp Verde, Arizona. Like any real science, it has specific limitations and applications. One limitation is that it can take significantly longer than other types of processing. High-volume production requires specific considerations, and the process may not work on some anodize treatments. "Degraded performance has been reported as a result of cryogenic processing on some anodized parts," Lenker revealed.
However, the process can be used in conjunction with coatings, and, in many cases, will actually improve the quality of the coating to be applied to a part. According to Charles Beresford, president of Cryogenics International, Scottsdale, Ariz., the process of applying coatings by electrochemical means is dramatically improved by the improved grain uniformity and higher electrical and thermal conductivity derived from cryogenic treating. "The coating will be more uniform and bond itself much better to the metal substrate," says Beresford. "Anodize will resist the acid bubble test by a factor of 5-10, and TiN coatings become longer lasting. The process can also be applied to tools that have already been coated."
Meeting Technical Challenges
One company in Ogden, Utah, Cryocon, Inc., uses a proprietary treatment process that it calls Deep Cryogenic Tempering (DCT). Cryocon uses the computer-controlled tempering process to significantly improve the wear and performance characteristics of engine components such as valves, rings, cylinders, intake manifolds, push rods, pistons, and connecting rods. According to company officials, the firm recently applied the process to solve premature wear and decreased performance in disk brake rotors and brake pads. Greening Testing Laboratories (GTL), of Detroit, Michigan is said to have performed independent testing on treated and untreated rotors.
GTL used a proprietary test procedure that had been specifically developed for evaluating the lining and rotor wear characteristics under simulated driving conditions. The procedure incorporates 25 unique braking events, repeated four times in a randomized order. The resulting group of 100 cycles is then repeated 10 times for a total of 1000 cycles. An inspection of pads and rotors, including mass and thickness measurements, was conducted after each 1000-cycle interval.
Based on measurements relating to loss of rotor thickness, the rotors treated with Deep Cryogenic Tempering were said to achieve 234% better wear than untreated rotors. Standard brake pads, which were not treated, reportedly achieved 361% better wear on treated rotors than the pads that were used on untreated rotors.
A number of Edwards Heat Treating's cryogenic jobs involve the processing of machine shop tools, including drill bits, hobbs, mills, and cutters. Recently, the company has been cryogenically treating stereo components, such as amps, tubes, connecting wires, and filters, with outstanding results, according to Lee Edwards, president. Unlike many cryogenic sources, Edwards Heat Treating can offer "a more complete metallurgical solution" that includes heat-treating services, Edwards noted. He says that the best cryogenic results are obtained only through "proper, high-quality heat treatment before the cryogenic process."
One of Edwards Heat Treating's customers, a gear cutter, had been paying for expensive, titanium-nickel-coated cutting tools. He found that normal high-speed cutters, when cryogenically treated, outlasted the Ti-Ni-coated tooling, and at less total cost, Edwards reported. Another customer, a screw machine company, was having a problem with tolerances on a stainless steel part.
"They were drilling a deep hole into a 304 rodbasically making a thin-walled tube," explained Edwards. "The drilling heat caused the part to go out of straight by 0.008 inch. After cryo-treating a high-speed drill bit, the resulting parts were out only 0.002 inch, well within their tolerance."
Leading Edge Cryogenics, Inc. has OEM customers in several product areas, mainly in the medical and automotive industries. The firm's primary product fields are orthopedic implants, brake pads, and spark plugs applications for which LEC has developed patents. The company also processes a variety of additional applications, including engines and cutting tools. A number of high-profile racing concerns ranging from Indy Lights race teams to nationally known motorcycle racers use LEC's treatment services.
LEC uses vacuum-insulated cryogenic processing systems that are built in-house. The systems are equipped with redundant process controls, running identical treatment controls in real time, and are backed up by uninterruptable power supplies. They are further supported by LEC's proprietary software and hardware control subsystem, which switches process control to the backup system, should the primary process control system experience a software or hardware failure.
Recently, LEC was approached by a manufacturer who was experiencing field failures of an aluminum product. The failures occurred due to stress fractures developing around drilled holes in the product, after about 50,000 stress cycles. The manufacturer's efforts to improve the fatigue life of the parts ranged from trying different aluminum alloys to changing the forming and machining processes, all of which were unsuccessful, according to Lenker.
After learning of the specific alloy in use for the application, LEC constructed and executed a cryogenic processing plan, which included five "screening" treatments on five groups of samples. These samples were subsequently tested on a test fixture that replicated the "real world" use of the product. Each "screening" cryogenic process produced increases in the fatigue life of the parts, Lenker said. "These improvements ranged from 80,000 cycles the least improvement to well over 200,000 cycles, for the best improvement," he explained. "The cryogenic process of choice increased the fatigue life of the parts by 400%, providing the manufacturer with a marketable product."
300 Below Cryogenic Tempering Services, Inc., cryogenically processes more than a million pounds of steel and materials per year, according to Pete Paulin, CEO. Having enlarged some of its equipment to permit processing of parts as long as 25 feet, the company recently processed a 27,000-pound die for a major aerospace company. "We now consume over 15,000,000 cubic feet of nitrogen, with an available capacity to process over 5 million pounds of steel per year," says Paulin. "Our plans are for expansion into core industries, implementing our new high-amplitude sonic, high-Gauss magnetic and deep cryogenic processing systems."
In addition to providing cryogenic tempering services, 300 Below makes equipment; the firm recently built a specialized processor for treating metallic mirrors for NASA's next generation of the space program. "It required eight temperature controls points on the mirrors themselves, as well as multiple temperature control points for the atmosphere of the processor," said Paulin. The company also recently developed a "new, fourth generation of software" for controlling its cryo processors, which are efficient, cascade compressor electric types "the most efficient processors we are aware of in the industry," Paulin says. The software is said to be more reliable and easier to use, and provides information about the cycles which was previously unavailable. "Graphing and data acquisition is now much easier," Paulin affirms.
One customer, an agricultural products manufacturer, cryogenically treats its M-2 horizontal nick and shear blades used in a Vogel tube cutting machine. The blade shears the tubing, and needs to be sharp so that the tubing is cut, rather than crushed. Both wear and breakage were previously a problem, according to Paulin. Now, after being cryogenically treated, the blades have doubled in wear life and run at least three times longer between breakage, he says. Another client, a maker of plastic packaging, uses carbide knives that must be run at less than full speed after two weeks of usage. "We switched them to HSS blades, which are 40% less expensive, and the cryogenically treated blades ran 10 weeks, of which 8 weeks were at full speed," Paulin said.
Thirty-six years of continuing research into the mechanisms of cryogenic processing have not even scratched the surface, according to Paulin. But as the technique becomes more popular, new applications for deep cryogenic processing are appearing every day, and still more uses promise to emerge tomorrow. "We are witnessing the genesis of a new industry," says Paulin. "It is apparent now that the commercial cryogenic processing industry is an industry with a future. Cryogenics is today where heat-treating was 100 years ago, in its infancy. And, oh, what a beautiful baby!"
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