By Steve Blankinship,
It could be said that Michael Stickney gave the coolest presentation at this year’s EPRI maintenance conference in Houston. He believes the time has come for the power industry to do what NASA, the U.S. Naval Nuclear Retrofit Facility, Lawrence Livermore National Laboratory, NASCAR, and the Chicago Symphony and Philadelphia Philharmonic – to name but a few – have already done: freeze stuff. NASCAR has frozen engines and chassis with liquid nitrogen to make them run better and longer. The orchestras believe freezing makes their horns sound better. Stickney says freezing power plant components will extend wear and reduce maintenance.
Stickney, with 22 years of commercial nuclear power plant experience, is manager of power generation services for 300degrees Below Cryogenic Tempering Services Inc. of Decatur, Ill. “The demands on today’s power plants are significantly different than the original designers and builders anticipated,” he says. “Generating capacities are being pushed to the limit. Deep cryogenic processing extends the service life of plant equipment to reduce planned and corrective maintenance and the subsequent loss of generation that can occur.”
A hundred years ago, Swiss watchmakers stored high wear parts for precision watches in high altitude caves over the winter to improve wear characteristics. Tool manufacturers have used the process in some applications for more than 50 years and NASA has specified cryogenic treatment for some parts since the early days of the space program. Early treatments used a variety of techniques to achieve a variety of results. Commonly, a temperature of -110 F was used because that is approximately the temperature of dry ice. The rate of cooling, the time at the low temperature and the subsequent warm-up rate all were variable or uncontrolled. As a consequence, results were unpredictable and generally discounted.
“A few individuals conducted serious research into the metallurgical phenomenon associated with deep cryogenic processing during the late 60s and early 70s,” says Stickney. “They used strict scientific methods and achieved predictable, repeatable results, thus validating the process as a useful tool in improving the physical properties of materials. That led to the current era of deep cryogenic treatment and the formation of a new industry.”
Founded in 1966, 300degrees Below currently processes more than a million pounds of steels and iron per year. Roughly a dozen U.S. companies perform cryo-freezing, employing various degrees of sophistication. Some bought their equipment from 300
“There are four main types of wear,” says Stickney. “Adhesive, abrasive, fatigue, and corrosive. The type of wear most common in industry is abrasive wear, which on a microscopic level is the penetration and gouging of material from one surface by another material.”
Stickney says that deep cryogenic treatment improves the ability of material to withstand abrasive wear by causing distinct changes in the microscopic structure. The most significant change is the transition of retained austenite to the harder and more durable martensite. In 52100 carbon steel, for example, normal heat treating results in the transformation of almost 90 percent of austenite to martensite. Deep cryogenic treatment increases that percentage to about 99 percent, providing a higher concentration of more durable material that is inherently more wear resistant.
In addition, the creation of more and smaller carbide particles and the improvement of their dispersion in the material provide a denser and more uniform crystalline structure. These changes create a denser, more uniform wear surface. As the quality of the surface at the microscopic level improves, the effect is the same as providing additional surface area. More wear area directly provides more resistance to wear. And although wear resistance alone is a significant improvement in the material properties, the reduction in internal stress can also be a major benefit of the process.
Cryogenic treatment is an extension of, but not a replacement for, proper heat treatment of metals to improve their physical properties. The process involves a controlled cooling of the material to -300 F, a prolonged soak at that temperature and a controlled heat-up to 375 F for stabilization. Critical components of this process are the rate of change of temperature and the length of time the material is held at -300 F.
Although the process involves the use of liquid nitrogen as a cooling medium, the liquid does not come in direct contact with the components, thus avoiding thermal shock and potential damage. The cryogenic portion of the process is performed in specialty cryo-processors, cooled by liquid nitrogen. As the temperature is increased and passes through ambient, the material is transferred to a natural gas fired furnace for the heating cycle. The temperature ramp rates are computer controlled, with programming specific to the material being treated. Unlike many other processes, the effects of cryogenic treatment are permanent and homogenous. Subsequent machining or tooling operations do not change the improved properties of the treated material.
“In most cases, it is not the cost of the replacement part that is of concern,” says Stickney. “The largest part of the expense is removing the equipment from service for the maintenance or repair.” He cites one case where, after cryogenic processing, a slurry pump went from two months to 12 months of service.
Cryogenic processing creates significant savings for the industry in unique areas, according to Stickney. “Primarily we realize savings on replacement parts with a cost of $0.20 on the dollar, or better stated, a 500 percent return on investment. Additionally and even more significantly for the power generation industry, reduced downtime and the savings recognized from newly increased throughput exceed the parts savings. By reducing downtime and replacement costs, millions of dollars are both saved in cost and also created by additional throughput generation allowing the company to efficiently produce energy at industry standards.”