Edited by Jessica Shapiro
"Supercooled tools last longer," MACHINE DESIGN, June 4, 2009
The manufacturing engineer couldn’t believe what the die-shop foreman was telling him: A die set that formed brass discs for automotive radiator caps had lasted 45,000 to 47,000 cycles before cryogenic processing but was now putting in 340,000 hits before it needed to be remachined. After that routine maintenance, it ran over 200,000 cycles more without needing an overhaul.
Cryogenic processing — treating parts at –300°F or colder — benefits not only tool-and-die sets. It has quadrupled the life of lumber mills’ log-chipper knives, let automakers’ resistance-welding electrodes run six times longer, and improved metal guitar-string harmonics. Audiophiles who have returned to vintage sound equipment powered by vacuum tubes swear cryogenically processed vacuum tubes produce discernible sound-quality improvements.
Other parts that have seen benefits from cryogenic processing include drill bits, milling cutters, shear blades, cold saws, metal-cutting band-saw blades, broaches, taps, thread chasers, gears, sprockets, roller-and-link chains, saw blades, granulator knives, carbide road-surface scarifier tips, injection molds, lumber-sorter slings, chain ways and guides, shafts, open-faced bearings, rock drills, and nail-gun driver bars.
Cryogenics reduces electrical resistance, but the major benefit is wear reduction. Unlike surface coating or case hardening, correctly performed cryogenic processing permeates part volume. The cryogenic effect is permanent for the use life of the item even with repeated resharpening, remachining, or resurfacing. Successful cryogenic processing should, at a minimum, double use life.
Those who cryogenically treat their parts can also expect reduced galling, better heat transfer for cooler operation, and tooling that stays sharper longer and resurfaces or resharpens with less time and material removal. Stress-related failures, especially those from compression stress, are reduced as well.
One particularly good cryogenic application is rollerchain treatment. Chain is said to stretch with use, but the chain components are, in fact, wearing at the pivot points. The pins get smaller, the holes get larger, and the chain effectively gets “longer.” Cryogenically reducing pivot-point wear can lengthen chain life by a factor of three while halving chain maintenance. Of course, proper chain lubrication is a necessity in all circumstances.
Cryogenically processed brake rotors, axles, transmissions, rear ends, and entire engine assemblies are giving several NASCAR teams the winning edge. Jeff Gary at the University of Texas at Austin has reported both empirical and anecdotal improvements in agricultural tillage implements, guns, guitar strings, metalworking tooling, and other items after cryogenic processing.
Cryogenics has numerous applications outside industry, too. Golf balls and clubs, composite baseball bats, gun barrels, panty hose, fishing line, lawnmower blades, grass trimmer line, and disposable razors are just the tip of the iceberg of hobby and household applications for cryogenics. Companies like Down River Cryogenics do thousands of items for customers’ personal use, often for free or at greatly reduced fees. However, golf balls still don’t float after cryogenic processing.
Engineers at NASA first noticed the cryogenic effect in metal recovered from the extreme cold of orbit. Aluminum parts in particular showed greater dimensional stability after exposure to the cold. For over 40 years scientists have documented that cryogenic processing could produce industrially beneficial changes in various material. Dr. Randall F. Barron at Louisiana Tech University confirmed the practical value of cryogenics in various tool steels. David N. Collins of the National Heat Treatment Centre, University College, Dublin, Ireland, demonstrated the molecular changes induced by cryogenic treatment. However, despite its promise, cryogenic processing was slow to transition from the lab to the shop floor.
A lot of nonlab experimentation led to occasional successes — perhaps two times out of 10 a cryogenic process would actually extend tool life. In most cases, results varied widely, were negative, or produced no discernible change. Only in the last decade have cryogenic processing providers begun to produce broadly repeatable, reliable results.
A look at any cryogenic processor’s Web site reveals myriad possibilities and applications. Unfortunately, many cryogenic processors fail to live up to the promise of the applications. In most cases, it isn’t the principle of cryogenics that has failed to work. Rather, many processors lack consistency in their techniques or are simply using faulty methods.
The manufacturers of cryogenic equipment have been the primary promoters of cryogenics. Very few cryogenic processing businesses are run by metallurgists, physicists, or scientists of any sort. Most cryogenic processors are dependent on the manufacturer of their equipment for technical expertise. Consequently, there has been an industry-wide disconnect between those who are technically trained and cryogenic owner-operators.
In addition, much of the cryogenic-processing equipment on the market hasn’t been engineered to produce the results industrial applications require. Every cryogenic-freezer manufacturer recommends a different process regimen. Many of the techniques have worked every now and then, but most haven’t produced consistent, high quality results.
However, this has slowly begun to change. Literature about cryogenics is disseminating more widely, and metallurgical curricula are adding cryogenics. Industrial cryogenic processors that consistently meet industry’s needs do exist, but finding them may require persistence.
Science tells us that low-temperature exposure “squeezes” a metal’s molecular structure. The contraction eases internal stresses that come from differences in grain density, improving dimensional stability and creating a denser microstructure. Within the microstructure, more atoms share electrons to join by covalent bonding. This bonding strengthens the microstructure and further contributes to wear resistance and stability.
The cryogenic process does not radically change the hardness, ductility, or dimensions of processed items. Still, it is good practice to cryogenically process close-tolerance items before machining them to working tolerances, just as it is with heat treating.
Because cryogenically processed metal tools only change ±2 Rockwell C hardness points, tool-maintenance personnel can use the same sharpening regimen and implements specified for parts that haven’t been treated. However, the treated parts will need to be sharpened less frequently and can go through more sharpening cycles before they must be thrown out.
One side benefit of cryogenics processing is that personnel who handle dangerously sharp items will have less-frequent exposure to these items when they have been cryogenically processed. Because cryogenics cuts down on the need to handle and change machinery components, it reduces the opportunities for injury.
Tool steels, titanium, hard chrome, carbide, stellite, nylons (including PA66 and Delrin), copper, brass, aluminum, cast steel, powder metals, and many more materials respond well to cryogenic processing.
In steels, exposure to cryogenic temperatures eliminates most of the retained austenite in favor of the formation of more wear-resistant martensite. It also initiates the precipitation of wear-resistant carbides from the martensite phase, enhancing the materials’ overall durability and toughness.
However, steels with high carbon, low-cobalt, or low-chromium content will realize little benefit from cryogenic processing. Heat-treatable items do better after heat treating.
Cryogenic processing greatly reduces curl memory in nylon fabrics and strands as well as improving their wear resistance. Cryogenically processed fishing line lays flat on the water. Solid nylons used as bearing surfaces, ways, guides, and other wear components generally double or triple their use life after cryogenic treatment.
High-density and ultrahigh-molecular-weight polyethylenes (HDPE and UHMWPE) also benefit from cryogenics. However, plastics molded with a lot of foreign matter or high recycled content don’t respond as well to this kind of treatment because the additives hinder reorganization and stress relief in the polymers’ crystalline phases.
Finding a freezer
How does one find a credible cryogenic-service provider? Recent trends show companies entering the growing field of cryogenic processing last less than five years and have less than a 20% success rate. So, it pays to ask a lot of questions. Any reputable processor should be able to provide answers that reflect a basic scientific and technical knowledge of the technique.
First, query a prospective processor on the process the company uses. Most cryogenic freezers look like a large chest-type deep freeze, although cylindrical vacuum-insulated freezers are also used. Sizes range from that of a footlocker to that of a small one-car garage.
Any cryogenic process should cool parts to at least –300°F. The closer the process gets to liquid nitrogen’s temperature of –316°F, the better. Liquid helium can also cool the parts, but it is much more expensive and produces similar results to liquid nitrogen. Anyone using dry ice or mechanical refrigeration isn’t getting parts cold enough.
A computer should control cooling so part temperature descends at no faster than 1°C/min. At that rate, parts starting at ambient temperature reach the deep soak temperature in 3 to 4 hr. Simply dipping a tool in liquid nitrogen produces rapid contraction and expansion that can quickly ruin the part.
Once parts reach the target temperature, they must be held there for 24 to 32 hr for the soak to be effective. Some processors cold soak less than 8 hr, but anything less than 24 hr is suspect. While faster may be less expensive, you get what you pay for.
Temperature must stay within ±5°C for the entire deep-soak period. Although some operations use thermal cycles that raise and lower temperature several times during a process, cycling induces stresses in the parts and doesn’t provide the same benefit as a long soak at a constant low temperature.
After cryogenic processing, most metallic items see at least two tempering cycles in a tempering oven. The tempering step is an absolute necessity; without it the cryogenic effect fades. Temper soaks must be at least 2 hr, although larger items may need additional time. A total tempering-oven time of 20 hr or more is desirable.
Some cryogenic-processing units have heating elements built-in to facilitate the temper soak. However, these usually accommodate shorter duration tempers that produce poor results. Look for a processor that uses two distinct pieces of equipment — one to freeze and one to temper.
In addition to these process-related details, get a sense of the processor’s confidence in its techniques. The company should be willing to supply two or three names of satisfied customers who have processed similar items over the course of a long-term business relationship. An agreement that the company will replace any items that are damaged or lost is also a must.
The company should be willing to process an item for free so you can evaluate its claims. Items whose operation depends upon each other — like shear sets, punch-and-die sets, or sets of chipper knives — must be processed as a complete setup. Processing two complete setups will provide a greater data pool for more accurate evaluation. If a free trial isn’t offered, the company should guarantee that if you aren’t satisfied, you don’t pay.
To evaluate the effects of cryogenic processing, start with a performance baseline collected under normal operating conditions. This can include an understanding of the impact misaligned, worn out, or poorly maintained equipment on an operation.
The cryogenic processor’s personnel may also be able to provide testing or data-collection recommendations. Examples include assessing power load with an amp meter, counting accumulated cycles with a stroke or revolution counter, and collecting machine operators’ opinions. However, be aware that human perceptions alone can be skewed and are not a substitute for hard data.
So what is a reasonable price for cryogenic processing? Most cryogenic processors charge by the pound. The price varies but, as of this writing, $5 to $7/lb is typical for cryogenically processing up to 250 lb. For example, cryogenically processing a granulator knife weighing 0.5 lb and costing $9 would typically cost $3.50. Because proper cryogenic treatment could quadruple the knife’s lifetime, the added processing could save $23.50.
Some processors use a sliding scale where the price per pound goes down as weight grows. Small items such as fly-cutter tips, carbide saw tips, carbide inserts, small cutters and bits, and very light items are usually priced by the piece. By-the-piece pricing is often based on the value of the item and the results achieved by cryogenically processing it. Extremely heavy items and low-initial-cost items are also usually priced by the piece.