The use of extremely low temperatures (cryogenic temperatures, which scientists define as below -244°F.) to boost the performance and service life of critical components is now commonplace in the racing industry and is becoming more and more prevalent in the manufacture of high quality components. What was once considered by many to be a questionable science is becoming a bedrock solid means of insuring the greater performance of materials.
Why has it taken this long to gain acceptance? It is extremely hard to get across to people that changes can be made using cold. People easily grasp the fact that heat can be made to modify solid materials. Heat treating is all around us. Humans have been modifying metals with heat for over 7,000 years, and archeologists have found evidence that humans heated the rocks to make better tools with them over 140,000 years ago. So, the use of heat is second nature to most of us. We’ve only had extreme cold available to us in commercial quantities for about 100 years.
When you try to explain to people that you can permanently modify the performance of metals and other solid materials with cold, they tend to give you a cold stare of incredulity. They will tell you that the only thing you can do with cold is to cause the conversion of retained austenite into martensite, and you really don’t need to go very cold to do that. (For those not familiar with the terms Austenite and Martensite, they refer to the crystal formation of hardened steels. Basically, you want only martensite for hardened steel but that is hard to get.) Martensite transformation is virtually finished at -140°F.
Going colder was considered just a waste of time. This attitude was (and is) so ingrained in people that when metallurgists started to create diagrams on how the atoms in a metal worked with each other at different temperatures, they went down to zero degrees Centigrade and stopped. But research into Deep Cryogenic Treatment started to show it works and that it works in materials where the old theories said it would not. This finding has caused even the naysayers to take a second look.
In the last five years, the pace of research into cryogenic processing by major companies and universities has increased considerably. The focus of the research is also starting to change. Where it once spotlighted whether the process worked or not, it is starting to focus on why cryogenic processing works and what processing parameters are best for a given material.
The research is being helped tremendously by the Cryogenic Society of America. CSA is a non-profit technical society serving all those interested in any phase of cryogenics, the art and science of achieving extremely low temperatures — almost absolute zero. For more information, visit their website at www.cryogenicsociety.org.
CSA has created a database of research papers and informational articles that is available to anyone on the internet. This is a definite boon to serious researchers.
Cryogenic processing has also caught the attention of other entities. Jay Leno did a webcast on the subject. It is available at www.jaylenosgarage.com or www.metal-wear.com/Jay%20Leno.htm.
Air Liquide, a noted manufacturer of the liquid nitrogen used in cryogenic processing has bought processing equipment and has assigned personnel to research the process in their French research center (www.airliquide.com). They are also sponsoring research at Wayne State University in Detroit.
ASM, which is the professional society of materials scientists (www.asminternational.org) is now holding webinars and classes on the subject as well as devoting time for research papers through its Heat Treating Society.
One of the major commercial uses of cryogenic processing came out of the racing industry. The treatment of brake rotors and pads has been shown to be an economic boon to commercial fleets. Considerable testing was done by independent laboratories. Lab tests indicate a three to seven times increase in brake rotor life. Recently a county in California did a field test of brakes for its patrol cars. The results were that the county will save over $560,000 per year by equipping its 550 Ford Crown Victoria patrol cars with brakes treated with cryogenic processing. The savings include reduced costs of parts and labor.
Observed changes include:
• Increased resistance to abrasion
• Increased resistance to fatigue.
• Precipitation of very fine carbides in ferrous metals that contain carbide forming elements.
• Transformation of austenite to martensite in ferrous metals.
• Change in vibrational damping.
• Increased electrical conductivity.
• Anecdotal evidence of changes in heat transfer.
• Stabilization of metals to reduce warping under heat, stress, and vibration.
In practice, cryogenic processing affects the entire mass of the part. It is not a coating. This means that parts can be machined after treatment without losing the benefits of the process. Additionally, cryogenics apply to metals in general, not just ferrous metals. For many years, it was assumed the only change caused by extreme cold was the transformation of retained austenite to martensite in steel and iron. Because of this, many misinformed engineers still believe that cryogenic processing is “just a fix for bad heat treat.” It is now known that cryogenic processing has a definite effect on copper, titanium, carbide, silver, brass, bronze, aluminum, both austenitic and martensitic stainless steel, mild steel, and others. It is also known that plastics such as nylon and phenolics show property changes.
Cryogenic processing can have a positive effect on virtually every engine, transmission, and drive line part, as well as many chassis parts. Increasing the durability of components in the vehicles is the main reason for using cryogenic processing. The great thing about cryogenic processing is that it allows an increase in durability without an increase in weight or major modifications to component design. In addition, the use of cryogenic processing has helped some racing teams reduce costs, enabling some expensive parts to survive the stresses of racing for use in subsequent races.
Brakes and Clutches. Brakes of a racing car take a real beating. It is not unusual for a racing vehicle to finish a race with the brakes totally worn out. This is especially true during road races and endurance racing, where brake rotors can get so hot they glow visibly at night. Cryogenic processing can be applied to both rotors and pads. The net result is two to three times the life of untreated components even under severe racing conditions. As a side benefit, the rotors are less prone to crack or warp. It is interesting that drivers report better braking action and feel. Some drivers are so sold on the concept that they have their street vehicle equipped with treated brakes. Clutches are a form of brake, and the results are very similar.
As an offshoot of racing development, cryogenically treated rotors and pads are making their way into fleet operations on the road. The U.S. Postal Service specifies cryogenic processing for their rotors and is experiencing up to five times as many miles as they were getting on the unprocessed rotors. Similarly, many police fleets are starting to adopt treated rotors and pads. They, too, are experiencing large maintenance savings on both parts and labor. What is metallurgically interesting is that the brakes are a gray cast iron that has a pearlitic structure. This rules-out the austenite to martensite transformation as the mechanism for increased life.
Springs fail in one of two modes. They either break or their spring constant starts to decline. Either way, it can have catastrophic effects on the performance of the vehicle. Most valve springs are made of specially made chrome silicon steel. The automotive valve spring is a fatigue failure waiting to happen. It typically can lose up to one third of its spring constant during a long race. In some forms of racing, it is just hoped that the valve springs will last through the race. Some drag racers routinely change the valve springs before every run down the drag strip to ensure consistent performance.
Typically valve springs exhibit a longer life after cryogenic processing. A recent doctoral thesis written by Ms. Debra Smith at Marquette University proved this. How much depends on the type of racing, the type of spring, the manufacturing lot of the spring and the criterion for a failure. Cryogenic processing of springs will usually triple the life before fatigue failure occurs, and it will reduce the amount of spring constant lost from 20 to 30% down to about 7%. This makes it easier to set up the engine, as there is not such a wide variation in the spring performance. It is difficult to determine absolute spring life increases, because the racers typically discard them long before they break. We do know one drag racer who used to change springs after each run: he now makes seven runs before changes. There is a caveat here. A further advantage for cryogenic processing of springs is that the process seems to eliminate or reduce harmonic vibrations. If you have ever seen a high-speed movie of a valve spring at high engine rpm, you will notice that the spring does not simply move up and down. It does a very complex hula dance because of the harmonic vibrations. Racers typically have to design the spring and valve trains so that harmonics do not interfere with the valve action.
Not unexpectedly, chassis springs are also affected by cryogenic processing. Chassis springs lose their spring constant during a race. This can cause the chassis to lose its cornering ability, which drastically slows the car. Loss of spring constant also alters the height or road clearance of the vehicle. The vehicle height is critical at high speeds because it has a big effect on the aerodynamics of the car, and hence on the handling and the top speed of the car. Other ramifications of springs sagging are evident. Watch the pit crew after a Sprint Cup race as the car is pushed up on to a platform for inspection. If the springs have settled too much, the car may be disqualified as it will sit too low. So the pit crew will often be lifting on the chassis as they roll it along to set it up a little higher. When they get the car to the measuring surface, they gently let it down so it does not bounce and settle farther than necessary.
The chassis itself is basically a very large, complex spring, having numerous welds and using not very precise tubing. The metals used here vary, depending on the type of racing. NASCAR frames are made from 1020 steel; other forms of racing use 4140 steel. Of course, other high strength, lightweight materials are also used. As the chassis experiences vibration during the race, residual stresses in the welds and the tubing can start to relieve. This causes the chassis to change shape during the race, affecting the handling of the vehicle and therefore its speed.
Gears, Shafts and Assemblies
A study for the U. S. Army Aviation and Missile Command, by the Illinois Institute of Technology Research Institute concluded that cryogenic processing of carburized 9310 steel increased the gear contact fatigue life by 100%, and the ability of the gear to handle load by 10% over the same material that had undergone a -84°C(-120°F) cold treatment per military specification. They also found that the conversion of retained austenite is only part of the effect on the gear. Most racing gears are 9310 carburized steel, although 8620 is also used. One major racing transmission maker, after inspecting numerous gearboxes after races, has ascertained that cryogenic processing cuts the gear wear dramatically. This also holds true for road racers of Porsches and BMW’s and other SCCA race cars who are now getting about three times the life on their gear boxes. Cryogenic processing also increases the life of other heavily loaded gears. We see a doubling of the life of ring and pinion gears in differentials, even under such severe usage as tractor pulls. Quick-change gears also show dramatic increases in life. Axle shafts, universal joints, and constant velocity joints all show dramatic increases in durability. As the racing of front wheel drive cars becomes more popular, we begin to see more and more constant velocity joints being processed, as this is one of the weak points of the drive line. Axles are treated to stave off fatigue failures in the splines.
Virtually every part of an engine will respond to cryogenic processing, with all components exhibiting life increases. Several component manufacturers are starting to take advantage of this and are treating their racing components as part of their production. Some of the main applications are:
• Connecting rods usually fail in fatigue. This occurs because of the high “g” loading of the piston and pin. Sprint Cup engines currently run around 9300 rpm. They have a stroke of around 86mm (3.375 in.) Pistons and pins typically have a mass of around 650 grams (23 oz.). Given these figures, it can be calculated that the upward force the piston and piston pin exerts on the connecting rod during the exhaust stroke is over 4800 g’s. Although this calculation ignores the weight of the small end of the connecting rod, it can be seen that there is a repeated stress on the rod, which has a cross sectional area of under 230 mm2 (0.35 in.2). Cryogenic processing increases the fatigue life of connecting rods considerably. We process steel, titanium and aluminum rods. The steel rods are generally AISI 4340 or 300M steel, aluminum rods are usually 7075 T6.
• Cylinder heads: Both aluminum and cast iron heads usually fail by cracking, which results from both thermal cyclic fatigue and the flexing of the head under combustion pressures. Further, the heads are often subjected to the extreme pressures created when the fuel mixture detonates. All these pressures can cause the head to flex so much that it is not unusual to find debris such as piston coatings under the heat gasket, blown there during a combustion stroke. Several Sprint Cup teams have concluded that 356 T6 aluminum heads yield about double the life after cryogenic processing. Other racers have the heads (both aluminum and cast iron) treated as a matter of routine. Of course, treating the heads increases the life of valve seats and valve guides. It is interesting to note that the heads can be treated with the valve guides and seats installed.
• Camshafts and lifters: Roller lifters usually fail by breaking, some of which is just poor design with sharp edges and stress risers all over. Even so, one customer reports that he gets about five runs down the drag strip unless he cryogenically processes his lifters. After cryogenic processing, he typically gets over 100 runs. Sprint Cup rules specify solid lifters. These cars are turning around 9300 rpm, so valve spring pressures have to be very high to slam the valve shut. The current practice is to create a cam profile that will actually loft the lifter. The lifter is thrown up in the air, forcing the valve to open very fast and then the spring slams the lifter down back against the cam. This creates extreme wear, but it gets the valve wide open as quickly as possible and leaves it wide open to the last possible microsecond. The lifters start with a slightly convex surface and wear into a concave configuration. Typically, they are cast iron and heat treated to the mid 50’s HRC. In use, any wear increases the valve lash and delays valve lift, creating a loss of power. It also leaves a lot of wear particles in the oil. It can take up to three sets of lifters to get an engine through dyno testing and the race due to the extreme wear caused by these radical cam profiles and high spring pressures. Cryogenic processing reduces this wear by about half. Camshaft wear is also a problem. Cryogenic processing has proven a boon to these racers because it reduces wear and therefore reduces camshaft replacement costs.
• Bearings: At least one racing bearing manufacturer cryogenically treats babbited bearings as part of their production process. They found it increased the life of the bearings and also of the steel backing, which tended to fail in fatigue. It is interesting that CRYOGENIC PROCESSING has an effect on the babbit metal of the bearings. Similarly, bronze bushings used on wrist pins also wear considerably less when treated. Many racers are processing ball bearings and roller bearings (typically 52100 steel) because they get a three to five fold increase in life. Rod ends used in steering and suspension systems get the same treatment and performance gains.
Cylinders, Pistons and Rings
Cryogenic processing of piston rings and cylinder walls has been shown to reduce wear substantially. One go kart racing customer claimed that he got a fivefold increase in engine life before he had to freshen the engine. Better ring seal was born out in pressure readings on a dynamometer. Apparently, this happens because the parts machine and hone better after treatment as a consequence of a more uniform hardness distribution over the surface of the part. CTP has done tests that show a significant reduction in the standard deviation of hardness readings taken before and after cryogenic processing. In some cases, the standard deviation is one third of what it was before the process. Processed piston rings typically wear both less and more evenly than untreated rings. More tribologically compatible with the cylinder walls, they tend to flutter less due to the vibrational damping the process imparts into the material and due to the more even hardness of both the rings and the cylinder walls. All these factors combine to give better ring sealing, and therefore more power.
Cryogenic processing of engine blocks also stabilizes the blocks and reduces warping and distortion due to vibration and heat during use. Caution is needed here because if the process is not done right, it can cause warping. Typically an untreated cast iron engine block will have readings that vary from 20 to 29 on the Rockwell D scale. After cryogenic treatment the readings will be 22 to 23. Blocks cast with CGI typically measure 19 to 31HRc before treatment and 25 to 26 afterward. Some of our Pro Stock drag racing customers tell us that they only need to hone the cylinders .0002” when freshening the engine due to the stability of the blocs after treatment.
The same is true for pistons. Several engine builders, who specify the process, have taken careful measurements of pistons before and after use, finding that cryogenically processed pistons distort less under use. Crankshafts benefit greatly from cryogenics. Several of the most respected names in the crankshaft business use cryogenics as a part of their thermal treatment. Cryogenic processing greatly decreases wear on crankshaft journals and stabilizes the crankshaft. We have treated everything from stock cranks through special racing nodular iron cranks and racing cranks made of 4340 steel. DCT is virtually mandatory in racing classes that specify production crankshafts if you want the crankshaft to survive.
Virtually all parts that are subject to stress or abrasion can benefit from cryogenic processing. Even head gaskets benefit because the armor around the combustion chamber is subject to both thermal cyclic fatigue and to flexing fatigue.
Keys to the Process
Success of cryogenic processing is critically dependent on the equipment in which the processing is done. There are companies that will dip your components in liquid nitrogen and pronounce them “treated.” For a good look at why this is a bad idea, go to www.youtube.com/watch?v=yg45ILXZ26w. This is a video where two Lincoln Laboratory scientists put a rubber stopper into liquid nitrogen. The result is that it explodes. Metal parts are stressed in much the same way by immersion. They may not explode, but they can crack and achieve very high levels of tensile residual stress.
The best cycles are those that reduce the temperature of the part slowly, typically going down to -300°F in eight hours or more. The part should then be held at -300°F for an extended time that depends on the material being processed. Research is indicating that the time at
-300°F is very dependent on the alloy being processes. The hold part of the cycle can be as short as 8 hours or as long as 60. The hold part of the cycle is followed by a slow rise in temperature to ambient. Many alloys need at least one heat tempering cycle to finish the process.
The quality and function of the machines available varies from very poor to excellent. (We’ve seen a Styrofoam beer cooler touted as a processing machine.)
So does the ability of cryoprocessor manufacturers to support their machines with technical and processing advice. The best machines are ones that use a heat exchanger to cool the parts. Vacuum insulated machines are more efficient in their use of liquid nitrogen. This keeps costs down as liquid nitrogen is a major expense in the process.
Cryogenic processing is an economic means of extending performance of metals. It has been proven on the race track, in the laboratory, on the road and on the production lines of industry.
Roger Schiradelly and Rick Diekman of Controlled Thermal Processing, Inc. have over 10 years of experience with cryogenic processing. Roger works out of the Mooresville, N.C. plant and is a specialist in racing technology, with over forty years of experience in racing. Rick works out of the Park City, Ill. plant and is the chairman of the ASM Cryogenic Processing Committee. For more information, visit www.metal-wear.com.
For a PDF of this article (complete with photos), go to: