What is the purpose behind deep freezing engine parts (and other components)? Though its roots go back to the 1800s, the cryogenic craze started about 20-plus years ago when news was that some Pro Stock teams were “freezing” engine parts. The only answer given when asked “why” was that the cold temperatures somehow moved the grain structure in the alloy making it more durable.
As time has passed, cryogenics has become a well-known process that has proven to be very effective. Today, there are quite a few shops that specialize in cryogenics and the process has become more affordable. In order to appreciate cryogenics let’s take a look at the process and the benefits associated with it.
Cryogenics is similar to heat-treating in that the metal is strengthened; but the process does not make it harder, just more durable. Cryogenic processing involves lowering the temperature of metal engine components to more than -300° F. This cooling process apparently rearranges the crystals within the metal, realigning the atomic structure and relieving stress.
According to experts, both on the treatment side and the end-user side, being “well known” and “well understood” aren’t the same thing. While the process itself has a proven track record, there is still a certain level of skepticism about the process’s legitimacy. Rather than just being “snake oil,” designed to quickly part customers from their money, cryogenics actually does something – whether you can see it or not.
The cryogenic process changes the microstructure of metals by subjecting them to super cold temperatures usually around -300 degrees F. This cold treatment creates changes in heat-treated steels, cast irons, and other alloys. Whether the steel is forged or cast, heat-treating is used to change the characteristics of the structure. The uniformity of the crystalline or micrograin structure of the metal is at an atomic level. When a piece of steel or cast iron is hardened, it is heated up into a range where the atoms of iron and the atoms of carbon form a particular type of crystal called austenite. This has a relatively soft grain structure with weak points. The cryogenic process causes this austenite to change to the more wear-resistant, yet more brittle martensite, which presents a more even crystalline structure.
Heat treatment going up to about 1,500° F removes some of the austenite but you have 10-15 percent retained austenite in the metal. These are literally voids and imperfections in the metal, and once they are converted to Martensite, it leaves the metal with only 1 percent Austenite. This gives a tougher, more durable metal without any imperfections.
All heat treated steels have a percentage of allowable Austenite that is retained during the heat treating process to form the Martensite. The Cryogenic process can help form additional transformation of the steel from Austenite into Martensite. This helps eliminate imperfections in the steel’s microstructure that can be seen using X-ray or microscope.
Stress Relief
Cryogenic treatment is also used as a form of stress relieving. Regardless of whether the metal is aluminum, copper, steel or cast all metals have some amount of residual stresses created when the metal changes from its molten state into its solid form. As the cooling process takes place, natural stress fractures occur. In addition, even after the metal is formed, cooled and then heat treated, it will usually undergo manufacturing processes such as grinding, cutting, machining, and forging where other stresses and fatigue are introduced. Cryogenic treatment helps eliminate and reduce fatigue and stress from the manufacturing process.
Precipitation of
Eta-Carbides
When steel is formed, it is made with iron and other elements such as carbon. The amount of carbon along with the other elements control the quality of steel such as hardness, ductility, and strength. The greater the amount of carbon contributes to how hard and strong the steel will be. The Cryogenic process helps modify the carbon structure into “Eta-carbides.” The formation of “Eta-carbides” visible with a microscope) causes the steel to be more wear resistant.
Experts say this process is less well understood than the others. More Eta-carbides give increased wear resistance. Scientists have theories about this anomaly but haven’t been able to really explain it – however, tests have been conducted to prove it happens.
Basically, how cryogenic treatment works is this: engine blocks, pistons, crankshafts or any other component is placed into a cryogenic treatment chamber. The chamber is sealed, and the temperature is then slowly lowered to -320° F using liquid nitrogen. The subfreezing condition is maintained for several hours; then the temperature is carefully and slowly raised again. Many materials then require a heat tempering cycle, so the temperature is raised even higher and then returned to ambient. These cycles may be repeated as needed.
Precise control of these processes is critical, and the computerized chambers must remain sealed during the cycles. To properly treat components may take from several hours to many days, after which the treated parts are stronger and more structurally stable.
Today’s treatment methods, which use liquid nitrogen flashed to vapor and computer-controlled dispersal systems, use no liquid in the treatment chamber, so parts are treated gently and completely.
But while cryogenics works with severe cold, the truth is, it’s actually an extension of the heat-treating process. A key component of the heat treating process is, of course, the cooling stage. As parts return to room temperature they develop their desired microstructure. However, certain high strength steels require very tight atmospheric control during the heat-treating process.
Cryogenic treatment does not take the place of heat treatment, rather, it completes what the heat treater started. When heat treating, the temperature is increased to the proper “high” temperature for the type of metal being treated. At the correct time, the parts being heat-treated are put in “the quench,” at which time the temperature begins to decrease. It is during the quench that the improvement actually takes place. Parts become cooler and cooler, and better and better. The heat treater considers the treatment completed when the temperature of the parts reach room temperature.
When cryogenic treatment is properly performed, the process begins at room temperature and drops via computer-control at the rate of one degree per minute until the temperature reaches 300° F. Although other metal products including engine blocks, machine tool cutting bits, rifle barrels, razor blades, sewing needles can all be cryogenically treated at the same time, the treatment time must be based on the biggest, thickest cross-section. The entire thickness is treated and receives the benefit which is normally improved performance, reduced wear and breakage, longer between rebuilds or replacement, reduced costs, and (hopefully) a more successful motorsports season. In other words, cryogenic treatment is not a surface treatment.
Cryogenic treatment is not a coating, nor is it a simple surface finish that can simply be “machined out.” The computerized treatment process ensures that the thickest cross-section of the metal is treated – in other words, the entire volume of the metal is structurally changed throughout. As such, a certain amount of machining can be done after the parts are treated.
Fantastic claims that cryogenically treated components are indestructible or that a treated engine will not break are, however, wishful thinking. According to experts, more than 75 percent of engine components fail because of fatigue. To the engine builder and racer alike, the benefits of this lengthy treatment process are three-fold: increased dimensional stability, stress relief and improved wear resistance.
As part of its core competency, Elgin Industries offers nine different types of commercial heat-treating options, including cryogenics, though not necessarily at the individual builder level.
“Let’s face it – it’s not an inexpensive process,” explains Elgin’s national sales and marketing manager Rick Simko. “We’ve treated blocks and other parts for certain teams we’ve sponsored, but the normal guy isn’t likely to want to spend the money with us for it unless he knows what he’s getting.”
Today, the cryogenic process is often overlooked when considering an expensive engine build. Maybe it is because it is something that you can’t see, feel or touch. After parts have been through the cryogenic process they come back looking virtually unchanged. The actual benefits lie underneath the surface unnoticeable by the naked eye.
In case you are wondering if it really works, as Simko points out, race teams practice the use of cryogenics on most parts for the entire car. If it did not work, especially on some of the small exotic pieces that see 9,000 rpm, it probably would not be practiced.
One word of advice would be to make sure that all of the machining is performed before having the cryogenic process performed. Whether you are considering a few pieces of an engine build or all of the components needed, make sure that the machining is performed and the assembly mocked up before having it done. This isn’t because subsequent machining is impossible, but it is certainly tougher because the metal becomes harder.
In fact, I have personal experience with this. Some time back I had an engine assembly treated for a spec class race engine. All of the machining was performed and the engine was mocked up to finalize everything before we sent the assembly off to be treated. Once the engine returned we then sent the components out to be coated with various other forms of engine coatings before the assembly. After the coatings were applied we reassembled the engine only to find out that the exhaust spring pockets now had to be machined an additional .060”. I’m not sure how that was missed but the problem had to be corrected. The solution was, of course, to cut the spring pockets and continue on. But that simple cutting of the spring pocket became something of an undertaking because the cryogenically treated cylinder head was now so hard and durable that it just kept dulling the cutter blade. We could machine the spring pockets but it was surprising to experience how the material had changed so aggressively through the cryogenic process.
It became evident that even though the process is something that you can’t actually see what you paid for sure does carry a lot of merit once you realize how it completely changed. Keep in mind that most shops that perform cryogenics keep their processes proprietary. As mentioned before, the process does bring temperature down to -300 degrees F. But how they arrive at that temperature and then bring the temperature back to ambient is their secret.
Still, at the end of the day it really doesn’t matter because you have pieces that are stress relieved, more wear resistant and more durable than they were new. ν
