5 Myths About Cryogenic Hardening, Explained
There are many different methods of treating metal to improve its durability. Heat treatment is one of the most common methods utilized for this purpose. It’s carried out either to increase the metal’s softness or increase its hardness, depending on the case. The same process is also utilized to make changes to the electrical and heat conductivity of metals.
Over the years, a need has been recognized to remove the imperfections that various treatment methods can leave behind. That happens more often than not regardless of how perfect the production process may have been.
Metals can have residual stress in them once they’re put through heat treatment, for example, so there is a need for removing those stresses.
That’s where the process of cryogenic hardening or cryogenic processing comes in. It enables a change in the crystalline lattice structure of the metal in order to increase the metal’s durability and subsequently, the lifespan of metal products like engine parts, brake rotors, industries dies, tooling, and more.
What is cryogenic hardening?
Metals are made up of atoms that are lined up in a crystalline lattice. This lattice is set by the alloys in the metal and its structure depends on the way the metal has been cast, forged, processed, or treated. Cryogenics treatment focuses on the manipulation of this lattice to improve the metal’s durability.
The flaws in the lattice structure aren’t visible to the naked eye. Nevertheless, they do create weaknesses and stress points in the metal.
To remove these weaknesses, the theoretical correct distance between the atoms in the lattice needs to be achieved. This can be done by lowering the temperature of the crystalline structure which draws the molecules closer together and collapses the empty spaces between them to achieve the correct distance.
Cryogenic processing takes things up step further. It lowers the temperature of the materials to as low as -450℉ in order to force the structure into an even tighter alignment. The result is a vastly improved structure that lends higher durability to the metal.
The most common misconception about this process is that some believe it to be a surface coating of some sort. Cryogenic hardening isn’t the result of some coating that’s applied to the surface of the metal. It actually affects the entire volume of the part. This means that the benefits of cryogenic processing don’t disappear when the material is machined or sharpened later on.
It strikes at the very core of the metal so it affects every single atom of the entire structure. When performed correctly, it results in a reduction in point defects, the redistribution of alloying elements, and the reduction of residual stress.
5 myths about cryogenic hardening, explained
- Cryogenic hardening increases brittlenessA common myth that people have about cryogenic processing is that they believe it will increase the metal’s hardness, thereby making it more brittle. It likely stems from the fact that while cryogenic processing can make metals more tough, people tend to equate toughness with hardness which in this particular sense would mean brittleness.The fact is that cryogenic processing isn’t a substitute for heat treating, as such, it can’t increase the hardness of metals and parts substantially because the surface and the core of the metal is cooled very slowly and they’re kept in equilibrium.
Since uniform residual stresses are imparted at the surface, this lends to increased durability without a significant increase in hardness.
- Cryogenic hardening can wear off with timeThere are many different types of surface treatments available for metals and plastics but that’s not what cryogenic processing is all about. It’s not a coating that can wear off after some time and would then need to be reapplied.Though it’s true that plating and other different coatings can be enhanced by cryogenic processing.
A coating isn’t going to make changes to the crystal structure of metals, which is what cryogenic hardening does. It would only protect the surface of the material against elements like rust.
It also wouldn’t do much to increase the durability of the material or to remove any residual stresses that may have been built up.
- Cryogenic treatment is a substitute for heat treatmentAnother common myth that many people believe is that cryogenic treatment is a replacement or substitute for heat treatment. That’s also not true. For example, the hardness of a soft tool steel can’t be increased with just the treatment. Heat treatment is a way of transforming the grain structure of the metal.Cryogenic treatment alone isn’t going to increase the hardness of the metal parts substantially. That’s why it’s utilized as an enhancement to heat treatment and not as a substitute. When it’s used in conjunction with proper heat treating, the resulting part would be better and more stable.
- Simply dipping the metal in cryogenic materials is enoughThe treatment process is a scientific one and as such, requires the use of specialized knowledge and advanced tools to achieve the results desired.A common myth that people have about the process is that they believe it can be performed fairly easily — that you just need to dip the material in a cryogenic material and take it out.
That couldn’t be further from the truth. In deep cryogenic treatment, the metal is submerged in cryogenic materials like liquid nitrogen for up to 24 hours. You cool the metal very gradually and then maintain the temperature at that level. Once done, you slowly bring it back up to room temperature.
- Cryogenic hardening is a simple process that anyone can doReading about the process does make it appear simple. What’s there to it other than basically a giant tub of liquid nitrogen or helium in which you drop a metal part and then forget about it for a day? Approaching cryogenic hardening from this perspective can be very harmful.First and foremost, cryogenic materials like liquid nitrogen require special care when being handled. That’s because they can instantly damage skin tissue because of the low temperatures.
Furthermore, highly specific equipment is required to carry out the process. This involves, among other things, regulating the temperature and the amount of material in the cryocooler, monitoring for any leaks or issues, then gradually raising the temperature to complete the process.
How cryogenic hardening and cryogenic treatment really works
When the steps mentioned above have been carried out, the changes in the crystalline structure become permanent when the temperature of the material is gradually brought back up to room temperature.
It goes beyond changing austenite to martensite as cryogenic processing works on materials other than steel. The change of retained austenite to martensite happens in steel and cast iron only.
Take brake rotors, for example. They are made from cast iron that’s pearlitic in structure. There’s no retained austenite but scientific tests have shown that cryogenic processing of brake rotors can increase their lifespan by up to seven times.
Most metals respond to cryogenic processing as do some plastics. There’s also evidence to support that it works on crystals like diamonds, cubic boron nitride, and aluminum oxide.
At Controller Thermal Processing Cryogenics, we have almost four decades of experience in performing deep cryogenic treatment of metals and plastics.
We use state-of-the-art equipment coupled with our immense knowledge in the field to deliver exceptional results for our customers.
CTP Cryogenics has a long list of satisfied customers which include the likes of NASA’s Space Shuttle program, Remington Firearms, the US postal service, and more. Our services are available coast to coast in the United States through three different processing locations.
Contact us today to get a free quote for any cryogenic processing needs that you may have. Our services also include metallurgical advising, cryogenic equipment sales as well as consulting.