A Primer on the Cryogenic Process
- September 24, 2020
- Controlled Thermal Processing
- Cryogenic Processing
Metals have long been subjected to thermal processing with heat treatment being one of the oldest methods of altering the characteristics of a metal. Such treatment is carried out to achieve positive change in tensile strength, compressive strength, and toughness.
It’s important to understand that the crystal structure of a metal is not uniform. Imperfections are introduced in its structure by way of the processes that are used to give metals a useful form, these processes can be rolling, forging or casting. The imperfections may include variations in spacing, discontinuities, and more.
Since parts that are heat treated have built up residual stresses, this can cause warping and dimensional change if enough heat and vibration is applied to the metal. Cryogenic processing helps relieve the residual stresses and subsequently reduces the risk of warping or dimensional change.
What is cryogenic processing?
Cryogenic processing or cryogenic hardening as it’s also called is simply the modification of a material or component using cryogenic temperature. It provides vast improvements over cold treatment processes as they utilize dry ice and alcohol. Conventional cold treatment can only achieve temperatures of 178ºK (-140ºF, -96ºC).
Studies performed by the Illinois Institute of Technology Research Institute for the US Army Aviation and Missile Command have shown that cryogenic treatments can provide an improvement in metal performance above that provided by cold treatment.
Regardless of just how perfect the production process may have been, flaws in the final material will remain. These imperfections in the metal are often caused by inconsistent treatment, variances in the raw materials, and a variety of other factors.
At the heart of cryogenics is the causation of change in the crystalline lattice of atoms that make up metals like steel. The shape of this lattice is set by the alloys in the metal and the manner in which it’s cast, forged or later treated.
The atoms in this lattice are bound together but they have empty spaces between them. In order to achieve the theoretical correct distance between these atoms, the temperature of the crystalline structure has to be lowered so that the crystalline structure can become more nearly ideal. Cryogenic processing requires gradual cooling and eventual reheating of the metals in a precisely controlled manner to force the flaws in the lattice to correct themselves. What isn’t cryogenic processing?
Conventional cold treatment of metals can’t be called cryogenic processing. . Dry ice and alcohol can only manage to achieve temperatures of -140ºF, these are not considered to be cryogenic temperatures.
Liquid nitrogen is one of the most commonly used cryogens It’s capable of reaching temperatures as low as −320°F. The cryogenic hardening process can only take place at such low temperatures which is why treatments that do not meet these specifics can’t be considered cryogenic.
Many are also under the assumption that cryogenic processing is a coating. This couldn’t be further from the truth. The treatment affects the entire volume of the part. Since it’s not a coating, cryogenic hardening doesn’t go away when the part is machined or sharpened.
A brief history of cryogenic processing
Cryogenic processing finds its roots in the sub-zero treatments of the past. It has long been known that Swiss watchmakers would bury newly made parts in the snow to enhance their durability and dimensional stability.
The word cryogenics comes from the Greek word “Kryos” which means cold and “Genes” which means born. Kamerlingh Onnes is believed to be among the first to use the term to describe his Leiden Cryogenic Laboratory. He was also the first physicist to liquefy helium.
Cryogenics took root in the United States during the second World War at Watertown Arsenal in Massachusetts under Clarence Zener. The method involved immersing steel cutting tools in liquid nitrogen for a set period of time, then removed and allowed to warm up before being placed on the production lines. The tools that survived the crude quenching were found to have significantly higher service life.
A step by step guide to cryogenic processing of metals and plastics
- Conventional Heat Treatment
The material needs to go through several stages before it can achieve the desired results. At the first stage, it’s exposed to heat through a conventional heat treatment process. Heating up the material followed by quenching to room temperature by immersion in oil, water or air produces microstructural changes that are responsible for improved mechanical properties of the material.
Gradual CoolingGradually bringing down the temperature of the material is crucial to the hardening process. The process thus starts by very slowly cooling the material with gaseous nitrogen which evolves from boiling nitrogen. This is an important step because the gradual decrease in temperature helps avoid thermal stress.
- Sustaining Temperature
Cryogenic hardening isn’t just dipping the materials in liquid nitrogen and calling it a day. It’s actually a very precisely controlled process that requires the use of specialized machinery and computers. The materials have to be kept at a stable temperature of around −320°F for up to 24 hours.
- Heat Tempering
Heat tempering follows next. The temperature of the materials has to be brought up slowly to around +300°F. This step is needed in order to reduce any brittleness that might have been caused by the formation of martensite during the cryogenic hardening process.
- Quality checks
Towards the end of the cryogenic treatment process, robust quality checks are performed to ensure that the materials have been hardened properly.
Enter CTP Cryogenics
You may have noticed by now that cryogenic hardening is something that requires expertise. It’s not something that everyone is capable of doing and the true benefits of this process can only be realized if it’s carried out by people who are extremely knowledgeable about the subject matter. This is where CTP Cryogenics comes in.
At Controlled Thermal Processing Cryogenics, we used state-of-the-art highly precise equipment to carry out cryogenic hardening. Our company has a highly experienced leadership at the helm that has been a part of this industry for almost four decades now.
Our company is a leader in cryogenic processing and treatment of metals and plastics. We operate from three locations across the United States, helping customers improve the quality and increase the lifespan of everything from boat propellers to street engines and CNC tools.
CTP’s mission is to use the most advanced research and technology in cryogenics to deliver exceptional results for our customers. Our services include the thermal processing of metal parts for transportation, marine, and industrial applications, micro polishing, cryogenic treatment equipment sales, and more. The results of our cryogenic processes have been verified by researchers affiliated with NASA, US Army, Illinois Institute of Technology, and more.
We provide our cryogenic processing and consulting services across the United States, coast to coast. Get in touch with the team at CTP Cryogenics today for a free quote and learn more about how we can help out.