Cryogenic Temperature: Everything You Need to Know
- October 29, 2020
- Controlled Thermal Processing
- Cryogenic Processing
There’s a common misconception about cryogenic temperatures. Low temperatures like ones that can be achieved by most common cooling equipment is nowhere near as low as it needs to be in order to be considered cryogenic.
The cryogenic temperature range is defined as from −238 °F to −460 °F, which is absolute zero, the point where molecular motion is as close to theoretically possible to stopping completely.
Temperatures above −238 °F can, therefore, not be considered cryogenic. Any treatments applied at such temperatures won’t qualify as cryogenic treatment. That’s because the changes in the metal’s crystalline structure that forms the basis of this treatment can’t happen unless the metal is treated at cryogenic temperatures.
The development of the field of cryogenics can be traced back to 1877 when oxygen was first cooled to the point that it became a liquid.
One of the most common uses of cryogenic gas liquefaction techniques is the storage and transportation of liquefied natural gas. It’s a mixture of ethane, methane, and other combustible gases. When liquefied, it can contract significantly at room temperature and can thus be easily transported in insulated tankers.
What temperatures qualify as cryogenic?
It’s not common for us to encounter cryogenic temperatures in normal physical processes. While the freezers at our home are perfectly capable of freezing anything we throw inside them, their lowest temperature is a long way off what can be considered cryogenic.
Specialized equipment and materials need to be utilized in order to reach temperatures that can qualify as cryogenic. These temperatures are easily achieved and maintained with the use of cryogens or liquefied gases. Liquid nitrogen and helium are among the most commonly used cryogens.
The −238 °F to −460 °F is a clearly defined range of cryogenic temperatures. As discussed previously, it’s not common to encounter such low temperatures in ordinary processes.
They can only be achieved when using specialized equipment and cryogenic liquids such as liquid nitrogen and helium. However, there are a lot of benefits to be had when treating metals and plastics at cryogenic temperatures.
How do you reach cryogenic temperatures?
It’s not possible to reach cryogenic temperatures without the use of specialized equipment and cryogenic gases like liquid nitrogen and liquid helium.
Only then does it become possible to lower the temperature to the range in which processes like cryogenic hardening of metals can be carried out.
Methods, tools, materials used
Tools
Two main types of tools are used to reach cryogenic temperatures. Cryocoolers and other equipment. This includes cryogenic storage racks and boxes, temperature controllers, cryogenic refrigerators, and tanks.
The cryogenic refrigerators are particularly important as they’re used to preserve the cryogenic materials.
In such a refrigerator, the refrigerant is circulated in a fluid flow patch between the first and second chambers. Tanks or dewards are used to store the cryogenic liquids which include liquid oxygen, nitrogen, argon, carbon dioxide, and methane. Large quantities need to be stored.
Liquids
It’s impossible to achieve cryogenic temperatures without using cryogenic gases. So what are cryogenic liquids? These are the liquids that have normal boiling points below –130°F.
These are actually some of the most widely used industrial gases that need to be handled, transported, and stored in liquid state at cryogenic temperatures.
Something to be very careful about when handling these liquids is that they can provide very large quantities of gas when they vaporize. A simple example is that of a liter of liquid nitrogen that can vaporize to 694 liters of nitrogen gas at 68°F and 1 atm.
They can’t be maintained as a liquid indefinitely, though, even if the containers are properly insulated.
The cryogenic liquids are all extremely cold gases with boiling points below -238°F. The boiling points for liquid nitrogen, helium, hydrogen, argon, and oxygen are -320.4°F, -452.1°F, -423.2°F, -302.5°F and -297.3°F respectively.
Processes
Handling these cryogenic liquids requires extreme care as they can immediately damage skin tissue because of the extremely low temperatures.
As such, a highly specialized process is used when working with these liquids to ensure safety for the workers while achieving the intended objectives.
The gases are used in a cryogenic refrigeration system for the cold station, this can either be a bath of cryogenic liquid or a conductive surface that’s cooled to the bath temperature. The materials that need to be processed are then fastened to the surface.
These systems have robust insulation in order to minimize heat leaks into the extremely cooled parts. High-vacuum technology is used in this process and so are cryostats that allow for the temperature to be adjusted by adjusting the rate at which the cold gas vapor removes the heat that flows in from the room temperature.
How do cryogenic treatments help harden materials?
The cryogenic treatment fundamentally changes the crystalline structure of the material being treated. This is a result of the extremely low temperatures that the material is subjected to. This process affects the entire volume of the material so the benefits of the treatment don’t go away when the material is machined or sharpened in the future.
The retained austenite becomes martensite when the treatment is completed. These are the patterns that are present in steel and cast iron.
Martensite is what gives steel its hard characteristic. It’s formed by heating the steel up until it forms an austenite structure and then cooling it rapidly.
When metal is cooled quickly, more of the steel changes to martensite. However, some austenite can remain and that’s called retained austenite.
Deep cryogenic treatment can convert most of this retained austenite to martensite.
The change of retained austenite to martensite is a result of the manipulation of the crystal structure of metals that’s caused by cryogenic processing. It helps harden materials by reducing residual stresses that may have built up during the forging or casting stages, reducing point defects, redistributing of alloying elements, and making the crystal lattice structure of the metal more uniform.
CTP Cryogenics has got you covered
If the benefits of cryogenic processing appeal to you and you want to utilize the process for your materials, there’s a much better way than going out and spending an insane amount of resources in building your own cryogenics facility.
Just work with a company that has decades of experience in this field and is widely considered to be an industry leader.
That’s where Controlled Thermal Processing Cryogenics comes in. We have almost four decades of experience in cryogenic processing, having worked with both influential clients like NASA’s Space Shuttle program and the US Postal Service, as well as local industrial clients that are just looking to increase the lifespan of their tools and dies.
We provide coast to coast services from our three locations across the United States and utilize the most advanced research and technology for cryogenics.
Our services include the processing of metal parts for industrial, transportation, marine, and electronic audio applications, micropolishing, sales of cryogenic equipment, and metallurgical advising.
We also partner with our customers to create and market superior cryogenics products.
Reach out to us for a free quote if you’re interested in any of the aforementioned services. We’ll be happy to have a chat with you and find a solution that best fits the needs of your business.
