The cryogenic treatment process uses extremely low, or cryogenic, temperatures to treat and strengthen metals. It is generally used to normalize, or eliminate, residual stresses in a metal work piece that has been welded or heat treated. The temperatures used in cryogenic treatment typically exceed 310° below zero Fahrenheit (about -190° Celsius). Treating metal at these extreme temperatures generally results in a work piece that is more durable, less prone to cracking, and easier to machine into a finished part. Cryogenic treatment is used in many industries, but is most common in the manufacture of automotive parts, castings and many types of cutting tools.
When a metal part is welded, heat causes the metal to expand in the area around the weld. As the part cools, it contracts unevenly, leaving behind residual stresses and potential weak spots. The same is true of heat treating and other processes that induce high temperatures in the metal. The extreme cold temperatures employed in cryogenic treatment relieve these stresses and eliminate weak spots in the part by promoting uniform contraction and altering the grain of the alloy. Common cryogenic treatment practices include cryogenic deburring and cryogenic hardening.
Deburring is the process of removing sharp edges caused by machining, or flash that builds up when a part is cast or forged. In cryogenic deburring, a work piece is usually cooled with liquid nitrogen to cryogenic temperatures. The extreme cold renders burs and flash brittle, making them easy to remove by shot blasting or tumbling. Since they are soft and difficult to machine at room temperature, plastic and rubber parts are often cryogenically deburred, as well.
When a metal part is heat treated, a crystalline structure, called austenite, is transformed to a different shaped grain, called martensite. Since some standards require more martensite than is usually produced by heat treating, the transformation is often extended by cryogenic hardening, which is usually done at temperatures of -300°F (-185°C) and below. This type of cryogenic treatment causes a rapid change in the structure of the alloy, resulting in a higher percentage of martensite.
The music and electronics industries have also found ways to improve the quality of their products through cryogenic treatment. It is believed that cryogenically treating a brass instrument, such as a trumpet or saxophone, relieves residual stresses created during manufacturing, resulting in better overall vibration and improved intonation. Even steel guitar strings and electronic components, like stereo cables and connectors, are said to show better performance after undergoing cryogenic treatment.