Friday, January 8, 2010

Carbon, Retained Austenite and Cryogenic Treatment

All heat treated steels have a certain percentage of retained austenite after heat treatment has been completed. The goal of heat treatment is to convert as much austenite into martensite. Ultimately, martensite is the most desirable crystal form as it is the toughest and strongest form. However, heat treatment does not remove all of the retained austenite from the steel.

Cryogenic treatment of heat treated steels will help improve the overall martensite crystal content in the steel that is treated. By lowering temperatures down to -120°F or below, it is possible to remove the retained austenite from the heat treatment process, while achieving a 100% martensite crystal structure.

As many of you know, steel is made up of carbon and iron. Carbon is the element that enhances wear resistance in steels. The higher the carbon content, the more wear resistant that steel will be. Tool steels such as A2 or D2 are rich in carbon. Cryogenic treatment enhances carbon clusters through the precipitation of eta-carbides. This helps an already wear resistant steel become even more resistant.

It is important to mention that the amount of retained austenite is directly proportional to the amount of carbon found in the chemistry of various steels. For example, a tool steel with a high carbon content will have significantly more retained austenite after heat treatment than a steel with a lower carbon content. A graph below shows the proportionality of retained austenite to carbon.


Therefore, cryogenic treatment will be of great benefit to high carbon steels as it will transform a greater amount of retained austenite while improving the wear resistance. Through the conversion of retained austenite to martensite, the high carbon steels will become more durable and will lack the voids and imperfections that untreated steels will suffer fatigue failures from.

The bottom line is cryogenic treatment will enhance high carbon steels micro-structure significantly while enhancing wear resistance factors. Due to the link between retained austenite and carbon, high carbon steels will benefit considerably more from the crystal structure transformations than steels with less carbon content. For more information, please visit our cryogenic treatment webpage. If you have any questions or comments, please call (508)-459-7447 or email rtaylor@nitrofreeze.com.

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