- The final case austenitizing temperature prior to quenching
- The alloy content of the carburized steel
- Surface carbon content
The first consideration to make before carburizing will be: What percentage of retained austenite can be tolerated in the component to be carburized? This consideration will apply to all of the various carburizing process techniques that are available.
It is reasonable to expect between 10-15% for surfaces that are in contact with each other. This will allow a small amount of deformation in the surface of the steel being carburized. In order to deal with retained austenite, one needs to fully understand the application of the part being carburized (all types of gears, sliding surfaces, etc.) Gears will demand a different “volume” of retained austenite to be present. Some applications will require an absolute minimum amount of retained austenite.
It can be said that retained austenite can be eliminated by utilizing very “tight” process parameters. Retained austenite is moderately soft even though it is saturated with carbon, and it exists with fresh hard martensite. Thus, lower than expected as-quenched hardness values will be observed.
The retained austenite is a failure of the “created” austenite (at case austenitizing) to completely transform into fresh martensite when quenched from the selected case austenitizing temperature. Thus, the amount of “softening” will be a direct relationship to the amount of austenite present in the formed case.
Once again, if there is to be a nearly complete transformation from austenite to martensite when quenching an austenitized carburized case, the process-control parameters need to be accurately set, monitored and maintained.
If it is necessary to decompose any retained austenite present in the formed carburized case, the decomposition procedure will be to cryogenically treat the component. Typical temperatures for the cryogenic treatment are approximately -300°F (-185°C) to -180°F (120°C). The procedure is, of course, time and temperature relevant.
Once the treatment is complete, the component should be allowed to rise naturally up to room temper. At that point, it is necessary to temper the component at 350°F (180°C) to 390°F (200°C). This is because the retained austenite has transformed into untempered martensite, which requires a temper. The cryogenic treatment will also induce dimensional stability to the component.