The retained austenite will also have another effect on the carburized steel – tensile strength (depending once again on the steel chemistry). A decline in the tensile strength as well as the impact strength will be seen.
One of the primary considerations for the use of carburized steel is the fact that it can be carburized and quenched to form a hard-wearing surface. The formed case is an excellent wear-resisting surface. Although the wear surface will not last forever, it will resist wear providing that the core hardness will be able to support the formed case.
The formed case will generally be comprised of martensite with some small spheroidized carbides. Any heat generation by friction that may develop during the face contact of the carburized component and the article that is likely to cause wear to it will begin to encourage carbide precipitation and some softening of the surface hardness. This can also be further aggravated by surface roughness. So, it becomes an important issue to decompose and stabilize the retained austenite.
How then do we control the formation of retained austenite?
It is very necessary to control as many of the process parameters as possible. Those parameters can include (but are not limited to):
- Carburizing temperature
- Carburizing time at temperature
- Process atmosphere carbon control
- Prior heat treatment
- Case austenitizing temperature
- Quench-medium temperature
- Final tempering temperature
For atmosphere carburizing, there are a number of ways to control the carbon potential. They are dew-point analysis, shim analysis and oxygen probe. The other two methods of carburizing with control would be plasma ion carburizing and low-pressure (vacuum) carburizing.
The last two methods will be reliant on acquired empirical data based upon surface area. The process of low-pressure carburizing is the current state of the art of carburizing. Atmosphere carburizing is still a viable process consideration, particularly when some of the industrial gas companies are now able to reduce the risk of grain-boundary oxidation at the carburized surface.
If retained austenite is considered to be a problem to a carburized component, refrigeration treatment should be introduced as a matter of course on completion of the carburizing procedure and the case austenitize and quench.