General Properties of Retained Austenite
Retained austenite (by nature) is soft and has a much lower strength and wear resistance than is seen in freshly formed plate martensite. In addition to this, the fatigue resistance will also have a lower value. Further, the retained austenite at the surface will most certainly reduce the residual compressive stresses.
One condition that is not always recognized is that whatever retained austenite is present within the formed case will progressively transform in two ways:
- Dimensional change in size will be seen, simply because the retained austenite is continually transforming itself to fresh untempered martensite and will only stop transforming when there is little or no retained austenite.
- A progressive change in surface hardness (an increase in surface hardness) will also occur should retained austenite be present as it transforms to martensite.
Control of the Formation of Retained Austenite
The critical areas of control of the formation of retained austenite lie primarily (but not totally) with the analysis of the process-gas carbon potential within the process chamber and to the steel surface. The ideal surface carbon content and atmosphere carbon potential should be anywhere between the eutectoid line on the Iron-Carbon equilibrium diagram (0.77%) up to an approximate surface carbon content of 1.1% maximum.
It is necessary for the heat treater to have an accurate method of carbon potential evaluation, such as a dew-point tester, shim analysis or oxygen probe. The next area of concern will be the selected case austenitizing temperature. Quenching from too high an austenitizing temperature will contribute to grain growth, untransformed austenite (retained austenite) and quench-medium temperature.
So, it can be seen that the potential for retained austenite can be problematic as far as repeatable metallurgy is concerned and an accomplishment of specific mechanical properties may not occur. Therefore, it should be mandatory that both analysis and control of the process conditions be of primary concern to the heat treater.
To be continued...