I work for a bearing manufacturer, and we are currently doing a lot of work on retained austenite(RA). I confirm that for X-ray diffraction is a reliable method for measuring RA (however the reliably of the technique cannot measure accurately below 1% RA).
It is also possible to do growth calculations before and after an additional temper. Internally we use an additional temper of 220°C for 6 hours, with a growth of 10 microns per 100 mm diameter is equivalent to 1% RA for rings (52100 material). Allow 24-48 hours before re-measurement. As when the austenite is transformed in martensite and volume increase occurs.
Typically 440C has a 6-8% of stable retained austenite due to the high levels of carbon and chromium.
I have a question related to transformation of retained austenite:
If 440C material undergoes an additional temper (150-200°C), after hardening, quenching and tempering cycles (including a deep freeze), why does the material shrink afterwards? Typically having a RA value in region of 6-12%.
If the retained austenite is transformed persumably into martensite, it should grow. Is the martensite temperature not being reached? Therefore changing the austenite into ferrite and cementite (not ever changing to martensite)?