When a steel part is quenched from austenitising temperature, the intention is to transform the austenite fully to martensite and achieve excellent hardness. However, the entire amount of austenite does not always transform to martensite and some portion of it remains as ‘Retained Austenite’. It is important to know what causes Retained Austenite (RA)?
The following are the main causes of Retained Austenite:
- The steel contains high % of austenite stabilising alloying elements such as Nickel, Manganese, Carbon, etc. Due to this reason retained austenite is usually encountered in high alloy steels (containing the above mentioned alloying elements), tools steels and also in the case of carburised parts having high surface carbon % and other austenite stabilising alloying elements.
- High austenitising temperatures cause higher amounts of carbon and other austenite stabilising alloying elements to dissolve in austenite and thus increases the stability of the austenite
- Slack quenching also causes retained austenite. Hence the steel should be quenched rapidly from austenitising temperature
- High quenching oil temperature also prevents the quenched part to reach the martensite finish (Mf) temperature and hence the martensite transformation is not completed.
In my forthcoming blogs, I will dwell on how to reduce the % RA after it has formed and how retained austenite affects performance of heat treated parts in different applications.
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