Evolution and thermal stability of retained austenite in SAE 52100 bainitic steel

In this work, the evolution of retained austenite during isothermal bainitic heat treatment in SAE 52100 steel, 1.01C-1.36Cr-0.32Mn-0.25Si (wt.%), is investigated with optical microscopy, X-ray diffraction and thermo-magnetic measurements. A significant amount of austenite is retained in the material. The dependence of the volume fraction of retained austenite on bainitic holding time at 503 K shows a maximum at 45 min. It has been demonstrated that the increase of volume fraction of retained austenite occurs as a result of the increased carbon concentration. The thermal stability of austenite is investigated. The temperature at which retained austenite starts to decompose to ferrite and carbides upon heating varies with bainitic holding time. The transformation of austenite to martensite during cooling till 10 K is found to be not complete, and a large amount of austenite remains untransformed.