Effect of Tempering on the Stability of Retained Austenite in Carbide-Free Bainitic Steel

In this study, a C-Mn-Si alloyed bainitic steel was treated by bainitic austempering (BAT) process to form a multiphase of bainitic ferrite, filmy retained austenite (RA), and blocky martensite/austenite. Tempering was employed to optimize the microstructure and mechanical response of steels after BAT treatment. Microstructural alterations because of tempering that affects the mechanical stability of RA was discussed via X-ray diffractometer, high resolution transmission electron microscopy, and auger electron spectroscopy. Results showed that an excellent combination of strength and ductility was achieved after tempering (ultimate tensile strength: 1486 MPa; yield strength: 1027MPa; total elongation: 32.4%), which was attributed to improved overall stability of RA and high fracture damage resistance. Interestingly, compared to normal BAT steel, the transformation ratio and rate of RA exhibit three stages with increase of strain in tempered BAT steel, leading to abnormally high stability of RA at strain of 2% -10%. It is suggested that the nano-sized η carbides precipitated after tempering could promote the dislocation multiplication in BCC matrix (i.e., bainitic ferrite and martensite) and obstruct the movement of dislocation from BCC matrix to RA, which results in a ductile but strong tempered matrix that provides a “shielding effect” on RA and improves RA’s stability.