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 diffrac-tometer, 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: 1489 MPa; yield strength: 1014 MPa; total elongation: 33.2%), which was attributed to improved overall stability of RA and high fracture damage resistance. Interestingly, compared to normal BAT steel, the transformation fraction 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 eta 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.

Automated summary

In this study, a C-Mn-Si alloyed bainitic steel was treated by bainitic austempering (BAT) process, which resulted in the formation of a multiphase microstructure consisting of bainitic ferrite, retained austenite (RA), and martensite/austenite. Tempering was employed to optimize the microstructure and mechanical response, leading to excellent combination of strength and ductility. The stability of RA was found to be significantly improved after tempering, attributed to the presence of nano-sized eta carbides that hinder the dislocation movement from the BCC matrix to RA, thus providing a shielding effect on RA.