Enhanced torsional fatigue performance of SAE 8660 steel via modified austempering: role of bainite-martensite microstructure
High-performance transmission components such as shafts and axles are frequently subjected to cyclic torsional loading, where fatigue resistance is critical for service reliability. In this study, a modified austempering process was developed to produce a bainite-martensite mixed microstructure in SAE 8660 steel for enhancing torsional fatigue performance. Microstructural characteristics were systematically examined using optical microscopy, scanning electron microscopy, and quantitative electron backscatter diffraction analysis. Mechanical properties were evaluated through hardness, tensile strength, torsional strength, and torsional fatigue tests. In addition, finite element analysis (FEA) was employed to compare the macroscopic deformation responses of the different heat-treatment conditions using experimentally measured constitutive data. Compared with the conventional quenching and tempering condition, the modified microstructure exhibited improved torsional fatigue resistance while retaining a relatively high torsional strength, demonstrating an effective balance between strength and deformation capability. Microstructural characterization and fractographic observations were consistent with improved strain accommodation and more tortuous crack propagation in the bainite-martensite mixed microstructure. The FEA predictions reproduced the experimentally measured maximum shear stresses with deviations below 3.3%, while the calculated damage was primarily localized within the central gauge section. These results indicate that controlled bainite-martensite microstructure design via modified austempering represents a promising approach for improving torsional fatigue performance in high-strength steels used in torque-transmission components.