Fig. 1 Schematic representation of the austempering treatment at 300 °C. The TTT data correspond to 0.55C-1.62Si-0.78Mn-0.77Cr steel [16]

Fig. 2 Schematic of rotating-bending fatigue setup showing specimen dimensions

Fig. 3 Microstructures of experimental steel after different heat treated: a austempered at 300 °C; b austempered at 350 °C; c quenched tempered at 460 °C

Fig. 4 S–N diagrams for all heat treatments

Fig. 5 Fractograph of a fractured fatigue specimen of the QT sample, the applied stress is 740 MPa

Fig. 6 Fractograph of a fractured fatigue specimen austempered at 300 °C (A1) and the corresponding single point EDX analysis results of the observed inclusion, applied stress is 975 MPa

Fig. 7 Fractograph of a fractured fatigue specimen austempered at 350 °C (A2) showing multiple initiation zones, applied stress is 740 MPa

Fig. 8 Detail view of the of crack initiation zone which is indicated by the arrow in Fig. 7, notice multiple initiation sites and radial ridges

Fig. 9 Fractographs of final fracture zone of sample QT a, sample A2 b, both showing a mixture of brittle and ductile fracture

Fig. 10 XRD patterns in the fracture surface and the bulk of A1 and A2 samples, note the disappearance of the austenite peak in the bulk

Fig. 11 Comparison between endurance limit (Rfat), crack surface hardness (HV Fat) and bulk hardness (HV Bulk) for the samples with different heat treatments