Fatigue behavior of bainitic and martensitic super clean Cr–Si high strength steels

• Austempered samples presented higher Nf for the highest stress amplitude (500 MPa).
• For the lower stress amplitudes (475 and 450 MPa), both samples showed similar Nf.
• Differences in behavior of HCF correlate with microstructure/local stresses.

This study compares the fatigue behavior of quenched/tempered and austempered super clean Cr–Si high strength steels (HSS). The microstructure of the quenched and tempered steel consisted of tempered martensite and that of the austempered steel, predominantly lower bainite. Steel samples that were given the two types of heat treatments were fatigue tested in the high cycle fatigue (HCF) regime (105 < Nf < 107) and just entering the very high cycle fatigue (VHCF) regime (Nf > 107). Fatigue crack initiation and propagation modes in samples given the two types of heat treatments were similar. However, those with the lower bainite structure performed better at the highest stress amplitude (500 MPa) used in the study and this was measured in terms of total fatigue life. At lower stress levels (475 MPa and 450 MPa) the total fatigue life of both samples tended to be similar.