Ultra-high cycle fatigue property of a multiphase steel microalloyed with niobium

The effect of microstructure on the ultra-high cycle fatigue behavior of a high strength steel microalloyed with niobium (Fe-0.23C-1.73Si-2.24Mn-0.68Cr-0.042Nb) was investigated through ultrasonic fatigue test. The ferrite and martensite (F/M) multiphase structure was obtained by air-cooling treatment, while the bainite and martensite (B/M) multiphase structure was obtained by oil-cooling treatment. The ultra-high cycle fatigue limit (109 cycles, σw9) of steel with F/M multiphase structure was 750 MPa, while the σw9 of steel with B/M multiphase structure was 900 MPa. In the high cycle regime, cracking occurred at the non-inclusion site in the steel with F/M structure. However, half the specimens cracked at the non-inclusion site, the others cracked at inclusions in the steel with B/M structure. The study clearly underscores that the microstructure had a distinct impact on the ultra-high cycle fatigue behavior of high strength steels. Moreover, the present work demonstrates that the fatigue properties of steel can be optimized by using appropriate heat treatment, which is related to the change in the F/M microstructure to B/M microstructure in an advanced Nb-microalloyed steel.