High cycle fatigue behaviour of a multiphase microalloyed medium carbon steel: a comparison between ferrite–pearlite and tempered martensite microstructures

To improve toughness and fatigue strength, a mutiphase (ferrite (F)–bainite (B)–martensite (M)) microstructure was developed in a V-bearing medium carbon microalloyed (MA) steel through a two-step cooling process that was followed by an annealing (two-step cooling and annealing (TSCA)) treatment. In the present paper, the high cycle fatigue (HCF) response determined in terms of the endurance limit, long crack fatigue threshold (ΔKth), crack closure and fatigue crack growth rate (FCGR) in a material that has a multiphase microstructure is presented and compared with those of the same material with a ferrite–pearlite (F–P) and a tempered martensite (T–M) microstructure obtained by air-cooling (AC) and quenching and tempering (Q&T), respectively. Long crack fatigue threshold (ΔKth) and crack closure were evaluated using a dynamic compliance (DYNACOMP) measurement technique. The fatigue limit of the F–B–M and the T–M microstructures (∼400 MPa) was greater than that of the F–P microstructure (∼340 MPa). At load ratios less than 0.5, the threshold for long crack growth was lower for the F–B–M microstructure compared with that of the F–P microstructure. This is attributed to the reduced roughness-induced crack closure (RICC) contribution to the threshold in the former multiphase microstructure. A quantitative analysis of the near-threshold fracture surfaces validated the above conclusion. Fatigue crack growth rate in the Paris regime was found to be independent of the microstructure but dependent on the load ratio.