High-cycle fatigue characteristics of non-heat-treated steels developed for bolt applications

High-cycle fatigue behavior of non-heat-treated steels for 900 MPa grade bolt applications, developed as substitutes for tempered martensite (TM) steel, was investigated. Four types of non-heat-treated steels with ferrite–pearlite (F–P) and granular bainitic ferrite (GBF) microstructures were developed by combining alloying and air cooling processes. The developed materials exhibited a similar tensile strength (840–890 MPa) as that of TM steel (910 MPa) but their fatigue ratios Rf = (fatigue strength at 107 cycles)/(tensile strength) were somewhat different from each other; in the F–P steels Rf was comparable to that of the TM steel but the GBF steels showed a somewhat reduced Rf (∼11%), due to a change in the crack initiation mechanism. For both TM and F–P steels, fatigue cracks initiated at the specimen surface, irrespective of the stress amplitude applied, and thus their Rf values became similar and conventional stress amplitude vs. fatigue life (S–N) curves were resulted. In the GBF steels, however, fatigue cracks initiated at inclusions, as the stress amplitude applied was low; this different crack initiation mechanism led to a reduction of fatigue strength and caused a duplex S–N curve. It was found that the F–P steels are a promising candidate to substitute for the TM steel in terms of the tensile strength and high-cycle fatigue resistance.