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

High-cycle fatigue behavior of non-heat-treated steels for 900MPa 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 (840-890MPa) as that of TM steel (910MPa) but their fatigue ratios R_f=(fatigue strength at 10⁷cycles)/(tensile strength) were somewhat different from each other; in the F-P steels R_f was comparable to that of the TM steel but the GBF steels showed a somewhat reduced R_f (∼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 R_f 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.