Gigacycle fatigue properties of a modified-ausformed Si-Mn steel and effects of microstructure

Abstract

Fatigue tests were conducted for modified-ausformed, oil-quenched, and water-quenched versions of a Si-Mn steel (JIS-SMn443) with a chemical composition of 0.42C-0.2Si-1.52Mn (in mass pct). The Si-Mn Steel has an advantage with respect to recycling, since it contains no Cr or Mo, although the steel has disadvantages with respect to tempering resistance and hardenability. Although the tempering resistance was not improved by modified ausforming, the problem of hardenability was minimized, since modified ausforming created fine and uniform martensite. The gigacycle fatigue properties of the Si-Mn steel were improved by the modified ausforming. The low-strength modified-ausformed steel (AF1400) was free of fish-eye fractures and had a fatigue limit of 770 MPa at 109 cycles. In spite of the occurrence of fish-eye fractures, the high-strength modified-ausformed steel (AF2000) achieved a fatigue limit of 830 MPa at 1010 cycles, higher than the 710 MPa limit of the water-quenched steel (QT2000W). The fatigue limits of the oil-quenched steels (QT1400 and QT2000), which contained numerous and large nonuniform structures, could not be determined because of the temperature increase of specimens during the 20 kHz tests. The fracture surfaces revealed optically dark areas (ODAs) even in the modified-ausformed steel when the specimens failed at over 107 cycles, since the steel was partially recrystallized during modified ausforming. The gap of fatigue limit between the AF2000 and QT2000W steels was not entirely explained by the difference of the ODA sizes. Also, the sizes of the nonuniform structures were likely to play an important role in the gigacycle fatigue properties, since the sizes in AF2000 were smaller than those in QT2000W.