Study of the effect of microstructure heterogeneity on fatigue crack propagation of low-alloyed PM steels

Powder metallurgy steels frequently withstand cyclic loading in service. Therefore, it is of high importance to thoroughly understand their fatigue properties. A detailed study of the effect of microstructure heterogeneity on fatigue properties of sinter-hardened PM steels was performed using admixed nickel PM steels (FN0208) as well as pre-alloyed PM steels (FL5208). Specimens were pressed and sintered to a density of 7.3 g/cm3 in order to specifically investigate the effect of microstructural constituents on fatigue properties. Fatigue crack growth rates were measured at four different R-ratios, 0.1, 0.3, 0.5 and 0.7 for both PM steels. The negative effect of increasing the R-ratio on fatigue properties was observed for both alloys. The crack propagation paths were studied using optical microscopy and the tortuosity of the crack paths were compared with their equivalent linear paths. This approach allowed us to determine the effect of microstructure in different fatigue conditions. Fatigue fracture surfaces were also studied using scanning electron microscopy.