Monotonic tensile and fatigue crack growth tests have been performed on AISI 1018 and Fe2Si0.1C steels with duplex ferrite-martensite (DFM) microstructures. The effects of microstructure on ambient temperature mechanical properties were examined. Two distinct martensite distributions were produced in AISI 1018 DFM. The primary differences between the two were the ferrite and martensite particle sizes. It was found that tensile fracture was sensitive to both martensile distribution and volume fraction. For a given volume fraction of martensite, large particle sizes increased strength but drastically reduced ductility. Threshold stress intensity ranges for fatigue crack growth in AISI 1018 DFM were found to be unaffected by the distribution or the volume fraction of martensite. However, crack growth rates in the range 10−5–10−3mm cycle−1 were increased in AISI 1018 DFM when the particle sizes were large. The increased crack growth rates were attributed to a crack extension mechanism involving cleavage fracture in ferrite. The Fe2Si0.1C DFM alloy was found to have a considerably higher fatigue threshold stress intensity than AISI 1018 DFM of comparable strength. The greater near-threshold fatigue crack propagation resistance of Fe2Si0.1C DFM was attributed to a meandering crack path, generated by the coarser microstructure in this steel, which promotes roughness-induced crack closure and crack deflection effects.
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