Fatigue Crack Growth of Sintered Steels with a Heterogeneous Microstructure

摘要

Powder metallurgy processing of steel alloys typically results in a material with heterogeneous microstructure and residual porosity. The fatigue crack growth behavior of these materials is strongly affected by the nature of porosity and heterogeneous microstructure. Notched fatigue specimens were prepared from a Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6%C. The alloys were tested at three different densities: 6.98 g/cm~3, 7.36 g/cm~3, and 7.53 g/cm~3. The microstructure at each density was characterized to determine the porosity, microconstituents, and phase fractions. Fatigue testing was performed at various R-ratios, ranging from -2 to 0.8. Increasing porosity and increasing R-ratio resulted in a decrease in ΔK_(th). In situ observation of crack growth showed that the cracks propagated through Ni-rich regions. It appears that pearlite regions, and, to some extent bainite regions, contributed to toughening and crack deflection. These findings are supported by quantitative measurements of crack growth rate, through the various microconstituents.