Fatigue-crack growth of Fe.0.85Mo-2Ni.0.6C steels with a heterogeneous microstructure

Abstract

Powder metallurgy (P/M) processing of steels typically results in a material with a heterogeneous microstructure and residual porosity. The fatigue-crack-growth behavior of these materials is strongly affected by the nature of the porosity and the heterogeneous microstructure. Notched fatigue specimens were prepared from a Fe-0.85 w/o Mo prealloy mixed and binder treated with 2 w/o Ni and 0.6 w/o C. The steels were tested at three different sintered densities: 6.98 g/cm 3 , 7.36 g/cm 3 , and 7.53 g/cm 3 . The microstructure was characterized at each density 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 D K th . In situ observations of crack growth showed that the cracks propagated through the Ni-rich regions. It appears that the pearlite regions, and to some extent the bainite regions, contributed to toughening and crack deflection. These findings are supported by quantitative measurements of crack-growth rates through the various microconstituents.