Importance of pore size and hardness on fatigue strength of a pre-alloyed powder metal with the density of ρ = 7.0g/cm³

In order to investigate the importance of pore size and hardness on the fatigue strength of a prealloyed powder metal (Fe-3.0 Cr-0.5 Mo-0.5C mass%) with the density of ρ= 7 .Og/cm³, ultrasonic fatigue tests were conducted at a test frequency of 20 kHz in air at room temperature. The fatigue limit of the powder metal was the maximum critical stress under which fatigue cracks initiating from the pore stop propagating. Therefore, the fatigue limit was accurately predicted by the -√area parameter model, σ_w.= 1.43(HV + 120)/(√area)"⁶, for the surface defect. Here, as √[area was employed the effective maximum pore size, -√areae//, max, in the control volume of the ultrasonic fatigue specimen which was estimated from the statistics of extremes distribution of the pore sizes at the polished surface. As HV was employed Vickers hardness, HV₉₈._{.1 N}, of the microstructure which included both the matrix and pores. The usage of -√Jarea_eff, _max and HV₉₈._{.1 N}, in the √/area parameter model is practical use and useful for predicting the fatigue limit of the powder metal. In addition, the usage of √■<jQ.YQQ.eff, max indicates that the dominating cause of fatigue strength for the powder metal is the maximum pore size rather than the porosity or density.