Effect of statistical distribution of iron powder size on fatigue strength of powder metals

Fatigue properties of two kinds of ferrous powder metals, which were produced from partially prealloyed iron powders and whose densities are 7.10 g/cm³ and 7.35 g/cm³, were investigated by tension-compression fatigue test. Main fatigue cracks did not initiate from pores but from slip bands in soft Fe particles in both these materials. The crack nucleated at the maximum Fe particle behaved as a defect larger than the maximum pore and accordingly the maximum size of the Fe particle existing on specimen surface was the crucial factor determining the fatigue strength. In order to confirm this fact more exactly, the maximum Fe particle size was evaluated by the statistics of extremes, and on the other hand an artificial hole with various sizes was introduced onto the specimen surface and the influence of the hole was investigated. The prediction of the maximum size of Fe particle by the statistics of extremes was in good agreement with the critical size of the artificial hole which was detrimental to fatigue strength. These facts explain that the difference in the fatigue strength between two materials was not caused by the difference in densities, i.e. pores, but by the strength properties of the microstructures such as Vickers hardness which was measured in details from the core to boundaries of grains.