EFFECT OF TENSILE MEAN STRESSES ON FATIGUE STRENGTH OF Fe-Cu-C STEELS IN AS-SINTERED AND HEAT TREATED CONDITIONS

摘要

Smooth and notched (K_t = 1.8) specimens according to ISO 3928 were compacted from Fe - 1.5 Cu - 0.6 C to different densities and sintered in industrial furnaces at 1120°C. The majority of the samples were fatigue tested under plane bending in the as-sintered condition at different stresses ratios R = σ_(min) / σ_(max) (R = - 1, -0.4, 0 and + 0.25) to establish Haigh diagrams. A smaller group of specimens was subjected to various heat treatments in order to study the effect of higher strength or brittle mechanical behaviour on the reaction to tensile mean stresses. In contrast to conventional structural steels or grey cast iron, porous PM steels seem to have linear Haigh diagrams, which permit us to determine the slope from only two fatigue strength values at different mean stresses or stress ratios. In this study the slope in the as-sintered state turned out to be rather well predictable for smooth and notched geometries, which enables fairly reliable inter- and extrapolations over a wide range of non-investigated loading conditions. After heat treatments, the behaviour turned out to deviate more from the anticipated functional relationships. Much more systematic work is needed to fully understand the material behaviour in the various heat treating conditions, because the scatter in the very few results of this study is too high to draw conclusions on quantitative relationships. Further areas of future research should be reliable axial tests also with compressive mean stresses, tests with stress ratios R > 0, the investigation of notched conditions with different notch geometries and the statistical size effect.