FATIGUE OF AN ULTRA-FINE GRAINED LOW CARBON STEEL

摘要

Fatigue strength, fatigue notch sensitivity and fatigue crack propagation behaviour of an ultra-fine grained low-carbon steel (grain size 1 μm) was investigated. The refining of ferrite grains was carried out by a thermo-mechanical treatment which consists in applying heavy deformation with a hot deformation simulator. Important improvement on fatigue strength was observed when the analysed steel was compared to ferritic steels with coarser grains. Results seem to follow the Hall-Petch relationship between the fatigue limit and grain size. The influence of tensile mean stress on propagation threshold and propagation behaviour was also analysed. The threshold for fatigue crack propagation is relatively low and experimental results are in good agreement with previous evidence that showed that the fatigue propagation threshold for long crack decreases with grain size. However, when compared at medium and high stress intensity range levels and equal applied mean stress, the ultra-fine grained steel showed lower crack propagation rates than a SM490 steel or a HT80 steel. Influence of the mean stress was observed up to a given stress ratio for the total analysed ΔK range. The combination of a high smooth fatigue limit, a small size of the non-propagating cracks that defines the smooth fatigue limit, and a relatively low threshold for fatigue propagation for long cracks gives rise to a high fatigue notch sensitivity. However, considering the great improvement of the smooth fatigue limit, the fatigue limit for blunt notches is still greater than that for coarse-grained ferritic steels. For sharp notches, whose fatigue limit is mainly given by the fatigue propagation threshold for long cracks, the analyzed ultra-fine grained steel showed no improvement in fatigue strength.