Fatigue behaviour of an ultra-fine grained low carbon steel

MIRCO DANIEL CHAPETTI; TAGAWA, T; MIYATA, T

Sao Pablo

Congreso; Fatigue 2004. SAE-Brasil Congress; 2004

SAE Brasil

Fatigue strength, fatigue notch sensitivity and fatigue crack propagation behaviour of ultra-fine grained low-carbon steels (grain sizes 0.8 and 1 μm) were investigated. Refining of ferrite grains was carried out by thermo-mechanical treatment applying heavy deformation by using a hot deformation simulator. Important improvement on fatigue strength was observed when compared with ferritic steels with coarser grains. Results seem to follow the Hall-Petch relation 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 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 level and the same applied mean stress, the ultra-fine grained steel showed lower crack propagation rate than a SM490 steel or a HT80 steel. Influence of the mean stress was observed until a given stress ratio for the total analysed ΔK range, including the near threshold crack growth. Remarkable correlation with fatigue crack propagation rate was obtained when the (ΔK Kmax)1/2 driving force parameter was applied.