The Effect of Low Temperature Quenching and Quench Ageing on the Fatigue Strength of Notched Specimens of Low Carbon Steels

概要

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The fatigue limits, σw_1 and σw_2, which are critical stresses for initiation and propagation of stage 2 cracks respectively, were compared among annealed, quenched and quench aged steels. To examine the effect of heat treatment on the fatigue life, crack propagation rate, d(2a)/dN, was measured at a stress above σw_2. The contribution of quenching and quench ageing to the increase in the fatigue limits is greater for σw_2 than for σw_1, and the relationship, ασw_1≃σw_0, is kept independent of the heat treatments, where σw_0 is the fatigue limit of unnotched specimens. Therefore, the notch sensitivity factor, ( σw_0/σw_2-1)/(α-1), is reduced by both treatments. The strengthening effect is more remarkable with quench aged steel than with quenched steel. At higher ΔK level, d(2 )/dN is independent of the heat treatment, and is correlated with the streas intensity factor ΔK. The increase in the fatigue life by quenching and quench ageing is due to the decrease in d(2a)/dN at lower ΔK level.