Effect of Grain-size on Fatigue Strength of Low Carbon Steel at Elevated Temperatures

概要

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Rotating beam fatigue tests over the temperature range 20-450℃ were made on low carbon steel in annealed and low-temperature-quenched conditions. The relationship σ_w=C_w+k_wd^<-1/2> which describes the dependence of the fatigue limit σ_w on the grain diameter d has been proved in this series of experiments to apply up to about 400℃. Both of the two constants C_w and k_w reached the maximum at 350-375℃ and the minimum at about 200℃. The existence of the maximum in the values C_w and k_w is found to be caused by a rapid strain ageing. The fatigue strength at 450℃ was independent of the grain size. Below 400℃, σ_w is increased by quenching. The increase in σ_w is induced by an increase in C_w, not in k_w, which is little influenced by quenching. The increase in C_w only may be explained as the consequence of precipitation hardening due to quench-ageing developed prior to fatigue stressing .