S45C焼入焼もどし材の高温疲労強度に及ぼすひずみ時効の影響 Effect of Strain Aging on the Fatigue Strength of Quenched and Tempered 0.45% Carbon Steel at Elevated Temperatures

Rotating bending fatigue tests were made on a quenched and tempered 0.45% carbon steel in the temperature range from room temperature to 450℃ at a machine speed of 3600 rpm. The fatigue strength σ_f at 4×10^4 to 3×10^7 cycles reached a maximum at about 350℃ due to cyclic strain aging in the same manner as the annealed material. For the quenched and tempered material the fatigue limit was clearly confirmed up to 200℃ but not observed at the peaking temperature of 350℃, while for the annealed material the fatigue limit had been confirmed even at 350℃. the fatigue strength of the quenched and tempered material was higher than that of the annealed material at every temperature, but the effect of cyclic strain aging on fatigue strength of the former was less than that of the latter. This suggests that this material shows different effect of cyclic strain aging at high temperatures when heat treated differently. The observation of fatigue crack propagation behavior revealed that the strain aging at 350℃ is more effective for the quenched and tempered material in preventing crack initiation rather than inhibiting crack propagation, contrary to the results of the annealed materials, which shows that strain aging is more effective in inhibiting crack propagation.