中高温における低炭素鋼のき裂進展下限界応力拡大係数範囲ΔK_thに及ぼす繰返しひずみ時効および高温酸化物の影響

抄録

Fatigue tests of 0.08% C Steel were made on in-plane bending at elevated temperatures up to 450°C (Room temperature, 200°C, 375°C and 450°C). the threshold stress intensity factor ΔK_th was determined by the critical condition of whether specimens broke or not at 10⁷ cycles. The threshold stress intensity factor reached a maximum at 375°C. The value of K_op/K_max increased with increasing temperature up to 450°C. Both ΔK_{eff th} and ΔK_{eff th}/E showed the same temperature dependence as described above. The oxide at 375°C was thinner than that at 450°C. The value of ΔK_th at 375°C, however, was larger than that at 450°C. It is concluded that the maximum ΔK_th is caused by strain aging rather than thick oxide induced closure.