繰返し応力のもとでひずみ時効した鋼の疲労強度

Abstract

To investigate the effect of cyclic-strain aging on the fatigue strength of carbon steel, two kinds of cyclic-strain-aged carbon steel were fatigued in rotating bending at room temperature as well as at elevated temperatures. The annealed and surface-rolled specimens were aged at 375°C and 250°C during cyclic stressing above or below the fatigue limits. The strengthening effect due to the aging treatment depends on the aging conditions; the temperature, the cyclic stress amplitude and the number of cycles.The strain-aging treatment at 375°C raises the fatigue strength at room temperature. The strengthening effect at room temperature through the aging treatment in the range of cycles 5.9 ×103∼3.2×106 under the stress just below the fatigue limit at 375°C is independent of the number of cycles at 375°C. In the range of stress 1.15Sw∼0.58Sw for 5.9×103 cycles at 375°C, the higher the stress level at 375°C the more effective the strengthening effects. The strengthening effect through the treatment at 250°C is less than at 375°C and depends on the number of cycles at 250°C.It can be considered that there is not a substantial difference in the strengthening effect through strain aging between annealed and surface-rolled conditions although the surface-rolled specimens have the strongly hardened layers and the residual stress, but that there is a large difference between the treatment at 375°C and 250°C. The strengthening effect seems to be based on dislocation-solute interaction which operates afresh at each cycle at 375°C, and on any cumulative effect of aging at 250°C.