抄録
Giga-cycle fatigue tests were conducted for two heats of induction hardened 0.40%C carbon steels. The fatigue testing types were rotating bending at 100Hz and ultrasonic at 20kHz. The fatigue test specimens were uniformly induction hardened from the surface to the center. The induction hardened 0.40%C carbon steels revealed fish-eye fractures whose origins were mostly a TiN inclusion in case of heat A and entirely an Al_2O_3 inclusion in case of heat B. In case of rotating bending fatigue tests of heat A, surface fractures also occurred at over 10^8 cycles, while that type of the surface fracture never occurred in case of the ultrasonic fatigue tests. When the ultrasonic fatigue tests were compared with the rotating bending, focusing on the results of fish-eye fracture, the ultrasonic type showed slightly higher fatigue strength in case of heat A in spite of good agreements in case of heat B. However, the difference in case of the heat A was so small as to be in scattering of the results of heat B. Moreover, effects of pre-treatments before the induction hardening were also investigated in heat B. In this case, the fatigue tests were additionally conducted for induction hardened specimens with a pre-treatment condition of only normalizing (N-IH), while other tests were of normalizing, quenching and tempering (Q-IH). As the result, difference between N-IH and Q-IH was negligible, showing no influence of the pre-treatment condition.
