Abstract
Giga-cycle fatigue properties were investigated forheats B and C of SUP7 spring steels tempered at 430℃. Rotating bending, high-speed servohydraulic and ultrasconicfatigue testing machines were used for 30 or 100Hz, 600 or 800Hz and 20kHz tests, respectively. S-N curves under each frequency were coincident in case of fish-eye fracture with an origin of Al_2O_3 and TiN inclusions and a matrix crack for each heat, when the control volumes were equal to each other. However, heat C, in which Al_2O_3 inclusions were smaller, revealed a higher fatigue limit than heat B at (10)^<10> cycles.
