Abstract
Giga-cycle fatigue tests were conducted for heats B and C of SUP 7 spring steels tempered at 430 and 500°C. Rotating bending, electromagnetic, high-speed servohydraulic and ultrasonic fatigue testing machines were used at 30 or 100 Hz, 120 Hz, 600 or 800 Hz and 20 kHz, respectively. The heat B developed fish-eye fractures only with an origin of an Al2O3 inclusion, while Al2O3 and TiN inclusions and matrix cracks appeared at the origins in case of the heat C. The Al2O3 inclusions in the heat B were larger than those in the heat C. Hence, the heat C revealed higher fatigue limit than the heat B. In case of fish-eye fracture, S-N curves under 100 Hz rotating bending, and 600 or 800 Hz and 20 kHz uniaxial loading were coincident each other in both heats, while those under 120 Hz uniaxial loading were lower. The frequency effects were negligible in case of fish-eye fracture and the difference between 120 Hz uniaxial tests and others were explained by the difference of the control volumes.
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