430℃と500℃で焼戻されたばね鋼SUP7の10<SUP>10</SUP>サイクル疲労特性

Abstract

Giga-cycle fatigue properties were investigated for SUP7 spring steels tempered at 430 and 500°C. Tensile strength levels were 1423 and 1730MPa. Rotating bending, electromagnetic, high-speed servohydraulic and ultrasonic testing machines were used for 30 or 100Hz, 120 Hz, 600Hz and 20kHz tests, respectively. 1010-cycles fatigue properties were obtained in 3 years under 100Hz rotating bending and in 7days under 20kHz uniaxial loading, using 6-and 3-mm-diameter hourglass specimens. 108-cycles fatigue properties were also obtained under 120 and 600 Hz uniaxial loading, using 6- and 3-mm-diameter hourglass specimens. Almost all specimens fish-eye-fractured at internal inclusions of Al2O3. Optically dark areas (ODAs) were formed around inclusions even at 20kHz. The data obtained was analyzed according to the predicted fatigue limit and control volume of specimen for fish-eye fracture proposed by Murakami and others. S-N curves for 430°C tempered steel under 100Hz rotating bending, and 600Hz and 20kHz uniaxial loading were coincident each other, because the control volume was almost the same of 34 and 33mm3. S-N curves under 120Hz unixial loading with the large control volume of 227mm3 shifted to the lower strength level. The inclusion size was nearly proportional to the control volume. Modified S-N curves, that is σa/σw' versus Nf curves, were independent on the test speed, loading type, specimen configuration and strength level. Here, σw' is the fatigue limite for fish-eye fracture predicted by Murakami and others. These results give us two findings. One is that fish-eye fracture is not the simple hydrogen embrittlement process. Another is that 20kHz ultrasonic machine is effective for accelerated testing method of giga-cycle fatigue.