430℃と500℃で焼戻されたばね鋼SUP7の10¹⁰サイクル疲労特性

抄録

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 σ_a/σ_w' versus Nf curves, were independent on the test speed, loading type, specimen configuration and strength level. Here, σ_w' 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.