430℃と500℃で焼戻されたばね鋼SUP7の1010サイクル疲労特性

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Giga-cycle fatigue properties were investigated for SUP7 spring steels tempered at 430 and 500℃. Tensile strength levels were 1423 and 1730 MPa. Rotating bending, electromagnetic, high-speed servohydraulic and ultrasonic testing machines were used for 30 or 100 Hz, 120 Hz, 600 Hz and 20 kHz tests, respectively. 10^<10>-cycles fatigue properties were obtained in 3 years under 100 Hz rotating bending and in 7 days under 20 kHz uniaxial loading, using 6- and 3-mm-diameter hourglass specimens. 10^8-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 Al_2O_3. Optically dark areas (ODAs) were formed around inclusions even at 20 kHz. 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℃ tempered steel under 100 Hz rotating bending, and 600 Hz and 20 kHz uniaxial loading were coincident each other, because the control volume was almost the same of 34 and 33 mm^3. S-N curves under 120 Hz unixial loading with the large control volume of 227 mm^3 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 20 kHz ultrasonic machine is effective for accelerated testing method of giga-cycle fatigue.
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430℃と500℃で焼戻されたばね鋼SUP7の1010サイクル疲労特性

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