マルエージング鋼の疲労特性に及ぼす時効条件と湿度の影響

概要

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In order to investigate the effects of aging condition and humidity on fatigue properties of 18%Ni maraging steel of grade 350, rotating bending fatigue tests were carried out in relative humidity of 25% and 85%. Aging conditions selected were under- and peak-aging ones at conventional aging temperature of 753K. In addition, double aging treatments, that is, under- peak- and over-aging treatments at 673K after the peak-aging at 753K were also examined. Both of static and fatigue strengths were increased by the double aging. Although the fatigue strength was markedly decreased by high humidity in all of the steels, the decrease in fatigue strength due to high humidity was suppressed by the double aging. In all of the steels, the change in growth behavior of a crack was yielded around the crack length of a few grain sizes of prior austenite regardless of humidity. The growth rate of a crack longer than the above stated length was hardly influenced by the aging condition and humidity, while both of an initiation and its early propagation of a fatigue crack were accelerated by anodic dissolution in high humidity. These results were discussed from a point of view of the difference in precipitates depending on the aging condition.