The relationship of the aging treatment conditions, hardness, and torsional fatigue strength of the
β type titanium alloy for the springs was examined, in addition observation of micro-structure change due to aging was conducted. As a result, the maximum hardness occurs at the aging treatment temperature of 450℃, while beyond this temperature it decreases. On the other hand, the torsional fatigue strength rises above 450℃, and reaches maximum at 540℃ aging treatment temperature. The notch sensitivity factor,
β of the specimen for 600
μm diameter-drilled hole is 1.29 at 480℃ and 1.24 at 540℃ aging treatment temperature, and the notch sensitivity factor,
β of the specimen treated at 540℃ shows a lower value. Furthermore,the increases in the amount of
α-phase precipitation due to aging increase the torsional fatigue, which becomes maximum when
α-phase precipitation amount reaches 35%.
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