Open Access

ARTICLE

Fatigue Properties of Nitrided Alloy 718 at Elevated Temperature

N. Kawagoishi¹, A. Ohkubo², S. Yoshimi³, K. Yamane⁴, K. Morino⁵

1 Kagoshima University, Kagoshima, Kagoshima, Japan; E-mail: hiro@mech.kogoshima-u.ac.jp
2 Kagoshima University, Kagoshima, Kagoshima, Japan
3 Fuiji Heavy Industries Ltd., Osawa, Japan
4 Kohan kogyou, Ltd., , Kudamatsu, Yamaguchi, Japan; E-mail: yamane@kohankogyo.co.jp
5 Tokuyama College of Technology, Shunan, Yamaguchi, Japan; E-mail: morino@tokuyama.ac.jp

Structural Longevity 2010, 3(3), 191-199. https://doi.org/10.3970/sl.2010.003.191

Abstract

Rotating bending fatigue tests were carried out using a radical nitrided nickel base superalloy, Alloy 718, at 500 ℃, to investigate the effect of radical nitriding on the initiation and propagation behavior of a fatigue crack at elevated temperature. The nitriding was conducted at 500 ℃ for 12h which was relatively low temperature compared with other nitriding technologies. Compound layer was formed on the specimen surface with the thickness of about 4µm. Hardness of the base alloy was hardly affected by nitriding. When stress levels were high, a crack initiated from the specimen surface in the aged alloy, and the base alloy beneath the compound layer in the nitride alloy. On the other hand, when stress levels were low, fracture origins changed to the interior of base alloy in both alloys, causing that S — N curves showed a step wise shape. The crack initiation was suppressed by the compound layer. As the result, fatigue strength increased by nitriding in the horizontal region in S — N curves where fatigue strength was controlled by the crack initiation and the propagation of a small crack, though the effect of nitriding on fatigue strength was very small in the region where fatigue life was occupied by the growth of a crack, i.e. in the region except for the horizontal one in S — N curves.

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