Fatigue fracture processes are divided into 7 stages. Promoting and resistance factor for fatigue damage of each stage were considered systematically. Based on this consideration, a new methodology to increase the fatigue limit of automotive components which subjected to R ≧0 stress was proposed. The methodology consists of following three points; (1) Maximize the yield stress (the hardness) of the component, (2) Introduce a large and deep compressive residual stress. Especially for fine grained steel, introduce a large compressive residual stress at the surface, (3) Decrease the grain size as fine as possible. To achieve the above three conditions, new surface treatment methods were developed by authors and the methods were applied to the gear. The surface treatments adopted were Vacuum Carburising, Contour Induction Hardening and Double Shot Peening. Using above three combined surface treatments, the stress range at the fatigue limit (R=0.1) of gear was increased up to 2207 MPa. 2 Isuzu Motor Ltd. 3Post-graduate student, Yokohama National University.
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