Abstract: Apparent fatigue limits of surface hardened machine parts (gears, for examples) are quantitatively studied according to comprehensive considerations to overcome the limitations caused by science divisions. Small specimens for three-point- bending fatigue test，as well as gear specimens made of low-carbon steel 20CrMnTi are carburized (and then quenched and low temperature tempered), and half of them were then shot-peened. Another group of small specimens used for comparison is pseudo-carburized. Compressive residual stress fields in the surface region of small specimens are determined. Fatigue tests are carried out on a high-frequency machine. The fatigue limit loads of the three-point bending specimens and the teeth of gear specimens at 5×106 cycles and stress ratio=0.05 are determined. The fracture surfaces of the specimens with longest fatigue lives in each groups are studied under SEM. Stress fields near the tooth root of gear specimens are calculated by using a finite-element method and the ANSYS software. The relationships between the fatigue limits of small specimens and the critical fatigue loads of gear specimens under both conditions are analyzed according to a “micro-meso-processes theory for fatigue source formation” and a concept of “surface / internal fatigue limits of metals” developed. A method for calculation of critical fatigue load of complex machine parts according to the fatigue limit determined with small specimens, as well as a judgment for optimization of surface hardening technology, is proposed.

Key words: Carburization, Fatigue limit, Modeling of mechanical behavior, Residual stress, Shot peening