Abstract
Axial fatigue tests of SNCM630 forgings, which are often used for high-strength components in diesel engines, have been carried out with various stress ratios (R) for fatigue life N≤107 cycles. S-N curves obtained from the experimental results have shown fatigue limits of 500, 400, 300 and 190 MPa for R=-1, 0, 0.3 and 0.6, respectively. The results suggest that the fatigue strength of SNCM630 is highly improved from a similar material, SNCM439. Study on the effect of stress ratio using a fatigue limit diagram with failure probability shows that modified Goodman diagram is applicable for the determination of fatigue strength of this material. However, under a stress condition of high R where the maximum applied stress may exceed the proportional limit, as for in the design it is recommended that the applied stress will not exceed the proportional limit. SEM observations of the fracture surfaces have revealed that fatigue cracks initiated from subsurface inclusions which are identified as alumina particles of 35μm in diameter in only two specimens while crack initiated from surface flaws in other samples. Using the √area parameter model for internal defects, the fatigue limit for N>109is estimated as approximately 410 MPa in case of R=-1.
