1977 年 26 巻 280 号 p. 62-67
To improve fatigue and wear resistances of machine parts, a low temperature gas carbonitriding process has recently come into practical use, in which steel parts are treated in ammonia and endothermic gas phase at about 570°C. The process provides a surface layer similar to that from tufftriding process.
In the present study, the effect of low temperature carbonitriding on fatigue strength was investigated by rotating bending tests on Cr-Mo structural steel specimens tempered at 600°C. The reason for fatigue resistance improvement was discussed, based on the hardness and residual stress distribution in the carbonitrided specimen.
The results obtained are as follows:
(1) The thickness of compound layer and nitrogen diffused zone in the low temperature carbonitrided specimen was approximately 18μm and 0.5mm, respectively. The hardness and compressive residual stress in the nitrogen diffused zone attained the maximum values near the surface and they were HV 575 and -30kg/mm2 at 0.025mm depth, respectively. The core hardness of the low temperature carbonitrided specimen was HV 313, which was equal to that of the quenched-and-tempered specimen.
(2) The improvement of endurance limit of plain and notched specimens by low temperature carbonitriding was about 30 and 90%, respectively, in terms of stress amplitude.
(3) The S-N relation of the plain carbonitrided specimen was evaluated from the relation between distribution of fatigue strength in the cross section and applied stress in rotating bending. The fatigue strength distribution in the specimen was estimated from the hardness distribution. The evaluated fatigue strength was 10∼20% lower than the experimental one. The difference can probably be ascribed to the fact that the fatigue crack initiated at about 0.5∼0.6mm depth from the surface and propagated to the surface in compressive stress field.
(4) In the case of the notched carbonitrided specimen, fracture initiated on the surface, and the S-N relation gave a knee point at about 105 cycles. In the stress level above the knee point, the fatigue strength is considered to be affected by the hardness distribution in the cross section. However, the endurance limit agreed well with the value estimated from the notch factor and the hardness and residual stress at 0.025mm depth from the surface.