Abstract
The effect of hardness on the fatigue behaviors of Cr-Ni-Mo high carbon low alloy steel for a hardness range from Hv 350 to Hv 650 has been discussed here. From the test results the following were clarified. (1) The fatigue strength of unnotched specimen under rotating bending and torsion increases with the increase of hardness, but the maximum fatigue strength is obtained at hardness Hv 550 and above this hardness the fatigue strength decreases on the contrary. (2) The fatigue strength of notched specimen under rotating bending increases with the increase of hardness, but the fatigue strength reduction factar decreases conversely above Hv 550. The rate of growth of fatigue crack in the specimen above hardness Hv 550 is considerably larger than that in the specimen below hardness Hv 450. (3) The fatigue strength of cracked specimen under rotating bending decreases gradually with the increase of hardness. This is similar to the case where the impact value of steel decreases with the increase of hardness. The fatigue strength reduction factor of cracked specimen increases proportionally to the decrease of the impact value. (4) Fish eye was found only in the fatigue fracutre of unnotched specimen of hardness Hv 650 under rotating bending, and the fracture appearance is of intergranular one. On the other hand, the fracture of specimen below hardness Hv 550 was a fatigue fracture. It is presumed that the drop in fatigue strength of steel of hardness Hv 650 is due to the existence of stress-raising elongated carbide particles.
