The effects of nitriding treatment on rotating-bending fatigue properties were investigated on nine kinds of pearlitic ductile cast iron samples with Mo (0.1%), Cr (0.1%), V (0.1%), Al (0.1, 0.3, 0.5%), Al (0.1%) & Cr (0.1%), Al (0.1%) & V (0.1%) and without alloying element. The white layer of Fe4N nitride formed on the surfaces of all the samples was about 0.01mm in thickness. The practical nitrided depth and micro-Vickers hardness at 0.03mm below the surface in the nitride layer of the sample without alloying element were 0.148mm and 549HV, respectively. The addition of alloying elements to nitrided samples increased the practical nitrided depth and hardness in the vicinity of the surface. In the nitrided samples, fatigue existing in the higher stress range from 500 to 650MPa was found to be longer in the order of no addition, single addition, and double addition of alloying elements. However, the fatigue limit at 107 cycles in the lower stress range ranged from 410 to 450MPa and no significant difference was seen among the nitrided samples. The improvement of fatigue characteristic by the addition of the alloying element is considered to be efficient only in the higher stress range. The fatigue strength in the high stress range is considered to be related to the difference in the initiation time of the fatigue crack because the spacings of the striation formed on the fracture surfaces are more or less the same in all the samples. This suggests that the larger the nitrided depth and/or the higher the hardness in the vicinity of surface promoted by the addition of alloying elements, the more delayed will the crack initiation be.
