Spheroidal graphite cast iron is widely used for various purposes due to its excellent mechanical properties. As the spheroidization ratio of graphite influences the mechanical properties, rare earth is used as a graphite nodularizer to improve spheroidization. However, given that the supply of rare earth mainly relies on imports from overseas, there is need for the development of spheroidal graphite cast iron with reduced rare earth. For this reason, this study investigated the fatigue strength of spheroidal graphite cast irons without using rare earth.

  Specimens of spheroidal graphite cast iron containing rare earth and those not containing rare earth were used. The matrices were all ferrite, pearlite ratio 30%, and all perlite. Fatigue test was carried out using a plane bending fatigue test machine. The stop condition of fatigue test was after 10⁷ time repetitions or fracture of specimen. The load stress of specimens that did not fracture was taken to be the fatigue strength.

  The results of the fatigue test showed that the σ_w of specimens not containing rare earth was reduced compared to those containing rare earth for each matrix. Casting defects existed inside the specimens. The effects of casting defects on fatigue strength were evaluated using a √area parameter model, stress intensity factor range ΔK, and threshold stress intensity factor ΔK_th. As a result, it was found that the difference between the ΔK and ΔK_th of specimens not containing rare earth increased, resulting in reduced fatigue strength.

  With increasing demands for the weight reduction of cast iron products in various industries, advanced processing techniques to make thin-wall ductile cast iron without chill structure were developed. This study investigated the tensile and fatigue properties of the thin-wall ductile cast iron and effects on the casting surface. Thin-wall ductile cast iron specimens of 2mm in thickness were prepared. The tensile test and fatigue test were carried out using test pieces with machined and casting surfaces. The tensile strength of the test pieces with casting surface was smaller than that of the machined test pieces. The measured thickness including the casting surface is thought to be larger than the effective thickness which supports the load. The authors estimated the effective thickness and it was approximately 0.25mm smaller than the thickness of the test pieces with casting surface. The S-N diagram indicated that the fatigue limit of the test piece with the casting surface was 217MPa, which is smaller than that of the machined test piece. Most of fracture origins were the concavity of the casting surface, and shrinkages and dross were also observed. The fatigue limit evaluated with the test pieces whose fracture origins were the concavity of the casting surface, was 252MPa. The surface roughness profile was evaluated by the two parameters in the altitudinal and horizontal directions, and the fatigue limit was estimated from the maximum concavity that was obtained by extreme statistics. Estimated fatigue limit was 259MPa, and it is close to the fatigue limit of the test pieces whose fracture origin was the concavity of casting surface. The authors also proposed a method to estimate the size of the fracture origin by roughness parameter.

  The effects of nitrocarburizing or nitriding treatments on rotating-bending fatigue properties were investigated on four kinds of pearlitic FCD700 (JIS G5502) -class ductile cast iron samples with V (0.1%), Al (0.1%), Al (0.1%) & Cr (0.1%) and Al (0.1%) &V (0.1%). Tensile and hardness characteristics of the nitrocarburized samples were compared to those of the nitrided ones and as-cast FCD700 without alloying element. Fe₄N nitride formed on the surfaces of the nitrided samples, while Fe₄N and Fe_2-3N nitrides formed on the surfaces of the nitrocarburized ones. The practical nitrided depth and micro-Vickers hardness at 0.03mm below the surface in the nitride layer of the nitrided samples were larger and higher than those of nitrocarburized ones, respectively. The addition of alloying elements to the nitrided and nitrocarburized samples increased the practical nitrided depth and hardness in the vicinity of the surface, compared to the samples without alloying element. The fatigue existing in the higher stress range from 500 to 650MPa was found to be longer in the order of as-cast FCD700, nitrocarburized and nitrided samples. 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 nitrocarburized and nitrided samples. The improvement of fatigue characteristics by nitrocarburing and nitriding treatments is considered to be efficient only in higher stress ranges. The fatigue strength in high stress ranges is considered to be related to the difference in the initiation time of the fatigue crack. 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.