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The purpose of this study is to investigate the effects of number of graphite nodules on fatigue limit and fracture origins in spheroidal graphite cast iron where carbon content and the number of graphite nodules are changed. As specimens, spheroidal graphite cast irons with 3.0, 3.2, 3.4, 3.6 and 3.8 mass% carbon content were produced respectively. Matrix was conducted pearlite (PDI) and ausferrite (ADI) by heat treatment (normalizing and austempering). The mean diameter of the graphite nodule decreases as the carbon content increases, and the number of graphite nodules per unit area increases. No differences were observed in tensile strength as a result of variations in the number of graphite nodules. The rotating bending fatigue test was conformed to JIS (Japanese Industrial Standards). Load frequency was 47 rpm, number of cycle to discontinue test was 1.0 © 10 7 cycles, and the specimen used was the 1 type of 8.00 mm in diameter. All the fracture surfaces were observed with a scanning electron microscope. The relationship between the characteristics of fatigue strength and the size of fracture origins (defect size) was investigated.No differences were observed in fatigue limit as a result of variations in the number of graphite nodules. Fracture origins were microshrinkage, unspheroidizing graphite nodule and aggregate graphite nodules. The percentage of fracture origin of aggregate graphite nodules was increased by cluster of the graphite nodules. The reason of this phenomenon was due to the decrease of the graphite nodules spacing. The mean defect size in each sample was constant irrespective of the change of number of graphite nodules, which was good agreement with the results of the fatigue limit.

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The purpose of this study is to investigate that effects of graphite spheroidization ratio and microstructure on the characteristics of fatigue limit in spheroidal graphite cast iron. Ferritic spheroidal graphite cast iron (FDI), pearlitic spheroidal graphite cast iron (PDI) and austempered spheroidal graphite cast iron (ADI) were used as specimens. The graphite spheroidization ratio was varied between 63³94% by the addition of a spheroidizing agent. Tensile test was carried out in air at room temperature. The experiment conformed to JIS (Japan Industrial Standards). Rotating bending fatigue test was also carried out using these materials. The experiment conformed also to JIS. Stress ratio R was ¹1, and the specimen used was type 1(JIS) with a diameter of 8 mm. The test was carried out in air at room temperature. The relationship between fatigue limit and graphite spheroidization ratio was investigated, as well as the correlation between fatigue limit and defect size.When the graphite spheroidization ratio was over 80%, the fatigue limit was not influenced. Fracture origins were micro-shrinkage, aggregate graphite, and unspheroidized graphite. When graphite spheroidization decreased, the ratio of unspheroidized graphite at the fracture origin increased. The defect size that transitioned from region I to II differed according to the microstructure: the defect size of PDI was the smallest, and that of FDI was larger than ADI. When there are no large defects in FDI, the fatigue limit can be estimated by tensile strength. However, in PDI and ADI, it must be estimated taking into account the size of existing defects in specimens.

Carbon equivalent of cast iron is known as the following equation: CE = [%C] ? (1/3) [%Si]. This equation is basically calculated from the carbon solubility in hypereutectic composition. On the other hand, liquidus from hypoeutectic to eutectic composition in Fe-C phase diagram is well known. Liquidus temperature (hereinafter referred to as T L ) can be measured easily with thermal analysis by focusing primary crystallization temperature. In this study, the effects of elements on T L and carbon equivalent in cast iron were examined from the view point of cooling curve, carbon flotation, and internal shrinkage, and a more accurate equation for calculating carbon equivalent is suggested. It is revealed by the experiment that the relationship between T L and elements from hypoeutectic to eutectic composition is as follows; T LDividing this equation by carbon coefficient, the carbon equivalent equation of liquidus (hereinafter referred to as CE L ) from hypoeutectic to eutectic composition is obtained as follows: CE L = [%C] ? 0.23 [%Si] -0.03 [%Mn] ? 0.32 [%P] ? 0.64 [%S] ? 0.02 [%Ni] ? 0.06 [%Cr]. This CE L is different from the commonly used CE since it is determined from liquidus. We investigated how the difference of carbon equivalent (CE/CE L ) application in the range of hypoeutectic to eutectic composition affected on the cast iron and revealed that CE L = [%C] ? 0.23 [%Si] should be applicable.