Relationship between Fatigue Limit and Defect Size in Spheroidal Graphite Cast Iron with Different Graphite Spheroidization Ratios and Microstructures

Abstract

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.

Influence of microscopic casting defects on fatigue endurance of ductile cast iron

“…It is well known, that fatigue tests of this category of materials show a significant scattering of the measured data. The magnitude of this scattering is most often justified by the presence of structural heterogeneities and the casting defects, which significantly affect the fatigue properties [1][2][3][4][5][6][7][8].…”

Estimation of Fatigue Limit of Spheroidal Graphite Cast Iron Using X-ray CT

“…Since the defects of the test material used in this study are small defects, we assumed that they belong to Region II of the 4-parameter model. 22) In estimating the fatigue limit using a 4-parameter model, we focused on the defects with maximum volume existing inside the specimen. Since defects are observed threedimensionally in X-ray CT, the projected area of the defect differs depending on the cross-section observed even for the same defect, and as a result, the measurement result of ffiffiffiffiffiffiffiffiffi area p differs.…”

“…23) The micro shrinkage present in spheroidal graphite cast iron have a complicated shape. 22,24) It will be necessary to study the definition and estimation method of defect dimensions, including eq. ( 4).…”

An Insight into the Defects-Driven Plasticity in Ductile Cast Irons

“…Defects and metallurgical discontinuities form during metallic alloys solidification and can affect significantly the magnitude and the variability of mechanical properties. In castings of Ductile Irons (DIs), for instance, the main defects and discontinuities can be degenerated graphite agglomerates, dross, inclusions, gas and solidification shrinkage porosities, etc., which can have a detrimental impact on several mechanical properties, such as the room-temperature tensile strength, ductility, fatigue resistance, and fracture toughness [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. In the continuous casting production of steels, for instance, the metallurgical discontinuities, such as cracks forming during solidification at the slab surface, influence appreciably the material quality resulting from the subsequent slab straightening [ 8 , 9 ].…”