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TESTING 1-2-3


Measuring the Effect of Surface Structure on Fatigue


Recent research explored the negative effect of casting skin on the dynamic property of fatigue in iron castings. A MODERN CASTING STAFF REPORT


Editor’s Note: Testing 1-2-3 is a new semi-monthly feature in MODERN CASTING summarizing experiments from the field of casting research.


determine how the phenomenon of casting skin affects the fatigue proper- ties of iron castings. Casting skin refers to the features


R


on or beneath the casting surface, including surface roughness, graphite degradation, graphite depletion, ferritic rim and pearlitic rim. Iron castings in particular usually retain the casting skin even after sand- or shot-blasting. While the surface of all iron cast-


ings retains some of these features, research has shown them to be par- ticularly harmful to ductile iron and compacted graphite iron (CGI) cast- ings. In a paper describing their work, Sarum Boonmee, Molly Moran and Doru Stefanescu explained the endur- ance limit that arises in the presence of casting skin, developed a “fatigue skin factor” to quantify the effect the skin


46 | MODERN CASTING March 2012


esearchers at Te Ohio State Univer- sity, Columbus, Ohio, recently set out to


has and evaluated the microsegrega- tion of the alloy surface.


Question What are the effects of the casting


skin on the fatigue properties of iron castings?


1


Background Casting skin on ductile


iron and CGI parts has been shown to have a negative effect on the mechanical


properties of the materials. Recent work has documented the reduction in tensile strength with the presence of the casting skin. According to research reports, the


casting skin effect is more pronounced during fatigue testing. Other research- ers have studied the fatigue strength of machined and as-cast samples of ductile iron. Te surface irregular- ity was found to have little effect on fatigue life compared to dross-type


defects, and the effect of the subsur- face features of the casting skin was not investigated. Still other research has shown that a reduction of 19% of the fatigue strength of ductile iron can occur due to casting skin. In this study, no characterization of the casting skin was provided. In more recent works, the char-


acteristics of the casting skin were correlated to static mechanical proper- ties. Quantitative correlations were proposed and verified. Te formation mechanism of


the casting skin is rather complex, and several mechanisms have been proposed. Te proposed mechanisms include magnesium depletion through oxidation or reaction with sulfur and carbon enrichment or depletion in the surface layer because of reactions with the molding materials. In this study, further investigation


of microsegregation within the casting skin was conducted.


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