Preventing Graphite Degeneration with Fluorine-Free Feeders

Kutz A., Martin P., Bührig-Polaczek A.

Graphite degeneration is the most prominent microstructural defect in the casting skin of ductile iron. Induced by either sulfur or oxygen contained in the molding material, its occurrence can be observed in a large range of iron castings, having substantial negative influences on the mechanical properties, especially fatigue resistance. Previous investigations predominately focused on the influence of the degenerated graphite layer, while accompanying changes of the iron matrix were neglected. The superposition of these effects hinders the evaluation of casting skin defects in state-of-the-art design of ductile iron components. The presented solution utilizes an experimental procedure, which enables the production of specimens with specific, individual microstructural configurations in the casting skin. This approach is based on the application of a modified sand core coating. By adding sulfurizing additives to the coating and adjusting its binder content and viscosity, a predominately homogeneous degenerated layer of 0.5 mm thickness was obtained. Using specific heat treatment steps assured a fully ferritic or pearlitic iron matrix, isolating the degenerated layer as the main microstructural defect. Fatigue testing of these specimens will further enable the numerical evaluation of the fatigue resistance in dependence of the casting skin microstructure.

Rosário A.M., Hupalo M.F.

Fish-eye defects are usually observed in the surface of ductile iron casting. Their formation mechanism is not fully understood and very little has been published concerning the subject. This paper investigates the sleeve material effect on fish-eye defect formation in ductile cast irons. Unburnt exothermic and insulating sleeves with different fluorine concentrations have been crushed and mixed in with green sand. These mixtures have been used as facing sands for the casting molds. The morphological and chemical characterization of the fish-eye defects have been performed by light optical microscopy (LOM) and scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS). Experimental results suggest that surface fish-eye defects on ductile cast irons are caused by the buildup of fluorine-containing sleeve materials in the molding sand.

Stefanescu D., Wills S., Massone J., Duncan F.

The mechanical properties of ductile iron (DI) and compacted graphite iron (CGI) are measured and reported on standard machined specimens (as per ASTM). However, most castings retain most of the as-cast surface. This surface layer (the casting skin) includes both surface and subsurface features. Because of the casting skin, the mechanical properties of the part are typically significantly lower than those found on standard ASTM machined specimens. The technical objectives of this research were to identify the individual features that together define skin quality in DI and CGI, to develop a method for the measurement of skin thickness, and to quantify the influence of the skin of thin wall (2 to 6 mm) DI castings on its tensile properties. The features of the casting skin include surface (roughness) and subsurface (graphite degradation, graphite depletion, pearlitic rim) elements. Graphite shape measurements were used to evaluate graphite degradation. Graphite area measurements were used to determine the thickness of the graphite depleted layer. Microhardness measurements are useful when a pearlitic rim occurs. The average thickness of the skin for thin wall DI castings ranged from 0.15 to 0.45mm, while for CGI it ranged from 0.7 to 2.5mm. It was found as expected that the strength decreased with thicker casting skin. The tensile and yield strength skin factor (ratio between the strength of as-cast and machined test plates) was about 0.93. This should be viewed as an upper limit, as only one of the surfaces of the mechanical properties test plate was as-cast. More significant reduction in strength should be expected. Diffusion calculations confirmed that graphite degradation, graphite depletion and the pearlitic rim are the result of magnesium and carbon depletion at the mold/metal interface because of their oxidation. Alternatively, carbon diffusion from the mold can also result in pearlitic rim formation.

Starkey M.S., Irving P.E.

The fatigue strength of machined specimens of ferritic SG iron has been compared with the strength of specimens having an as-cast surface. Micropores, exposed by machining, initiated failure in the machined specimens, whereas surface irregularities or dross defects initiated failure from the as-cast surface. While the endurance of specimens in which failures initiated at surface irregularities was only marginally less than that of machined spec imens, dross defects reduced fatigue life by a factor of ten. It is demonstrated that the fatigue life of SG iron is dominated by micro-crack growth and that the effect of all types of SG -iron defects on fatigue endurance is related to defect size. Specimen endurances of less than 10 5 cycles can be predicted using a fracture mechanics approach and integrating the Paris Law from the appropriate defect size.