Abstract
The main objective of the present paper is to evaluate by thermal (cooling curve) analysis the solidification pattern and the occurrence of the layer of degenerate graphite at the surface of ductile iron castings (3.15% Si, typically as 450-18 grade, ISO 1563/2011), with or without a mold coating, including S or O, and different agents (carbonic material, iron powder), supposed to act to block their diffusion into the iron melt. It is found that the mold coating materials temperately influence the parameters of the solidification cooling curves and, more visibly, the occurrence and the thickness of the undesired skin layer. Different graphite morphologies comparable to the casting body are present, at a large range of thicknesses, from 50 up to 200 µm. The sulfur presence in the mold coating will promote a higher skin thickness compared to oxygen (up to 50% by oxygen and 2.5–3.3 times for sulfur action), despite the fact that in the casting body, the graphite nodularity undergoes a limited decrease (from 85% up to 82%–83% level). Carbonic material or iron powder supplementary addition decreases these undesired effects, but the solidification undercooling compared to the equilibrium system is increased. It is found that carbonic material is more efficient at limiting oxygen than iron powder is at limiting the negative effects of sulfur on the casting skin thickness. More experiments are necessary to quantify their capacity to block the oxygen or sulfur transfer into the iron melt.
Highlights
Ceramic molds, used in metal castings production, are usually coated in order to control the metal–mold chemical interaction and to decrease the casting surface roughness
The main objective of the present paper is to evaluate the solidification pattern and the occurrence of graphite degeneration in the surface layer of relatively-high-silicon ductile cast iron (3.15%Si, typically as 450-18 grade, ISO 1563/2011), solidified in a ceramic mold without sulfur contribution, as the influence of the mold coating (with or without S or O-content, and with or without supplementary addition)
The experiments could be grouped in the following variants: (a) Uncoated mold, without oxygen or sulfur available to diffuse into the iron melt, as reference variant; (b) coated mold, including oxygen (Fe2 O3 -bearing coating) or sulfur (FeS2 -bearing coating), as potential sources of these active elements to react with iron melt, before solidification, at least in the superficial layer of casting; (c) supplementary addition of other substances on the previous Fe2 O3 - or
Summary
Ceramic molds, used in metal castings production, are usually coated in order to control the metal–mold chemical interaction and to decrease the casting surface roughness. In Mg-treated iron castings, the coating is important to control the graphite degeneration process in the surface layer. The surface layer with degenerated graphite causes stress raisers in the casting, similar to a notch, so all the mechanical properties are reduced. The casting skin effect on fatigue properties is expected to be more pronounced than the static properties because in thin-wall ductile iron castings, a reduction of 16.3% in the fatigue strength is observed. A deleterious effect of the casting skin in compacted graphite iron casting is identified, suggesting that both the graphite degradation layer and surface roughness were responsible for the reduction in tensile properties
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