Prediction of reoxidation inclusion composition in casting of steel
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TRODUCTION
INCLUSIONS are a major cost factor in the production of steel castings. Removing inclusions and refilling the defect areas with the weld metal account for approximately 20 pct of the direct cost of steel castings.[1] Reoxidation inclusions are the most common form of inclusions that are found in steel castings. Based on a survey of steel foundries, they account for approximately 83 pct of the inclusions in carbon and low-alloy steel castings and approximately 48 pct of the inclusions in high-alloy and stainless steel castings.[2] Reoxidation is defined as the reaction of elements in steel with oxygen during pouring of the (deoxidized) steel from the ladle into the mold. Oxygen may come from the surrounding atmosphere, refractories, slag, or the sand mold.[3] The atmosphere is generally believed to be the primary source of oxygen, and limiting the exposure of the steel to the atmosphere is an important consideration in designing pouring systems. Large reoxidation inclusions often accumulate on the cope surface of steel castings, where they may be visible to the naked eye. They have a detrimental effect on machining and mechanical performance of steel castings and may cause casting rejection. An image of a typical reoxidation macroinclusion cut from a low-alloy steel casting is shown LIANG WANG, Graduate Research Assistant, and CHRISTOPH BECKERMANN, Professor, are with the Department of Mechanical and Industrial Engineering, The University of Iowa, Iowa City, IA 52242. Contact e-mail: [email protected] Manuscript submitted September 20, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
in Figure 1.[2] The inclusion has a diameter of about 2 mm and consists of 34 wt pct Al2O3, 46 wt pct SiO2, and 20 wt pct MnO. Its globular shape indicates that a relatively large fraction of the oxide mixture was in liquid form during formation of the inclusion. Inside the inclusion, smaller particles that were presumably solid during formation are visible as well. The objective of the current study is to predict the composition of reoxidation inclusions in the casting of steels in sand molds. This study is part of a larger project to model the entire process of reoxidation inclusion formation and transport in steel casting.[4] It is shown that the inclusion composition is very sensitive to the steel composition and the amount of oxygen absorbed at the steel-atmosphere interface. Available thermochemical analysis software is used, which enables parametric studies to be conducted that would be difficult to perform experimentally. Reoxidation of liquid steel has been investigated extensively in the past.[5–14] All of these studies conclude that the oxidation rate is primarily controlled by the rate of oxygen transfer through the atmosphere to the surface of the steel. Only for very low alloy element concentrations, or in the presence of stable oxide films, do internal diffusion processes become important.[6,7,9–11] The fact that the oxidation rate is limited by atmospheric transport of oxygen is established in the above
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