Influence of Mg on Solidification of Hypereutectic Cast Iron: X-ray Radiography Study
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s been found that small addition of Ce or Mg to hypereutectic cast iron changed the morphology of graphite particles from a flake shape to a more compacted/spheroidal shape.[1,2] A number of studies have been done to investigate the mechanism of spheroidal graphite formation, to control the microstructure for commercial application.[3,4] In general, the formation of spheroidal graphite particles has been attributed to nucleation events and growth modes of graphite particles. This paper focuses on the influence of Mg additions, which are commonly used for producing spheroidal graphite particles, on solidification and melting behaviors of hypereutectic cast iron. One important consideration in morphological transitions during solidification is nucleation. It has been proposed that both inclusions and bubbles act as
K. YAMANE, Researcher, is with the IHI Master Metal, Aioi, Hyogo 678-0041, Japan. H. YASUDA, Professor, and K. MORISHITA, Assistant Professor, are with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-0851, Japan. Contact e-mail: [email protected] A. SUGIYAMA, Professor, is with the Department of Mechanical Engineering for Transportation, Osaka Sangyo University, Daito, Osaka 574-8530, Japan. T. NAGIRA, Assistant Professor, and M. YOSHIYA, Associate Professor, are with the Department of Adaptive Machine Systems, Osaka University, Suita, Osaka 565-0871, Japan. K. UESUGI, A. TAKEUCHI, and Y. SUZUKI, Researchers, are with the JASRI/SPring-8, Sayocho, Hyogo 679-5198, Japan. Manuscript submitted September 22, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleation sites. The bubbles in the melt can be produced by vaporization of Mg.[5–7] Sulfides and oxides are produced by the reaction of S/O with added Mg in the melt.[8–10] Silicates such as MgO-SiO2 and 2MgOSiO2 were also confirmed to be present in the ductile cast iron.[11] The chemical composition and crystal structure of the inclusions produced by adding ferrosilicon containing Mg were examined.[12] The observed inclusions were (Ca, Mg)—sulfides surrounded by (Mg, Al, Si, Ti)—oxides. A confocal scanning laser microscopy also observed the inclusions inside the spheroidal graphite particles.[13] The core of the MgS was surrounded by sulfides and oxides. It is obvious that the inclusions promote the graphite nucleation events. However, it is still not clear how the nucleation sites influence the morphology of the graphite particles. Spheroidal graphite particles have been produced using the cast iron made from high-purity metals. The spheroidal graphite particles did not contain any impurity elements around the core region.[14] The results proved that the spheroidal shape was attributed to the nature of graphite growth. The role of added Mg is to remove some trace elements solved in solution in the melt, such as S and O. The influence of S concentration and cooling rate on the morphology of graphite particles were also investigated in the high-purity cast iron.[15] No spheroidal graphite particles were formed in the sp
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