Relationship Between Inoculants and the Morphologies of MnS and Graphite in Gray Cast Iron
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THE nucleation of graphite in cast iron has been researched for many decades and, consequently, many theories regarding graphite nucleation have been proposed. Warrick[1] suggested that complex oxides and sulfides serve as the nuclei for flake graphite. Many other researchers came to similar conclusions, including Muzmudar et al.,[2,3] Jacobs et al.,[4] and Gadd et al.[5] Kusakawa et al.[6] have reported the presence of nonmetallic inclusions that exhibited the highest inoculation effect. Campbell[7] introduced the concept of a silica-rich oxide bifilm that nucleates on oxysulfide particles in the melt. To precipitate graphite as a stable iron graphite eutectic structure, inoculants are added to the melt that typically consist of rare earth metals or strong deoxidizers such as Sr and Al. These inoculants produce heterogeneities in the form of oxides and sulfides to enhance the nucleation of graphite on oxysulfide particles. Each type of inoculant influences the graphitizing efficiency differently, as discussed by Riposan et al.[8]; for example, Sr has a higher graphitizing efficiency than Ba and Ca. Researchers have found that the (Mn,X)S particle (X = Al, Sr, Ca, …) acts as a nucleation site for graphite.[9–12] Velichko et al.[13] performed a 3D visualization of the eutectic cell and found many small MnS inclusions within a thick graphite flake. Skaland[14] proposed a mathematical model for the heterogeneous
nucleation of graphite in cast iron based on the concept of interfacial energies and lattice disregistry. According to Skaland, undercooling decreases in a parabolic manner with increasing values of the planner lattice disregistry. Hua Qu et al.[15] analyzed the lattice structure of graphite and compared it to those of CaS and MnS. Hua Qu found a close match between the lattice parameters of MnS and CaS and those of graphite. Sommerfeld et al.[16] concluded that Mn/S ratio influences graphite nucleation. Similarly, the content of Mn in cast iron has been investigated many times by different researchers, whereby the ratio of Mn/S was explored with respect to the graphite precipitation process and morphology along with its relationship to the tensile strength of cast iron (Muzmudar et al.,[2] Goodrich et al.,[17] Sommerfeld et al.[16] and Fras´ et al.[18]). The role of MnS with a varying sulfur concentration in the alloy, the nucleation of oxides in the melt due to inoculation, and the influence of MnS and oxides on the growth morphology of graphite were investigated experimentally and by thermodynamic analysis. The samples were analyzed by energy-dispersive X-ray spectroscopy (EDX) to verify different types of oxide/sulfide inclusions formed during inoculation. One of our goals was to observe the morphological differences in graphite and MnS with respect to inoculation.
II. H. M. MUHMOND, Ph.D student, and H. FREDRIKSSON, Professor, are with the Materials Science and Engineering Department, Royal Institute of Technology, Brinellva¨gen 23, 10044, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted May 26, 2012.
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