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ology to measure surface finish. PRESENTATION Influence of Mn and S on the


Microstructure of Cast Iron AUTHOR Richard B. Gundlach, B. Gundlach,


Element Materials Technology (Wixom, Michigan)


RESEARCH FOCUS Sulfur is generally considered a


tramp element in cast iron, and its level must be controlled. Manganese will react with sulfur to form MnS inclu- sions and, thereby, tie up sulfur before solidification occurs. When manganese is not present at sufficient concentra- tions, sulfur reacts with iron to produce a low-melting phase that can produce hot-shortness and embrittlement in iron castings. Consequently, the in- dustry has always added manganese to control sulfur in cast iron. Various approaches for balancing manganese and sulfur have been pro- mulgated in the industry. To avoid FeS formation, many look to the stoichio- metric ratio of MnS (1.7 to 1), and rely on adding excess manganese to control sulfur. Some employ a manganese to sulfur ratio (such as in the range 5 – 7). Others advocate that the sulfur content must simply be at, or above 0.04%S to obtain adequate inoculation response. With the exception of a few investiga- tors, none has considered the solubility of MnS (from thermodynamic prin- ciples) as a guide to balance manganese and sulfur. While the concept of “excess manganese” suggests that manganese can completely tie up sulfur prior to the solidification of the melt, thermo- dynamic calculations show that the reaction manganese + sulfur o MnS is weak and does not go to completion before the metal reaches the eutectic temperature (typically around 2,150F), see Figure 1. Based on thermodynamic calculations, there can be significant amounts of sulfur in solution when eutectic solidification begins. At the eutectic temperature for


typical commercial cast iron chemis- tries, the equilibrium constant, Keq, for the reaction Mn + So MnS is


Figure 1. Depiction of the solubility of MnS in cast iron at temperatures near and well above the eutectic temperature.


Figure 2. Estimation of amounts of MnS precipitation and soluble sulfur in cast iron containing 0.78%Mn and 0.15%S.


around 0.03. That is, the following relation is true: Keq ≈ %Mn x %S = 0.03 (at 2,150F) When the value %Mn x %S exceeds


0.03, the solubility of MnS is exceeded and some MnS will precipitate in the liquid metal before the metal reaches the eutectic temperature. Te precipitation of MnS will reduce the level of sulfur


in the melt until the solubility limit is attained (Figure 2). Consequently, some free sulfur is always present when eutectic solidification begins, and the “free sulfur” content in the liquid metal is primarily controlled by the manganese concentration. Some “free” sulfur is beneficial. As a surface active element, sulfur promotes


May 2018 MODERN CASTING | 47


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