Microstructure and Dimensional Stability in Si-Mo Ductile Irons for Elevated Temperature Applications

The demands for high temperature metallic applications requires increased thermal (i.e., microstructural and dimensional) stability due to ever tightening tolerances, restrictive designs and higher Internal Combustion Engine (ICE) operating temperatures. Furthermore, these demands have caused the industry to place restrictive microstructure specifications on ductile iron production to ensure dimensional stability for certain applications. The presence of a distribution of precipitate phase was identified in Si-Mo iron as that with the Fe₂MoC-type structure, or less predominantly with a two-phase Fe₂MoC/M₆C-type structure, which is often mistaken for pearlite. This structure may provide Si-Mo irons with an inherent microstructure advantage with respect to dimensional stability at elevated temperatures, relative to alternative ductile iron materials containing a pearlitic microstructure. This paper describes the results of the microstructural characterization of a Si-Mo ductile iron and its influence on the thermal stability of this material. Complete microstructure analysis, semi-quantitative chemical analysis and dilatometer results have shown the absence of pearlite and the existence of a more thermally stable moly-rich precipitate distribution.