Abstract
This paper considers the most important quality factors in processing spheroidal graphite cast iron; namely, primary grains and graphite nodules in thin-walled ductile iron castings (TWDI). In the present study, the effect of grain refinement (by means of Ti, Nb and Zr) and of the holding time after spheroidization and inoculation on effecting the primary grains and eutectic structure in TWDI castings was investigated. Moreover, metallographic examinations (including electron backscattering diffraction, EBSD) were carried out to reveal the macro- and micro-structural features during the primary and eutectic solidification of the cast iron. EBSD results indicate that, within a single dendritic grain, there are numerous boundaries that split the grain into numerous smaller areas. In particular, it is found that the graphite nodules are in contact with the boundaries inside the primary dendritic grain. In turn, crystallization of highly branched dendrites is observed, which seems to “push” the graphite nodules into the interdendritic regions during their growth. The present work investigates the dominant mechanism that gives rise to the primary spheroidal graphite cast iron (SGI) structure. In addition, this work shows that the melt quality is closely associated with the resultant morphology and number of austenite dendrites, graphite nodules, and matrix structure.
Highlights
Spheroidal graphite cast iron (SGI) belongs to a group of important engineering materials due to its excellent mechanical properties and significant savings in cost and weight [1,2,3]
The excellent combination of properties found in thin-walled ductile iron castings (TWDI), including thin-walled austempered and alloyed iron, make them highly viable materials to be employed as substitutes for steel castings and forgings in various engineering applications
The fading effects of grain refinement and inoculation of the primary structure and eutectic are accompanied by changes in the morphology and number of austenite dendrites and eutectic grains
Summary
Spheroidal graphite cast iron (SGI) belongs to a group of important engineering materials due to its excellent mechanical properties and significant savings in cost and weight (as compared to equivalent steel and aluminum alloys) [1,2,3]. This, in combination with excellent castability and machinability, including a less-sensitive microstructure to cooling rates (vs flake graphite cast iron, FGI) makes it the base material for the production of “high tech” austempered ductile iron (ADI). The excellent combination of properties found in thin-walled ductile iron castings (TWDI), including thin-walled austempered and alloyed iron (e.g., ferritic and austenitic SGI), make them highly viable materials to be employed as substitutes for steel castings and forgings in various engineering applications. From the point of view of economics and ecology, thin-walled iron castings compete with “light” aluminum alloy castings in terms of mechanical properties [1,4,5,6,7]
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