As part of a study devoted to the effect of trace elements on graphite degeneracy, near-eutectic ductile iron melts were prepared to which minute amounts of lead and of both lead and cerium were added. The melts were cast into an insulated Y4 mould, giving a solidification time of about 1 hour and a cooling time to room temperature of about 15 hours. In the thermal centre of the Pb containing sample graphite spheroids as well as intergranular lamellar graphite have been found. At the same location of the casting containing both Pb and Ce, exploded as well as chunky graphite could be observed, while the formation of intergranular lamellar graphite has been suppressed. Deep etching of the samples allowed reaching the following conclusions: i) intergranular graphite in the SG-Pb sample often, if not always, originates on graphite nodules and extends towards the last to freeze areas; ii) in one location of the SG-PbCe sample, chunky graphite strings were observed to originate on an exploded nodule, thus confirming the close relationship between these two forms of graphite. Because of the over-treatment in cerium of the SG-PbCe sample, other unusual degenerate graphite was observed which appears as coarse aggregates of "porous" graphite after deep etching.
Abstract. High life expectancy of cast components and good material performance at dynamic load are a prerequisite to cater for future trends in wind energy generators. To remain competitive in this ever evolving sector challenges reside in alloy development. In this work fractional factorial design has been applied to ferritic ductile iron with varying contents of silicon (1.6-2 wt%), nickel (0-1 wt%), cobalt (0-3 wt%) and copper (0-0.2 wt%). The minimum criteria the new alloy should meet were a minimum yield strength of 240 MPa and an impact work of minimal 8 J at a temperature of -20 °C for wall thicknesses of 60-200 mm. To obtain these mechanical properties thick-walled castings with additional insulation were produced to achieve a higher thermic module. They provided the material for test specimens to perform static tensile tests, Charpy impact tests at varying temperatures and a microstructure analysis. With these results, a sweet spot plot has been created. That way, an optimum alloy composition could be found and has been proven by validation experiment.The optimum alloy for thick-walled castings is composed of Si = 1.6 wt%, Cu = 0.2 wt%, Ni = 0 wt% and Co = 0 wt%. It offers an enhancement in yield strength and acceptable impact work at low temperatures for massive castings in as cast state. The heat treated, full ferritic material could even improve these results.
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