Effect of Bismuth on Preventing Chunky Graphite in High-Silicon Ductile Iron Castings

Abstract

The effect of Bi level on the macro- and microstructure of thick-walled high-silicon ductile iron castings has been investigated, as well as the influence of chunky graphite on the mechanical properties. Influence of cooling rate on graphite morphology was also investigated. High silicon level has caused the formation of chunky graphite, in very high amounts, along the entire cross section of the casting. Microstructure reveals that the addition of Bi affects not only the reduction of chunky graphite amount, but also the distribution along the casting cross section which is not uniform any more as in the casting without Bi. When Bi is added, the zone with chunky graphite appears independently of the cooling rate. Positive effect of Bi on the reduction of chunky graphite has been shifted towards higher levels, from 0.006 to 0.01 wt% Bi. Higher levels of Bi are more efficient also for the improvement in spheroidal morphology of graphite. The addition of 0.01 wt% Bi gave the best results, while the Ce/Bi ratio amounted to 0.6. This paper has proven that Bi acts on the increase in the strength values also in the high-silicon ductile iron casting. According to microstructure observations, it can be concluded that eutectic cells of chunky graphite are formed during bulk eutectic reaction. First, the primary graphite nodules are precipitated, followed by the initial phase of eutectic reaction during which the austenite dendrites precipitate and grow encapsulating at the same time primary graphite nodules.