Effect of Bismuth on Preventing Chunky Graphite in High-Silicon Ductile Iron Castings
- PDF / 1,932,509 Bytes
- 11 Pages / 595.276 x 790.866 pts Page_size
- 74 Downloads / 250 Views
Copyright 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00419-0
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.
Introduction
and there are an increasing number of large nodules with low degree of nodularity.1 If there are elements present in the melt that stimulate the formation of CHG, such as Ce, Ca, Si, Ni and RE, it is very likely that CHG will be formed, particularly in the absence of tramp elements.2–8 Recent researches have focused on the effect of the RE and trace elements and their interaction in preventing the formation of CHG. The majority of MgFeSi master alloys contain a certain level of RE, especially cerium, which is added in order to counteract the negative effect of tramp elements, increase the number of nodules and improve nodularity. However, an excessive level of cerium can cause the formation of CHG, especially in the case of thick-walled castings. Since it is always better to work with the addition of cerium due to its positive effect (increase in nodularity, number of nodules, level of ferrite, and reduced
There are several types of degenerated graphite forms, and one of the most frequent and most deleterious is the chunky graphite (CHG). CHG appears locally, mainly in the thermal centre of thick-walled ductile iron castings, and causes the deterioration of mechanical properties, especially of tensile strength, elongation, and fatigue strength. This defect can be seen by visual method of inspection on the cross section of the casting after saw cutting as the darker area. CHG appears because of the reduction of th
Data Loading...
