On the solidification sequence of ductile iron castings containing chunky graphite

The macro- as well as the microstructures in ductile iron castings containing chunky graphite have been investigated. Instrumented castings with wall thicknesses ranging from 10 to 200 mm were studied using optical and scanning electron microscope. The graphite morphology was revealed by the deep etching technique. Information about the solidification sequence could be obtained by combining thermal analysis results with a color etching technique revealing the segregation pattern. The amount of chunky graphite generally increases with an increasing wall thickness. However, chunky graphite occasionally appeared even in 10 mm castings. After a relatively sharp transition, following the macroscopic isotherms, chunky graphite starts to grow as eutectic cells. When approaching the outer regions of the cells, the graphite suddenly became significantly coarser, before a complete transition took place into ordinary graphite nodules. Further on, by color etching it was found that the major part of the graphite nodules in many cases was located in the most segregated areas. This suggests that chunky graphite forms early during eutectic solidification, before formation of the nodular graphite. The growth rate of the chunky graphite eutectic cells was of the same order of magnitude as in gray iron but slower compared to supercooled D-graphite cells.