Microstructure of As-Cast High-Silicon Ductile Iron Produced via Permanent Mold Casting

Abstract

Permanent mold casting of ductile iron is an environmentally friendly alternative to sand mold casting. The high cooling rate leads to a homogeneous microstructure with low grain size, but can also lead to cementite formation. In the work presented, the as-cast microstructure of solid solution-strengthened ductile iron (SSDI) cast in permanent mold made of gray iron is analyzed. The high silicon content in SSDI decreases chilling tendency and promotes the formation of ferrite. Silicon contents in the range 3.2–4.3 wt% and mold preheating temperatures in the range 300–450 °C were analyzed for section thicknesses of 10, 20 and 30 mm. Solidification time and cooling rate at the casting sections analyzed were measured. The results show that an as-cast microstructure free from carbides is achieved in sections of 20 and 30 mm for all castings analyzed, whereas for 10 mm a partial chilled microstructure is produced. As expected, the pearlite content decreases with increasing silicon content and increasing preheating temperature. A higher cooling rate increases eutectoid undercooling and thus promotes pearlite formation. From the experiments carried out, a quantitative correlation is deduced for the pearlite content as a function of eutectoid undercooling and Si content. The nodule count is approximated as a linear dependency on solidification time.