Static Mechanical Properties of Ferritic and Pearlitic Lightweight Ductile Iron Castings

摘要

In a previous research report (Stefanescu et al, in print), the static mechanical properties of horizontal-plates thin ductile iron castings (1.5 to 6 mm) were measured. It was found that the properties of these castings have the potential of being equivalent or superior to current ASTM specifications for regular size (12.7 mm) ductile iron castings. However, because of solidification anomalies, the data were widely scattered, with many plates exhibiting substandard properties. Consequently, the pattern was redesigned and sound vertical-plate castings (2.5 to 6 mm) were produced. The cooling rates of these plates ranged from 4.6 to 12 deg C/s. Copper additions were used to cover a microstructure ranging from 80 to 20 percent ferrite. To obtain a predominantly pearlitic matrix, copper additions in excess of 0.2 percent were required. The results of the testing of the as-cast and fully machined tensile samples produced from the thin plates are reported in this paper. The fully machined samples were always superior to the minimum mechanical properties specified by the ASTM standard for regular section ductile iron. After detailed metallographic analysis, statistical analysis was performed to correlate the mechanical properties of thin wall ductile iron to the processing variables and the microstructure. To establish the base line for regular section ductile iron, several ASTM U-block mechanical testing specimens were also produced and tested. Good correlation was found between cooling rate and nodule count for both ferritic and pearlitic irons, as well as between the copper and ferrite content and the mechanical properties. It was concluded that the high nodule count associated with the increased cooling rates of thin wall castings is a major factor in matrix evolution. Indeed, somehow surprisingly, it was found that because of the high nodule count produced by the rapid cooling rate, the ferrite content was higher in the thinner plates. The surface quality of the samples affected their mechanical properties. Typically, as-cast samples exhibited lower properties than their fully machined counterparts did. The ferritic samples were more severely affected by surface quality than the pearlitic samples. To fully document the role of surface quality, the as-cast surfaces were analyzed and quantified with a profilometer. It was found that the surface roughness of the vertical-plate castings depends on the metallostatic pressure and the pouring temperature. Higher pouring temperatures and metallostatic pressures produced rougher surface finish. The properties decreased significantly above a certain critical roughness.