CHEMICAL-COMPOSITION AND STRUCTURAL IDENTIFICATION OF EUTECTIC CARBIDE IN 1 PCT MN DUCTILE IRON

Manganese, which may be used in ductile cast iron as a potent hardenab ility promoter, segregates in the intercellular region. This segregati on becomes more severe as a consequence of poor inoculation, low cooli ng rate, or increasing of nominal Mn content in the alloy. In severely Mn-segregated regions, Mn eutectic carbide may be formed, which has a deteriorating effect on the mechanical properties of casting. In this study, a 1 pct Mn ductile iron was used to investigate the chemical c omposition and crystal structure of the Mn eutectic carbide by electro n microscopy (scanning and transmission), X-ray, and an electron probe microanalyzer (EPMA). Transmission electron microscope (TEM) and X-ra y studies show that the crystal structure of carbide is orthorhombic w ith lattice parameters of a = 14.825, b = 11.415, and c = 8.880 (angst rom). The concentrations of Mn, Si, and Cr in carbide, analyzed by sca nning electron microscope-energy-dispersive X-ray spectrometer (SEM-ED X) and transmission electron microscope-energy-dispersive X-ray spectr ometer (TEM-EDX), were 5.0 to 7.0, 0.5 to 2.8 and 1.5 to 2.2 (wt pct), respectively. The ratio of Fe plus other metal atoms to C was calcula ted from EPMA experiments to be 2.5-2.9. It was shown that by diminish ing Mn segregation, precipitated eutectic carbide can be reduced. It i s expected that this can be achieved by reducing nominal content of Mn or by increasing nodule count.