CARBIDE CHARACTERIZATION IN SINGLE AND DOUBLE STEP INOCULATED THIN WALL DUCTILE IRON

Abstract

Heterogeneous nucleation of graphite is an important aspect of cast iron metallurgy. This is adequately achieved by inoculation treatments, however due to thickness reduction in Thin Wall Ductile Iron (TWDI), carbide precipitation with poor nodularity and nodule count occur in microstructure. Understanding the characteristics of these carbide precipitates regarding type, composition, morphology, and hardness are necessary if use of TWDIs for automotive application is to be sustained. This study produced 3 mm TWDIs castings subjected to pre (single step) and pre/post (double step) inoculation treatments. The ferrosilicon (FeSi) inoculant was varied during melt treatment to capture a wide range of carbide precipitates. Microstructure of samples was investigated using Optical and Scanning Electron Microscopy (SEM). Vickers microhardness tester was employed to evaluate hardness properties. Spot Energy Dispersive Spectroscopy (EDS) was conducted on carbide precipitates to determine composition, also nodularity and nodule counts were estimated. The results indicated that cast samples contained higher volumes of carbides in pre-inoculated melts than those subjected to both pre and post inoculated treatments. Significant portion of carbide precipitates appeared plate-like in shape. Higher Vickers hardness (HV) values correspond to higher volumes of carbide precipitates, 310HV and 382HV for 2.5 and 1.5 wt. % Si respectively for the pre-inoculated samples; while 299 HV and 263 HV for 2.7 and 3.7 wt. % Si respectively for pre and post inoculated samples. The as-cast 3 mm TWDI samples contained M₃C type eutectic carbides (Fe₃C) with plate-like morphology.