Abstract
This work is designed to better understand the influence of cooling rate on the nature and morphology of intercellular precipitates in Silicon-Molybdenum ferritic ductile iron (SiMo). Plates of 3, 6, 9 mm thickness were cast in greensand and investment casting molds to give a wide spectrum of cooling rates. It was found that at higher cooling rates, the intercellular regions have a lamellar structure typical of pearlite. With decreasing cooling rates, the precipitate contains complex (Fe-Mo-Si) carbides of fine spheroidal or rod-like structure surrounding the eutectic carbides.Intensive Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) and Optical Microscopy (OM) investigations showed that the eutectic carbides are mainly (Fe, Mo, Si) C containing up to 48% Mo, whereas the fine precipitates contain lower Mo-contents. Both carbide types did not show to have a strict stoichometric composition. The solidification and solid-state transformation path was determined using both phase diagram calculated from Thermo-Calc software as well as Differential Scanning Calorimetry (DSC).
Highlights
Over the past few decades, Silicon-Molybdenum ferritic ductile iron (SiMo) ductile irons have been finding increasing applications for high temperature automotive components such as exhaust manifolds and turbocharger housings [1,2,3]
Casting experiments and solidification modeling have shown, that the shrinkage tendency of high Si-Mo containing 4-6% Mo can be controlled in the same level as that of regular SiMo, using suitable C.E (4.35-4.85), depending upon the critical thickness of the casting [3]
Thermo-Calc phase diagram of 0.7% Mo section of the Fe-Mo-C is shown in fig (1-a,b), which illustrates the formation of carbide phases in equilibrium with austenite (FCC) and ferrite (BCC) on cooling from the liquidus line (~1270°C) to the room temperature
Summary
Over the past few decades, SiMo ductile irons have been finding increasing applications for high temperature automotive components such as exhaust manifolds and turbocharger housings [1,2,3]. The impact of the section thickness and cooling rate of the castings on the cell boundary carbide precipitation in SiMo alloys has been scarcely dealt with in the literature. The main objectives of this work, is to investigate the influence of the cooling rate of thin-wall SiMo castings of section thickness ranging from 3-9 mm and cast in greensand or investment casting molds.
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