Mechanical and microstructural effects of different niobium addition in ductile cast iron (DCI) for turbocharger applications
Ductile cast iron (hereafter called DCI) is a vastly used material in casting engineering and its behavior is extremely compelling for the manufacture of components for the automotive industry, such as turbine housings, exhaust manifolds and other components due to its low cost and good castability as well as its high oxidation resistance and high-temperature strength. However, the recent trend of increasing combustion engine performance due to new targets of lowering CO2 emissions limits highlights the need for improvements to this alloy, not only in regard to its maximum temperature range but also in regard to its thermal and mechanical properties. The maximum temperature limit of this DCI commercially known as Si Mo and SiMoCr is about 850°0. Alternatives for reaching a higher cyclic temperature are austenitic stainless steels and ferritic stainless steels. However, they both are high cost and have low castability as well as issues regarding machinability. Therefore, there is a need for modification to the standard DCI, Si Mo, using some alloying elements such as niobium (Nb). Niobium was observed to promote the formation of niobium carbides of the type NbC. Such carbides increased in size and number as niobium content increased in the alloys.Two different thickness specimens were casted (13 mm and 25 mm) to simulate and understand the comportment of these Nb additions on a large range of turbine housing wall thicknesses. Hardness (HB) at room temperature, yield strength, tensile strength and elongation have been determined at room temperature, 425°0, 800°0 and 850°0. Finally, the niobium recovery rate after remelting 5 times was studied to determine the percentage of niobium lost during the remelting process. This new alloy, “SiMoCr+Nb,” demonstrated good mechanical properties at room and high temperatures. It also presented a relatively small increase in hardness (HB) at room temperature. However, it showed a significant increase in strength and elongation at almost all high temperatures tested. Last but not least, the niobium recovery rate was observed to be very close to one hundred percent which is excellent news for foundries.Download the full technical report now