Nishikawa, L., Melado, A., Goldenstein, H., Bauri, L. et al., "Enhancing Mechanical Properties of Ductile Cast Iron Conrods through Different Heat Treatments," SAE Technical Paper 2016-36-0360, 2016.
The Austempering heat treatment is a well-known solution to improve the mechanical properties of ductile cast irons, therefore being referred as 'ADI' (Austempered Ductile Iron). The improved mechanical properties of ADI's with respect to conventional ductile iron is attributed to its resulting microstructure, which contains mainly carbide-free bainite with stabilized retained austenite. More recently, ductile cast irons were submitted to another heat treatment, known as 'Quenching and Partitioning' (Q&P). In this case, the ductile cast iron is austenitized, quenched to a temperature between Mf and Ms temperatures and subsequently heated to a temperature above Ms in order to partition the carbon from the martensite to the remaining austenite. The resulting microstructure comprises mainly low carbon martensite, austenite (stabilized by the carbon partition) and carbide-free bainite. Such microstructure resulted in equal or better properties than ADI. In the present study, these two heat treatments, ADI and Q&P, were applied to specimens extracted from small block engine connecting rods made of ductile cast iron. The resulting microstructure, as well as the original as-cast microstructure, have been characterized using optical and high resolution scanning electron microscopy (FEG-SEM). In addition, mechanical properties were comparatively evaluated. The results were analyzed and discussed considering the available literature. The Q&P process showed to be a promising solution, enhancing the properties of ductile cast iron connecting rods for automotive applications.