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It is well known that 4 to 6% silicon spheroidal irons are suitable for use at high temperature. This paper describes solidification behavior, microstructure, mechanical properties, high temperature oxidation, and thermal fatigue of high silicon SiMo cast irons. Cooling curves of cast irons were recorded using a thermal analysis apparatus to correlate with the solidified microstructures. Uniaxial constrained thermal fatigue testing was conducted in which the cycling temperatures were between 500°C and 950°C. Oxidation behavior was studied by measuring the specimen weight and the penetration depth of oxides from laboratory cyclic oxidation testing. The coefficient of thermal expansion and critical temperature of the phase transformation A1 during heating were determined through dilatometry testing.

Both spheroidal graphite (SG) iron and compacted graphite (CG) iron are currently used to produce engine exhaust manifolds and turbocharger housings. The graphite morphology nodularity is typically specified lower than 30 or 50% for CG and higher than 80% for SG. The so-called mixed graphite (MG) high-Si cast iron has been proposed and developed in this work, in which the nodularity was defined from 30 to 80%. It was also referred to as hybrid graphite (HG) iron. A series of material and casting evaluations were conducted for MG high-Si cast iron, including melt treatment, solidification curves, microstructures, tensile properties, hot oxidation, thermophysical properties, and engine exhaust simulator (EES) testing. Middle temperature brittleness (MTB) from 400 to 500 C of MG was improved over CG and SG specimens. The EES testing showed that the cycles to failure for MG parts were equal or longer than those of CG and SG cast iron parts for three different manifold applications.

There is a wide spectrum of cast ferrous heat resistant alloys available for exhaust component applications such as exhaust manifolds and turbocharger housings. Generally speaking, the ferrous alloys can be divided into four groups including: ferritic cast irons, austenitic cast irons, ferritic stainless steels, and austenitic stainless steels. Selection of a suitable alloy usually depends on a number of material properties meeting the requirements of a specific application. Ferritic cast irons continue to be an important alloy for exhaust manifolds and turbocharger housings due to their relatively low cost. A better understanding of the alloying effects and graphite morphologies of ferritic cast irons are discussed and their effect on material behavior such as the brittleness at medium temperatures is provided. The nickel-alloyed austenitic cast irons, also known as Ni-resist, exhibit stable structure and improved high-temperature strength compared to the ferritic cast irons.

The cyclic plasticity of a typical exhaust manifold material has been successfully characterized using a unified constitutive model based on the models developed by Sehitoglu and his co-workers [1-2]. Both monotonic tensile and cyclic strain controlled fatigue tests were performed at temperatures ranging from ambient to 800 °C to evaluate the material constants in the model. In this model, the isotropic hardening is ignored due to its minor effect upon the selected cast iron, which simplified the data processing. The model predictions are satisfactory for a wide temperature range (23-800 °C), and the strain rates covered (0.5-50 %/min).