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Technical Paper

Application of a Unified Plasticity Model for an Exhaust Manifold Material

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).
Technical Paper

Simulation of Thermomechanical Fatigue of Ductile Cast Iron and Lifetime Calculation

In this paper, both standard and constrained thermomechanical fatigue (TMF) tests were conducted on a high silicon ductile cast iron (DCI). The standard TMF tests were conducted with independent control of mechanical strain, out-of-phase (OP) and in-phase (IP) strain, and temperature in the range from 300 to 800°C. The constrained TMF tests were conducted with various constraint ratios of 100%, 70%, 60% and 50% at the temperature ranges of 160 to 600°C and 160 to 700°C. Based on a material model as calibrated with low-cycle fatigue (LCF) data of DCI, finite element analyses (FEA) of the above TMF tests were carried out with Abaqus. A damage mechanism-based lifetime model was integrated into a C++ API code to post-process the Abaqus output results. Simulation predictions show good agreement with experiments for stress-strain responses and lifetime under different TMF conditions.
Technical Paper

High Silicon Cast Iron with Mixed Graphite (MG) Shapes for Elevated Temperature Applications

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.
Technical Paper

Solidification Behavior, Microstructure, Mechanical Properties, Hot Oxidation and Thermal Fatigue Resistance of High Silicon SiMo Nodular Cast Irons

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.
Technical Paper

Microstructure and Dimensional Stability in Si-Mo Ductile Irons for Elevated Temperature Applications

The demands for high temperature metallic applications requires increased thermal (i.e., microstructural and dimensional) stability due to ever tightening tolerances, restrictive designs and higher Internal Combustion Engine (ICE) operating temperatures. Furthermore, these demands have caused the industry to place restrictive microstructure specifications on ductile iron production to ensure dimensional stability for certain applications. The presence of a distribution of precipitate phase was identified in Si-Mo iron as that with the Fe2MoC-type structure, or less predominantly with a two-phase Fe2MoC/M6C-type structure, which is often mistaken for pearlite. This structure may provide Si-Mo irons with an inherent microstructure advantage with respect to dimensional stability at elevated temperatures, relative to alternative ductile iron materials containing a pearlitic microstructure.
Technical Paper

Quantification of the Internal Surface Roughness of Cast Iron Exhaust Manifolds

A study has been conducted to quantify the typical internal surface roughness of a cast iron exhaust manifold. In addition, the range of surface roughness values that can be obtained with various manufacturing methods was measured. Initial investigations were conducted to measure the effect of a range of surface roughness values on the performance of the engine system, specifically torque and the thermal losses through the exhaust manifold walls. Several manifold geometries were used to represent a variety of actual manifold applications, including designs that were subjected to tight packaging constraints. Physical tests were used to show that large variations in surface roughness resulted in modest changes in manifold component pressure losses. A simulation tool was used to predict that modest improvements in manifold pressure losses have little impact on engine output.