Ductile iron optimization approach for mechanically and thermally loaded components
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Copyright Ó 2020 The Author(s) https://doi.org/10.1007/s40962-020-00529-9
Abstract The mechanical and thermal properties of ferritic–pearlitic ductile irons vary widely according to their silicon and pearlite contents. Thus, different combinations of silicon and pearlite affect components’ lifetime under mechanical and thermal stress. An excellent example of the usage of such irons is combustion engine cylinder heads. They experience transient thermal loading (heating and cooling) during starting and stopping in addition to mechanical loading (combustion) during engine operation. An optimization approach and calculation models for the
estimation of the optimal ductile iron composition are presented in this study. The approach allows selection of the most suitable base composition for subsequent analyses, such as casting simulation and final accurate finite element modelling and fatigue calculations.
Introduction
Depending on engine size, the combustion load of such engines (high-cycle load) is between 250 and 600 cycles per minute (0.5 times the revolution speed). At engine start, the cylinder head heats to its operating temperature and cools to the ambient temperature when the engine stops. This causes substantial thermal gradients and stresses and is known as low-cycle loading. Variable temperature fields not only determine critical locations but are also decisive in reducing the number of cycles before failure, both in the high- and low-cycle fatigue regimes. Fast ramp-ups and ramp-downs result in high thermal gradients and stresses, generally making low-cycle fatigue the limiting factor in final component life.
Factors such as reliability and durability are the primary targets for the structural analysis of components. Current design processes take account of integral boundary conditions, such as component geometry, and optimize them in order to achieve the set objectives.1–3 Large combustion engines contain many components that operate under combined thermal and mechanical loads. Cylinder heads are one of the most loaded of these, experiencing static, thermal and dynamic loads during operation. Maximum combustion pressures of modern large engines are well over 200 bar (20 MPa). Static loads arise from pre-tensioning forces. Such pre-tension is used, for example, to ensure cylinder head sealing tightness during combustion.4
International Journal of Metalcasting
Keywords: ductile iron, thermal conductivity, elevated temperature properties, optimisation
Under these kinds of loads, material selection is a critical part of the structural design. An optimal selection process considers both thermal and mechanical properties carefully. This study aims to develop a methodology that can be used for selection of an optimized ductile iron
Figure 1. Schematic picture of the cylinder head, studied areas (1) flame plate, (2) coolant channel area, (3) exhaust channel.
Figure 2. Optimization loop.
composition for a thermally and mechanically loaded component. The method is finally applied to the analysis of a cylinder
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