Abstract
Grey iron alloyed with molybdenum and niobium in seven different compositions has been casted using three, in industrial components viable, solidification times which resulted in 21 different samples. The samples have been investigated with respect to microstructure, static properties and thermo-mechanical fatigue performance. It was found that the solidification time is very important for both the static and thermo-mechanical performance. If the solidification time is long the properties are controlled entirely by the large graphite flakes and there is no influence of the alloying elements. On the other hand if the solidification time can be kept short the need for alloying elements may be removed. For the shorter solidification times an influence from the matrix and thus the alloying elements can be seen. It was found that molybdenum enhances TMF-life while no such effect was found for niobium. Niobium, on the other hand, has a larger effect on static strength than molybdenum and also on the cyclic stress in the thermo-mechanical fatigue experiments.
Highlights
Demanding environmental legislation as well as requirements of decreasing fuel consumption and increased specific power for combustion engines inevitably lead to increased combustion pressure and temperature
The results indicate that niobium has a larger impact on the static properties compared to molybdenum the small number of experiments makes it impossible to draw any definite conclusions
It is emphasized that the investigated solidification times are rather typical for industrial castings, for example the short solidification time used in this investigation is close to what can be found at the top of a cylinder head
Summary
Demanding environmental legislation as well as requirements of decreasing fuel consumption and increased specific power for combustion engines inevitably lead to increased combustion pressure and temperature. The enhanced high frequency and low amplitude pressure pulses increase the elastic high cycle fatigue (HCF) load that affects part of the cylinder head. For the cylinder head the TMF-load is critical as it affects the area between the valves. On heating it experiences high compressive stresses giving plastic deformation and when cooled down, tensile stresses develop initiating cracks and crack propagation in the subsequent cooling cycles. This damage process occurring close to the fire deck is often life limiting for the cylinder head. In some cases the HCF-load is superimposed to the TMF-load which may lead to a large decrease in life, [1,2,3]
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