Abstract
Grades of high silicon ductile iron offer excellent combinations of static strength and ductility as well as good machinability due to their fully ferritic, solution strengthened matrix. As a result of elevated silicon contents, however, the ductile-to-brittle transition temperature in the Charpy impact test is significantly increased. Thus, minimum required Charpy impact energies cannot be met for many applications by using high silicon ductile iron. Therefore, alloys with lower strength and higher toughness properties are commonly used for many technical applications. The enormous lightweight construction potential of high silicon ductile iron can therefore not be fully exploited. The present investigation pursues the metallurgical approach of partially substituting silicon with molybdenum as an alternative strengthening element in order to improve the toughness properties while maintaining similar static mechanical properties. Molybdenum serves as a carbide-stabilising element in ductile iron, while simultaneously promoting ferrite formation and is therefore regarded to be suitable alloying element. In Charpy impact tests, the ductile-to-brittle transition temperature could be reduced by about 55 °C by reducing the silicon content to 2.95 wt% and adding 0.21 wt% molybdenum compared to a high silicon alloy. Additionally, it was possible to mathematically describe the transition behaviour of the studied alloys using nonlinear regression functions and to achieve a sufficient correlation of empirically determined and calculated data. This present metallurgical concept offers a promising metallurgical tool for further improving the toughness properties of alloyed ductile iron.
Highlights
Back in the 1950s and 60s, studies were carried out by White et al and Peleg on ductile iron with elevated silicon contents between 4 and 5 wt%.1,2 They found that a very good combination of strength and elongation can be achieved by solid solution strengthening of the metallic matrix
In Charpy impact tests, the ductile-to-brittle transition temperature could be reduced by about 55 °C by reducing the silicon content to 2.95 wt% and adding 0.21 wt% molybdenum compared to a high silicon alloy
It was in the 1990s that the idea of solid solution strengthening by alloying with silicon was taken up again by Bjorkegren et al, resulting in the incorporation into the Swedish Standards SS 140720 and SS 140725 and the European Standard DIN EN 1563 in 2012.3-6 In contrast to conventional strengthened grades that are characterised by a significant amount of pearlite, elevated silicon (Si) contents in the range of 3.2 to 4.3 wt% lead to the formation of a fully ferritic matrix with good machinability on the one hand, and to excellent static mechanical properties on the other
Summary
Back in the 1950s and 60s, studies were carried out by White et al and Peleg on ductile iron with elevated silicon contents between 4 and 5 wt%.1,2 They found that a very good combination of strength and elongation can be achieved by solid solution strengthening of the metallic matrix. Back in the 1950s and 60s, studies were carried out by White et al and Peleg on ductile iron with elevated silicon contents between 4 and 5 wt%.1,2 They found that a very good combination of strength and elongation can be achieved by solid solution strengthening of the metallic matrix. Elevated amounts of Si promote a stable eutectic solidification of carbon, resulting in higher tolerances against carbide formation, in thin wall castings.[7,8] This advantageous combination of static mechanical properties and good machinability has led to a large number of casting components in a wide range of technical applications such as rotor hubs and other wind power components as well as various automotive applications like wheel suspensions and suspension arms made of high silicon ductile iron in the last years.[9,10,11]
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