Abstract
In this paper, we explore the effect of inoculants and solidification time on the mechanical properties of an EN-GJS-400-type ferritic ductile cast iron (DCI). For this purpose, static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests are carried out on three different material conditions. They are produced under fast cooling (solidification time 2h45min), representative of thin walled castings, and very slow cooling (solidification time 10—13h), representative of thick walled castings, this latter with and without the addition of Sb. It has been found that the long solidification time leads to an overgrowth and degeneration of the spheroidal graphite nodules. The addition of Sb avoids the formation of chunky graphite observed in the slowly cooled condition but results in large exploded graphite nodules. These effects impact negatively on tensile strength, total elongation and fatigue strength. Conversely, the resistance to fatigue crack growth is even superior and the fracture toughness comparable to that of the fast cooled condition. Metallurgical and fractographic analyses are carried out to explain this behaviour.
Highlights
Low production cost and excellent castability make ductile cast iron (DCI) the preferred material choice when low-to-moderately stressed mechanical components of complicated shape and large dimensions must be manufactured
The ferritic matrix displays usually the lower tensile and fatigue strength and a noticeable ductility, which have promoted the DCI in the production of thick walled parts undergoing in service low/medium levels of mechanical stresses
The fatigue life is exclusively thought of as the propagation of cracks up to a critical size leading to structural collapse or functionality loss. They require a detailed characterization of fracture toughness and fatigue crack growth resistance of the castings, in order to assess the critical crack size and the time to propagation to this critical value. We address this latter issue by investigating the crack growth resistance of a DCI under three different microstructural conditions
Summary
Low production cost and excellent castability make ductile cast iron (DCI) the preferred material choice when low-to-moderately stressed mechanical components of complicated shape and large dimensions must be manufactured. While the effect of chunky graphite on static and cyclic properties has been largely investigated in the literature, its effect on fatigue crack growth rate and fracture toughness is not yet clear. This is of paramount importance when designing high-added value components of large dimensions, such as those employed in MW-series wind turbines, which cannot rely on the knowledge of the fatigue strength only. Damage tolerant design and structural health monitoring approaches have been proposed to overcome these limitations According to these approaches, the fatigue life is exclusively thought of as the propagation of cracks up to a critical size leading to structural collapse or functionality loss.
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