PSI - Issue 18

Kim Bergner et al. / Procedia Structural Integrity 18 (2019) 792–801 Author name / Structural Integrity Procedia 00 (2019) 000–000

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functionality of these components is retained. Therefore, the prerequisite for lightweight component design is to better understand the cyclic material and fatigue behavior of cast iron components. However, to describe the fatigue behavior of cast iron components as accurately as possible, the casting skin has to be considered in the design process, because the formation of casting skin during the casting process is unavoidable and casting skin cannot always be removed [Bauer (1982)]. It is important to acknowledge that post-processing, such as machining or shot peening of remaining casting skins, not only leads to higher costs, but also cannot be performed for inaccessible component surfaces. Until now, the material behavior considered in existing design concepts is usually gained from tests done with machined specimens and the casting skin is taken into account by a reduction factor such as the roughness factor K R,σ in the FKM guideline [FKM (2012)]. In the literature, casting skin describes not only surface roughness, but also decarburization or graphite degeneration and a ferritic or pearlitic rim [Bauer (1982), Kutz (2018)]. To assess the influence of casting skin on the fatigue behavior of EN-GJS-400-15 and EN-GJS-700-2, an inquiry was conducted among foundries providing typical cast components with irremovable casting skin, to determine the most common casting skin conditions. Subsequently casting processes were designed to produce casting blanks from which specimens for fatigue tests were manufactured. In addition to bending specimens, flat specimens for axial strain tests were manufactured. The results gained from the axial strain tests will be used to determine cyclic material properties for FE simulation. In this paper, only the results from the bending tests will be presented. The inquiry and the fatigue tests were performed during the research project Gusshaut, funded by the German Federation of Industrial Research Association (AiF). The results gained from the fatigue tests will be used to develop a design concept that considers the influence of the casting skin on the fatigue life of cast iron components. The design concept will be validated with fatigue tests on one cast iron component. 2. Materials and Methods 2.1. Materials Casting skin is a natural consequence that results from the reaction of the melt with elements in the molding or core material. Its most prominent form of expression is a degenerated graphite layer consisting of graphite lamellae, which form due to a reaction of magnesium from the melt with sulfur and / or oxygen from the molding, meaning that it is thus no longer able to form nodular graphite. Therefore, in this project, three types of casting blanks of EN-GJS 400-15 and EN-GJS-700-2 were made, one with only surface roughness and two with rim zones consisting of a degenerated graphite layer and a deviating microstructure [Kutz (2018)]. The work in this paper focuses on EN-GJS 400-15 (Table 1). The foundry institute of the RWTH Aachen designed a circular casting geometry, to produce 14 cast blanks (Fig. 1). The gating system of the cast geometry prohibited Dross or erosion formation through a slow and smooth flow of the melt. The degenerated graphite layer develops at the bottom interface of the casting between the melt and the sand core. A detailed description of the casting process can be read in Kutz (2018).

Table 1. Overview of the casting blank type

Type of casting blank

Bulk material EN-GJS-400-15 EN-GJS-400-15 EN-GJS-400-15

Rim zone

Surface roughness

1 – PT_III 2 – GE_I 3 – GE_II

No

Yes Yes Yes

Pearlite, DGL Ferrite, DGL

To differentiate between the influence of the surface roughness and the rim zone, type 1 casting blanks (Table 1) were cast containing only bulk material and surface roughness (Fig. 1, top right). The influence of a changing microstructure, accompanying the degenerated graphite layer (DGL), was investigated by type 2 casting blanks consisting of graphite lamellae in a pearlitic matrix (GE_I, Fig. 1, bottom left) and type 3 casting blanks consisting of graphite lamellae in a ferritic matrix (GE_II, Fig. 1, bottom right). The chemical composition of the three types of casting blanks can be found in Table 2.