PSI - Issue 19

Kim Bergner et al. / Procedia Structural Integrity 19 (2019) 140–149 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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goal is the use of design concepts that describe the material behavior as precisely as possible. Cast iron offers some lightweight design advantages, because components can be cast close to final shape, which not only saves material, but also presents a high innovative freedom in the design process [1]. In the case of cast iron components, however, the casting skin is often depicted in a simplified way, by reducing it to the consideration of surface roughness only. This method is applied, for example, in the FKM guideline [2], where a roughness factor K R,σ is used to account for the casting skin, regardless of its microstructure or thickness. The literature on cast iron components, however, shows that casting skin not only consists of surface roughness, but also decarburization or graphite degeneration and a ferritic or pearlitic rim [3, 4]. Therefore, a prerequisite for the optimization of a cast iron component design is to understand the cyclic material and fatigue behavior of the bulk material and the casting skin, since post-processing, such as machining or shot peening of remaining casting skin, not only leads to higher manufacturing costs, but also cannot be performed on inaccessible component surfaces. In order to be able to describe the cyclic material behavior of cast iron components with casting skin holistically, the influence of the casting skin on the fatigue strength is investigated. To determine the most common types of casting skin, an inquiry among foundries, providing typical as cast components with casting skin, was carried out [4]. Subsequently, casting processes were designed to produce casting blanks of EN-GJS-400-15, from which specimens for fatigue tests were manufactured. The casting process of the cast blanks is described in detail in [4]. From the cast blanks, bending specimens with and without casting skin, as well as flat specimens from the bulk and the rim zone for axial fatigue tests, were manufactured. The cyclic material properties determined in this way were used for a numerical simulation of the bending specimens and cast components. The results of fatigue tests were used for three different fatigue approaches; on the one hand, by the FKM guideline [2] and, on the other hand, by two variations of the strain-life approach. The results gained from the axial strain-controlled tests were used to determine cyclic material properties for an FE-simulation of the bending specimens. 2. Materials and Methods 2.1. Materials In the industrial production of cast iron components, casting skin cannot be avoided, since it is caused by a reaction of the melt with elements in the mold or the core material. For nodular cast iron, its most prominent form of appearance is a degenerated graphite layer (DGL) consisting of lamellar graphite. Accordingly, three types of cast blanks of EN GJS-400-15 were manufactured at the foundry institute of the RWTH Aachen University to investigate the influence of casting skin on the fatigue strength. Further details on the manufacturing process can be read in [4]. Fig. 1 shows the phase analysis of these castings, which are composed of bulk material and surface roughness (W1) or bulk material, rim zone and surface roughness. The rim zone shows the degenerated graphite layer accompanied by a pearlitic (Fig. 1, GE_I) or ferritic (Fig. 1, GE_II) matrix. A detailed overview of the cast blank composition can be found in Table 1. W1 cast blanks serve as a reference, to be able to distinguish between the influence of surface roughness and the influence of the rim zone on the fatigue strength. To consider the influence of the cyclic material behavior of the rim zone in the FE analysis, cast blanks W3, with a pearlitic microstructure corresponding to the rim zone of GE_I, were produced (W3). The specimens for the fatigue life assessment were removed from the cast blanks, as can be seen in Fig. 2.

Table 1. Overview of the cast blank types Type of casting blank

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

Rim zone

Surface roughness

1 – W1 2 – GE_I 3 – GE_II

No

Yes Yes Yes

Pearlite, DGL Ferrite, DGL

4 – W3

DGL 1 in pearlite matrix

No

No

1) DGL – degenerated graphite layer