PSI - Issue 18

Christoph Bleicher et al. / Procedia Structural Integrity 18 (2019) 46–62 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Thick-walled cast iron components, which are mainly made of nodular cast iron, are utilised in many different components in wind energy turbines and normal mechanical applications, for example for press frames, due to their high material strength combined with a comparably high ductility. Furthermore, casting offers the necessary freedom to design load-optimised components with a minimum of weight. Nevertheless, to reduce the mass and structural stiffness to the lowest possible value, special knowledge of the quasi-static and cyclic material strength needs to be at hand. This increases in importance when additional material defects, such as pores and shrinkages are present in the cast component. In this context, the material defect of Dross needs special treatment, during the design and, especially, the quality assurance process. Dross, as a sulphidic, oxidic and siliceous material defect, needs to be precisely detected in the component after the casting process and evaluated during the quality assurance process in the foundries. During the casting process, especially for large castings, Dross is hard to avoid and leads foundrymen to problems in assessing the lifetime of each component, since exact methods for detecting Dross and determining its influence on the fatigue strength of nodular cast iron are currently missing. Due to this, Dross in components, especially for wind energy applications, is not accepted by the customers and by certifying companies, such as DNV GL (2016). This, furthermore, leads to efforts in rework or rejection of components containing Dross. Also, reworking has its difficulties, in addition to the effort and the loss of time and money. If the Dross is removed, but the nominal wall thickness due to the removal is reduced to a value lower than the nominal thickness, finite element analysis and welding processes will take place in order to save the component. To optimise the non-destructive detection techniques as well as the lifetime assessment methods for Dross, quasi static and cyclic material tests on Dross-afflicted and Dross-free, thick-walled EN-GJS-400-15 were performed during the research project unverDROSSen, funded by the Federal Ministry for Economic Affairs and Energy (BMWi). For this purpose, large cast blocks and components with Dross were non-destructively investigated with magnetic particle inspection and also with mechanised ultrasonic testing. The main aim is to detect different shapes of Dross and to determine their specific cyclic material behaviour with the help of strain-controlled fatigue tests. 2. Materials and Methods 2.1. Dross Dross is one of the major material defects occurring more or less exclusively in thick-walled nodular cast iron and consists of large fractions of silicon, magnesium, sulphur and oxygen. This reduces the local material quasi-static and cyclic strength and leads to a certain brittleness in the affected parts of components described by Hasse (1999) and Röhrig (2015). Dross occurs during the casting process, when a high magnesium content is present in the melt, induced, for instance, during pouring with a lot of turbulence. Especially, the low solidification times in thick-walled cast components and its lower density compared to the melt offer Dross the possibility of forming and rising in the cast component to the upper surface of the component, thus causing a metallurgical notch in the component [Best et al. (2002), Rödter (2018), Kaufmann (1998) and Gangé et al. (2009)]. While the formation of Dross is not easy to prevent, foundries add material at the upper surface where the Dross is likely to occur, in order to catch the Dross and then to remove it by chipping. For Dross, no real definition exists. However, a literature review shows that different formations occur, such as lines, chunks or pits at the surface, as well as strings and clusters [Best et al. (2002), Rödter (2018), Gangé et al. (2009) and Bouvet et al. (2012)]. Best et al. (2002) differentiate between Dross with a high content of sulphur, oxygen and sulphur and oxygen. Since Dross needs high contents of magnesium to be formed, the nodular graphite next to the Dross mainly shows degeneration to a lamellar or vermicular form [Rödter (2018), Bartels et al. (2007) and Wehling (2016)]. If Dross occurs, quasi-static and cyclic strength decrease. In this context, Kaufmann (1998) determines a loss of fatigue strength at the knee point of the S-N curve of about 44 % for thick-walled EN-GJS-400-18-LT in comparison to the sound material. For the same material, Kainzinger et al. (2013) determine a loss of 29 % in fatigue strength. During fatigue tests on EN-GJS-400-15, Bouvet et al. (2012) determine stress amplitudes at a number of cycles of 1ꞏ10 7 of σ a = 158 MPa for the sound and σ a = 94 MPa for the Dross-affected material. Additional investigations, made