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clay-bonded sand

Pitted surfaces

Characteristic features Formation of craters on castings of SG cast iron, particularly at very hot sections. Can appear individually but also extensively. Graphite degeneration occurs beneath the crater. Incidence of the defect The defect occurs exclusively in SG iron castings when poured into bentonite-bonded sand, predominantly at the positions in the sand which are subjected to high thermal loading. Explanations The defect occurs only in the presence of fluorides, which are either introduced into the sand through exothermic feeders or are used for the treatment of the melt. Since oxide formation and graphite degeneration are typical characteristics in the deeper parts of the pits and craters, an oxidizing influence can be assumed. In our opinion, fluorides destroy the protective magnesium oxide layer on the surface of the molten iron. Water vapour from the green sand mould then reacts on the surface with the residual magnesium content of the melt. Possible causes Clay-bonded sand  Proportion of exothermic feeder residues too high.  Insufficient replenishment of moulding sand, consequently, fluoride content too high.  Moisture content too high. Metallurgical  Residual magnesium content of the melt too high Remedies Clay-bonded sand  Reduce the fluoride content in the moulding sand by screening out the feed residues. Use feeders which adhere to the metal, replenish with new sand.  Use insulating feeders or those with reduced fluoride content.  Reduce water content through improved bentonite development, reduce bentonite content, use high quality bentonite with a high montmorillonite content.  Use highly active lustrous carbon producer with rapid release of gas. Gating and pouring practice  Reduce the number of exothermic sleeves, use insulating materials. Background information The defect is barely mentioned in literature, the mechanism is unknown outside our company. The single reference describes the increased susceptibility of SG cast iron with a high silicon content (3%) (1). Since the actual defect is caused by the reaction of water vapour with the residual magnesium content of the melt after the protective oxide layer has been destroyed by fluorides, any action which reduces the fluoride and water content in the moulding sand will be effective. Feeders In order to achieve the reaction 4 Al+ 3 O2 -> 2 Al2O3all exothermic feeders contain fluorides which destroy the aluminium oxide layer on the aluminium powder. Depending on the binder, the aluminium used and the oxidant, it is possible to work with different amounts of fluoride. The user should know the fluoride content of the feeder. There is also the option of using sodium silicate-bonded feeder sleeves which adhere to the metal feeder and are removed with it. However, it should be noted that the fluoride content in sodium silicate-bonded feeder sleeves is normally higher but it is often possible to use insulating sleeves that are free of fluoride. Moulding sand The higher the water content in the moulding sand, the faster can a reaction with the surface of an SG iron casting occur when a critical fluoride content is exceeded. All measures for reducing the water content and, above all for reducing the bentonite content, are effective. Here, reference is made to the use of bentonite with a high montmorillonite content and a high cohesive capacity. In addition, improved bentonite development will result in a reduction in crater formation. The use of carbon carriers with a high capacity for forming lustrous carbon also leads to a reduction of crater formation. Reduction of the water content in the moulding sand as well as suppression of water vapour from the mould cavity could also play a part. References [1] Labbé, D.; Arliguie, J.-P. Eigentümlicher Oberflächenfehler an Gußstücken aus Gußeisen mit Kugelgraphit Fonderie, Fondeur aujourd'hui 1985, S. 33-34 (franz.) Fig.22 Heavy SG iron casting. Pronounced surface pitting at so-called "hot spots".   Fig.23 GSG iron casting. Pronounced craters in the top part through use of cryolite for slag liquefaction.

metal casting

Pengertian Pengecoran

A. Mengenal Proses Pengecoran Logam

1. Pengertian

Pengecoran (casting) adalah suatu proses penuangan materi cair seperti logam atau plastik yang dimasukkan ke dalam cetakan, kemudian dibiarkan membeku di dalam cetakan tersebut, dan kemudian dikeluarkan atau dipecah-pecah untuk dijadikan komponen mesin. Pengecoran digunakan untuk membuat bagian mesin dengan bentuk yang kompleks.

Gambar 2 1. Logam cair sedang dituangkan ke dalam cetakan

Pengecoran digunakan untuk membentuk logam dalam kondisi panas sesuai dengan bentuk cetakan yang telah dibuat. Pengecoran dapat berupa material logam cair atau plastik yang bisa meleleh (termoplastik), juga material yang terlarut air misalnya beton atau gips, dan materi lain yang dapat menjadi cair atau pasta ketika dalam kondisi basah seperti tanah liat, dan lain-lain yang jika dalam kondisi kering akan berubah menjadi keras dalam cetakan, dan terbakar dalam perapian. Proses pengecoran dibagi menjadi dua, yaitu : expandable (dapat diperluas) dan non expandable (tidak dapat diperluas).

Gambar 2 2. Proses pengecoran logam