Evaluation of Ductile Iron for Offshore Application
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The possibility of using ductile iron as a construction material for offshore applications have been investigated. If ductile iron is considered as being a substitute material forwelded steel, it can create a larger freedom for engineers in a design process as well as provide benefits regarding strength and cost.The issue of getting ductile iron approved as a reliable substitute material for steel is a statement made by DNV GL, being that cast irons normally should not be used with minimum design temperature below0 C. This statement is mostly based on the perception of cast iron being brittle i lower temperatures. Based on this statement ductile iron has been evaluated with regard to toughness properties determined by both a Charpy impact test and a fracture toughness test. This is mainly done to investigate whether the values gathered froma impact test give a realistic picture of ductile irons toughness, when compared to a selected grade of steel. This is done of three materials, named 400-18LT, 500-14, and 500-7. Included is also a study concerning fracture behavior in various ductile irons microstructures. To declare ductile iron as sufficient for offshore applications have a value of minimum fracture toughness been set. The results show that mechanical properties in ductile iron is highly dependent on microstructural mechanics and chemical composition. Achievable by awell planned and correctly performed production process, especially when it comes to quality and holding time of inoculants. It is also apparent that the Charpy impact test is not suitable when evaluating ductile irons, mainly due to the material’s notch- and strain rate sensitivity. It is concluded that the 500-14 material is the only material declared being sufficient according to the listed requirements. This result clearly illustrates the positive effects of using Silicon as a ferrite strengthener. Pearlite was observed being damaging to toughness properties, especially at lower temperatures, and should be kept at a minimum for the reviewed area of use. 400-18LT displayed lower than expected fracture toughness results, despite performing well regarding impact energy. The reason for this has been concluded being connected to a degenerated inoculation effect.
Master's thesis in Mechanical engineering