On the Solidification and Structure Formation during Casting of Large Inserts in Ferritic Nodular Cast Iron
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TION
SVENSK Ka¨rnbra¨nslehantering AB (SKB) investigated a method to store nuclear waste material. The method consists of a Cu canister with an insert in ferritic nodular cast iron (NCI). The insert has a height of 5 m and a diameter of 1 m with 4 or 12 steel channel tubes inside. It is tricky to cast such a large casting with a homogeneous structure. A large number of casting tests have been performed, and to date, molds have been made in furan sand with the use of an inert atmosphere in the mold during casting. The filling time is 60 seconds, which provides a 5 m s1 streaming rate for the melt. This streaming rate causes waves and a splashing of the melt surface and a strong convection in the melt during the filling operation. The strong convection and the complex construction with steel tubes result in large variations in the cast structure. Two different types of inserts are produced: one insert has 4 steel channel tubes and another has 12. The insert with 4 steel channel tubes is referred to as pressurized water reactor (PWR), and the insert with 12 steel tubes is named boiling water ABEL TADESSE and HASSE FREDRIKSSON are with Materials Science and Engineering, Royal Institute of Technology (KTH), Brinellva¨gen 23, S-100 44 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted January 24, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
reactor (BWR). In this report, a structural analysis of a cross section of the two different inserts was performed. A computer analysis of the structure was also performed. More than 30 years of computer analyses of cast structures were used.[1-20] In the beginning, rather simple simulations were performed. Later, more extensive programs have been developed.[17-20] In this work, the COMSOL Multiphysics computer module for solidification was used. A new COMSOL module based on the kinetic models for the solidification of NCI was developed and combined with the solidification module for the calculation and analysis of the solidification structure. The calculations are compared and discussed in relation to the casting conditions and casting structure.
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SAMPLE PREPARATION
Figure 1(a) shows a 3D sketch of the BWR insert with 12 steel channel tubes, and a cross section of the sample was investigated. The cross section was taken at a height of 2160 mm. Similarly, Figure 1(b) shows a sketch of the PWR insert with four steel channel tubes. The cross section locations of the investigated samples are also shown in the figure. The samples were taken at a height of 1150 mm. Test bars from the mechanical[21,22] tests were used for a metallographic analysis of the microstructure using scanning electron microscopy and light microscopy. The
Fig. 1—(a, b) The 3D cross section of BWR and PWR inserts with sampling positions where the temperature simulation and microstructural analysis were done, respectively.
microstructure was investigated on the polished surface of the samples, which were cut approximately 5 mm behind the fractured surface. The graphite fraction, the graphite nod
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