38 The value of MTR represents the Modulus value below which feeding of the casting from risers is no longer effective and the iron becomes selffeeding due to graphite expansion. MTR is critical in designing the risering system. The basic premise in all design work for feeding iron castings is that expansion pressure must be controlled. Assuming the mold is rigid enough, all contacts with the casting (gates and riser contacts) should be solid enough to ensure that the expansion pressure is contained in the casting after the onset of the graphite expansion. This leads to another simple rule: The Modulus of the feeder contact neck should be equal to MTR. This ensures that feeding of the liquid contraction will be able to occur, and also that the expansion pressure will be contained in the casting due to freezing of the feeder contact at just the correct point in solidification. CASE STUDY As an example of both the incorrect and correct approach to feeding cast irons, we examine a 210 Kg ductile iron ring casting, shown in Figure 1. This casting is a bearing connector for a wind power generator. Figure 1. Ductile iron bearing connector (210 Kg). The foundry making this casting approached riser design as a steel casting rather than an iron casting. Figure 2 shows two alternate riser designs which were being used to produce this casting. The original design specified five risers with insulating sleeves. When the results of this design were unsatisfactory, the design was changed to include six risers. software, solidification of a casting can be predicted, often in a matter of minutes. Data from this simulation can be converted to Modulus values in the casting. This means that Modulus data is now available at every point in a 3D representation of the casting; this also means that the Modulus data is more accurate, as effects such as local superheating of the mold material are accurately taken into account by the simulation, which is not possible with manual methods. With the Modulus data for the casting, as well as the chemistry and temperature data, the point at which expansion begins can be calculated. Castings which have a higher Modulus (heavy section castings) will begin to expand earlier and will undergo more expansion than castings with low Modulus (light section castings). The expansion start point is expressed as a percent of full solidification and is called the Shrinkage Time (ST) point. Knowing the ST point for the iron in a casting, it is possible to calculate an equivalent Modulus at which contraction of the iron stops and expansion begins. This is known as the Transfer Modulus (MTR), because it defines for us the areas of the casting where liquid metal transfer is possible. The calculation of MTR is as follows: MTR = SQR ( ST /100) * MC By plotting the value of MTR we can visualize the feed zone(s) in the casting. This allows us to determine the number of required feeders, using the rule of one feeder per feed zone. Figure 2B. Redesigned process with six risers. This is typical of the approach to design and problem solving that one might find in a steel foundry; if a casting cannot be successfully produced with a given set of risers, the typical decision is to add more risers. This approach did not resolve the problem, instead the quality of the casting was worse. This casting represented the most expensive scrap problem of all production castings in the foundry. Examination of the defective casting showed that porosity was exposed on the top surface of the casting after machining 6 mm of iron off the surface, as shown in Figure 3. Figure 2A. Original design with five risers
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