Q We see primary carbides in a ductile iron 80-55-06 casting with relatively thin cross-sections (0.25-0.0375 in.). Is it normal to anticipate a certain amount of primary carbides in a casting like this? What is the acceptable amount that can be tolerated? What specific practical measures can be taken to minimize the formation of carbides?Background: Carbides occur when undissolved carbon combines with iron to form iron carbide Noun 1. iron carbide - a chemical compound that is a constituent of steel and cast iron; very hard and brittle cementite chemical compound, compound - (chemistry) a substance formed by chemical union of two or more elements or ingredients in definite ([Fe.sub.3]C), a hard and brittle phase that is not desirable in ductile iron castings. The absence or presence of carbides is determined by the process of crystallization Crystallization The formation of a solid from a solution, melt, vapor, or a different solid phase. Crystallization from solution is an important industrial operation because of the large number of materials marketed as crystalline particles. or freezing. A section thickness of 0.0375 in. is thin, and carbides in 80-55-06 ductile iron in sections this thin are not uncommon due to the high cooling rate. Recommendations: Typical ASTM ASTM abbr. American Society for Testing and Materials specifications do not have a specified level of acceptable carbides, and acceptable amounts of carbides are typically negotiated between the customer and the metalcasting facility. There are things that can be done to minimize or eliminate the formation of carbides. Late inoculation inoculation, in medicine, introduction of a preparation into the tissues or fluids of the body for the purpose of preventing or curing certain diseases. The preparation is usually a weakened culture of the agent causing the disease, as in vaccination against (in-stream or in-the-mold) may prove helpful. Increasing the carbon and/or silicon content of the iron also might help. Caution is required because if carbon or silicon contents are too high, they can cause other problems and result in castings that do not meet required tensile properties. As 80-55-06 is a pearlitic grade of ductile iron, the method chosen to create the pearlite pearl·ite n. 1. A mixture of ferrite and cementite forming distinct layers or bands in slowly cooled carbon steels. 2. Variant of perlite. Noun 1. can impact the tendency to form carbides. Choosing a charge and alloy configuration that will minimize the presence of elements that are known to promote carbide formation is important. The supplier of treatment alloys, inoculants or other charge materials might be of help in this area. The design of the component and how the iron is being specified also needs to be examined. Are the thin sections the critical area for properties? If properties are being achieved in a normal ductile iron test bar, the strength achieved will be far greater in the thin sections. The softer the iron can be made, the less likely carbides will be a problem. Information in this column was supplied by the AFS A distributed file system for large, widely dispersed Unix and Windows networks from Transarc Corporation, now part of IBM. It is noted for its ease of administration and expandability and stems from Carnegie-Mellon's Andrew File System. AFS - Andrew File System Cast Iron Div. 5 |
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