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Segregation
Segregation should be analysed and predicted to be within limits of the specification, or out-of-specification compositional regions should be agreed with the customer. Channel segregation formation should be avoided if possible.
At regions in which the local cooling rate of the casting changes, such as at a change of section or at a chill or at a feeder, it is to be expected that a change in composition of the casting will occur. One of the best understood segregations of this type is inverse segregation, which the author prefers to call simply 'dendritic segregation'. In this case the partitioned solute is segregated preferentially to the face of the mould, especially if this is a chill mould. A similar effect will occur, of course, at the junction with a thinner section which will act as a cooling fin. However, in a complex thermal field, and where the geometry of the casting is requiring a complex distribution of residual liquid to feed shrinkage, these chemical variations can be complex in distribution, and not always easily predicted, except perhaps by a sophisticated computer simulation.
Other segregations are driven by gravity, and account for the concentration of carbon and other light elements in the tops of large ferrous castings, and the concentration of heavy elements such as tungsten and molybdenum at the base of large tool-steel castings. Strong concentrations of segregated solutes and inclusions are found in channel segregates which are once again a feature of large, slowly cooled castings.
When extensive and/or intensive, such changes in composition of the
casting may cause the alloy of the casting to be locally out of specification.
If this is a serious deviation, the coincidence of local brittleness in a highly stressed region of the casting might threaten the serviceability of the product. The possibility of such regions therefore needs to be assessed prior to casting if possible, and demonstrated to be within acceptable limits in the cast product.
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