Machinery's Handbook, 31st Edition
SOLIDIFICATION AND COOLING OF METALS 1495 Table 1. Volumetric Solidification Contraction or Expansion for Various Metals
Defects in Casting The International Committee of Foundry Technical Associations has developed a stan- dardized nomenclature, consisting of eight basic categories of casting defects: Metallic Projections: Such defects can include fins, flash, or massive projections, such as swells and rough surfaces. Cavities: Variously rounded or rough internal or exposed cavities include blowholes, pinholes, and shrinkage cavities. Discontinuities: These can include cracks, cold or hot tearing, and cold shuts. If the solidifying metal is constrained from shrinking freely, cracking and tearing can occur. Coarse grain size and the presence of low-melting-point segregates along the grain boundaries (intergranular) increase the tendency for hot tearing. Cold shut is an interface in a casting that lacks complete fusion because two streams of liquid metal meet from dif- ferent gates and do not completely fuse. Defective Surface: Surface defects include folds, laps, scars, adhering sand layers, and oxide scale. Incomplete Casting: Misruns can occur due to premature solidification, the molten metal being at too low a temperature, pouring metal too slowly, insufficient volume of metal being poured, or runout (loss of metal from a mold after pouring). Incorrect Dimensions or Shape: These undesirable results may be caused by factors such as an improper shrinkage allowance, pattern-mounting error, deformed pattern, irregular contraction, or warped casting. Inclusions: Inclusions form during melting, solidification, and molding and generally are non-metallic. They increase stress and reduce strength of the casting. Inclusions may form during melting, when the molten metal reacts with the environment (usually oxygen) or with crucible or mold material. Chemical reactions among components in the molten metal itself may produce inclusions. Spalling of the mold and core surfaces also produces inclusions, indicating the importance of the quality and maintenance of molds. Porosity: Porosity may be caused by shrinkage or gases or both. Thin sections in a cast- ing solidify sooner than thicker regions. As a result, molten metal can flow into thicker
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