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9/8/98 AC 43.13-1B (4) Porosity is holes in a material’s sur- face or scattered throughout the material, caused by gases being liberated and trapped as the material solidifies. (5) Inclusions are impurities, such as slag, oxides, sulfides, etc., that occur in ingots and castings. Inclusions are commonly caused by incomplete refining of the metal ore or the incomplete mixing of deoxidizing materials added to the molten metal in the furnace. (6) Shrinkage cracks can occur in cast- ings due to stresses caused by the metal con- tracting as it cools and solidifies. c. Primary Processing Flaws. Flaws which occur while working the metal down by hot or cold deformation into useful shapes such as bars, rods, wires, and forged shapes are primary processing flaws. Casting and weld- ing are also considered primary processes al- though they involve molten metal, since they result in a semi-finished product. The follow- ing are brief descriptions of some primary processing flaws: (1) Seams are surface flaws, generally long, straight, and parallel to the longitudinal axis of the material, which can originate from ingot blowholes and cracks, or be introduced by drawing or rolling processes. (2) Laminations are formed in rolled plate, sheet, or strip when blowholes or inter- nal fissures are not welded tight during the rolling process and are enlarged and flattened into areas of horizontal discontinuities. (3) Cupping is a series of internal metal ruptures created when the interior metal does not flow as rapidly as the surface metal during drawing or extruding processes. Segregation in the center of a bar usually contributes to the occurrence. (4) Cooling cracks can occur in casting due to stresses resulting from cooling, and are often associated with changes in cross sections of the part. Cooling cracks can also occur when alloy and tool steel bars are rolled and subsequently cooled. Also, stresses can occur from uneven cooling which can be severe enough to crack the bars. Such cracks are gen- erally longitudinal, but not necessarily straight. They can be quite long, and usually vary in depth along their length. (5) Flakes are internal ruptures that can occur in metal as a result of cooling too rap- idly. Flaking generally occurs deep in a heavy section of metal. Certain alloys are more sus- ceptible to flaking than others. (6) Forging laps are the result of metal being folded over and forced into the surface, but not welded to form a single piece. They can be caused by faulty dies, oversized dies, oversized blanks, or improper handling of the metal in the die. They can occur on any area of the forging. (7) Forging bursts are internal or exter- nal ruptures that occur when forging opera- tions are started before the material to be forged reaches the proper temperature throughout. Hotter sections of the forging blank tend to flow around the colder sections causing internal bursts or cracks on the sur- face. Too rapid or too severe a reduction in a section can also cause forging bursts or cracks. (8) A hot tear is a pulling apart of the metal that can occur in castings when the metal contracts as it solidifies. (9) A cold shut is a failure of metal to fuse. It can occur in castings when part of the metal being poured into the mold cools and does not fuse with the rest of the metal into a solid piece. Par 5-5 Page 5-3PDF Image | AFS-640
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