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Metal Casting Technologies : March 2007
METAL Casting Technologies March 2007 49 NAME & DESCRIPTION OF DEFECT CAUSES Expansion defects Stage one is rattails which are irregular lines caused by low temperature • Too large flat surface on casting. expansion of the mold surface resulting in a fault in the mold surface. • Slow pouring of mold. Stage two is buckles which are V-shaped indentations which may occur • Interrupted metal flow during pouring. by themselves or under an expansion scab. • Excessive moisture in the molding sand. Stage three is expansion scabs which are rough layers of metal connected • Low hot deformation of molding sand. to the main body of the casting by a vein of metal. • Incorrect sand grain distribution. Stage four is cope spalls or pull downs which are indent-ations in the cope surface of the casting. Depending on the time of formation they may have the appearance of a buckle, rattail, shrink, or blow. Fusion • Protruding sections. Fusion is a defect having a rough glossy appearance resulting from metallic • Too soft ramming of mold. oxides acting as flux on the sand or low fusion binders which melt leaving • Too high cope height. voids on the surface. As fusion progresses, it results in metal penetration. • Low fusion-point of molding sand. • Localized over-heating of sand in mold. Gas defect Blows or gas holes in castings are either spherical, flat-tened or elongated • Inadequate venting of mold. cavities. They are related to localized gas (internally trapped air) pressure that • Cope too shallow. exceeds metal pressure in any locality during solidification of the metal. • Head pressure too low. Core blows may be confused with mold blows. Since gas travels upward in • Too high moisture in the molding sand. the mold, it is quite possible for a gas bubble to form from a core and then • Under-baked oil-sand cores. travel toward the cope surface. Pinholes, blisters, and body scars, as well as • Gassy metal. certain types of porosity, are variations of gas holes. Hard and chilled spots • Unbalanced section thicknesses. Hard spots, hard areas, and chilled spots in a casting are localized zones of • Wet spot in the mold excessive hardness. • Tramp metal in the core. • Cold metal due to cold or wet ladles. • Excessive use of chill blocks. • Bad metal chemistry. Hot tears • Inadequate fillets. A hot tear is a crack in the casting which occurs while it is hot, or either • Inadequate use of chill blocks. solid or semisolid. • Too hard ramming of mold. • Occurrence of internal stresses in casting. • Poor mold collapsibility. Inclusions • Dropping sand into the mold. Inclusions are surface or subsurface particles of sand, slag, dross, oxides, • Improper gating. or other materials imbedded in metal. They may originate from the molds, • Lack of fillets. cores, metal, ladles, or careless practices. • Low fusion materials in molding sand. • Slag or dross in the metal. Inverse chills • Thin sections adjacent to heavy sections. Inverse chill, also known as reverse chill, is common in gray iron, ductile • Excessive gas generation during pouring. iron, and malleable iron castings. This is a condition wherein the interior • Hydrogen atmosphere in the mold. of the casting section is chilled or white, while the outer sections • Contaminated materials in the charge. are mottled or gray. Mass hardness Mass hardness is a condition in which the entire casting is too hard for • Poor heat transfer to metal composition the application. relationship. • Oxidized metal. • Too fast cooling rate after pouring. • Incorrect metal composition.