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Metal Casting Technologies : June 2007
54 www.metals.rala.com.au Back to the NAME & DESCRIPTION OF DEFECT CAUSES Fins • Loose assembling or fitting together of Bits of metal that entered unplanned cavities in the mold. Usually found at the various mold components. parting lines between flasks or at positions of core prints. • Smoothing of surfaces after pattern removal. • Mismatching of patterns. Hot Tears or Surface Cracks • Mold is too hard offering too high resistance Cold cracks are straight line cracks of few branches resulting from large inherent preventing contraction of the metal. contraction stresses produced during the solidification process. • Cores are too hard offering too high Hot-tear cracks have an irregular jagged appearance with an oxidized fracture resistance to contraction of the metal. surface, starting from the surface and proceeding inwards. The crack is wider at • Design of the casting leading to high the surface, is uninterrupted in depth, and may traverse the entire section. stresses in solidification. Inclusions • High iron oxide content in melt before Metallic or non-metallic material embedded in the casting body due to the deoxidation produces iron-manganese-silicate. solubility of certain furnace additions (metallic) or entrapped slag, refractory, • Aluminum deoxidizer exceeded 1kg/1000kgs. or molding material (non-metallic). • Poor skimming practice in top-pour ladles. • Avoid slag streamers in teapot-pour ladles. • Poor grade of refractory materials, ladle sleeves, and nozzles. • Erosion of sprues and gates of the mold. Misruns • Low fluidity of the molten metal. Failure of molten metal to reproduce mold contours resulting in incomplete • Low pouring temperature. filling of mold cavity. • Section is too thin, less than 5 mm. thickness. Penetration • Too high permeability of the mold due to Results when molten metal penetrates into the mold. Penetration may be to too coarse molding sand. the extent of only about three sand grains, or could be more pronounced. • Too low mold hardness due to soft ramming. • Improper application of mold wash. Pinholes or Pinhole Porosity • Gases in the molten metal, esp. hydrogen. Small, elongated, smooth-walled gas-holes which occur immediately under the - Hydrogen introduced into the furnace skin of a casting and are brought to the surface by heat treatment due to the charge by wet, rusty, or oily scrap. loss of oxide scale. Sometimes these appear only after heat treatment when - Water vapor or steam in the the layer of scale formed is removed. furnace atmosphere. - Moistened slag-making materials or wet refractory materials. - Holding of heats in the furnace after the final deoxidizers have been added. - Moist ferroalloy additions in the furnace. - Insufficient aluminum deoxidizer added. • Poor pouring practice. - Volatile matter in the furnace spout. - Insufficient dryness of ladles. - Lack of steam vents in ladles. - Volatile matter in the ladle lining. - Old skulls in the ladles receiving the metal. - Too many ladle transfers being made. - Moist ladle additions of alloys. - Dipping of used skimming rods in the metal. • High moisture condition in green sand molds. • Low permeability of the facing sand. • Casting contours. • Excessive organic bonding materials. Rat Tail • High-expansion spalling of the mold wall. A long irregular depression of little width in the casting surface. • Too high mold hardness. Sometimes has the appearance of a fissure or crack, but it is not formed • Poor type and quality of the facing sand. because of stress action on the casting. • Not enough cereal binders.