CASTING DEFECTS:-
a) Gas defects
b) Shrinkage cavities
c) Moulding material defects
d) Pouring metal defects
e) Metallurgical defects
(a) Gas defects:
A condition existing in a casting caused by the trapping of gas in the molten metal or by mould gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.
Causes
The lower gas-passing tendency of the mould, which may be due to lower venting, lower permeability of the mould or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mould mixture, and excessive moisture present in the mould.
Metal contains gas
Mould is too hot
Poor mould burnout
(b) Shrinkage Cavities
These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mould. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities.
(c) Molding Material Defects
The defects in this category are cuts and washes, metal penetration, fusion, and swell.
Cut and washes
These appear as rough spots and areas of excess metal, and are caused by erosion of moulding sand by the flowing metal. This is caused by the moulding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of moulding sand and the latter can be overcome by the proper design of the gating system.
Metal penetration
When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mould wash was applied on the surface of the mould. The coarser the sand grains more the metal penetration.
Fusion
This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.
Swell
Under the influence of metallostatic forces, the mould wall may move back causing a swell in the dimension of the casting. A proper ramming of the mould will correct this defect.
Inclusions
Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mould can prevent this defect.
a) Gas defects
b) Shrinkage cavities
c) Moulding material defects
d) Pouring metal defects
e) Metallurgical defects
(a) Gas defects:
A condition existing in a casting caused by the trapping of gas in the molten metal or by mould gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.
Causes
The lower gas-passing tendency of the mould, which may be due to lower venting, lower permeability of the mould or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mould mixture, and excessive moisture present in the mould.
Metal contains gas
Mould is too hot
Poor mould burnout
(b) Shrinkage Cavities
These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mould. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities.
(c) Molding Material Defects
The defects in this category are cuts and washes, metal penetration, fusion, and swell.
Cut and washes
These appear as rough spots and areas of excess metal, and are caused by erosion of moulding sand by the flowing metal. This is caused by the moulding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of moulding sand and the latter can be overcome by the proper design of the gating system.
Metal penetration
When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mould wash was applied on the surface of the mould. The coarser the sand grains more the metal penetration.
Fusion
This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.
Swell
Under the influence of metallostatic forces, the mould wall may move back causing a swell in the dimension of the casting. A proper ramming of the mould will correct this defect.
Inclusions
Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mould can prevent this defect.
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