With regard to gas porosity defects in a casting, which one of the following sta

Question

With regard to gas porosity defects in a casting, which one of the following statements is false? Gas porosity defects occur during the liquid-solid shrinkage phase Gas porosity defects are typically less of a problem with molds with lower thermal conductivity than those with higher thermal conductivity Gas porosity defects are due in large part because the solubility of gas in a metal increases as metal temperature decreases Gas porosity defects are typically more of a problem with molds with less permeability than those with higher permeability None of the above a) b) c) d) e)

Explanation / Answer

Gas porosity defect in a casting is caused by the trapping of gas in the molten metal or by mold 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.
The main causes of the gas porosity defects are a lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability or high thermal conductivity of the mold or improper design of the casting. The lower permeability is caused by the finer grain size of the sand, a high percentage of clay in mold mixture, and excessive moisture present in the mold.
The defects occurring during liquid-solid shrinkage phase,i.e., solidification phase are called shrinkage defects. Therefore, statement (a) is FALSE.

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