In sand casting, Gas porosity occurs when the metal traps gas (most often nitrogen, oxygen or hydrogen) during casting.
When the casting cools and solidifies, bubbles form because the solid form of the metal cannot hold as much gas as the liquid form. These bubbles appear on a casting as rounded, circular cavities or holes.
There are three types of casting defects related to gas porosity:
Pinholes, also sometimes referred to as porosities, are very tiny holes (about 2 mm) usually found in the cope (upper) part of the mold, in poorly vented pockets.casting defects
A subsurface blowhole appears on the inside of a cast and usually isn’t visible until after machining.
Subsurface blowholes can be difficult to detect before machining, requiring harmonic, ultrasonic, magnetic or x-ray analysis
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