Usually, shrinkage occurs during solidification. But, gray cast iron expands upon solidification.
what is the science behind it ?
Sathish Kumar
https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=grey+cast+iron+expansions&oq=grey+cast+iron+expa#d=gs_qabs&u=%23p%3D2XjZHboY8ksJ
Hope this paper is of some use.
Martin
hello dear, becuase of graphite in cast iron, during solidification we have expantion instead of contruction
Gray iron does not always have an expansive behavior. It depends on the geometric module and the graphite behavior.
Yes and no. The casting actually shrinks as it solidifies loosing 1 to 2 percent of volume overall. The maximum stress occurs just before solidus at a point we call the end of eutectic growth. The iron actually contracts 10%, but feeding and graphite growth makes up for most of this contraction. The graphite will continue to grow down to the eutectoid temperature adding another 1 to 2% volume but still about 1% less than the original mold volume. The problem is void formation sometimes also referred to as shrinkage. This void formation, if it happens, will occur in the zone between the maximum stress and the solidus. With internal voids, the volume problem is satisfied and stress reduced. But because the graphite continues to grow, the casting increases in volume. Restated, if there is late oversize growth of the casting, then there is shrink in the casting. If there is no shrink, then the casting will not be over-sized. I hope this is clear.
During solidification of some grades of graphitic cast irons, contraction followed by expansion occur due to the graphitization reaction which is accompanied by swelling. This swelling mainly depend on some factors like cast iron chemical composition (high carbon Equivalent), casting modulus ( high modulus, thick section), pouring temperature.
A phase transformation happens from carbon into graphite, which results in volumn expansion in case of grey cast iron.
Beside all above it is always appear in High Carbon Equvalant and high thickness of graphitic cast iron
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