Late rising in feeders of ASTM B148 C95500 castings.

[mmcastahd]

Castings weight 120 Kg, Pouring Temp - 1187^o C.
Melting in Induction medium frequency furnace.
Ceramic filter of 10 ppi was used. Marble grit plunging was used to remove gas for 4 min approximately in a melt of 170 Kg and Mg was used in ladle for degassing.
Furnace sample was good but late rising seen in the feeders resulting in rejection of castings.
Initially my doubt was on temperature, but I observed that I do not face such difficulty in pouring C95200 and C95800 at same temperature although the castings were of less weight (around 90 Kg).
I am practicing CO₂ molding process in my foundry. Oil core was used in forming cavity.
Just don't get it why this happens. Any clue?

A M CONTRACTOR

 

[EdStainless]

Pictures?

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P.E. Metallurgy, Plymouth Tube

 

[racookpe1978]

Why do you suspect late rising in the feeders?
Were the castings below the risers well-filled with no voids?
Any chance the flow path from the main casting volume up to the riser-to-casting intersection was slowed or momentarily stopped up by out-gassing or a bubble of gas? (I'm thinking like a bubble of air in honey or maple syrup rises very slowly past an obstruction, but - once past the obstruction, leaves no trace of its previous passage.)

 

[mmcastahd]

EdStainless Please find attached pics.
racookpe1978 Sir because the furnace sample was good enough to pour. The cavity was filled smoothly and feeders were in action of feeding creating a cone cavity, but within in a few seconds they sparkled and right from the center of the feeder metal started bulging. You can see the attached pics.

A M CONTRACTOR

 

[racookpe1978]

Moisture or a chemical trapped in the higher levels of sand that began bubbling out - but only once the liquid metal got that high above the casting itself?

 

[mmcastahd]

racookpe1978 - I had a talk with an industrial consultant recently and he has confirmed that this is nothing but mold gas evolving while the metal is yet to freeze completely. I have considered his advice and will take all necessary steps to overcome the defects. I will update here my results soon.

A M CONTRACTOR