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Subject: Re: Internal Shrinkage in SG iron-how to control?
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Date: 03/23/02 at 6:28 AM
Posted by: Rudy Sillen
In Reply to: Internal Shrinkage in SG iron-how to control? posted by Vikas Goel on 01/24/02 at 11:35 PM:
There are several types of shrinkages. Outer sunks and macro shrinkages happen during the early part of solidification. These types of shrinkages can be cured by using correctly designed feeders and feeder necks.
The other type is micro shrinkage or porosity. In ductile iron this type occurs at the later parts of solidification. The mechanism is that the precipitation of carbon into graphite is not enough to compensate for the shrinkage of the austenite phase. Therefore small cavities are formed usually close to heat centers.
A feeder can supply feed metal approximately until the time when the iron reaches the eutectic temperature in the section to be fed. After that time the amount of solids in the iron restricts flow from the feeder. Thus after that the eutectic temperature has been reach shrinkages can only be avoided if the precipitation pattern of graphite is favourable. Ideally there should be an even precipitation of graphite til the end of freezing.
This can be achieved by preparing the melt properly (charge composition, temp/time) and by using proper inoculation.
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Thermal Analysis and ATAS for more details.
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