· IngenierÃa ·
Rudolf Val. Sillén*
* NovaCast Technologies AB, Sweden
67th World Foundry Congress
The paper focuses on the mechanisms behind shrinkages in cast iron and
how they can be avoided. Grey and ductile iron are unique casting alloys
due to their solidification behaviour. During solidification the alloys initially
contract when the temperature drops from the pouring temperature to the
liquidus temperature. This contraction must always be compensated for by
supply of feed metal from the gating system and often also from a feeder
(riser). The paper will demonstrate that this type of feeding can only be
functional until the amount of solid phase reaches a certain level. This
usually happens slightly after reaching the low grey eutectic temperature.
When this stage has been reached a balanced precipitation rate and
amount of graphite will make it possible to compensate for contraction of
remaining liquid and the eutectic austenite. The proper balance is crucial
as the precipitation of graphite is associated with a volume expansion,
which if too high will cause mould wall movement and increase shrinkage
tendency. Too low precipitation will lead to micro-shrinkages. The paper
describes how optimization of the gating system in many cases makes it
possible to eliminate feeders for supply of the initial feed metal. After the
low eutectic temperature is reached the contraction can be eliminated by
careful control of graphite precipitation using a combination of chemical
and adaptive thermal analysis. Through proper control of the feeding
sequences it is possible to produce certain ductile iron castings in green
sand moulds without feeders!
