Typical Defects of Ductile Iron Castings
The following typical defects of
ductile iron castings are
based on the experiences of process engineers from
Dandong Foundry in China. We hope
these information could be helpful for other iron foundries and
1. abnormal and degraded nodularizing
If the ductile iron castings have
defects of abnormal and degraded nodularizing, the fracture surfaces
will show the grey color with the small black particles. There will
be large amount of flaky graphite in the microstructure.
It is mainly because the too much sulphur in melted iron, or other
anti-nodularizing elements. So, we suggest to use the low sulphur
coal, or use the desulphite treatment. If necessary, please try to
add more nodularizing agents.
2. shrinkage holes and porosity
Shrinkage holes show the concaved
surfaces, and the inside surfaces of shrinkage holes are very rough.
Shrinkage porosity shows many dendritic small holes.
These defects are because of low content of carbon and silicon, too
Using hard molding, such as resin sand casting process. Increasing
the content of Si and C.
3. graphite floatation
This defect is usually happen at the end
of castings, especially at the gating head. You could see many
graphite balls gathering together.
This defect is because of high content of carbon and silicon, and
too much residues of nodularzing agents.
The solving method is reducing the content of C and Si, and control
the nodularzing agents.
4. inverse chill
This defect will show the clear white
blocks on the surfaces of fractures. Some are just like the white
The reason is the too quick cooling for some positions, or
foundries need to control the usage of nodularizing
agents, not too much. At the same time, enhance the temperature of
5. slag inclusion
Slag inclusion will show the inclusion
with dark color and different shapes on the surfaces of fractures.
From macrostructures, you could see the obvious inclusion.
One reason is because of too much sulphur in the melted iron.
Another reason is too much magnesium residues.
So, iron foundry needs to reduce the content of sulphur in the pig
iron, and try to reduce the residues of magnesium, and set the more
gating heads at the positions of easy slag inclusion.
6. deformation and cracks
When the shrinkage stress becomes higher
than resistance to deformation, then there will be deformation or
even cracks to ductile iron castings. The hot cracks are brown, and
cold cracks are light yellow.
The main reasons are low content of carbon, insufficient
nodularizing and too quick cooling.
So, you need to increase the content of carbon and silicon, reduce
the content of sulphur and improve the nodularizing.