How to eliminate sand hole and skin pores in ductile iron castings?
Skin pores and sand hole are common
metal casting defects,
which has a serious impact on their performance. How to eliminate
sand hole and skin pores in ductile
iron casting? According to the experience of Dandong
Foundry, we will start from factors to look for the answers.
1. Water content of sand
The main factors affecting the skin pores
is the water content of sand which should be controlled lower. 3 to
5% is the reasonable range (to ensure the performance of wet sand).
2. The content of Sulfur
The second major factor is the S content. S
in liquid iron reacts with Mg in nodulizer we can get MgS. MgS is
reduced by the water in sand generating H2S gas. If the casting
speed is too fast, the gas is not completely discharged in time. It
will produce the skin porosity under the epidermis in the casting.
So, firstly iron foundry should try to use iron ball that has
low-sulfur. Secondly, after processing it is better to slag quickly
cleanly, to prevent the return of sulfur.
3. Reasonable gating
The third factor is reasonable the gating
system. There are many ways to form pores, such as involved pores
and immersed porosity and so on. If the gating system is
unreasonable, the air will involve in the
casting process. Because
the solidification method is fast and the thin-walled castings
solidify quickly, it is difficult to emptying air completely and the
skin pores generate.
4. The content of
aluminum in nodulizer and iron
The dangerous residual aluminum content of
wet-type ductile iron is
0.030-0.050%. It will make the skin pores. If
iron foundries add more
than 0.2% of aluminum in the ladle before casting (without affecting
the microstructure), it can eliminate the skin pores in ductile iron
5. Pouring temperature
Iron foundries should base their casting of
the main wall to choose a suitable pouring temperature in order to
discharge the gas in the molten iron before solidification. If the
thickness of wall is less than 10, the pouring temperature should be
controlled at 1380 ~ 1430. If the thickness of wall is more than 10,
temperature can be controlled at 1320 ~ 1380.
6. Machining parts
If the casting is machining parts, some
small pores of the skin are allowed within a reasonable amount of
processing. Otherwise, if iron foundry wants to completely avoid the
skin pores, it will need strict measures of multi-party and an
increase in cost. That will be more harm than good; if the surface
requirements of the cast is strict, it need to take great efforts to
prevent the skin pores.
A qualified cast iron product
requires a wide range of needs, the attention of all aspects iron
foundry should take the cost, the requirement and production process
into account, only in this way, can
iron foundry obtain harvest.