How To Prevent Defects such As Air Pore In Castings?

In the production process of iron castings, defects such as air pore often occur. How to prevent these defects from occurring has always been a concern for casting manufacturers.

The pores in the casting refer to the lubrication holes of varying sizes that exist inside the casting, on the outside or near the outside. The pore walls often have an oxidized color, because the origin and composition of the gas are different, and the manifestations of the pores are also different. There are invasive air pore, separating air pore, and subcutaneous air pore.

1. separating air pore

This kind of pores are numerous and scattered, generally located on the surface of the casting, and most of the castings poured in the same batch are found. The main reason for this kind of pores is that in the smelting process, the gas absorbed by the molten metal cannot be completely separated before condensing, so many small scattered pores are formed in the casting.

Preventive measures:

1. Use clean and dry furnace burden to restrict the use of furnace burden with more gas content.
2. Guarantee “Three Drying”: that is, the tapping launder, tapping hole, and bridge must be completely dried.
3. The ladle must be dried. It is best to scald with molten iron before use. If there is molten iron in the ladle, a masking agent must be placed on the surface of the molten iron.
4. Various additives (spheroidizing agent, inoculant, masking agent) should be kept dry from time to time. When the humidity is high, it can be used after drying.

2. invasive air pore

The number of such pores is small and the scale is larger. They mostly occur in certain parts of the outer surface of the casting, which are pear-shaped or spherical. The main reason is that the gas from the mold or sand core invades the molten metal and fails to escape.

Preventive measures:

1. Reduce gas generation: control the content of gas-producing substances in molding sand or core sand. The moisture content of green sand should not be too high, and the amount of release agent and water should not be too much when modeling and repairing. The sand core should be dried. The sand core after drying should not be stored for too long. The sand core used the next day should be dried in the oven before use to prevent the sand core from absorbing moisture. Do not use damp, rusty cold iron and core.
2. Improve the air permeability of the molding sand, select the appropriate molding tightness, and arrange the air outlet position reasonably to facilitate the exhaust, and to ensure that the sand core ventilation pores are unblocked.
3. Properly increase the pouring temperature, open vent holes and vent risers, etc. to facilitate the floating and discharge of the gas invading the molten metal.

3. subcutaneous air pore

This kind of pores mainly appear at 2 to 3mm below the surface of the casting, with a diameter of about 1 to 3mm. Moreover, the number of castings is large, and the castings will be clearly revealed after the outer skin is removed by heat treatment or rough machining.

Preventive measures:

1. Properly increase the pouring temperature, strictly control the amount of various additives, and shorten the pouring time as much as possible.
2. The participation amount of inoculant should be controlled within (mass fraction) 0.4%～0.6%. At the same time, the mass fraction of A1 in the inoculant should be strictly controlled. Under normal circumstances, the Al content of inoculant should not exceed 1.5%.
3. Avoid oxidation of molten iron, add relay coke appropriately, and strictly control air intake.