Prevention method of porosity in cast iron

Cast iron castings often have various casting defects. How to prevent these defects has always been the concern of casting manufacturers. This paper introduces the author’s knowledge and practical experience in this field.

Characteristics of casting porosity: porosity in casting refers to the smooth pores with different sizes existing in, on or near the surface of casting. The pore wall often has oxidation color. Because of different gas sources and formation reasons, the expression forms of pores are also different. There are invasive pores, precipitating pores, subcutaneous pores, etc.

1 Invasive pore

This kind of air hole is small in number and large in size, which is mostly produced in some parts of the outer surface of the casting, in the shape of pear or ball. It is mainly caused by the gas produced by the mold or sand core invading the liquid metal and not escaping.

Precautions:

(1) Reduce gas generation: control the content of gas generation substances in molding sand or core sand. The moisture content of green molding sand should not be too high. The amount of mold release agent and water should not be too much during molding and mold repair. The sand core shall be dried. The dried sand core shall not be stored for a long time. The sand core used the next day shall be dried in the oven before use to prevent the moisture absorption of the sand core. The damp and rusty cold iron and core support shall not be used.

(2) Improve the air permeability of molding sand, select the appropriate air compactness, reasonably arrange the position of air outlet to facilitate the exhaust, and ensure that the air outlet of sand core is smooth.

(3) Properly raise the pouring temperature, open the vent hole and the vent riser, etc., so as to facilitate the upward floating discharge of the gas invading the liquid metal.

2 Precipitated pores

This kind of porosity is many and dispersive, which is usually found on the surface of the casting. This kind of porosity is mainly due to the fact that in the melting process, the gas absorbed by the molten metal does not completely precipitate before solidification, so many small dispersed pores are formed in the casting.

Precautions:

(1) Clean and dry charge shall be used to limit the use of charge with more air content.

(2) Ensure that “three dry”: that is, the iron trough, the iron outlet and the bridge should be dried thoroughly.

(3) The ladle shall be dried, and it is better to scald it with molten iron before use. If there is molten iron in the ladle, the covering agent must be placed on the surface of the molten iron.

(4) All kinds of additives (spheroidizing agent, inoculant and covering agent) should be kept dry from time to time. When the humidity is high, they can only be used after drying.

3 Subcutaneous stomata

This kind of porosity mainly occurs in the surface layer of the casting, which is 2-3 mm under the skin, and the diameter is about 1-3 mm. And a large number of castings, after heat treatment or rough machining to remove the skin, it will be clearly exposed.

Preventive measures;

(1) The pouring temperature should be raised properly, the amount of various additives should be strictly controlled, and the pouring time should be shortened as much as possible.

(2) The amount of inoculant should be controlled between 0.4% and 0.6%, and the mass fraction of A1 in inoculant should be strictly controlled. The higher w (AL) is easy to react with the water on the surface of mold cavity: 2Al + 3H2O = Al2O3 + 3h2 โ†‘, generally, the amount of Al in inoculant should not exceed 1.5%.

(3) Prevent the oxidation of molten iron, properly add relay coke, and strictly control the air intake.

(4) On the premise of ensuring spheroidization, try to reduce the amount of spheroidizing agent.

(5) During pouring, the surface of molten iron shall be covered with cryolite powder to prevent oxidation of molten iron.

(6) Try to reduce the moisture content of molding sand.

(7) Improve pouring speed.

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