Iron castings often have various casting defects […]
Iron castings often have various casting defects. How to prevent these defects from occurring has always been a concern for casting manufacturers. This article introduces the author's knowledge and practical experience in this area.
1 Pores Features
Pores in castings refer to smooth pores of varying sizes that exist on or near the surface of the casting. The pore walls often have an oxidized color. Due to the different sources and formation reasons of the gas, the manifestations of the stomata are also different, including intrusive stomata, exudative stomata, and subcutaneous stomata.
1.1 Intrusive pores The pores are small in number and larger in size. They are mostly produced in some parts of the outer surface of the casting, which are pear-shaped or spherical. It is mainly caused by the intrusion of the gas generated by the mold or sand core into the molten metal and the failure to escape.
(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 dried sand core 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 or rusty cold iron and core Hold and wait.
(2) Improve the air permeability of the molding sand, choose a suitable cavity compactness, and arrange the position of the air outlet to facilitate the exhaust, and to ensure the smooth flow of the sand core vent.
(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.
1.2 Precipitating pores
This kind of pores are many and scattered, and they are generally found on the surface of castings in most castings cast in the same batch. This kind of pores is mainly due to the fact that in the smelting process, the gas absorbed by the molten metal cannot be completely precipitated before solidification, so many scattered small pores are formed in the casting.
(1) Use clean and dry charge and limit the use of charge with more gas content.
(2) Ensure the "three dry": that is, the tapping trough, tapping hole, and bridge must be dried.
(3) The ladle must be dried, and it is easy to scald with molten iron before use. There is molten iron in the ladle, and a covering agent must be placed on the surface of the molten iron.
(4) Various additives (spheroidizing agent, inoculant, covering agent) should be kept dry from time to time. When the humidity is high, they should be dried before use.
1.3 Subcutaneous pores
This kind of pores mainly appear at 2 to 3mm below the surface of the casting, with a diameter of about 1 to 3mm. And the number is large, the casting will be clearly revealed after heat treatment or rough machining to remove the skin.
(1) Properly increase the pouring temperature, strictly control the amount of various additives added, and shorten the pouring time as much as possible.
(2) The amount of inoculant added 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. If w(Al) is too high, it will easily react with the moisture on the surface of the cavity: 2Al＋3H2O =Al2O3＋3H2↑, under normal circumstances, the Al content of the inoculant should not exceed 1.5%.
(3) Prevent the oxidation of molten iron, add relay coke appropriately, and strictly control the air intake.