Causes of subcutaneous blowholes in castings

The formation of subcutaneous blowholes is a comprehensive reaction of improper operation of each link and process in the casting process. The causes are complex, and there are many influencing factors, involving everyone. Everyone should pay high attention to each process.

1、 Raw materials

1. Raw materials with high content of titanium (Ti) and aluminum (AI), as well as high alloy steel and stainless steel are prohibited.

2. Raw materials that are seriously rusted and oxidized and have a lot of oil, coal sludge and waste sand shall be used after cleaning.

3. Wet raw materials with water shall be used after drying.

4. Thin iron sheet (pipe fittings) less than 3mm shall not be used.

5. The length of raw materials shall be controlled at about 300-400mm.

2、 Hot metal smelting

1. When the raw materials are loaded into the furnace, they must be compact and the space must be minimized to reduce the hot metal suction and oxidation.

2. In the process of molten iron melting in each furnace, the slag shall be thoroughly removed for at least 2-3 times. After the slag is removed, the hot metal shall be timely covered with slag accumulation and thermal insulation materials to prevent the hot metal from being inhaled and oxidized in contact with the air for a long time.

3. The waiting time for molten iron at high temperature shall not exceed 10-15 minutes, otherwise, the quality of molten iron will deteriorate seriously and become “stagnant water”.

4. The tapping temperature shall not be lower than 1540 ± 10 ℃. After tapping, the slag shall be removed in time and covered with slag gathering insulation agent to prevent the molten iron from cooling and oxidation.

5. It is prohibited to use the unqualified carburetor.

3、 Inoculant

1. Before use, it should be baked at 300-400 ℃ to remove the absorbed water and crystal water.

2. Inoculant particle size 5-10mm

3. Inoculant aluminum content<1%

4、 Pouring

1. The large and small bags must be dried thoroughly. It is prohibited to use wet bags, and it is prohibited to use hot metal bags instead of drying.

2. Increase pouring temperature, high temperature quick pouring. Pouring principle: slow fast slow. The practice shows that the porosity can be greatly reduced when the pouring temperature is increased by 30-50 ℃. During pouring, the molten iron shall always be filled with the sprue and flow continuously in the middle to quickly establish the static pressure of molten iron and resist the invasion of interface gas.

3. If the temperature of molten iron in small ladle is lower than 1350 ℃, it shall not be used, and it shall be used after being returned to the furnace and temperature raised. 4. Strengthen slag blocking and covering, and timely remove oxide skin to prevent it from entering the mold cavity..

5、 Sand mixing

1. Strictly control the moisture of molding sand not more than 3.5%

2. The air permeability of molding sand is controlled at 130-180, the wet compression strength is 120-140KPa, the compaction rate is 35-38%, and the surface hardness of molding sand is>90

3. High quality bentonite and pulverized coal are selected.

4. Add new sand as required.

5. At the end of sand mixing every day, the excess molding sand shall be recycled, and the sand mixer shall be thoroughly cleaned and cleaned.

6、 Mold and modeling

1. The parting surface of the mold shall be provided with an exhaust hole groove or an exhaust passage and a dark gas chamber to reduce the gas pressure.

2. A dark gas chamber is added on the die to reduce the gas pressure.

3. Sprinkle cryolite powder on the runner or sand core (the amount shall be determined after the test).

7、 Core making

1. The water content of silica sand shall be less than 0.2%, and the mud content shall be less than 0.3%.

2. Core making process: Before sand mixing, silica sand needs to be heated to 25-35 ℃. First, add component 1 to the sand, mix for 1-2 minutes, then add component 2, and continue to mix for 1-2 minutes. The addition amount of the two components is 0.75% of the sand mass fraction respectively

3. Due to the high sensitivity of polyisocyanate to water, the storage time of the prepared sand core should not exceed 24 hours.

4. Too high concentration and residue of triethylamine will easily cause subcutaneous blowholes in castings.