The general summary of the causes of subcutaneous porosity in castings

The formation of subcutaneous pores 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 and every working procedure, which should be attached great importance to.

I. raw materials

  1. It is forbidden to use raw materials with high titanium (TI) and aluminum (AI), high alloy steel, stainless steel, etc.
  2. Raw materials with serious corrosion and oxidation, oil, slime and waste sand shall be used after cleaning.
  3. Raw materials with moisture and water shall be used after drying.
  4. Thin iron sheet (pipe fitting) less than 3mm is forbidden to be used.
  5. The length of raw materials shall be controlled at about 300-400mm.

II. Molten iron smelting

  1. When the raw materials are loaded into the furnace, they must be compact and the space shall be reduced as much as possible to reduce the suction and oxidation of molten iron.
  2. In the process of molten iron melting of each furnace, the slag shall be completely cleaned at least 2-3 times, and after slag cleaning, the slag shall be covered in time and the thermal insulation material shall be covered with molten iron to avoid the long-term contact between molten iron and air for inspiration and oxidation.
  3. The waiting time of molten iron at high temperature shall not exceed 10-15 minutes, otherwise, the quality of molten iron will be seriously deteriorated and become “dead water”.
  4. The tapping temperature shall not be lower than 154 ± 10 ℃. After tapping, the slag shall be removed in time and the slag accumulating heat preservation agent shall be covered to prevent the molten iron from cooling and oxidation.
  5. Do not use carburizers that do not meet the standard.

III. inoculants

  1. Before use, it should be baked at 300-400 ℃ to remove the adsorbed water and crystal water.
  2. The particle size of inoculant is 5-10 mm
  3. Inoculant aluminum content < 1%

IV. pouring

  1. Large and small bags must be dried thoroughly. Wet bags are forbidden to be used. It is forbidden to use hot iron bags instead of drying.
  2. Raise pouring temperature, high temperature and fast pouring. Pouring principle: slow fast slow. It has been proved that the porosity can be greatly reduced by increasing the pouring temperature by 30-50 ℃. During pouring, the molten iron should always be filled with the sprue and flow continuously in the middle, so as to quickly establish the static pressure of molten iron and resist the invasion of interface gas.
  3. If the temperature of ladle iron is lower than 1350 ℃, it is forbidden to use it. It shall be used after the temperature is raised in the furnace. 4. Strengthen slag blocking, slag shielding, timely remove oxide skin to prevent it from entering the mold cavity..

V. sand mixing

  1. Strictly control the moisture content 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 compactness is 35-38%, and the surface hardness of molding sand is more than 90
  3. High quality bentonite and coal powder are selected.
  4. Add new sand as required.
  5. At the end of mixing every day, the surplus molding sand shall be recycled, and the sand mixer shall be thoroughly cleaned and cleaned.

Vi. mold and modeling

  1. The mold parting surface shall be provided with an exhaust hole slot or exhaust channel and a dark air chamber to reduce the gas pressure.
  2. A dark gas chamber is added on the mould to reduce the gas pressure.
  3. Sprinkle the ice crystal powder on the runner or sand core (the amount shall be determined after the test).

VII. Core making

  1. The water content of silica sand is less than 0.2%, and the mud content is less than 0.3%.
  2. Core making process: before sand mixing, the silica sand needs to be heated to 25-35 ℃, first add component 1 into the sand, mix for 1-2 minutes, then add component 2, continue mixing for 1-2 minutes. The amount of the two components is 0.75% of the mass fraction of sand
  3. Due to the high sensitivity of polyisocyanate to water, the storage time of the sand core should not exceed 24 hours.
  4. Too high concentration and residue of triethylamine may cause subcutaneous porosity in castings.