Characteristics, causes and prevention of casting porosity

Features:
  1. Invasive pore: the pore size is generally large, in the shape of round, ball or pear; the pear shaped small head points to the direction of the external gas source; the pore wall is smooth, and the pore wall is blue when the composition of the invasive gas is mainly Co, and it is metallic and bright when the composition of the invasive gas is mainly hydrogen, and it is oxidized and dark when the composition of the invasive gas is mainly water vapor. It usually appears in the upper part of the casting, single distribution or local aggregation
  2. Packing air hole: generally round, globular or oblate, with dimensions up to several millimeters, smooth hole wall, oxidation color of inner wall, distributed in the casting section in the action area of inner gate dispersedly
  3. Precipitated pores: generally round and globular, the pore diameter is smaller than 1mm, the pore wall is smooth and shiny, and it is metallic, and the pore wall is covered with a layer of carbon film or graphite film, which is generally distributed dispersedly
Causes:

A. There will be o, h, n three gas elements in the molten iron. The solubility of the three gas elements suddenly decreases during the solidification process. After the precipitation, the gas forms to form pores

B. Gas is drawn into the liquid metal by the bubble or impact of the liquid metal flow. When the gas can not be discharged, it will form a pore

C. The air content of sand core is large, and pores are easy to form when the sand core is wet or the coating is not dry

D. The burden is rusted, and the Al content of inoculant is high, which is easy to form pores

E. The scheme design is unreasonable, and the hot metal forms vortex and air entrainment in the pouring process

F. Poor exhaust of the scheme system

G. Molding sand has poor permeability or high moisture content

Plan improvement measures:

A. Set the exhaust passage or overflow riser to make the gas discharge smoothly

B. Rapid punching and simultaneous punching to avoid the formation of pores due to excessive water content in local sand mold caused by “water migration”

C. The flow path area should be correctly selected to avoid the formation of vortex and air entrainment during the pouring of molten iron. After the completion of the scheme, the Reynolds formula should be used to check whether the metal in the runner is stratified or turbulent

The Reynolds number (RE) is calculated as follows: Re = g × 105 / T × p

<G = weight of molten iron below the calculated cross section (kg)>

<T = filling time of this part of molten iron (seconds)>

<P = perimeter of calculated sprue section (CM)>

<10 = several physical constants including molten iron, such as kinematic viscosity and density of molten iron at pouring temperature>

Re < 2300 (laminar flow) 2300 < Re < 13800 (non turbulence) re > 13800 (turbulence)

Experience shows that the Reynolds number at F1 should be less than 13800, and that at F2 should be less than 6000. It is impossible to make the metal flow be laminar in the casting system, but it can be non turbulent.

D. Increase the temperature of air hole position (increase the water inlet, increase the pouring temperature, etc.). Create conditions for gas to float up.

spacer