Blowhole defects and preventive measures in steel casting

Inside, on or near the surface of the casting, there are smooth holes of different sizes, round, long and irregular in shape, single or compact. Having a white or dark color, sometimes covered with an oxide skin. Because of the different sources and formation reasons of gas, the forms of pores are different, including invasive pores, precipitated pores and reactive pores.

The intrusive air hole is large in volume and similar to pear shape, which often appears near the core wall or the pouring position at the upper part of the casting. It is mainly caused by the gas generated in the sand core invading into the metal and not escaping. The small end position of pear shaped air hole indicates that the gas enters the casting from there.

There are many and dispersive blowholes on the surface of castings, which often appear almost in the same furnace. The main reason is that the gases absorbed in the melting process of metals do not precipitate completely before solidification, so many small dispersed blowholes are formed in the castings.

Reactive porosity is a kind of porosity produced by chemical reaction of metal mold interface. It is also called subcutaneous porosity because of its uniform distribution and often 1-3 è below the surface of casting (sometimes under a layer of oxide skin). Because this kind of porosity is mostly elongated pinhole, its long axis is perpendicular to the surface of the casting, it can also be called pinhole.

Precautions for air holes:

  1. Adopt clean and dry charge, limit the use of charge with more air content, reduce the air absorption of metal during smelting; dry and scald the ladle; properly increase the pouring temperature to facilitate gas diffusion.
  2. Control the pressure head and speed during pouring to ensure that the steel is filled horizontally and stably in the sand mold cavity, avoid turbulence and prevent gas entrapment.
  3. Reduce the gas generation, control the moisture content of mold (core) sand and the content of gas generation raw materials, reduce the gas generation of sand mold in the pouring process, and do not use the cold iron and core support which are damped, rusted or greasy.
  4. Improve the permeability of sand mold, select the appropriate compactness of sand mold, improve the permeability of sand mold and core; arrange the air hole reasonably, so that the air in the mold (core) can be discharged smoothly.
  5. Increase the resistance of gas entering the molten metal, design the pouring system reasonably, avoid the gas involved in the pouring, and brush the coating on the mold (core) surface to reduce the interface effect of the small metal mold.
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