Sand Casting Gas defects Reasons and Solutions

Any irregularity in the moulding process causes defects in castings which may sometimes be tolerated, some times eliminated with proper moulding practice or repaired using methods such as welding and metallization.

The following are the Gas defects which are likely to occur in sand castings

Gas Defects

The defects in this category can be classified into blow holes and open blows, air inclusion and pin hole porosity. All these defects are caused to a great extent by the lower gas-passing tendency of the mould which may be due to lower venting, low permeability of the mould and/or improper design of the casting. The lower permeability of the mould is, in turn, caused by finer grain size of the sand, higher clay, higher moisture, or by excessive ramming of the moulds.

Blow Holes and Open Blows

These are the spherical, flattened or elongated cavities present inside the casting or on the surface as shown in Fig. On the surface, they are called open blows and inside, they are called blow holes. Because of the heat in the molten metal, the moisture is converted into steam, part of which when entrapped in the casting ends up as blow or as open blows when it reaches the surface. Apart from the presence of moisture, they occur due to the lower venting and lower permeability of the mould. Thus, in green sand moulds it is very difficult to get rid of the blow holes, unless proper venting is provided.

Air Inclusions

The atmospheric and other gases absorbed by the molten metal in the furnace, in the ladle, and during the flow in the mould, when not allowed to escape, would be trapped inside the casting and weaken it. The main reasons for this defect are the higher pouring temperatures which increase the amount of gas absorbed; poor gating design such as straight sprues in unpressurized gating, brupt bends and other turbulence-causing practices in the gating, which increase the air aspiraton and finally the low permeability of the mould itself. The remedies would be to choose the appropriate pouring temperature and improve gating practices by reducing the turbulence.

Pin Hole Porosity

This is caused by hydrogen in the molten metal. This could have been picked up in the furnace or by the dissociation of water inside the mould cavity. As the molten metal gets solidified, it loses temperature which decreases the solubility of gases, thereby expelling the dissolved gases. The hydrogen while leaving the solidifying metal would cause very small diameter and long pin holes showing the path of escape. These series of pin holes cause the leakage of fluids under high operating pressures. The main reason for this is the high pouring temperature which increases the gas pick-up.