Sand Casting Pouring Metal Metallurgical Reasons and Solutions

1. Pouring Metal Defects

The likely defects in this category are misruns and cold shuts, and slag inclusions.

Misruns and Cold Shuts

Misrun is caused when the metal is unable to fill the mould cavity completely and thus leaves unfilled cavities. A cold shut is caused when two metal streams while meeting in the mould cavity do not fuse together properly, thus causing a discontinuity or weak spot in the casting. Some times a condition leading to cold shuts can be observed when no sharp comers are present in a casting. These defects are caused essentially by the lower fluidity of the molten metal or that the section thickness of the casting is too small. The latter can be rectified by proper casting design. The remedy available is to increase the fluidity of the metal by changing the composition or raising the pouring temperature. This defect can also be caused when the heat-removal capacity is increased such as in case of green sand moulds. The castings with large surface-area-to-volume ratio are more likely to be prone to these defects. This defect is also caused in moulds which are not properly vented because of the back pressure of the gases. The remedies are basically improving the mould design.

Slag Inclusions

During the melting process, flux is added to remove the undesirable oxides and impurities present in the metal. At the time of tapping, the slag should be properly removed from the ladle, before the metal is poured into the mould. Otherwise, any slag entering the mould cavity will be weakening the casting and also spoil the surface of the casting. This can be eliminated by some of the slag-trapping methods such as pouring basin screens or runner extensions.

2. Metallurgical Defects.

The defects that can be grouped under this category are hot tears and hot spots.

Hot Tears

Since metal has low strength at higher temperatures, any unwanted cooling stress may cause rupture of the casting. The main cause for this is the poor casting design.

Hot Spots

These are caused by the chilling of the casting. For example, with grey cast iron having small amounts of silicon, very hard white cast iron may result at the chilled surface. This hot spot will interfere with the subsequent machining of this region. Proper metallurgical control and chilling practices are essential for eliminating hot spots.

As seen from earlier paragraphs, the remedies of some defects are also the causes of others. Therefore, foundry engineer has to analyze the casting from the view point of its final application and thus arrive at a proper moulding procedure to eliminate or minimize the most undesirable casting defects.