Common casting defects and preventive measures

1.Porosity

Defect characteristics

Inside, on or near the surface of the casting, there are smooth holes with different sizes, round, long and irregular shapes, single and compact. Having a white or dark color, sometimes covered with an oxide skin.

Preventive measure

Reduce the air absorption of gossip metal during smelting. It can reduce the gas emission of the sand mold in the process of pouring, improve the structure of the casting, improve the permeability of the sand mold and the core, and make the gas in the mold be discharged smoothly.

2.Shrinkage hole

Defect characteristics

In the inner part of the thick section, the inner part of the two interfaces and the inner part or the surface of the joint of the thick section and the thin section of the casting, the shape is irregular, the hole is rough and uneven, and the grain is coarse.

Preventive measure

The castings with small and uniform wall thickness shall be solidified at the same time. The castings with large and uneven wall thickness shall be solidified from thin to thick in sequence, and the cold iron with reasonable riser shall be placed.

3.Shrinkage porosity

Defect characteristics

There are small and incoherent shrinkage cavities in the casting, which gather in one or more places. The grains are coarse, and there are very small holes between the grains. The water seeps during the hydrostatic test.

Preventive measure

The hot spot should be minimized at the joint between walls, and the pouring temperature and speed should be minimized.

4.Slag blowhole

Defect characteristics

An irregularly shaped hole in the interior or surface of a casting. The hole is not smooth, and all or part of it is filled with slag.

Preventive measure

Increase the temperature of molten iron. Reduce slag viscosity. Improve the slag retaining capacity of the gating system. Increase the casting fillet.

5.Sand holes

Defect characteristics

There are holes filled with molding sand inside or on the surface of the casting.

Preventive measure

Strictly control the molding sand performance and molding operation, and pay attention to cleaning the cavity before molding.

6.Hot cracking

Defect characteristics

There are penetrating or non penetrating cracks on the casting (note if it is curved), and the metal surface at the crack is oxidized.

Preventive measure

Strictly control the content of S and P in molten iron. The wall thickness of castings shall be as uniform as possible. Improve the yield of molding sand and core. The riser shall not hinder the casting shrinkage. Avoid sudden changes in wall thickness. Do not open too early. Castings shall not be quenched.

7.Cold cracking

Defect characteristics

There are penetrating or non penetrating cracks (mainly straight) on the casting, and the metal surface at the crack is oxidized.

Preventive measure

Strictly control the content of S and P in molten iron. The wall thickness of castings shall be as uniform as possible. Improve the yield of molding sand and core. The riser shall not hinder the casting shrinkage. Avoid sudden changes in wall thickness. Do not open too early. Castings shall not be quenched.

8.Bonded sand

Defect characteristics

On the surface of the casting, all or part of it is covered with a layer of metal (or metal oxide) and sand (or coating) mixture (compound) or a layer of burning structure molding sand, resulting in rough surface of the casting.

Preventive measure

Reduce sand gap. Lower the pouring temperature of metal properly. Improve the fire resistance of molding sand and core sand.

9.Sand inclusion

Defect characteristics

On the surface of the casting, there is a layer of metal tumor or sheet, and a layer of molding sand is sandwiched between the metal tumor and the casting.

Preventive measure

Strictly control the properties of molding sand and core sand. Improve the pouring system to make the liquid metal flow smoothly. Large flat casting should be inclined.

10.Cold separation

Defect characteristics

There is a kind of incomplete fusion gap or pit on the casting, and its boundary edge is smooth.

Preventive measure

Improve pouring temperature and speed. Improve the gating system. The flow is not interrupted during pouring.

11.Less than casting

Defect characteristics

Lack of flesh in castings due to incomplete filling of the cavity with liquid metal.

Preventive measure

Improve pouring temperature and speed. Don’t cut off the flow and prevent running fire.

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