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How to prevent the appearance of reaction pores in casting production

Issuing time:2021-06-18 15:59Author:Momo

How to prevent the appearance of reaction pores in casting production

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In casting production, the causes of reaction pores are complicated, and reaction pores appear in different processes and different materials.

The gas source of the reaction pores is not only brought in during the smelting of molten metal, but also produced by the high-temperature roasting of the molten metal.

The gas generated in the mold is divided into two sources. One is that the moisture content of the mold is too high, and the exhaust hole is insufficient to exhaust gas, which causes the gas to invade the casting. Another source is the production of paint and chemical additives under high temperature baking. The gases that cause the reaction pores are different, such as carbon monoxide, hydrogen, and nitrogen.

Reactive stomata characteristics

The reaction pores are generally evenly distributed in groups, and are often produced under the skin of the casting to form subcutaneous pores, and because of their needle-like shape, they are also called pinholes.

Reaction pores often appear in gray cast iron, ductile iron, and cast steel.

The production rate of steel castings is higher than that of cast iron.

This is because the temperature of the molten steel in cast steel is high, and the water glass sand and resin sand used to make the cast steel mold contain large amounts of gas.

Reactive porosity between metal and mold

The reactive pores generated between the molten metal and the mold are usually distributed 1-3mm below the surface of the casting, sometimes only under a layer of oxide scale, and the surface is exposed after machining or cleaning. Such pores are spherical or plow-shaped, with a diameter of about 1-3mm; some are elongated, with a length of up to 7-10mm, and most of them are perpendicular to the surface of the casting.

Another type of reactive pores are honeycomb-shaped pores produced by chemical reactions between chemical components in the molten metal or between components and non-metallic inclusions (including slag), which are pear-shaped or spherical, and are more uniformly distributed.

Preventive measures for reactive porosity between metal and mold

(1) Take drying, dehumidification and other measures to prevent and reduce the gas entering the molten metal.

(2) Strictly control the content of strong oxidizing elements in the alloy, such as magnesium and rare earth elements in ductile iron; aluminum for deoxidation in cast steel.

(3) Sand molds (cores) should be strictly controlled with moisture. Important castings can be dry or surface-dried. Nitrogen-containing resin sand should minimize the urea content and control the amount of urotropine curing agent added to ensure that the mold is good. Air permeability.

(4) Appropriately increasing the pouring temperature can reduce the solidification rate and facilitate the removal of gas.

(5) In the process plan design, try to ensure that the molten metal enters the mold smoothly to reduce the oxidation of the molten metal.

In fact, in order to prevent reactive pores in castings, control the charge, adjust the moisture of the mold, understand and adjust the additives that are prone to gas molding sand, and control the composition within a reasonable range. Can effectively prevent reactive pores.

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