the porosity defect of castings

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Various defects will appear in the production process of castings, which affect the quality of the castings, and even make the castings directly scrapped. Porosity are relatively common among them. No matter iron castings, steel castings or alloy castings, the problem of porosity may occur.

Characteristic:

In most cases, subcutaneous pores are composed of multiple small pores with a diameter of 1-3mm, which are arranged in series at 1-3mm below the surface of the casting. The inner wall of the pores is smooth and uniformly distributed on the upper surface of the casting or away from the inner runner, but occasionally exists on the side and bottom of the casting. It is generally spherical, spherical, teardrop-shaped, or long needle-shaped. In the as-cast state, the subcutaneous pores are not easy to be found; however, the porosity are exposed after heat treatment or mechanical processing.

Control the quality of molten iron for porosity

(1)Control the amount of residual aluminum

The dangerous residual aluminum content of wet ductile iron castings is 0.03~0.05%, and subcutaneous pores will appear at this time. Generally, they will not appear when it is less than 0.03%. Without affecting the metallographic structure, adding more than 0.2% aluminum before pouring can eliminate subcutaneous pores. However, the aluminum in cast iron mainly comes from inoculants. For wet-type spheroidal graphite cast irons, when too much ferrosilicon inoculant is added to the molten iron, it is one of the reasons for the subcutaneous pores in the castings.

(2)Control the amount of titanium

There are both residual aluminum and residual titanium in cast iron. Excessive residual titanium will cause serious subcutaneous pores in cast iron. When the residual aluminum content of ductile iron is less than 0.03%, subcutaneous pores generally do not appear. If the residual titanium content exceeds 0.01% at this time, subcutaneous pores will occur. Under the combined action of titanium and aluminum, the reduction of interfacial water vapor is intensified, making the hydrogen content of the interfacial molten iron higher, and it is easier to form subcutaneous pores. Residual titanium mainly comes from smelting furnace charge pig iron ingots. Pay attention to the titanium content of pig iron ingots. Use the high and low titanium content in combination to control the titanium content.

(3)Reduce the content of sulfur

Manganese and sulfur for wet ductile iron castings, sulfur is a harmful element from the perspective of preventing subcutaneous pores. When the sulfur content exceeds 0.094%, subcutaneous pores are prone to occur. The higher the sulfur content, the more serious the defect. In addition, the generated H2S may make the defect more serious. The oxide and sulfide slag generated after the spheroidizing treatment should be cleaned up. Otherwise, the H2S gas generated at the interface will also form subcutaneous pores, and the graphite around the subcutaneous pores will not be spheroidized.

(4)Add RE elements

The addition of RE elements can deoxidize and desulfurize, increase the surface tension of the cast iron liquid, and can effectively prevent the generation of subcutaneous pores. Nodular cast iron generally uses rare earth magnesium ferrosilicon alloy as spheroidizing agent, and tellurium (Te) 0.002% (20ppm) is added to the molten iron before pouring, and a small amount of addition can achieve obvious effects.

(5)Control the original hydrogen content of molten iron

Practice has proved that when the hydrogen content of molten iron reaches 4-5ppm, subcutaneous porosity are likely to occur. Generally, the hydrogen content of the original molten iron is controlled below 2-2.5ppm. Within 1.5-3.5 hours after the furnace is opened, the hydrogen content is relatively high and not suitable for pouring.

Anyang Huatuo Metallurgy Co., Ltd has in the raw materials for more than 10 years, own professional knowledge of the inoculant, spheroidizing agent, recarburizer for castings, if any interests just consult us.

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