Shrinkage porosity of casting defects

What is shrinkage porosity?

Shrinkage porosity refers to that the final solidification area of the casting is not fed by liquid metal or alloy to form dispersed and small shrinkage cavity. Often scattered in casting The axis area of wall thickness, the thick part, the root of riser and the vicinity of inner gate. When the volume of shrinkage porosity and shrinkage cavity are the same, the distribution area of shrinkage porosity is much larger than that of shrinkage cavity. Shrinkage porosity is hidden in the inside of the casting and is not easy to be found on the appearance.

Classification

Shrinkage porosity is divided into macro shrinkage porosity and micro shrinkage porosity (also known as porosity).

• a) Macroporosity is a small dispersed shrinkage cavity that can be seen by naked eye or magnifying glass.
• b) The micro shrinkage is distributed between the grains.

Causes of formation

When the alloy crystallizes in a wide range of temperature, there is a wide solidification region of liquid and solid phases coexisting in the casting surface. As the solidification continues, the solid phase increases. In the later stage of solidification, the dendrites contact each other and divide the alloy liquid into many small closed areas. When the solidification shrinkage of the alloy liquid in the closed area is not supplemented, shrinkage porosity is formed. Shrinkage porosity can be seen as many dispersed small shrinkage pores. The wider the range of crystallization temperature, the easier shrinkage porosity is. Shrinkage porosity usually occurs in the axial area of casting wall, hot spot, riser root and near the inner gate, and also under the concentrated shrinkage cavity.

Causes

• a) The feeding effect of riser is poor;
• b) Too much air content in charge;
• c) Overheating near runner;
• d) There is too much moisture in the sand mold and the sand core is not dried;
• e) The grain size of the alloy is coarse;
• f) Improper position of casting in mold;
• g) The pouring temperature is too high and the pouring speed is too fast.

The following four points are the experience summarized by the author for reference.

• (1) The forming factors of casting shrinkage porosity are permeability of molding sand, dryness, pouring temperature, pouring speed, runner arrangement and molten iron composition;
• (2) When the molten iron reaches the high temperature, the molten iron is not enough to ensure the fluidity;
• (3) The preheating temperature before casting, the flow capacity of pouring channel, the pouring temperature and pouring speed of molten iron are not well controlled;
• (4) Or if the liquid iron is pouring multiple parts, a long time, cooling, no heating or adding high-temperature molten iron, direct low-temperature pouring, it must appear the probability of shrinkage.

Prevention methods

• a) To improve the design of riser by pouring liquid metal from riser;
• b) The charge should be clean and free of corrosion;
• c) To place or place with a chill iron riser;
• d) Control the moisture content of molding sand and drying of sand core;
• e) Take measures to refine grain size;
• f) Improving the position of casting in mold and reducing pouring temperature and pouring speed.

Actual case sharing: the macro morphology, micro morphology and electron microscope morphology of casting shrinkage porosity can be seen in the figure.

Source: China Butt Weld Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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