Cause analysis of shrinkage porosity of nodular iron castings

Nodular cast iron solidifies in “paste”. Compared with gray cast iron, eutectic solidification time is long, the number of eutectic clusters is large, and the solidification expansion pressure is large. These solidification characteristics are the fundamental reason for shrinkage porosity. The formation of shrinkage porosity of nodular iron castings is related to the casting structure, sand mold quality, gating system design, melting chemical composition, pouring temperature and so on.

1. Structure of casting itself

The casting of wind turbine has complex shape and large size, with wall thickness from 60 ~ 120 mm and weight of more than 30 T. shrinkage porosity is easy to occur at the place with large wall thickness and the transition of different wall thickness. In the casting process, where the wall thickness is large and the external surface cooling conditions are good, it will give priority to solidification and form a layer of hard shell. The internal metal liquid has poor heat dissipation conditions and the temperature is still very high, which increases the liquid shrinkage and is easy to produce shrinkage porosity at the hot joints of the wall thickness; In addition, if the section thickness of the casting changes suddenly, the isolated rear end face will not be fed, and the shrinkage porosity defect will also occur in the final solidified area.

2. Quality of sand mold

Insufficient fire resistance, low high temperature strength and low compactness of sand mold will increase its thermal deformation. Under the action of pouring pressure, hydrostatic pressure of metal and graphite precipitation expansion force, the volume of mold cavity will become larger, resulting in insufficient feeding of original metal liquid, resulting in shrinkage and porosity of castings. In addition, if the water content of the sand mold is high, the thickness of the dry layer on the mold wall surface will be reduced and the water in the water condensation area will be increased, so as to increase the moving capacity of the mold wall and lead to shrinkage.

3. Gating system design

When the design of gating system is unreasonable, which is not conducive to the sequential solidification of castings, the tendency of shrinkage porosity increases. If the ingate is opened at the wall thickness of the casting, and the size of the ingate itself is thick and large, the liquid metal near the ingate cannot solidify for a long time after pouring. Under the action of graphite precipitation expansion force, the liquid iron will flow back to the right angle channel, resulting in shrinkage and looseness of the casting; If the position, quantity and size of riser are unreasonable, it can not promote the sequential solidification of castings, and it will also lead to shrinkage and looseness of castings; If no air outlet riser is set at the top of the concealed riser, or the cold iron is improperly used, the casting will also shrink and loosen.

4. Melting

The gas content of liquid metal is too high, which precipitates in the form of bubbles when the casting is cooled, and the adjacent liquid metal cannot flow here for feeding, resulting in shrinkage porosity.

The carbon content in molten metal has a great influence on the fluidity of ductile iron and the precipitation of graphite. In a certain range, increasing carbon content is beneficial to increase the fluidity of molten iron and the number of graphite balls, and reduce the tendency of shrinkage porosity; However, when the carbon content exceeds a certain value, it will reduce the fluidity of liquid metal and increase the shrinkage tendency.

When the content of impurities such as phosphorus or sulfur in the molten metal is high, it will also cause shrinkage and porosity. Phosphorus is an element that expands the solidification temperature range, forms a large number of low melting point phosphorus eutectic at the same time, and reduces the feeding capacity of molten iron; Sulfur is an element that hinders graphitization and reduces the fluidity of molten iron.

Magnesium and rare earth elements are elements that hinder graphitization and promote the formation of cementite, so as to increase solidification shrinkage and produce shrinkage porosity. The spheroidization rate of nodular cast iron is required to be greater than 90%. Inoculants such as ferrosilicon are required for inoculation treatment before pouring. Sufficient inoculation will increase the number of graphite balls, reduce the ball spacing, reduce the diffusion distance of carbon, accelerate the transformation of austenite to ferrite and graphite, increase the ferrite content in the structure, and reduce the interface roughness of eutectic cluster after the spheroidization rate increases, which is conducive to the flow of feeding liquid and reduce the tendency of shrinkage porosity. If the inoculation is poor, a large amount of cementite will be precipitated during the solidification of molten iron, which will increase the solidification shrinkage of molten iron and produce shrinkage porosity.

5. Pouring temperature

If the pouring temperature is too low, the feeding capacity of the riser will be reduced and the casting is easy to shrink loose; When the riser is not filled or the metal liquid is not used to supplement the open riser, the feeding capacity of the riser will also be reduced, resulting in shrinkage porosity of the casting. If the pouring temperature is too high, the shrinkage of liquid metal increases and the heat load of the mold wall increases, resulting in the movement of the mold wall and the increase of the cavity space, resulting in the increase of the shrinkage supplement required by the casting, which may lead to shrinkage porosity.