Shrinkage porosity and shrinkage cavity in steel castings

During solidification, the steel casting will produce liquid shrinkage and solidification shrinkage. When the feeding system cannot provide sufficient liquid steel to supplement and shrink the final solidification area of the steel casting, the steel casting will have shrinkage defects with irregular shape and rough surface, and dendrite endings can be observed on its internal surface.

The liquid shrinkage caused by solidification of steel castings can be expressed as:

According to the size and distribution characteristics of shrinkage cavity, it can be divided into macroscopic shrinkage cavity visible to the naked eye and microscopic shrinkage cavity that can be observed only by section corrosion or microscope. Micro shrinkage cavity will occur in the solidification process of steel castings, but it has little effect on the properties of steel castings. Its existence needs to be considered only in steel castings with high requirements for air tightness and physicochemical properties. Macro shrinkage cavity can be divided into shrinkage cavity with small volume and shrinkage cavity with large volume. Shrinkage cavity is mostly concentrated in the final solidification area of steel casting, and shrinkage porosity is mostly distributed in the axis area of steel casting wall.

The existence of shrinkage defects will reduce the effective stress area of steel castings, and stress concentration is easy to occur in the shrinkage area, which will reduce the mechanical properties of steel castings. When the shrinkage area of ZG35 steel castings reaches 4% of the total area, its tensile strength will be reduced to 30% of the original. Shrinkage defects can also reduce the air tightness and physicochemical properties of steel castings. When the steel castings are under high pressure, leakage is easy to occur. Therefore, shrinkage defects do great harm to steel castings, and their formation should be avoided in production.

In the production of cast steel castings, the methods of setting riser and adding cold iron are often used to prevent shrinkage defects. The function of the riser is to transfer the shrinkage cavity on the steel casting body to the riser by establishing the feeding channel. The use of cold iron can change the local solidification rate of steel castings, make the steel castings better realize sequential solidification, and ensure the transfer of shrinkage defects to the riser.