Laizhou Dingtai Machinery Co., Ltd.

　　The main reason for shrinkage porosity in ductile iron castings is that the liquid and solidification shrinkage during alloy solidification are not effectively compensated, coupled with improper casting structure, composition design, or process control. Specific reasons can be summarized as follows:

　　I. Material Composition and Metallurgical Factors

　　Insufficient Carbon Equivalent

　　Too low carbon equivalent (C%+1/7Si%) will reduce graphitization expansion and weaken the self-compensation ability. The empirical formula requires C%+1/7Si%>3.9%, but graphite flotation should be avoided.

　　High Phosphorus Content

　　Phosphorus eutectic is difficult to compensate during final solidification, expanding the solidification range and increasing the risk of shrinkage porosity. Phosphorus content should typically be controlled to <0.08%.

　　Excessive Residual Magnesium and Rare Earth

　　Residual magnesium and rare earth elements will hinder graphitization, reduce the spheroidization rate, and increase the tendency of white iron. Residual magnesium (0.03%-0.06%) and rare earth (0.02%-0.04%) should be controlled.

　　II. Casting Structure and Process Design

　　Heat Nodes and Wall Thickness Variations

　　Thick sections, concave corners, or areas with significant wall thickness variations in castings cool slowly, forming a "last solidification zone." Liquid shrinkage easily leads to shrinkage porosity. For example, dendritic crystals at heat nodes partition the molten iron, blocking compensation channels.

　　Improper Gating System

　　Unreasonable number, location, or size of risers, or excessively small pouring gates, will all weaken the compensation effect. Improper use of chills may also disrupt sequential solidification.

　　Pouring Temperature and Speed

　　Temperatures that are too high (>1350℃) increase the amount of liquid shrinkage, while temperatures that are too low (<1250℃) hinder compensation. The ideal range is 1300-1350℃.

　　III. Mold and Operating Conditions

　　Insufficient Mold Compactness

　　The mold cavity expands under the static pressure of the metal, resulting in insufficient molten iron to compensate for shrinkage.

　　High Hydrogen Content in Molten Iron

　　When the hydrogen content >4ppm, subcutaneous pores are easily generated, indirectly increasing the risk of shrinkage porosity. The original hydrogen content of the molten iron should be controlled to ≤2.5ppm.

　　Solidification Method and Mold Rigidity

　　The pasty solidification characteristics of ductile iron result in a weak surface solidified layer, and the internal graphite expansion cannot effectively compensate for shrinkage. Mold rigidity needs to be improved.

　　IV. Improvement Measures

　　Optimize composition: Increase carbon equivalent, reduce phosphorus and control sulfur, balance magnesium/rare earth residuals.

　　Process adjustments: Adopt directional solidification design, reasonably arrange risers and chills, and control pouring temperature.

　　Mold strengthening: Improve mold compactness and reduce mold cavity displacement.

　　Key control points: Through solidification shrinkage compensation and heat node elimination, combined with composition optimization and process control, shrinkage defects can be significantly reduced.