Interpretation of common defects such as shrinkage cavities, shrinkage porosity, slag inclusion, and graphite floating in cast iron components

Ductile iron, widely developed in China over the past 40 years, is a strong, tough, and cost-effective casting material. Its spherical graphite structure reduces stress concentration, giving it better tensile strength, plasticity, and toughness than other cast irons. While its plasticity is lower than steel, it offers comparable fatigue strength and a high yield strength ratio (0.7–0.8), which is almost twice that of ordinary carbon steel. However, ductile iron also faces unique casting defects, such as:

1　Shrinkage cavity and porosity

2　Slag inclusion

3　Subcutaneous porosity

4　Graphite floating

5　Poor spheroidization and recession

These defects affect performance and increase scrap rates. To improve quality and reduce waste, it's essential to analyze causes, control influencing factors, and apply precision casting techniques.

1. Shrinkage and porosity

1.1. Several factors influence the formation of shrinkage cavities and porosity in ductile iron castings:

1. Carbon Equivalent (CE)

• Higher carbon content increases graphitization expansion and improves fluidity, reducing shrinkage.

• Ideal formula: C% + 1/7Si% > 3.9%

• Overly high CE may cause graphite floating.

  
  

2. Phosphorus (P)

• Increases the solidification range and weakens the casting shell, leading to more shrinkage defects.

• Should be controlled below 0.08%.

3. Rare Earth & Magnesium (RE & Mg)

• Excessive RE and Mg can reduce graphite spheroidization and promote white structure, increasing shrinkage risks.

4. Wall Thickness

• Thick sections retain heat longer, increasing liquid shrinkage.

• Sudden changes in wall thickness can lead to isolated hot spots and poor feeding.

5. Pouring Temperature

• Must balance fluidity and shrinkage:
  1300–1350°C is generally appropriate.

• Too high increases liquid shrinkage; too low risks poor filling.

6. Sand Mold Compactness

• Low or uneven compactness can cause cavity expansion, leading to inadequate feeding and shrinkage.

7. Pouring & Feeding System Design

• Improper riser, gating, or chill setup disrupts sequential solidification and reduces feeding effectiveness.

1.2. To reduce shrinkage cavities and porosity in ductile iron, the following preventive strategies should be implemented:

1. Control Molten Iron Composition

• High carbon equivalent: > 3.9%

• Low phosphorus: < 0.08%

• Low residual magnesium: < 0.07%

• Use rare earth-magnesium alloy and control residual rare earth oxides at 0.02–0.04%

2. Optimized Process Design

• Ensure continuous feeding from risers during solidification.

• Use proper riser size and quantity to enable sequential solidification.

3. Use of Chills and Subsidies

• Apply as needed to adjust temperature distribution, promoting directional solidification.

4. Pouring Parameters

• Pouring temperature: 1300–1350°C

• Complete pouring within 25 minutes to avoid spheroidization recession.

5. Improve Mold Quality

• Sand compactness: ≥ 90

• Ensure uniform ramming and proper moisture content to maintain mold rigidity.

2. Slag inclusion

2.1. Slag inclusion is a common defect in ductile iron, mainly caused by chemical and physical conditions during melting and pouring. Key influencing factors include:

1. Silicon (Si)

• Silicon oxide is a major slag component.

• Excess silicon increases slag formation risk.

2. Sulphur (S)

• Forms low-melting-point sulphurate that increase molten iron viscosity, hindering slag separation.

• Sulphur should be kept below 0.06%.

• Levels above 0.09–0.135% sharply increase slag inclusions.

3. Rare Earth & Magnesium (RE & Mg)

• Oxidation of these elements leads to slag formation.

• Excess residual RE and Mg should be avoided.

4. Pouring Temperature

• Too low: High viscosity prevents slag and oxides from rising.

• Too high: Surface slag becomes thin and flows into the mold.

• Proper temperature should consider carbon-silicon balance.

5. Pouring System Design

• Must include slag-blocking features.

• Should minimize splashing and turbulence for smooth mold filling.

6. Molding Sand Quality

• Adhered sand or coating can combine with oxides to form slag.

• Uneven compactness leads to metal erosion, creating low-melting-point compounds that cause slag inclusion.

2.2. To minimize slag inclusion defects, the following measures should be implemented:

1. Molten Iron Composition Control

• Keep Sulphur content < 0.06%

• Add 0.1–0.2% rare earth alloy to purify molten iron

• Reduce silicon and residual magnesium levels

2. Optimized Smelting Process

• Increase tapping temperature to aid inclusion separation

• Perform sedimentation (sedation) to allow inclusions to float

• Remove surface slag and apply covering agents (e.g., perlite or plant ash) to prevent oxidation

• Use a pouring temperature ≥ 1350°C

3. Improved Pouring System Design

• Ensure smooth metal flow

• Incorporate slag collecting bags and slag-blocking devices (e.g., slag filters)

• Prevent sand erosion in the sprue

4. Mold Quality Assurance

5. Maintain uniform compactness and adequate strength

• Clean mold cavity thoroughly before closing the box to prevent sand or debris-related slag

3. Graphite Floating

3.1. Graphite floating occurs when excess graphite separates from molten iron and rises to the upper part of the casting, leading to non-uniform structure and weakened mechanical properties. Key factors include:

1. Carbon Equivalent (CE)

• Main cause of graphite floating.

• High CE → excessive graphite precipitation → graphite rises due to lower density and magnesium vapor.

• Larger and thicker castings are more prone to this defect.

2. Silicon (Si)

• Lowering silicon content (at a fixed CE) helps reduce floating tendency.

3. Rare Earth Content (RE)

• Low RE reduces carbon solubility, leading to more graphite precipitation and aggravated floating.

4. Spheroidizing & Inoculation Temperature

• Optimal range: 1380–1450°C

• Higher temperature within this range improves Mg and RE absorption, reducing segregation.

5. Pouring Temperature

• Higher pouring temperature increases graphite floating due to prolonged molten state and more graphite formation.

6. Retention Time After Inoculation

• Delays between inoculation and pouring allow graphite to form and float.

• Should be kept under 10 minutes to minimize risk.