With the rapid growth of the global wind power industry, the production of wind power castings has gained significant momentum in China's foundry sector. However, the stringent quality standards required for these components pose a major challenge to the technical and managerial capabilities of foundry workers. Wind power castings operate in harsh environments, enduring prolonged exposure to wind, sun, and rain. Once in service, they must remain defect-free for at least 20 years, making their quality requirements comparable to those of nuclear and aerospace castings.
Wind power castings typically include blade hubs, gearboxes, mechanical frames, and base components. Each 1–2 MW unit requires approximately 15 tons of castings, while units between 3–5 MW need 35 to 50 tons. The materials used—such as QT400-18AL, QT700-2, and QT350-22AL—must meet strict criteria for tensile strength, yield strength, elongation, impact toughness, and hardness. These castings must also exhibit high internal and external integrity and density, with no welding repairs allowed. They undergo rigorous testing, including ultrasonic and magnetic particle inspections. Additionally, QT400-18AL must withstand low-temperature impacts at -20°C, while QT350-22AL must perform at -40°C.
The smelting and processing of molten iron are critical steps in producing large-section ductile iron castings for wind power applications. This article focuses on the control of smelting technology in the production of QT400-18AL and QT350-22AL castings.
**1. Raw Material Selection**
**1.1 Pig Iron**
To ensure that the mechanical properties of wind power casting materials meet standard requirements, Q4–Q10 grade pig iron is typically selected. The chemical composition must be strictly controlled, as shown in Table 1.
**2. Target Molten Iron Composition Selection**
The main elements in wind power ductile iron castings include carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), magnesium (Mg), and rare earth (RE). Their selection ranges are as follows:
**2.1 Carbon (C)**
Given the large size of wind power castings and the need for low-temperature performance, the C content must ensure full graphitization and prevent graphite floating. The raw material C range is typically 3.5–3.8%, with lower limits for thick sections and higher limits for thinner ones.
**2.2 Silicon (Si)**
Si promotes graphitization but should be carefully controlled. Higher Si levels increase brittle transition temperatures, so wind power castings requiring low-temperature impact resistance must maintain tight Si control. For example:
- QT400-18AL: 1.8–2.1%
- QT700-2: 2.0–2.3%
- QT350-22AL: 1.7–2.0%
**2.3 Manganese (Mn)**
High Mn increases pearlite content and can cause segregation, reducing ductility. Mn levels are generally kept below 0.3%, with QT350-22AL requiring less than 0.2%.
**2.4 Phosphorus (P)**
P forms harmful eutectics and reduces low-temperature impact properties. P content must be below 0.03%.
**2.5 Sulfur (S)**
S is anti-spheroidizing and reduces spheroidization efficiency. While S < 0.02% is ideal, it should not be too low (not less than 0.006%).
**2.6 Magnesium (Mg)**
Mg is the primary spheroidizing agent. To avoid adverse effects on toughness and shrinkage, Mg content is usually maintained between 0.040% and 0.060%.
**2.7 Rare Earth (RE)**
RE helps remove impurities and supports spheroidization. Light RE should be used cautiously, especially when S is low, to avoid inclusions. Heavy RE is preferred. RE levels should be below 0.025%.
**3. Smelting Technology Control**
The quality of the original molten iron significantly affects the performance of ductile iron and the occurrence of casting defects. Key factors include chemical composition control, equipment selection, and process optimization.
**3.1 Original Molten Iron Composition**
Based on the casting requirements, the chemical composition of the original molten iron is outlined in Table 4.
**3.2 Smelting Equipment Selection**
Two common options are cupola + induction furnace or direct induction melting. Cupola-based systems offer high efficiency and better graphite refinement, making them suitable for pig iron-based production. Induction melting is simpler and stable, ideal for scrap steel-heavy processes.
**3.3 Covering, Inoculation, and Treatment**
Tapping temperature should be between 1520–1540°C to maintain activity. Spheroidizing packages must be clean and preheated to 600°C or more. Spheroidizing agents are added in stages, followed by inoculants and covering agents. Temperature is maintained at 1420–1460°C during treatment, with multiple inoculations during pouring to refine microstructure.
**4. Conclusion**
As wind power casting production scales up, competition intensifies. Improving foundry technology and reducing costs and environmental impact are essential for sustainable development. This article highlights key aspects of smelting control in wind power ductile iron castings, aiming to support industry professionals.
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