With the rapid growth of the wind power industry worldwide, the production of wind power castings has gained significant momentum in China's foundry sector. However, the stringent quality requirements for these components pose a major challenge to the technical and managerial capabilities of foundry workers. Wind power castings operate in harsh environments, exposed to wind, sun, and rain over long periods. Once in service, they must function without any quality issues for at least 20 years. As a result, their quality standards are comparable to those of nuclear and aerospace castings.
Wind power castings include critical parts such as blade hubs, gearboxes, mechanical frames, and base members. A typical 1–2 MW unit requires approximately 15 tons of castings, while a 3–5 MW unit demands 35 to 50 tons. The materials used—such as QT400-18AL, QT700-2, and QT350-22AL—must meet strict mechanical properties, including tensile strength, yield strength, elongation, impact toughness, and hardness. These materials also need to have high internal and external integrity and density. Importantly, no welding repairs are allowed, and all castings must undergo rigorous ultrasonic and magnetic particle testing. Additionally, QT400-18AL must perform well at -20°C, while QT350-22AL must withstand temperatures as low as -40°C.
The smelting and processing of molten iron is a crucial and technically challenging step in producing large-section ductile iron castings for wind power applications. This paper focuses on the control of smelting technology in the production of QT400-18AL and QT350-22AL castings.
**Raw Material Selection**
**Pig Iron**
To ensure that the mechanical properties of wind power casting materials meet standard requirements, pig irons of grades Q4–Q10 are typically selected. The chemical composition of the alloy must be strictly controlled, with specific ranges outlined in Table 1.
**Target Molten Iron Composition**
The main chemical elements in wind power ductile iron include carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), magnesium (Mg), and rare earth (RE). Each element plays a vital role in determining the final properties of the casting.
- **Carbon (C)**: To ensure full graphitization and prevent graphite floating, the C content should be carefully controlled. The range is typically 3.5–3.8%, depending on the section thickness.
- **Silicon (Si)**: While Si promotes graphitization, excessive amounts can increase the brittle transition temperature. The Si content varies by grade: 1.8–2.1% for QT400-18AL, 2.0–2.3% for QT700-2, and 1.7–2.0% for QT350-22AL.
- **Manganese (Mn)**: High Mn levels can lead to pearlite formation and reduce ductility. Generally, Mn should be kept below 0.3%, with stricter limits for QT350-22AL (below 0.2%).
- **Phosphorus (P)**: P is harmful, as it reduces low-temperature impact performance. It must be controlled below 0.03%.
- **Sulfur (S)**: S affects spheroidization and must be kept below 0.02%, but not too low (minimum 0.006%) to support inoculation.
- **Magnesium (Mg)**: Mg is essential for spheroidization, but excessive amounts can affect toughness and shrinkage. The ideal range is 0.040–0.060%.
- **Rare Earth (RE)**: RE helps purify the molten iron and assist spheroidization. However, its level must be carefully managed to avoid inclusions. The recommended range is less than 0.025%.
**Smelting Technology Control**
The quality of the original molten iron significantly impacts the performance of ductile iron and the occurrence of casting defects. Key factors include chemical composition control, equipment selection, and the melting process.
**Chemical Composition of Original Molten Iron**
Based on the required casting specifications, the chemical composition of the original molten iron is detailed in Table 4.
**Selection of Smelting Equipment**
Two common options are cupola plus induction furnace or direct induction furnace smelting. Cupola-based systems offer high efficiency and better graphite refinement, making them suitable for pig iron-based production. Induction furnace systems are simpler and more stable, ideal for scrap steel-heavy processes.
**Covering, Inoculation, and Treatment**
Proper covering, inoculation, and treatment are essential for achieving desired microstructure and mechanical properties.
- **Tapping Temperature**: The tapping temperature should be maintained between 1520–1540°C to ensure optimal spheroidization.
- **Spheroidizing Package**: The package must be properly prepared, baked, and covered to improve absorption and reduce impurities.
- **Spheroidization and Inoculation**: Multiple inoculation steps are often used to refine the structure. The spheroidizing agent is added in stages, followed by inoculant to enhance nucleation and ensure uniform microstructure.
In conclusion, with the growing demand for wind power castings, improving foundry technology and reducing environmental impact are essential for sustainable development. This article provides insights into the smelting process and aims to assist foundry professionals in enhancing their production practices.
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