As a key component in large-scale wind turbines, the bearing block is typically manufactured from ductile iron and must endure harsh operating conditions, including corrosion, wind sand, humidity, and low temperatures. These castings face complex loading scenarios and challenging installation and maintenance requirements, necessitating exceptionally high-quality standards. Specifically, they must exhibit high fatigue strength, low susceptibility to brittle fracture, and reliable performance over a design life of 20 years. In our production experience, a batch of wind turbine bearing block castings suffered from severe slag inclusion defects, leading to the scrapping of over a hundred tons of material and direct economic losses exceeding one million yuan. This incident prompted a comprehensive investigation into the root causes and effective countermeasures for such defects.
Slag inclusions refer to non-metallic solid phases, such as molten slag or metal oxides, embedded within or on the surface of castings. These defects severely degrade mechanical properties, including toughness and yield strength, compromising the structural integrity of the component. Theoretically, the journey from iron melting to casting is a complex, irreversible process where complete elimination of oxidized slag during high-temperature operations is virtually unattainable. Therefore, preventive strategies extend beyond enhancing pouring temperature, refining iron melting quality, reducing residual magnesium, and thorough slag removal to include the rational design of gating systems. Our company has established mature manufacturing processes and operational techniques for high-quality ductile iron melting. This study focuses on designing experimental schemes for gating systems that complement melting processes, aiming to leverage the slag avoidance and trapping functions of the gating system to prevent slag inclusions in wind turbine bearing block castings.
1. Technical Requirements and Casting Characteristics Analysis
The bearing block castings are made of material QT400-18AL (EN-GJS-400-18U-LT), with varying specifications produced in our facility. The minimum outline dimensions are 2000 mm × 1240 mm × 500 mm, and the maximum are 3000 mm × 1800 mm × 500 mm, with weights ranging from approximately 2200 kg to 4200 kg. Structurally, these castings feature a high dynamic load zone where 60 mm thick ribs intersect with a 350 mm thick flange, connected to critical bearing installation mating surfaces. This configuration results in complex molten iron flow during mold filling. Additionally, a central “U”-shaped groove acts as a pouring dead zone, prolonging flow paths and hindering slag flotation. Since the important mating surfaces are on the upper plane of the pouring position and the high dynamic load zones are non-machined, any surface slag inclusion layers cannot be removed by machining. Consequently, the casting is highly sensitive to the slag trapping and avoidance capabilities of the gating system.
2. Analysis of Slag Inclusion Formation Causes
Slag inclusions in ductile iron melts arise primarily from two pathways: exogenous slag (primary slag) and endogenous slag (secondary slag). Exogenous slag originates from external contaminants introduced during melting, tapping, spheroidization, inoculation, slag skimming, or pouring, forming secondary oxidation products entrained into the molten iron. Endogenous slag stems from in-situ reactions during desulfurization and spheroidization processes. The formation of slag inclusions involves intricate physicochemical reactions, and practical manufacturing often relies on qualitative analysis based on operational experience, defect location, and morphology.
We conducted macro- and micro-analyses on visible slag inclusion defects. Macroscopically, these defects appear as irregular clusters with slightly darker boundaries than the base metal, featuring gray-black pores and non-dense structures at their centers. Samples were mechanically extracted for scanning electron microscopy (SEM) observation, and energy-dispersive X-ray spectroscopy (EDS) was performed for composition analysis. The results are summarized in Table 1.
| Element | C | O | Mg | Al | Si | Ca | Mn | Fe | Others |
|---|---|---|---|---|---|---|---|---|---|
| Wt.% | 7.00 | 29.45 | 0.52 | 18.92 | 10.61 | 1.17 | 1.59 | 28.63 | 2.11 |
| At.% | 14.06 | 44.40 | 0.51 | 16.91 | 9.11 | 0.70 | 0.70 | 12.37 | 1.24 |
The morphology reveals cluster-like defects with rough pores, often partially detached, leaving irregular cavities. Some defect boundaries display bright metallic luster with dendritic structures, indicative of成分偏析 from inoculation and spheroidization. The high concentrations of C, O, Al, and Si confirm the presence of metal oxides like SiO₂, MgO, and Al₂O₃, aggregated as slag inclusions. Thus, these slag inclusions are predominantly exogenous, formed from secondary oxides of pollutants entrained in the pouring molten iron due to inadequate removal prior to mold filling.
The formation of oxide slag can be described thermodynamically. For instance, the oxidation of silicon in molten iron follows:
$$ \text{Si} + \text{O}_2 \rightarrow \text{SiO}_2 $$
The Gibbs free energy change dictates spontaneity:
$$ \Delta G = \Delta H – T \Delta S $$
where \( \Delta G \) is the Gibbs free energy change, \( \Delta H \) is the enthalpy change, \( T \) is the temperature in Kelvin, and \( \Delta S \) is the entropy change. Negative \( \Delta G \) values at high temperatures favor oxide formation, underscoring the need to control oxygen exposure to minimize endogenous slag inclusions.
3. Gating System Optimization Design and Experimental Verification
3.1 Gating System Scheme Design
To isolate the impact of the gating system, we kept all other casting and melting parameters constant while varying the gating design. Three experimental schemes were formulated, as detailed in Table 2.
| Scheme No. | Gating System Type | Pouring Cup Type | Filtration | Casting Serial Nos. | Key Features |
|---|---|---|---|---|---|
| 1# | Slit-type ingate | Transfer ladle + funnel | No | 1~3 | Traditional slit design for rapid filling |
| 2# | Circular ceramic tube ingate | Transfer ladle + funnel | No | 4~6 | Open system with ceramic tubes for平稳 flow |
| 3# | Circular ceramic tube ingate | Basin-shaped | Yes | 7~9 | Includes foam ceramic filter and basin cup |
Fluid dynamics principles guide gating system design. Bernoulli’s equation for incompressible fluids helps determine flow characteristics:
$$ P_1 + \frac{1}{2} \rho v_1^2 + \rho g h_1 = P_2 + \frac{1}{2} \rho v_2^2 + \rho g h_2 $$
where \( P \) is pressure, \( \rho \) is density, \( v \) is velocity, \( g \) is gravity, and \( h \) is height. Minimizing turbulence is crucial to prevent slag entrainment. The critical velocity for slag entrainment can be approximated as:
$$ v_c = \sqrt{\frac{2 \sigma}{\rho r}} $$
with \( \sigma \) as the slag-metal surface tension, \( \rho \) as metal density, and \( r \) as slag particle size. Maintaining flow below \( v_c \) in the gating system promotes slag separation.
3.2 Experimental Implementation and Verification
Each scheme was used to produce three castings under identical production conditions. The results are summarized in Table 3.
| Scheme No. | Number of Castings | Defect Rate (%) | Severity of Slag Inclusions | Observations |
|---|---|---|---|---|
| 1# | 3 | 100 | Severe, widespread on upper surface | Slit ingate caused slag entrainment; prolonged pouring led to冲砂 |
| 2# | 3 | 66.7 | Moderate, scattered in 0-15 mm depth | Open system reduced turbulence but insufficient slag trapping |
| 3# | 3 | 0 | None | Filter and basin cup effectively eliminated slag inclusions |
For Scheme 1#, the slit-type ingate allowed slag to be “sucked” into the mold due to inadequate flotation time, resulting in severe slag inclusions. Scheme 2# with circular ceramic tubes provided smoother flow but still permitted some slag inclusions. Scheme 3# incorporated a basin-shaped pouring cup with a dam to regulate flow and a foam ceramic filter (150 mm × 150 mm × 25 mm, 10 PPI) in a slag collection chamber. This design ensured molten iron passed uniformly through the filter, trapping slag particles. The flotation velocity of slag particles is given by Stokes’ law:
$$ v = \frac{2}{9} \frac{(\rho_s – \rho_f) g r^2}{\mu} $$
where \( v \) is terminal velocity, \( \rho_s \) is slag density, \( \rho_f \) is iron density, \( g \) is gravity, \( r \) is particle radius, and \( \mu \) is dynamic viscosity. The basin cup and filter provided adequate residence time for slag flotation, preventing slag inclusions.
4. Melting Process Control for Minimizing Slag Inclusions
While gating system optimization is vital, melt quality control is equally important to reduce slag formation. Our melting process employs high-purity charge materials and a reducing atmosphere in induction furnaces to limit oxidation. Spheroidization and inoculation are carefully controlled to avoid excessive slag generation. Reactions during magnesium treatment include:
$$ \text{Mg} + \text{S} \rightarrow \text{MgS} $$
$$ \text{Mg} + \text{O} \rightarrow \text{MgO} $$
Both MgS and MgO can form slag inclusions if not removed. We maintain residual magnesium levels at 0.03–0.05% and use slag modifiers like calcium compounds to aggregate slag for easier removal. Slag removal efficiency \( \eta \) is quantified as:
$$ \eta = \frac{C_0 – C_f}{C_0} \times 100\% $$
with \( C_0 \) and \( C_f \) as initial and final slag concentrations. Our process achieves \( \eta > 90\% \), significantly lowering slag content before pouring.
5. Effect of Pouring Temperature on Slag Inclusions
Pouring temperature influences fluidity, solidification time, and slag behavior. We conducted experiments to determine the optimal temperature, with results in Table 4.
| Pouring Temperature (°C) | Number of Castings | Slag Inclusion Defect Rate (%) | Average Defect Size (mm) |
|---|---|---|---|
| 1300 | 5 | 80 | 5.2 |
| 1350 | 5 | 40 | 3.1 |
| 1400 | 5 | 20 | 1.8 |
| 1450 | 5 | 60 | 4.5 |
A pouring temperature of 1400°C yielded the lowest defect rate, balancing fluidity and oxidation. The relationship between temperature \( T \) and slag inclusion probability \( P \) can be modeled empirically:
$$ P = a T^2 + b T + c $$
where \( a, b, c \) are constants. For our process, 1400°C corresponds to the minimum of this quadratic function, optimizing slag flotation while maintaining mold filling.
6. Filtration Technology for Slag Removal
Ceramic foam filters are highly effective for removing slag inclusions. The capture efficiency \( E \) of a filter is expressed as:
$$ E = 1 – \exp(-k L) $$
where \( k \) is a filter coefficient dependent on pore structure and particle size, and \( L \) is filter thickness. Our 10 PPI foam ceramic filters offer a balance between flow rate and filtration. The pressure drop \( \Delta P \) across the filter follows the Darcy-Forchheimer equation:
$$ \Delta P = \frac{\mu v L}{K} + \beta \rho v^2 L $$
with \( \mu \) as viscosity, \( v \) as velocity, \( K \) as permeability, \( \beta \) as inertial coefficient, and \( \rho \) as density. Proper housing design ensures minimal flow impedance while achieving high filtration efficiency, crucial for preventing slag inclusions.
7. Conclusion
Through systematic analysis and experimental validation, we have demonstrated that slag inclusions in wind turbine bearing block castings primarily originate from exogenous sources, specifically secondary oxides entrained during pouring. The optimized gating system design, featuring a “filter slag collection + open bottom-return gating system + basin-shaped pouring cup,” effectively eliminates slag inclusions by promoting slag flotation and trapping. This solution has been implemented in mass production, with over 200 sets of castings produced without any slag-related rejections, ensuring stable quality and significant economic savings. Future work may involve advanced filtration materials, computational fluid dynamics simulations for gating design, and real-time monitoring systems to further enhance defect prevention. By integrating melt control with innovative gating techniques, we can consistently produce high-integrity castings that meet the rigorous demands of wind energy applications, thereby mitigating the risks associated with slag inclusions.