In this study, we investigate the sand casting process for high-pressure steam chamber steel castings, which are critical components in steam turbines operating under extreme conditions of high temperature and pressure. The objective is to design an optimized casting process that minimizes defects such as shrinkage porosity and cavities, ensuring the integrity and longevity of the castings. We employ numerical simulation using ProCAST software to analyze the filling and solidification stages, and based on the results, we propose modifications to the initial sand casting design. The focus is on leveraging sand casting techniques to achieve high-quality castings, with repeated emphasis on the sand casting methodology throughout the research.
The high-pressure steam chamber is subjected to service temperatures of approximately 565°C and water pressures of 8.28 MPa, requiring leak-free operation for extended periods. The material used is ZG15Cr2Mo1 alloy steel, which offers excellent high-temperature strength and corrosion resistance. Traditional sand casting approaches often involve iterative trial-and-error methods, leading to increased costs and extended development cycles. By integrating numerical simulation into the sand casting process, we aim to streamline the design phase, reduce material waste, and enhance the overall efficiency of sand casting production for complex steel components.
Structural Characteristics and Requirements of the Casting
The high-pressure steam chamber casting features a complex geometry with dimensions of 1648 mm × 620 mm × 1077 mm, a volume of 0.251 m³, and a mass of 1957.8 kg. The wall thickness varies significantly, with a maximum of 153 mm and a minimum of 30 mm, which aligns with standard sand casting guidelines for steel castings where the minimum wall thickness is typically 20 mm. This variation necessitates careful design to prevent defects inherent in sand casting processes, such as shrinkage and porosity in thicker sections. The material composition of ZG15Cr2Mo1 is critical for performance, and its properties influence the sand casting parameters. Table 1 summarizes the chemical composition of the alloy, which dictates the liquidus temperature of 1501°C and a linear shrinkage rate of 1.8% during solidification in sand casting.
| Element | C | Mn | Si | Cr | Mo | S | P |
|---|---|---|---|---|---|---|---|
| Content (%) | ≤0.18 | 0.40–0.70 | ≤0.60 | 2.00–2.75 | 0.90–1.20 | ≤0.030 | ≤0.030 |
Wall thickness analysis confirms that the casting meets the minimum and critical thickness requirements for sand casting, but the presence of thick sections, such as the base and top bosses, introduces challenges in achieving uniform solidification. These areas are prone to thermal hotspots, which can lead to defects if not properly addressed through sand casting design optimizations. The sand casting process must account for these variations to ensure dimensional accuracy and mechanical properties, as per casting standards like CT13 for dimensional tolerances and MT12 for weight tolerances in sand casting.
Casting Process Design for Sand Casting
In sand casting, the selection of the pouring position and parting plane is crucial for minimizing defects and simplifying mold assembly. After evaluating vertical and horizontal pouring options, we opted for a horizontal pouring position in the sand casting process. This choice facilitates the use of a parting plane at the maximum cross-section, easing pattern removal and reducing the risk of misalignment during mold closing. Additionally, the horizontal orientation allows for effective placement of feeders and chills in the sand casting mold to address shrinkage issues in thick sections.
The mold design in sand casting utilizes phenolic resin-bonded sand for both the mold and cores, with a high silica content (≥97%) to withstand the high pouring temperatures of steel. The cores are designed to form the internal cavities of the steam chamber, consisting of three separate sand cores: two for the main chamber and one for the external shell openings. This core arrangement in sand casting ensures precise internal geometry and reduces the need for additional supports, such as chaplets, which could introduce weaknesses. The coating applied is alumina-based alcohol paint, applied via spraying for uniform coverage and enhanced surface quality in the sand casting process.
Key process parameters in sand casting include the pouring temperature, pouring time, and gating system design. The pouring temperature is set at 1600°C to ensure fluidity while minimizing thermal shock to the sand mold. The pouring time is calculated based on the total molten metal weight, including allowances for mold expansion. The initial weight of the casting is 1957.8 kg, with a 7% allowance for mold dilation, resulting in a total molten metal weight \( G_L = 2094.846 \, \text{kg} \). Using one ladle with one nozzle of 70 mm diameter, the pouring rate \( v_{\text{ladle}} \) is 120 kg/s. The pouring time \( t \) is derived from the equation:
$$ t = \frac{G_L}{N \cdot n \cdot v_{\text{ladle}}} $$
where \( N = 1 \) (number of ladles) and \( n = 1 \) (number of nozzles). Substituting the values, we get:
$$ t = \frac{2094.846}{1 \cdot 1 \cdot 120} \approx 17.46 \, \text{s} $$
In practice, we round this to 18 s for the sand casting process. To verify the metal rise velocity \( v_L \), we use the casting height \( h_C = 620 \, \text{mm} \):
$$ v_L = \frac{h_C}{t} = \frac{620}{18} \approx 34.44 \, \text{mm/s} $$
This meets the required rise velocity of 35 mm/s for complex steel castings in sand casting, ensuring adequate mold filling. The gating system is designed as an open type with a ratio of cross-sectional areas for the ladle gate, sprue, runner, and ingate as \( 1.0 : 1.9 : 1.9 : 2.2 \). Three ingates are evenly distributed at the parting plane to promote uniform temperature distribution during the sand casting filling stage. The minimal residual head \( h_M \) is calculated to ensure sufficient pressure for filling distant sections:
$$ h_M = L \tan \alpha $$
where \( L = 790 \, \text{mm} \) (horizontal distance) and \( \alpha = 6^\circ \). Thus, \( h_M = 790 \cdot \tan(6^\circ) \approx 83 \, \text{mm} \), which satisfies the condition for preventing mistruns in sand casting.
Numerical Simulation Setup in ProCAST for Sand Casting
We use ProCAST software to simulate the sand casting process, focusing on filling and solidification behaviors. The model is meshed with finite elements to capture the complex geometry, as illustrated in the grid division of the casting system. The initial parameters for the sand casting simulation include a pouring temperature of 1600°C, pouring time of 18 s, and a heat transfer coefficient between the metal and sand mold of \( h = 750 \, \text{W/(m}^2 \cdot \text{K)} \). These settings replicate real-world sand casting conditions, allowing us to predict potential defects and optimize the process.
The simulation begins with the filling phase, where the molten metal flows into the mold cavity. The results show a stable filling process without any short runs, confirming the effectiveness of the gating design in sand casting. However, the solidification analysis reveals significant shrinkage defects in the thick sections of the steam chamber base and top bosses, as well as dispersed microporosity in other areas. These defects are common in sand casting due to uncontrolled solidification in isolated liquid zones. The shrinkage porosity volume is quantified, highlighting the need for modifications in the sand casting process.
Analysis of Initial Simulation Results in Sand Casting
The initial sand casting simulation indicates that while filling is complete, the solidification stage leads to volumetric shrinkage cavities and dispersed porosity. The defects are primarily located in regions with high thermal mass, where cooling rates are slower, causing liquid metal to shrink without adequate compensation. In sand casting, this is addressed by implementing feeders and chills to promote directional solidification. The simulated defect distribution is summarized in Table 2, which categorizes the types and locations of defects observed in the sand casting process.
| Defect Type | Location | Severity | Probable Cause in Sand Casting |
|---|---|---|---|
| Shrinkage Cavity | Base Thick Sections | High | Isolated Liquid Zones |
| Shrinkage Porosity | Top Bosses | Medium | Insufficient Feeding |
| Microporosity | General Areas | Low | Rapid Solidification |
The presence of these defects underscores the limitations of the initial sand casting design, particularly in handling thermal gradients. The sand casting process must be refined to ensure that solidification proceeds from the extremities toward the feeders, minimizing shrinkage. The simulation results provide a basis for quantifying the improvements, with the initial total shrinkage porosity measured at approximately 3.5% of the casting volume, which exceeds acceptable limits for high-integrity sand casting components.
Process Optimization in Sand Casting
Based on the simulation findings, we modify the sand casting process by adding feeders and chills to the defect-prone areas. Feeders are placed at four key locations: the thick sections of the base and the top bosses. The feeder dimensions are determined using the modulus method, which calculates the feeder size based on the cooling characteristics of the casting in sand casting. The modulus \( M \) for a section is given by:
$$ M = \frac{V}{A} $$
where \( V \) is the volume and \( A \) is the surface area. For the base thick section, \( M \approx 0.05 \, \text{m} \), leading to feeder dimensions of 200 mm diameter and 300 mm height. Additionally, chills are installed in the inner cavities of the base and near the partition walls of the steam inlets to accelerate cooling in these regions. The chills are made of cast iron with high thermal conductivity, enhancing heat extraction in the sand casting process.
The modified sand casting design is re-simulated in ProCAST, and the results show a significant reduction in defects. The shrinkage cavities are now concentrated in the feeders, with the casting itself exhibiting minimal porosity. The total shrinkage porosity decreases to 2.71%, demonstrating the effectiveness of the optimizations in sand casting. However, the use of chills in the partition walls causes earlier solidification, leading to increased porosity in the top bosses due to reversed feeding patterns. This indicates that further refinements, such as adjusting feeder sizes or incorporating insulating materials in sand casting, may be necessary to achieve optimal results.
Discussion on Sand Casting Improvements
The integration of numerical simulation into the sand casting process allows for a systematic approach to defect reduction. By identifying thermal hotspots and modifying the sand casting design proactively, we minimize the need for physical prototypes. The use of feeders and chills in sand casting is a well-established technique, but their placement and sizing require precise calculation to avoid overcompensation or new defect formations. In this study, the sand casting process benefits from the iterative simulation, which highlights the interplay between cooling rates and solidification sequences.
Future work in sand casting could explore advanced materials for molds and cores, such as zirconia-based sands, to further enhance thermal resistance and reduce defects. Additionally, optimizing the gating system ratios or employing vacuum-assisted sand casting could improve metal flow and reduce turbulence. The sand casting process for high-pressure components like the steam chamber demands continuous improvement to meet stringent quality standards, and simulation tools like ProCAST play a pivotal role in achieving this goal.
Conclusion
In this research, we successfully designed and optimized a sand casting process for high-pressure steam chamber steel castings using numerical simulation. The initial sand casting方案 revealed significant shrinkage defects in thick sections, which were mitigated through the addition of feeders and chills. The modified sand casting process resulted in a casting with reduced porosity and no major shrinkage cavities, confirming the value of simulation-driven design in sand casting. The sand casting methodology proved effective in handling the complexities of large steel castings, and further enhancements can be pursued to eliminate residual microporosity. Overall, this study underscores the importance of sand casting optimizations for producing high-integrity components in demanding applications.