In the context of global carbon reduction strategies, the steel casting industry faces increasing pressure to enhance efficiency and reduce material consumption. As a key component in heavy equipment manufacturing, steel castings often rely on risers to compensate for shrinkage during solidification. Traditional riser design methods, however, may not fully account for the influence of thermal convection in molten steel, leading to over-designed risers and unnecessary material waste. This study investigates the impact of thermal convection on riser shrinkage in steel castings using numerical simulation, with the aim of optimizing riser design and improving material utilization rates. Through a combination of simulation analysis and physical validation, we explore how thermal convection affects shrinkage cavity formation in risers of varying sizes, providing insights for more sustainable steel casting practices.
Thermal convection in molten steel is driven by temperature gradients within the casting. After mold filling and before solidification, hotter, less dense steel at the core rises, while cooler, denser steel at the edges sinks, creating a convective flow. This phenomenon can significantly alter the morphology and height of shrinkage cavities in risers. In steel casting, which typically exhibits directional solidification, risers are placed at the thickest sections to ensure sound casting quality. Understanding and leveraging thermal convection can thus lead to more precise riser sizing and reduced material usage. This research employs advanced numerical simulation software to model thermal convection effects, comparing scenarios with and without convection calculations for different riser specifications.
The numerical simulation approach used in this study is based on finite difference methods, which solve the heat transfer and fluid flow equations during solidification. The governing equations for thermal convection include the energy equation and the Navier-Stokes equations. The energy equation accounts for heat transfer due to conduction and convection:
$$ \frac{\partial T}{\partial t} + \mathbf{u} \cdot \nabla T = \alpha \nabla^2 T $$
where \( T \) is the temperature, \( \mathbf{u} \) is the velocity vector of the molten steel, and \( \alpha \) is the thermal diffusivity. The momentum equation incorporates buoyancy forces driven by density variations:
$$ \rho \left( \frac{\partial \mathbf{u}}{\partial t} + \mathbf{u} \cdot \nabla \mathbf{u} \right) = -\nabla p + \mu \nabla^2 \mathbf{u} + \rho \mathbf{g} $$
where \( \rho \) is density, \( p \) is pressure, \( \mu \) is dynamic viscosity, and \( \mathbf{g} \) is gravity. In the simulation, these equations are coupled with a mass balance that includes feeding flow to model shrinkage compensation. The software tracks virtual tracer particles to visualize flow patterns, as shown in the simulation results for different riser sizes.
For this study, three riser sizes made of low-alloy steel (ZG20Mn) were selected, representing common specifications in hydroelectric valve steel castings. The riser dimensions are summarized in Table 1. Each riser was simulated with and without thermal convection calculations, and the results were validated through physical dissection to measure actual shrinkage cavity heights. The simulation parameters included a pouring temperature of 1550°C, with insulation sleeves or bricks modeled using appropriate thermal properties. The heat transfer coefficient between the riser and insulation was defined as a function of temperature, decreasing from 940 W/(m²·K) near the pouring temperature to 104 W/(m²·K) below 1000°C.
| Riser ID | Diameter (mm) | Height (mm) |
|---|---|---|
| 1 | 300 | 450 |
| 2 | 600 | 900 |
| 3 | 1014 | 1100 |
The simulation results for the 300 mm diameter riser revealed that thermal convection had a modest effect on shrinkage cavity height. Without insulation, the shrinkage height increased from 10 mm (without convection) to 24 mm (with convection), a gain of 14 mm. With insulation, the height increased from 100 mm to 135 mm, a 35 mm improvement. Physical dissection confirmed these trends, showing that insulation had a more pronounced impact than convection for small risers in steel casting. This suggests that for smaller risers, the benefits of thermal convection are secondary to insulation effects.
For the 600 mm diameter riser, thermal convection played a more significant role. With insulation bricks applied, the simulated shrinkage height rose from 370 mm (without convection) to 480 mm (with convection), an increase of 110 mm. Physical examination showed an actual shrinkage height of 450 mm, closely matching the convection simulation. The error rate was only 6.7%, indicating that convection calculations improve accuracy for medium-sized risers in steel casting. The shrinkage volume remained similar in both simulations, but the cavity morphology shifted from a deep “V” shape to a shallower one, reducing the risk of defects in the casting body.
| Condition | Actual Height (mm) | Simulated with Convection (mm) | Simulated without Convection (mm) | Height Gain (mm) |
|---|---|---|---|---|
| Without Insulation | 20 | 24 | 10 | 14 |
| With Insulation | 250 | 135 | 100 | 35 |
The 1014 mm diameter riser demonstrated the most substantial convection effects. With insulation, the shrinkage height increased from 290 mm (without convection) to 460 mm (with convection), a 170 mm gain. Physical dissection yielded an actual height of 450 mm, with an error rate of just 2.2%. The shrinkage volumes were nearly identical (0.14477 m³ with convection vs. 0.14427 m³ without), confirming that convection redistributes the cavity rather than altering the total volume. This highlights the importance of thermal convection in large risers for steel casting, as it elevates the final solidification point and enhances casting integrity.
To quantify the relationship between riser size and convection effects, we analyzed the data across all risers. The height gain due to convection can be modeled empirically. For risers with diameters greater than 600 mm, the gain \( \Delta h \) correlates with riser diameter \( D \) and weight \( W \). A linear regression based on the data gives:
$$ \Delta h = k_1 \cdot D + k_2 \cdot W + c $$
where \( k_1 \), \( k_2 \), and \( c \) are constants derived from experimental data. For instance, using the values from Table 3, we can estimate gains for other riser sizes in steel casting. Additionally, the solidification time \( t_s \) for a riser can be approximated using Chvorinov’s rule:
$$ t_s = C \cdot \left( \frac{V}{A} \right)^n $$
where \( V \) is volume, \( A \) is surface area, \( C \) is a mold constant, and \( n \) is an exponent typically around 2 for sand molds. Longer solidification times in larger risers allow more prolonged convection, amplifying its effects.
| Riser ID | Diameter (mm) | Actual Height (mm) | Simulated with Convection (mm) | Simulated without Convection (mm) | Height Gain (mm) | Weight (kg) | Error Rate (%) |
|---|---|---|---|---|---|---|---|
| 1 (No Insulation) | 300 | 20 | 24 | 10 | 14 | 232 | 20 |
| 1 (With Insulation) | 300 | 250 | 135 | 100 | 35 | 232 | 46 |
| 2 | 600 | 450 | 480 | 370 | 110 | 1857 | 6.7 |
| 3 | 1014 | 450 | 460 | 290 | 170 | 6484 | 2.2 |
The discussion centers on the implications of these findings for steel casting optimization. Thermal convection significantly influences shrinkage cavity height in risers larger than 600 mm, with greater diameters yielding higher gains. This is attributed to the larger thermal mass and extended solidification times, which sustain convective flows. In contrast, for smaller risers, insulation materials dominate the effects. By incorporating convection into numerical simulations, designers can achieve more accurate predictions, enabling riser height reductions without compromising quality. For example, reducing the height of a 1014 mm riser by 100 mm could save approximately 0.59 tons of steel per riser. In a foundry producing 50,000 tons of steel annually, this could translate to savings of 750–1000 tons of riser steel, reducing costs by $300,000–$400,000 based on a steel cost of $4000 per ton (accounting for recycling).
Furthermore, the study underscores the role of simulation software in advancing steel casting sustainability. By refining thermal properties of insulation materials in models, foundries can enhance prediction accuracy. Future work could explore dynamic adjustments to convection parameters based on real-time data, further optimizing riser design. The integration of these approaches supports the broader goal of reducing the environmental footprint of steel casting processes while maintaining high product quality.
In conclusion, thermal convection is a critical factor in riser shrinkage for steel castings, particularly for large-diameter risers. Numerical simulations that account for convection provide reliable predictions, facilitating material savings and cost reductions. This research demonstrates that for risers above 600 mm, convection increases shrinkage height substantially, allowing for downsized risers. As the steel casting industry moves toward greener practices, leveraging such insights will be essential for achieving efficiency and sustainability targets.