In the evolving landscape of the casting industry, there is an increasing demand for steel castings that exhibit superior comprehensive properties, higher dimensional accuracy, reduced machining allowances, and smoother surface finishes. Concurrently, the imperative for energy conservation, emission reduction, and green foundry practices has become paramount. The inherent challenge in steel casting production lies in the inability to directly observe the flow and solidification processes of molten metal within the mold cavity. This opacity often leads to significant expenditures of time and trial costs when addressing casting defects, with outcomes frequently falling short of ideal. Consequently, the rapid and accurate analysis of defect formation mechanisms and locations, along with the shortening of trial cycles and reduction of production costs, has emerged as a critical issue. Among various factors, the rational design of the gating and riser system is a key element in controlling defects.
The application of advanced casting simulation technologies enables designers to visually analyze the flow and solidification processes of molten metal under different gating systems. This capability allows for the prediction of potential defect locations and probabilities in steel castings before actual production, thereby effectively shortening trial cycles and lowering costs. Necking risers are widely employed in the production of iron castings; however, their application in steel castings, particularly low-alloy steel castings, is less documented. Through literature review, it is noted that necking risers have been utilized in the production of high-manganese steel castings. In our research, we introduced a necking heating riser to optimize the process for low-alloy steel castings of drilling rig pulleys. Simulations and experiments confirmed the feasibility of this approach, leading to its broader application and achieving the goal of reducing production costs for steel castings.
Pulleys are critical functional components in the lifting systems of drilling rig derricks and bases, facilitating energy transmission, conversion, and control. These steel castings must withstand high pressure and abrasion while maintaining high precision. The pulley is a rotational symmetry component with uniform wall thickness at the rim and a thicker central axle hole. Traditionally, to accommodate oval heating risers, a riser base and padding were designed. The material used is ZG35Cr1Mo, selected for its high strength and compactness. In actual production, the large padding associated with side risers results in low yield, extensive subsequent cutting, and poor cutting quality on the riser padding surface. This leads to increased machining time and tool wear, elevating the overall manufacturing cost for these steel castings.
In our study, we employed ProCAST finite element simulation software to analyze the temperature field, flow field, and predict shrinkage porosity defects in the steel casting of drilling rig pulleys. The traditional process scheme was optimized, and the new design was validated through physical production. The results demonstrate that the necking heating riser scheme is feasible for steel castings, offering significant production benefits. Specifically, the yield of the pulley steel casting increased by 5%, the weight decreased by approximately 13%, the cutting efficiency for risers and padding improved by over 40%, and machining efficiency rose by more than 30%.
Simulation and Optimization of Pulley Process Scheme
The traditional empirical design for the pulley casting process involved a total poured steel weight of 1405 kg for a rough casting weight of 805 kg, yielding a casting yield of 57%. This scheme utilized two oval heating risers at the central thick section and a circle of oval heating risers at the distal rim. In the optimized scheme, we introduced necking heating risers to replace the oval ones, resulting in a rough casting weight of 720 kg and a total poured steel weight of 1160 kg, which improves the yield to 62%. The use of necking heating risers eliminates the need for riser bases and padding, reduces the contact area between the riser and the pulley rim, and mitigates the adverse effects of riser padding on subsequent cutting and machining processes for steel castings.
We constructed three-dimensional models using UG and imported the Prt files into ProCAST software for pre-processing in the simulation. The initial conditions were set as follows: (1) Mesh division: 1,000,000 elements; (2) Simulation conditions: Pulley material as ZG35Cr1Mo, risers set as exothermic insulation materials, target process as sand casting, analysis type including filling and solidification processes, pouring temperature at 1570 °C, and pouring speed at 47 kg/s; (3) Solution conditions: Definition of solving equations, termination conditions, and data saving parameters; (4) Solution: Initiating the solver for steel casting simulation.
Temperature Field Simulation Results
The temperature field simulation results for the traditional scheme and the optimized scheme with necking heating risers are compared. The temperature gradient distribution indicates that the solidification process in both schemes follows a sequential solidification pattern without forming isolated liquid phase regions. This is crucial for ensuring the integrity of steel castings. The governing heat transfer equation during solidification can be expressed as:
$$ \rho C_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + L \frac{\partial f_s}{\partial t} $$
where \( \rho \) is density, \( C_p \) is specific heat, \( T \) is temperature, \( t \) is time, \( k \) is thermal conductivity, \( L \) is latent heat of fusion, and \( f_s \) is solid fraction. The simulations confirmed that both schemes maintain favorable temperature gradients for steel castings.
Flow Field Simulation Results
The flow behavior of molten metal during filling significantly impacts the quality of steel castings. Improper filling can lead to defects such as gas pores, slag inclusion, and cold shuts. By studying the flow patterns and stability, potential issues can be predicted. The Navier-Stokes equations for incompressible flow are applied:
$$ \frac{\partial \mathbf{u}}{\partial t} + (\mathbf{u} \cdot \nabla) \mathbf{u} = -\frac{1}{\rho} \nabla p + \nu \nabla^2 \mathbf{u} + \mathbf{g} $$
where \( \mathbf{u} \) is velocity vector, \( p \) is pressure, \( \nu \) is kinematic viscosity, and \( \mathbf{g} \) is gravitational acceleration. The simulations for both the traditional and optimized schemes showed stable filling processes without jetting, splashing, or air entrapment, ensuring high-quality steel castings.
Shrinkage Porosity Prediction
During the solidification of steel castings, shrinkage porosity and cavities may form due to temperature variations and physical state changes, affecting the denseness of the casting structure. The Niyama criterion is often used to predict these defects:
$$ G / \sqrt{\dot{T}} $$
where \( G \) is temperature gradient and \( \dot{T} \) is cooling rate. A threshold value helps identify regions prone to shrinkage. The simulation results indicated that both process schemes produce dense structures in the steel castings, with no shrinkage defects in critical areas such as the rope groove.
In summary, while the traditional design yields sound steel castings internally, it is conservative, resulting in low yield, poor surface quality, and high manufacturing costs. The optimized scheme with necking heating risers maintains internal soundness, improves yield, enhances surface quality, and reduces costs for steel castings.
Production Application and Validation
Based on the optimized casting process with necking heating risers, we modified the patterns and trial-produced the pulley steel castings. The finished pulleys were machined and inspected via magnetic particle testing, meeting the requirements of GB/T9444. Compared to the traditional process, the new scheme for steel castings demonstrated complete removal of riser padding at the rim, improved yield, reduced weight, and enhanced cutting and machining efficiencies. The following table summarizes the key performance metrics:
| Parameter | Traditional Scheme | Optimized Scheme | Improvement |
|---|---|---|---|
| Rough Casting Weight (kg) | 805 | 720 | ~13% reduction |
| Total Poured Steel Weight (kg) | 1405 | 1160 | — |
| Casting Yield (%) | 57 | 62 | 5% increase |
| Riser and Padding Cutting Efficiency | Base | — | >40% improvement |
| Machining Efficiency | Base | — | >30% improvement |
The implementation of the necking heating riser process in steel castings not only reduced the weight of the rough casting but also minimized the difficulties in cutting and machining, improved efficiency, and enhanced the appearance quality of the pulleys.
Conclusions
Through the use of ProCAST casting simulation software, we conducted a solidification simulation analysis of the necking heating riser process for steel castings of pulleys. Compared with the traditional process, the necking heating riser scheme is feasible for application in steel castings. Production validation confirmed that the steel castings produced with the new process exhibit dense internal structures and excellent appearance quality, meeting design requirements. The new process for steel castings achieved a 5% increase in yield, approximately 13% reduction in weight, over 40% improvement in riser and padding cutting efficiency, and more than 30% enhancement in machining efficiency. This optimization underscores the potential of advanced riser designs in improving the economics and quality of steel castings in industrial applications.