In the field of heavy machinery, track-type steel castings play a critical role as components in mining equipment, particularly in electric shovels where they serve as walking mechanisms. These steel castings operate under harsh conditions with complex and variable loading, leading to issues such as wear, deformation, and potential fracture. For large-scale equipment, track plates are typically replaced in sets, resulting in high replacement costs. Therefore, it is essential that these steel castings exhibit a combination of high strength, wear resistance, impact toughness, and fatigue resistance. In this article, I will discuss the casting process design for a specific track-type steel casting, incorporating numerical simulation to analyze filling and solidification behaviors. The simulation results guided the trial production, leading to successful outcomes where the castings met all technical requirements, including the absence of defects like sand sticking and cracks, as well as compliance with ultrasonic testing standards.
The steel casting material used is GS32CrNiMo6V, a low-alloy steel known for its high hardenability and mechanical properties. The chemical composition requirements are detailed in Table 1, which ensures the material’s suitability for demanding applications. This steel casting undergoes heat treatment involving normalizing and tempering to achieve a surface hardness of ≤260 HB and a grain size of ≥6. Non-destructive testing is rigorously applied, including 100% ultrasonic inspection and magnetic particle inspection across all surfaces, adhering to specific standards for critical and non-critical areas. Additionally, dimensional accuracy is verified through 3D scanning. The structural complexity of the steel casting, with internal hollow sections and external pin holes, presents significant challenges in the casting process, particularly in avoiding defects such as shrinkage, porosity, and cracks.
| Element | Min (%) | Max (%) |
|---|---|---|
| C | 0.32 | 0.36 |
| Si | 0.30 | 0.60 |
| Mn | 0.90 | 1.20 |
| P | – | 0.015 |
| S | – | 0.010 |
| Cr | 0.90 | 1.10 |
| Ni | 1.30 | 1.50 |
| Mo | 0.30 | 0.40 |
| V | – | 0.05 |
| N | – | 0.015 |
| Al | – | 0.07 |
The casting process for this steel casting involves several critical steps to address its complex geometry and material properties. The single casting weight is approximately 916 kg, with overall dimensions of 1400 mm × 735 mm × 415 mm. The wall thickness varies significantly, ranging from 30 mm to 200 mm, with a predominant thickness of 80 mm. This variation, combined with the alloy’s high hardenability, increases the risk of cracking, especially in areas of abrupt thickness changes and constrained contraction. To mitigate these issues, the casting process design focuses on optimizing the molding scheme, core design, riser and chill placement, and gating system. Numerical simulation software was employed to model the filling and solidification processes, ensuring that the design achieves sequential solidification and minimizes defects in the steel casting.
One of the primary challenges in producing this steel casting is the prevention of cracks in stress concentration zones, such as the pin ears and their connections to the main body. The high hardenability of GS32CrNiMo6V can lead to martensite formation during cooling, making cracks difficult to repair, especially in internal cavities. To address this, chromite sand was used in critical areas to accelerate solidification and reduce the risk of sand sticking and cracking. The design of the gating system aimed to ensure smooth filling with minimal turbulence, reducing oxidation and gas entrapment. The cross-sectional area ratio of the gating system was calculated as A_vertical : A_horizontal : A_inner = 1 : 2 : 2.2, with a vertical sprue diameter of 70 mm. This ratio promotes steady flow and minimizes冲刷 of the mold.
The filling process simulation showed that the steel casting filled smoothly, starting from the bottom and progressing upward without splashing. The initial flow velocity was approximately 0.5 m/s, and the filling was completed in a layered manner, ensuring stability. The solidification simulation demonstrated that the steel casting achieved sequential solidification from the bottom up, with no isolated liquid regions formed in the casting itself. Shrinkage porosity was concentrated in the gating system and risers, confirming the effectiveness of the design. The use of chills and risers helped consolidate the thermal zones, with three risers sufficient to meet the feeding requirements. Necked-down risers were employed to reduce the cutting area during removal, lowering the risk of heat-affected zone cracks.
In terms of core design, the internal cavity core was shaped as an L-structure with large core heads for positioning and stability. The pin hole cores were cylindrical for simplicity. Chromite sand, with a refractoriness above 1900°C, was used for the cores to prevent sintering and sand sticking defects. The molding scheme placed critical regions like the track surface and pin ears in the lower mold, while larger planes were positioned in the upper mold to facilitate riser placement and cleaning. The parting plane was set at the pin hole center to simplify core assembly.
To further elaborate on the theoretical aspects of steel casting design, the solidification time can be estimated using Chvorinov’s rule, which is expressed as: $$ t = B \left( \frac{V}{A} \right)^2 $$ where \( t \) is the solidification time, \( V \) is the volume of the casting, \( A \) is the surface area, and \( B \) is a constant dependent on the mold material and casting conditions. For this steel casting, the modulus method was applied to determine riser sizes, ensuring adequate feeding. The modulus \( M \) is given by: $$ M = \frac{V}{A} $$ For the thermal zones, chills were used to modify the solidification pattern, promoting directional solidification.
The gating system design involved calculations based on fluid dynamics principles. The flow rate \( Q \) can be described by: $$ Q = A \cdot v $$ where \( A \) is the cross-sectional area and \( v \) is the flow velocity. To minimize turbulence, the gating system was designed to maintain a critical velocity below which splashing occurs. The Reynolds number \( Re \) was considered to ensure laminar flow: $$ Re = \frac{\rho v D}{\mu} $$ where \( \rho \) is the density of molten steel, \( v \) is the velocity, \( D \) is the hydraulic diameter, and \( \mu \) is the dynamic viscosity. For this steel casting, the values were optimized to keep \( Re \) low.
During production, the steel casting was poured and subjected to standard post-casting processes including shakeout, cutting, and cleaning. The castings were inspected for surface defects, with no sand sticking observed in the pin holes or internal cavities. Magnetic particle and ultrasonic testing confirmed the absence of cracks and other discontinuities. The ultrasonic testing results showed no defects exceeding the specified levels in critical areas, complying with the required standards. Additionally, the castings were sectioned into 19 blocks for density analysis, revealing no visual defects and confirming internal soundness.
The mechanical properties of the steel casting were evaluated after heat treatment. Table 2 summarizes the typical properties achieved for GS32CrNiMo6V steel casting, highlighting its suitability for heavy-duty applications. The combination of high strength and toughness ensures reliable performance in service.
| Property | Value |
|---|---|
| Tensile Strength | ≥ 1000 MPa |
| Yield Strength | ≥ 800 MPa |
| Elongation | ≥ 12% |
| Impact Toughness (at 20°C) | ≥ 40 J |
| Hardness | ≤ 260 HB |
In conclusion, the successful production of this track-type steel casting demonstrates the importance of integrated process design and simulation. By dividing the casting into zones and using risers and chills to achieve sequential solidification, internal soundness was ensured. The use of chromite sand in stress concentration areas effectively reduced the risk of cracks and sand sticking. This approach can be applied to other complex steel castings to enhance quality and reliability. The numerical simulation provided valuable insights into the filling and solidification behaviors, allowing for optimization before actual production. Future work could focus on further refining the gating system and exploring alternative materials for improved performance in steel casting applications.
Overall, the design and production of this steel casting involved a holistic approach that considered material properties, geometric constraints, and process parameters. The application of fundamental principles, such as heat transfer and fluid dynamics, played a key role in achieving the desired outcomes. For instance, the heat conduction during solidification can be modeled using Fourier’s law: $$ q = -k \nabla T $$ where \( q \) is the heat flux, \( k \) is the thermal conductivity, and \( \nabla T \) is the temperature gradient. In this steel casting, the temperature distribution was controlled to prevent hot spots and ensure uniform cooling.
Additionally, the feeding efficiency of risers can be evaluated using the feeding distance concept, which depends on the geometry and cooling conditions. For steel castings, a common formula is: $$ L = 5 \sqrt{T} $$ where \( L \) is the feeding distance and \( T \) is the thickness. In this case, the riser placement was optimized based on such calculations to eliminate shrinkage defects. The successful trial production validates the design methodology and underscores the value of simulation in modern steel casting practices.
In summary, this project highlights how advanced techniques in steel casting can address complex challenges, resulting in high-quality components for critical applications. The integration of simulation, material science, and practical engineering ensures that the steel casting meets all performance criteria, paving the way for more efficient and reliable manufacturing processes in the future.