In the production of large-scale agricultural machinery, ductile iron castings play a critical role due to their excellent mechanical properties and durability. One such component is the drive wheel used in crawler-type tractors, which requires precise casting to meet stringent quality standards. This article details the challenges faced in manufacturing a drive wheel made of GGG60-DIN1693 ductile iron and the iterative improvements made to the casting process. The drive wheel has an outer diameter of 916 mm, a height of 526 mm, and a mass of 260 kg. Its gear-like structure features 19 evenly spaced teeth around the circumference, each with isolated hot spots at the root, and no draft angle is permitted on the teeth to ensure proper engagement with the track. Internal quality demands include Level 2 radiographic inspection and dissection validation allowing shrinkage porosity less than 12.7 mm in diameter within a 38.1 mm × 38.1 mm area. Surface quality requires no more than five gas holes with diameters up to 2 mm in the same area. Through first-person experience, I will describe how we addressed issues like shrinkage cavities and gas defects using advanced simulation, core assembly techniques, and process optimizations, emphasizing the importance of ductile iron castings in heavy-duty applications.
The mechanical properties of the ductile iron material are crucial for ensuring the drive wheel’s performance under operational stresses. Below is a summary of the required properties based on the standard specifications:
| Property | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HBW) |
|---|---|---|---|---|
| Standard Requirement | ≥ 600 | ≥ 370 | ≥ 3 | 187–269 |
These properties ensure that the ductile iron castings can withstand high loads and abrasive conditions, making them ideal for drive wheels. However, achieving these in practice requires meticulous process design. Initially, we adopted a symmetrical parting line perpendicular to the wheel’s axis and employed a core assembly method to handle the 19 teeth without draft angles. The core assembly involved multiple steps: first, individual teeth were formed by combining 1# and 2# cores into 3# cores; then, three 3# cores were assembled into 6# cores using 4# and 5# positioning cores. Finally, six 6# cores were placed in the bottom mold. This approach minimized flash at the teeth and maintained precise contours, which is essential for the functionality of ductile iron castings. The upper and lower molds used resin sand, while the 1#, 2#, 4#, and 5# cores were made from shell-coated sand via hot-box core shooting to ensure dimensional accuracy.
To validate the initial process, we utilized ProCAST simulation software, which revealed significant shrinkage cavities at the center of the teeth, exceeding the allowable limits. The simulation results indicated that these defects were primarily due to inadequate feeding during solidification. After multiple iterations, we found that adding risers above each tooth could provide effective feeding, reducing the shrinkage size and shifting its location from the tooth root to the upper-middle section. Additionally, incorporating chills at the tooth bases helped minimize hot spots and extend the riser’s feeding range. This combination is particularly beneficial for ductile iron castings, as it controls solidification patterns and enhances soundness.
The riser design was based on modulus calculations to ensure optimal performance. For a single tooth with a mass of $G_c = 4.44 \, \text{kg}$, volume $V_c = 6.03 \times 10^{-6} \, \text{m}^3$, and surface area $A_c = 5.58 \times 10^{-4} \, \text{m}^2$, the modulus $M_c$ is given by:
$$ M_c = \frac{V_c}{A_c} = 0.0108 \, \text{m} $$
The mass circumference quotient $Q_m$ is calculated as:
$$ Q_m = \frac{G_c}{M_c^3} = 3.52 \times 10^3 \, \text{kg/m}^3 $$
Using control pressure riser design, the riser modulus $M_R$ is derived from factors $f_1 = 1.19$, $f_2 = 0.76$, and $f_3 = 1.2$:
$$ M_R = f_1 f_2 f_3 M_c = 0.0117 \, \text{m} $$
The riser neck modulus $M_N$ is determined with $f_p = 0.65$ and $f_4 = 0.9$:
$$ M_N = f_p f_4 M_R = 0.0068 \, \text{m} $$
This results in a riser neck diameter of 52 mm. For a cylindrical blind riser with a height-to-diameter ratio $K = 1.8$, we selected a standard riser size of ϕ80 mm × 120 mm. The gating system was modified to connect 19 such risers via a horizontal runner, with ingates linking the risers to the casting. Pouring temperature was maintained between 1,355 °C and 1,365 °C to ensure proper fluidity for ductile iron castings. The improved process layout included these risers and chills, which significantly reduced internal shrinkage, as confirmed by dissection tests showing only minor micro-porosity in some upper tooth sections, meeting the technical requirements.
However, surface quality issues emerged in the form of gas holes, attributed to high gas evolution from the shell-coated sand cores and inadequate venting in the thin-walled sections. To address this, we implemented several measures focused on reducing gas generation and improving exhaust. First, we switched to coarser 50/100 sand for the 1# and 2# cores to lower gas evolution. Second, assembled 6# cores were baked in a drying oven at 180 °C for 4 hours to remove moisture and volatile compounds. Third, additional venting channels were incorporated into the mold design to facilitate gas escape during pouring. These steps proved effective, as subsequent production of over 120 drive wheels showed no gas defects, resulting in high-quality surface finishes for the ductile iron castings.
The benefits of these improvements are multifaceted. The core assembly technique eliminated the need for draft angles on critical areas like teeth, reducing flash and ensuring dimensional accuracy. Using risers and chills effectively tackled shrinkage defects in isolated hot spots, which is common in complex ductile iron castings. The baking process for cores minimized gas-related issues, highlighting the importance of process control in achieving defect-free components. In summary, this case study demonstrates how iterative design and simulation can optimize casting processes for ductile iron components, ensuring they meet rigorous industrial standards. Future work could explore advanced materials or automated systems to further enhance the efficiency and quality of ductile iron castings.
In conclusion, the drive wheel project underscores the critical role of ductile iron castings in agricultural machinery and the need for continuous process refinement. By leveraging simulation tools like ProCAST, adopting modular core designs, and implementing targeted improvements such as risers, chills, and core baking, we successfully produced drive wheels that satisfy all mechanical and quality requirements. This experience not only highlights the versatility of ductile iron castings but also provides a framework for addressing similar challenges in other heavy-duty applications. As industries evolve, the demand for high-performance ductile iron castings will continue to grow, driving further innovations in casting technology.