In my research, I focused on the application of shell casting technology for producing ductile iron castings, specifically crankshafts for diesel engines. The study aimed to optimize the process using domestic equipment and materials, addressing key challenges such as shrinkage defects, dimensional accuracy, and production efficiency. Ductile iron castings are critical components in engines due to their high strength and durability, but their production often faces issues like micro-shrinkage and porosity during solidification. Through systematic experimentation, I explored melting, molding, shell-making, and pouring processes to enhance the quality and reliability of these ductile iron castings.
The background of this work stems from the increasing demand for high-performance crankshafts in automotive and industrial applications. Traditional methods like green sand molding and iron mold coated sand processes have limitations in terms of dimensional precision and defect rates. Shell casting, with its use of resin-coated sand and iron shot filling, offers superior surface finish and mechanical properties. However, its adoption has been hindered by high costs and technical complexities. My investigation sought to bridge this gap by developing a cost-effective approach for producing ductile iron castings, emphasizing the role of process parameters in minimizing defects.
In the process design phase, I analyzed the solidification behavior of ductile iron castings. The volume changes during cooling include liquid contraction, solidification shrinkage, graphite expansion, and mold deformation. To model this, I used the following formula for total volume change: $$\Delta V = V_l + V_s + V_g + V_m$$ where \(V_l\) is liquid contraction, \(V_s\) is solidification shrinkage, \(V_g\) is graphite expansion, and \(V_m\) is mold expansion. For ductile iron castings, the carbon equivalent (CE) plays a crucial role, calculated as: $$CE = C + \frac{1}{3}Si$$. Maintaining a CE of 4.4–4.5% was found to optimize fluidity and reduce shrinkage in these ductile iron castings.
I designed the gating and riser system to enhance feeding during solidification. The riser weight was determined using the formula: $$W_r = \frac{W_c}{6.5} + \frac{R^3}{25}$$ where \(W_r\) is riser weight, \(W_c\) is casting weight, and \(R\) is riser radius. For the crankshaft, with a weight of 16.5 kg, the riser weight was set to 2.49 kg, achieving a casting-to-riser ratio of 6.6:1. This ensured adequate liquid feeding for the ductile iron castings, minimizing shrinkage defects. Additionally, chills were incorporated at critical sections like the main journal and web areas to accelerate cooling and prevent micro-shrinkage.
| Process Parameter | Shell Casting | Green Sand | Iron Mold Coated Sand |
|---|---|---|---|
| Dimensional Accuracy (ISO) | 7–8 | 8–11 | 8–9 |
| Surface Roughness (Ra, μm) | 12.5–25 | 25–50 | 25–50 |
| Mechanical Properties | Excellent | Poor | Good |
| Investment Cost (Relative) | Medium | High | Low |
For equipment selection, I utilized a domestic K9407EK shell molding machine for producing the resin-coated sand shells. This machine operated with a cycle time of 150–240 seconds per shell, ensuring high productivity for ductile iron castings. The shell thickness was designed to be 6–8 mm at the casting sections for rapid cooling and 12–14 mm at the risers for thermal insulation. To address filling issues in narrow web gaps, I modified the shell design to eliminate deep recesses, improving iron shot compaction. The iron shot used for mold filling had a mixed grain size distribution: 40% Φ5.5 mm, 30% Φ4 mm, and 30% Φ3 mm, which enhanced cooling efficiency and shell rigidity for these ductile iron castings.
In the experimental phase, I conducted multiple trials to optimize the melting and treatment processes. The chemical composition of the ductile iron castings was carefully controlled, as summarized in Table 2. Using a medium-frequency induction furnace, I melted charges consisting of 90% pig iron and 10% steel scrap. The base iron composition was adjusted to achieve a final carbon content of 3.7–3.9% and silicon of 2.0–2.3%, with magnesium and rare earth additions for nodularization. The球化处理 involved a sandwich method with 1.5% nodularizer, and inoculation was done using 75% ferrosilicon. The pouring temperature was maintained at 1340–1360°C to ensure proper fluidity and reduce gas defects in the ductile iron castings.
| Element | Base Iron | Final Composition |
|---|---|---|
| C | 3.78 | 3.8–3.9 |
| Si | 1.21 | 2.2–2.4 |
| Mn | 0.42 | 0.4–0.5 |
| P | 0.050 | ≤0.05 |
| S | 0.028 | ≤0.02 |
| Mg | – | 0.04–0.06 |
| RE | – | 0.02–0.04 |
The shell-making process involved using phenolic resin-coated sand with a resin content of 3%, which provided a tensile strength of 4.0–4.8 MPa and low gas evolution of 14–16 mL/g. I optimized the shooting parameters, such as a shooting pressure of 0.3–0.4 MPa and a curing time of 4–6 minutes, to produce consistent shells for the ductile iron castings. For the core-making, a Z9405E core shooter was employed to produce oil hole cores, with a cycle time of under 180 seconds. The shells were then bonded using a hot adhesive applied at multiple points to prevent distortion during handling and pouring.
During molding and pouring, I implemented a环形浇注线 with iron shot filling. The shells were placed in sand boxes and filled with iron shot in three stages: a base layer, pre-filling up to one-third height, and final filling to the pouring cup. Compaction was achieved using a vibrating table with a short duration of 5–8 seconds to avoid shell cracking. The pouring process was rapid, with a pouring time of 8–10 seconds per mold, to prevent slag inclusion and gas defects. After pouring, the ductile iron castings were cooled for 20–30 minutes before shakeout. The iron shot was recycled through a separation and cooling system to maintain consistent properties.
To analyze the results, I performed ultrasonic testing and metallographic examination on the produced ductile iron castings. The volume change during solidification was critical, and I used the formula for graphite expansion: $$V_g = 2 \times \%C_{graphite}$$ where each 1% of graphite carbon expands the volume by 2%. Combined with liquid contraction of approximately 1.6% per 100°C superheat, the net volume change could be negative, leading to shrinkage. By optimizing the CE and using chills, I achieved a shrinkage-free rate of over 80% in the final ductile iron castings. Table 3 summarizes the experimental results from multiple batches, showing the impact of process adjustments on defect rates.
| Batch No. | Pouring Temp. (°C) | Carbon Equivalent (%) | Defect Rate (%) | Remarks |
|---|---|---|---|---|
| 1 | 1400 | 4.3 | 64 | Shrinkage in webs |
| 2 | 1380 | 4.3 | 71 | Gas porosity |
| 3 | 1380 | 4.4 | 83 | Improved with chills |
| 4 | 1400 | 4.3 | 43 | Reduced leakage |
| 5 | 1380 | 4.3 | 71 | Optimized gating |
| 6 | 1420 | 4.3 | 25 | Best results |
Through iterative testing, I identified that increasing the pouring speed and ensuring proper venting significantly reduced subsurface gas holes in the ductile iron castings. The use of chills at thermal junctions helped achieve directional solidification, minimizing shrinkage. The mathematical model for cooling rate was expressed as: $$\frac{dT}{dt} = k \frac{(T – T_m)}{\delta^2}$$ where \(k\) is thermal conductivity, \(T_m\) is mold temperature, and \(\delta\) is shell thickness. This guided the design of shell sections for optimal cooling of the ductile iron castings.
In conclusion, my research demonstrates that shell casting with iron shot filling is a viable method for producing high-quality ductile iron castings, particularly crankshafts. Key findings include the importance of maintaining a carbon equivalent of 4.4–4.5%, optimizing riser design, and controlling iron shot compaction. The process offers advantages in dimensional accuracy, surface finish, and production efficiency for ductile iron castings. Future work should focus on multi-station shell-making machines and computer simulation of solidification to further enhance the process. This study provides a foundation for widespread adoption of shell casting for ductile iron castings in various industrial applications.
The success of this approach underscores the potential of domestic equipment in achieving international standards for ductile iron castings. By addressing technical challenges such as shell distortion and feeding efficiency, the method can be extended to more complex components. Continuous improvement in resin-coated sand properties and automation will drive the evolution of shell casting for ductile iron castings, ensuring sustainable and cost-effective production in the foundry industry.