This study focuses on addressing casting defects in a ZG25CrNiMo alloy lifting box through systematic process optimization. The component, measuring 1,040 mm × 760 mm × 933 mm with wall thickness ranging from 45 mm to 187 mm, presented significant solidification challenges due to its structural complexity and demanding service requirements in petroleum machinery applications.
1. Component Analysis
1.1 Structural Characteristics
The box features asymmetric geometry with thicker sections (187 mm) at the mounting flange transitioning to thinner walls (45 mm) at the load-bearing surfaces. Thermal analysis identified three critical heat accumulation zones requiring special attention during solidification control.
1.2 Material Properties
The ZG25CrNiMo alloy composition was optimized for fatigue resistance and impact toughness:
| Element | Content (wt.%) |
|---|---|
| C | 0.22–0.30 |
| Cr | 0.60–0.90 |
| Ni | 0.60–1.10 |
| Mo | 0.35–0.50 |
| Mn | 0.70–1.00 |
| Si | 0.30–0.60 |
| Fe | Bal. |
2. Gating System Design
Two gating configurations were evaluated to minimize casting defects:
2.1 Top Gating System
Utilized ceramic tubes with calculated cross-sections:
$$
\Sigma A_{\text{runner}} = 39.2\ \text{cm}^2,\ \ \Sigma A_{\text{ingate}} = 43.1\ \text{cm}^2
$$
2.2 Bottom Gating System
Combined ceramic sprue with sand-formed runners:
$$
\Sigma A_{\text{spure}} = 19.6\ \text{cm}^2,\ \ \Sigma A_{\text{ingate}} = 43.1\ \text{cm}^2
$$
3. Numerical Simulation
ProCAST analysis revealed critical differences in defect formation:
| Parameter | Top Gating | Bottom Gating |
|---|---|---|
| Filling Time | 39 s | 40 s |
| Shrinkage Volume | 2.8% | 3.1% |
| Surface Defects | 12 locations | 8 locations |
The bottom gating system demonstrated superior metal flow stability despite slightly higher shrinkage tendency, particularly in controlling casting defects at critical load-bearing sections.
4. Process Optimization
4.1 First-Stage Optimization
Implemented exothermic riser system using modulus calculations:
$$
M_C = \frac{V}{A} = 5.8\ \text{cm},\ \ M_R = 1.2M_C = 7.0\ \text{cm}
$$
Designed riser dimensions: Ø300 mm × 300 mm with 21.2 dm³ volume, achieving 14.1 dm³ effective feeding capacity.
4.2 Second-Stage Optimization
Strategic chill placement addressed residual shrinkage defects:
| Location | Chill Type | Dimensions (mm) |
|---|---|---|
| Lower Flange | Formed Chill | 150 × 75 × 40 |
| Bearing Surface | Curved Chill | Ø120 × 35 |
Cooling efficiency enhancement:
$$
Q_{\text{removed}} = kA\frac{\Delta T}{d} \approx 15.8\ \text{kJ/cm}^3
$$
5. Defect Reduction Results
Final process validation showed remarkable casting defect reduction:
| Defect Type | Initial | Optimized | Reduction |
|---|---|---|---|
| Macroshrinkage | 3.2% | 0.4% | 87.5% |
| Microporosity | 12/cm² | 2/cm² | 83.3% |
| Surface Defects | 8 locations | 0 | 100% |
The optimized process achieved directional solidification with progressive cooling from lower sections to the exothermic riser, effectively eliminating casting defects in critical areas while maintaining production efficiency.