High chromium cast iron (HCCI) is extensively utilized in pumps and hydraulic machinery due to its exceptional wear and corrosion resistance. However, its inherent brittleness and poor castability often lead to defects like shrinkage cavities, porosity, and cracks during the production of complex components such as impellers. V-process casting, characterized by smooth mold filling, high dimensional accuracy, and superior surface finish, offers significant advantages for manufacturing intricate geometries. This study focuses on the simulation and optimization of the V-process casting process for a Cr26 HCCI impeller, addressing critical defect formation mechanisms through numerical modeling and experimental validation.
1. Technical Specifications and Initial Casting Process Design
The impeller features a complex geometry with a maximum diameter of 1040 mm, height of 250 mm, and flow channel height of 100 mm. Critical sections include front/rear cover plates (38 mm thickness), four main blades (75 mm), and four auxiliary blades (45 mm), with a total weight of 730 kg. Material specifications for Cr26 HCCI are stringent:
| C | Si | Mn | Cr | Mo | Ni | Cu | S | P |
|---|---|---|---|---|---|---|---|---|
| 2.6–2.9 | 0.6–0.8 | 0.6–0.8 | 25–27 | 0.4–0.6 | 0.8–1.0 | 0.8–1.0 | ≤0.06 | ≤0.10 |
| Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Impact Energy (J) | Hardness (HRC) |
|---|---|---|---|---|
| ≥400 | ≥500 | ≥8 | ≥30 | 55–62 |
The initial casting process employed dry quartz sand for molds and resin-bonded sand for cores. An open gating system was designed with cross-sectional area ratios:
$$ \Sigma F_{\text{choke}} : \Sigma F_{\text{runner}} : \Sigma F_{\text{ingate}} = 1 : (1 – 1.5) : (1 – 1.5) $$
Four risers (two Ø150 open risers and two Ø150 blind risers) were positioned on the rear cover plate. Process parameters included:
- Pouring temperature: 1380–1400°C
- Mold vacuum: ≥0.05 MPa (pre-pour), ≥0.035 MPa (post-pour)
- Solidification time: 24 hours minimum
2. Numerical Simulation of Initial Casting Process
Finite Element Method (FEM) simulations with 4 million mesh elements analyzed temperature fields, solidification sequences, and defect formation. Key boundary conditions were:
$$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T + \frac{\dot{q}}{\rho c_p} $$
where \( T \) = temperature (K), \( \alpha \) = thermal diffusivity (m²/s), \( \dot{q} \) = heat generation rate (W/m³), \( \rho \) = density (kg/m³), \( c_p \) = specific heat (J/kg·K).
Heat transfer coefficients:
- Metal-mold interface: 1200 W/m²·K
- Metal-air interface: 400 W/m²·K
- Mold-air interface: 1000 W/m²·K
Simulation results identified critical issues:
- Thermal isolation at the rear cover plate hub (Fig. 4)
- Premature riser solidification causing interrupted feeding
- Shrinkage porosity risk (Niyama criterion < 1.0 °C0.5·min0.5/mm):
$$ G/\sqrt{\dot{T}} < K_{\text{critical}} $$
where \( G \) = temperature gradient (°C/mm), \( \dot{T} \) = cooling rate (°C/min).
3. Process Optimization Strategies
To address the deficiencies in the initial casting process, two modifications were implemented:
3.1 Exothermic Riser System
Conventional risers were replaced with exothermic sleeves. The exothermic reaction extends solidification time, improving feeding efficiency. The modified riser effectiveness \( \eta \) is given by:
$$ \eta = \frac{V_{\text{feed}}}{V_{\text{riser}}} \times 100\% $$
where \( V_{\text{feed}} \) = volume of metal fed to casting, \( V_{\text{riser}} \) = riser volume. For exothermic risers, \( \eta \) increased from 14% to 32%.
3.2 Strategic Chill Placement
External chills (steel, 40 mm thickness) were positioned at the rear cover plate hub to accelerate solidification:
$$ t_{\text{solidification}} = B \left( \frac{V}{A} \right)^n $$
where \( V/A \) = modulus (cm), \( B \) = mold constant (min/cm²), \( n \) ≈ 2 (Chvorinov’s rule). Chills reduced local modulus by 35%, eliminating isolated liquid zones.
| Parameter | Initial | Optimized |
|---|---|---|
| Riser Type | Conventional | Exothermic |
| Chill Implementation | None | 4 Steel Chills |
| Predicted Shrinkage (%) | 3.7 | 0.2 |
| Solidification Gradient (°C/mm) | 0.8 | 2.3 |
4. Validation and Production Results
The optimized casting process eliminated shrinkage defects in critical zones, with porosity fully transferred to risers. Post-production inspection confirmed:
- X-ray inspection: ASTM E802 Level B compliance
- Mechanical properties: \( R_m \) = 540 MPa, \( R_p \) = 620 MPa, \( A \) = 9.4%, \( KV \) = 45 J, HRC = 61
- Static balance: 50 g imbalance (< 80 g requirement)
The successful implementation enabled batch production with defect rates below 2%, demonstrating the robustness of the optimized V-process casting process.
5. Thermal Dynamics and Solidification Control
The fundamental heat transfer equation governing solidification is:
$$ \rho c_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + \rho L_f \frac{\partial f_s}{\partial t} $$
where \( k \) = thermal conductivity (W/m·K), \( L_f \) = latent heat (J/kg), \( f_s \) = solid fraction. For HCCI, the high chromium content (25–27%) significantly reduces thermal conductivity (\( k \approx 20 \text{W/m·K} \)) compared to gray iron (\( k \approx 50 \text{W/m·K} \)), exacerbating shrinkage tendencies. The V-process further complicates thermal management due to the insulating effect of the vacuum-sealed film. Our simulations quantified the thermal resistance (\( R_{\text{film}} \)):
$$ R_{\text{film}} = \frac{\delta_{\text{film}}}{k_{\text{film}}} \approx 0.0025 \text{m²·K/W} $$
This additional resistance delays heat extraction, necessitating active thermal control strategies like chills.
6. Advanced Riser Design Methodology
The efficiency of exothermic risers was maximized through modulus extension calculations:
$$ M_{\text{exo}} = M_{\text{std}} \times K_e $$
where \( M_{\text{std}} \) = geometric modulus, \( K_e \) = exothermic efficiency factor (1.8–2.2 for Cr26). The required riser volume was calculated using:
$$ V_{\text{riser}} = \frac{V_{\text{casting}} \times \beta}{ \eta \times (1 – f_{\text{exo}})} $$
where \( \beta \) = solidification shrinkage (6.8% for Cr26), \( \eta \) = feeding efficiency, \( f_{\text{exo}} \) = fraction of exothermic heat contribution (0.35). This methodology reduced riser weight by 22% compared to conventional safety margins.
7. Multi-Objective Process Optimization
A response surface methodology (RSM) optimized three critical parameters:
$$ \begin{aligned}
\text{Minimize: } & \Phi = w_1 \cdot S + w_2 \cdot P + w_3 \cdot T_c \\
\text{Subject to: } & 1360^\circ \text{C} \leq T_{\text{pour}} \leq 1420^\circ \text{C} \\
& 0.04 \text{MPa} \leq P_{\text{vac}} \leq 0.07 \text{MPa} \\
& 8 \leq t_{\text{fill}} \leq 20 \text{seconds}
\end{aligned} $$
where \( S \) = shrinkage index, \( P \) = porosity index, \( T_c \) = cycle time, \( w_i \) = weighting factors. The Pareto-optimal solution yielded:
- Pouring temperature: 1395°C
- Vacuum pressure: 0.055 MPa
- Fill time: 12 seconds
| Parameter | Range | Optimal Value | Effect on Defects (%) |
|---|---|---|---|
| Pouring Temperature (°C) | 1360–1420 | 1395 | -38% shrinkage |
| Vacuum (MPa) | 0.04–0.07 | 0.055 | -27% porosity |
| Fill Time (s) | 8–20 | 12 | -19% turbulence |
8. Industrial Implementation and Quality Metrics
Production monitoring over six months demonstrated consistent quality improvement:
$$ \text{Process Capability Index } C_{pk} = \min \left( \frac{\text{USL} – \mu}{3\sigma}, \frac{\mu – \text{LSL}}{3\sigma} \right) $$
Key metrics achieved:
- Hardness uniformity: \( C_{pk} = 1.83 \) (HRC 58–62)
- Dimensional accuracy: IT13 grade maintained
- Surface roughness: \( R_a \leq 12.5 \mu \text{m} \) without machining
The optimized V-process casting process reduced total manufacturing costs by 17% through improved yield and reduced rework.
9. Conclusion
This comprehensive study demonstrates that integrating exothermic risers and strategic chill placement fundamentally alters the solidification dynamics in V-process casting of high chromium iron impellers. The optimized casting process achieves directional solidification with a thermal gradient exceeding 2.3°C/mm, eliminating shrinkage defects while maintaining mechanical properties beyond specification requirements. The methodology establishes a scientific framework for defect minimization in complex HCCI castings, with broad applicability across wear-resistant component manufacturing. Future work will explore real-time thermal monitoring coupled with adaptive control systems for Industry 4.0 implementation of this casting process.