Steam turbine components operating under extreme thermal and mechanical stresses require exceptional material integrity. This study focuses on the sand casting process optimization for ZG15Cr2Mo1 steel castings used in high-pressure steam chambers, combining theoretical analysis with ProCAST-based numerical simulations to address shrinkage defects.
1. Material Characteristics and Casting Requirements
The chemical composition of ZG15Cr2Mo1 steel casting is detailed in Table 1. With liquidus temperature at 1501°C and linear shrinkage rate of 1.8%, proper riser design becomes critical for defect mitigation.
| C | Mn | Si | Cr | Mo | S | P |
|---|---|---|---|---|---|---|
| ≤0.18 | 0.40–0.70 | ≤0.60 | 2.00–2.75 | 0.90–1.20 | ≤0.030 | ≤0.030 |
①For every 0.01% reduction in C content, Mn content may increase by 0.04% up to 1.2% maximum.
2. Casting Process Design Fundamentals
The casting geometry (1648×620×1077 mm) requires careful thermal management. Key process parameters include:
Pouring temperature: 1580–1600°C
Mold material: Phenolic resin-bonded sand
Coating: Alumina-based alcohol coating
The critical pouring time is calculated using:
$$ t = \frac{G_L}{N \cdot n \cdot v_{\text{package}}} $$
Where:
\( G_L \) = Total molten steel weight (2094.8 kg)
\( v_{\text{package}} \) = Pouring rate (120 kg/s)
This yields theoretical pouring time \( t = 17.46 \, \text{s} \), rounded to 18 s for practical implementation.
3. Solidification Analysis and Defect Prediction
Initial ProCAST simulation revealed significant shrinkage defects in heavy sections (Figure 1). The shrinkage volume (\( V_s \)) in critical regions follows:
$$ V_s = \beta \cdot V_c \cdot (1 – f_s) $$
Where:
\( \beta \) = Solidification shrinkage coefficient (4.2% for ZG15Cr2Mo1)
\( V_c \) = Component volume
\( f_s \) = Solid fraction
4. Process Optimization Strategy
Modified gating system and chilling strategy achieved defect reduction through:
- Riser optimization using modulus method:
$$ M_r = 1.2 \cdot M_c $$
Where \( M_r \) = Riser modulus, \( M_c \) = Casting modulus - Strategic chill placement:
$$ t_{\text{chill}} = 0.24 \cdot Q \cdot \left(\frac{V}{A}\right)^2 $$
Where \( Q \) = Chill efficiency factor
| Parameter | Initial | Optimized |
|---|---|---|
| Riser Volume (L) | 42 | 68 |
| Chill Mass (kg) | 0 | 156 |
| Shrinkage Porosity (%) | 5.34 | 2.71 |
5. Thermal Gradient Control
The optimized temperature gradient (\( \nabla T \)) ensures directional solidification:
$$ \nabla T = \frac{T_{\text{riser}} – T_{\text{chill}}}{d} \geq 25 \, ^\circ\text{C/cm} $$
Where \( d \) = Distance between riser and chill
6. Industrial Implementation
The final steel casting process demonstrates:
- Zero macroshrinkage defects
- UT inspection compliance with ASTM A609 standards
- Production yield improvement from 68% to 83%
This systematic approach combining numerical simulation with fundamental casting principles provides a reliable methodology for complex steel casting production, particularly for high-integrity components in thermal power systems.