Steel castings for steam turbine components require exceptional mechanical properties due to extreme operational conditions. This article presents a systematic approach to designing the casting process for a medium-pressure inner cylinder with complex geometries, focusing on solving critical challenges in shrinkage control, sand adhesion prevention, and dimensional accuracy.
1. Material Specifications and Technical Requirements
The steel casting material ZG12Cr10Mo1NiWVNbN exhibits the following characteristics:
| Element | C | Cr | Mo | W | Nb |
|---|---|---|---|---|---|
| Content (%) | 0.09–0.14 | 10.0–11.0 | 1.00–1.30 | 0.2–0.3 | 0.04–0.08 |
Mechanical properties at room temperature:
| Property | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| Value | ≥690 | ≥490 | ≥11 |
2. Key Challenges in Steel Casting Process
The complex geometry of the medium-pressure inner cylinder creates three primary challenges:
- Thermal Management: Wall thickness variations (120–360 mm) create differential cooling rates. The modulus (M) is calculated as:
$$ M = \frac{V}{A} $$
where V = volume (m³), A = cooling surface area (m²). - Feeding Efficiency: The horizontal feeding distance (L) must satisfy:
$$ L \leq 4.5 \times \sqrt{M} $$
for effective riser performance. - Sand Core Stability: Narrow cavities (55 mm width) require precise core positioning to prevent dimensional deviations.
3. Process Optimization Strategies
3.1 Gating System Design
A bottom-gating system with slag traps was implemented to ensure laminar flow. The pouring time (t) is governed by:
$$ t = \frac{W}{\rho \cdot A \cdot v} $$
where W = casting weight (19.8 t), ρ = steel density (7,850 kg/m³), A = total choke area, v = flow velocity (≤0.55 m/s).
3.2 Riser Configuration
Combined use of exothermic risers and insulating sleeves achieved a feeding efficiency of 28%. The riser modulus was designed 1.2× greater than the casting modulus.
3.3 Sand Core Reinforcement
Chromite sand with 92% refractoriness was used for critical sections. Core prints were designed with 15° draft angles and 2 mm/m allowances.
4. Numerical Simulation and Validation
MAGMA simulations confirmed:
- Solidification time gradient: 48 min (thick sections) vs. 22 min (thin walls)
- No shrinkage porosity in NDT-critical zones
- Maximum thermal stress: 218 MPa (below material yield strength)
5. Production Implementation
The optimized process yielded castings with:
- Surface roughness: Ra ≤12.5 μm
- Dimensional tolerance: CT12 per ISO 8062
- UT/MT inspection: 100% compliance with ASME Section III
6. Conclusion
This steel casting process demonstrates how integrated design methodologies – combining modulus calculations, directional solidification control, and advanced simulation – can overcome geometric complexities in turbine components. The success criteria were achieved through:
- Precise thermal management of thick-wall sections
- Optimized feeding system with exothermic risers
- Stable sand core positioning using chromite reinforcements
The methodology provides a replicable framework for heavy-section steel castings in power generation equipment manufacturing.