This article details the comprehensive process design, simulation analysis, and production methodology for a critical high-pressure steam chamber casting part. This component is a vital element in fossil-fuel steam turbines, serving to equalize steam flow after it passes through the main stop valve. The inherent complexity of the part’s geometry, featuring numerous internal passages, presents significant challenges in molding and core-making, often leading to various casting defects.
The casting part, characterized by a typical cylinder-block shape, has approximate outer dimensions of 1640 mm × 560 mm × 1070 mm. The wall thickness varies considerably, from a maximum of approximately 200 mm at the outer flange of the U-shaped opening and at the lowest point of the U-shaped arc, down to a minimum of about 60 mm at the exhaust port locations. This casting part is subjected to stringent technical requirements. All non-machined surfaces must adhere to the GB/T 6414-2017 dimensional tolerance standard (CT10 grade). Furthermore, the entire casting part requires 100% Ultrasonic Testing (UT) and Magnetic Particle Testing (MT). A critical pressure test is also mandated, where the part must hold a hydrostatic pressure of 8.28 MPa for 10 minutes without leakage. Consequently, the casting part must be free from porosity and shrinkage defects like cavities and macro/micro-shrinkage, demanding exceptionally high density.
The material specified for this demanding application is ZG15Cr2Mo1 heat-resistant cast steel. Key chemical composition ranges are summarized in Table 1.
| Element | C | Cr | Mo | Si | Mn | P | S |
|---|---|---|---|---|---|---|---|
| Range | 0.12-0.18 | 2.00-2.75 | 0.90-1.20 | ≤0.60 | 0.50-0.70 | ≤0.025 | ≤0.025 |
Foundry Process Design and Analysis
1. Molding Strategy and Process Parameters
To meet the high-density and surface quality requirements, a hybrid sand system was employed. The facing sand (30 mm thick) uses chromite sand (40-70 mesh) bonded with alkaline phenolic resin, mixed using a sand mullet to ensure strength. The backing sand is standard alkaline phenolic resin self-hardening sand. The molds and cores are coated with a zirconium-based alcohol-borne paint, applied by brushing once and spraying twice to achieve a coating thickness of 3-4 mm.
Considering the technical specifications, the casting part was oriented with the two exhaust ports (critical surfaces for subsequent welding and NDT) at the bottom. The thicker flange mating surface was positioned at the top to facilitate the placement of feeding risers. The parting line was conveniently set at the largest cross-section through the flange, enabling a two-flask molding process. The internal cavity is formed by two assembled sand cores. Core I is positioned using vertical core prints from the exhaust port cylinders and horizontal semi-circular prints at the U-opening. Core II is a semi-ring placed during the final assembly sequence before closing the mold.
The key casting process parameters were selected as follows:
- Dimensional Tolerance: CT10 Grade.
- Weight Tolerance: MT10 Grade.
- Machining Allowances: 20 mm for top surfaces, 15 mm for sides and bottom.
- Solidification Shrinkage: 2.1%.
- Draft Angle: Corresponding to 8 mm of taper.
2. Gating and Feeding System Design
A bottom-gated, anti-shower (reverse rain) gating system was chosen to ensure a tranquil fill and minimize air entrapment and mold erosion for this steel casting part. The system is designed as an open type with a gating ratio of:
$$ \sum F_{\text{sprue}} : \sum F_{\text{runner}} : \sum F_{\text{ingate}} = 1 : 1.4 : 2 $$
The total pouring weight of the steel casting part was estimated at G = 2523 kg.
a) Pouring Time Calculation:
The pouring time (τ) is determined using an empirical formula:
$$ \tau = A \sqrt{G} $$
Where A is a coefficient typically between 1.0 and 1.5 for large steel castings. Taking A = 1.35:
$$ \tau = 1.35 \times \sqrt{2523} \approx 36 \text{ seconds} $$
b) Choke Area Calculation:
The choke area (A_choke) at the bottom of the sprue is calculated using the Osborne formula:
$$ A_{\text{choke}} = \frac{G}{\rho \tau \mu \sqrt{2g H_p}} $$
Where:
- ρ = density of liquid steel ≈ 7000 kg/m³.
- μ = discharge coefficient. Considering the large size and resistance, and the presence of open risers, μ was selected as 0.35.
- H_p = average effective metal pressure head during filling ≈ 0.6 m.
- g = gravitational acceleration (9.81 m/s²).
Substituting the values:
$$ A_{\text{choke}} = \frac{2523}{7000 \times 36 \times 0.35 \times \sqrt{2 \times 9.81 \times 0.6}} \approx 0.0070 \text{ m}^2 = 70 \text{ cm}^2 $$
Therefore:
- $$ \sum F_{\text{sprue}} = 70 \text{ cm}^2 $$
- $$ \sum F_{\text{runner}} = 1.4 \times 70 = 98 \text{ cm}^2 \ (\text{designed as } 100 \text{ cm}^2) $$
- $$ \sum F_{\text{ingate}} = 2 \times 70 = 140 \text{ cm}^2 $$
c) System Verification:
The minimum metallostatic pressure head (H_min) must be sufficient to fill the furthest and highest point of the casting part cavity, defined by the required pressure angle (α). For a wall thickness around 200 mm, α is approximately 6°.
$$ H_{\text{min}} \geq L \cdot \tan(\alpha) $$
Where L is the horizontal projected distance from the sprue center to that point (≈1000 mm).
$$ L \cdot \tan(6^\circ) \approx 1000 \times 0.1051 = 105 \text{ mm} $$
The designed pressure head of 300 mm is greater than 105 mm, confirming the sprue height is adequate.
d) Riser Design (Modulus Method):
The main thermal centers (hot spots) requiring feeding are located at the top flange and the bottom of the U-arc. Open top risers were designed for the flange, and blind side risers for the U-arc bottom.
The casting modulus (M_c) at the flange (wall thickness T ≈ 194 mm, considered as a plate) is:
$$ M_c = \frac{V}{A} \approx \frac{T}{2} = \frac{19.4}{2} = 9.7 \text{ cm} \ (\text{approximated as 9 cm for calculation}) $$
The riser modulus (M_r) should be 1.2 times the casting modulus:
$$ M_r = 1.2 \times M_c = 1.2 \times 9 = 10.8 \text{ cm} $$
A cylindrical open riser with a modulus of ~11 cm was selected, resulting in dimensions of Ø550 mm × 300 mm with a neck of Ø500 mm × 40 mm.
Similarly, a blind riser was designed for the U-arc hot spot, resulting in a riser with dimensions of 400 mm × 160 mm × 230 mm.
Riser adequacy was verified using the volume feed requirement formula:
$$ \varepsilon (V_c + V_r) \leq V_r \cdot \eta $$
Where:
- ε = volumetric solidification shrinkage of alloy steel (≈5% or 0.05).
- V_c = volume of casting section fed.
- V_r = volume of riser.
- η = riser efficiency (12% for open, 15% for blind risers).
The calculation confirmed the designed risers provide sufficient feed metal volume for the specific casting part sections.
e) Chills Application:
To promote directional solidification and reduce the local modulus of certain hot spots, external chills were applied. Based on the modulus calculations at these specific locations, four chills of different sizes were designed and placed strategically.
| Chill Designation | Approximate Surface Area (mm²) | Thickness (mm) | Purpose |
|---|---|---|---|
| Chill c & d | 217,200 | 200 | Modify solidification at major hot spots |
| Chill e & f | 102,800 | 50 | Modify solidification at secondary hot spots |
Numerical Simulation and Optimization
The designed process for the ZG15Cr2Mo1 casting part was analyzed using AnyCasting simulation software to predict and optimize the filling and solidification behavior.
1. Simulation Parameters
The casting part geometry was discretized into a mesh of approximately 5 million tetrahedral cells. The key boundary conditions and material properties used in the simulation are listed in Table 3.
| Parameter | Value |
|---|---|
| Pouring Temperature | 1570 °C |
| Initial Mold Temperature | 20 °C |
| Heat Transfer Coefficient: Metal/Sand | 800 W/(m²·K) |
| Heat Transfer Coefficient: Metal/Chill | 1200 W/(m²·K) |
| Heat Transfer Coefficient: Chill/Sand | 400 W/(m²·K) |
| Heat Transfer Coefficient: Sand/Core | 200 W/(m²·K) |
2. Filling Analysis
The simulation of the filling process confirmed the efficacy of the bottom-gated system. The metal entered the mold cavity smoothly from the bottom, with a total fill time of approximately 35 seconds, closely matching the calculated 36 seconds. The analysis showed no signs of severe turbulence, air entrapment, or premature cooling, indicating a stable fill suitable for this complex casting part. The open risers were completely filled, confirming no misrun concerns.
3. Solidification and Shrinkage Prediction
The solidification sequence analysis demonstrated that the combination of risers and chills successfully established a favorable thermal gradient. The casting part sections solidified progressively from the thinner, chilled areas towards the risers, which remained molten longest. The risers functioned as effective thermal and feed metal reservoirs.
The shrinkage porosity analysis module was employed to predict the location of potential shrinkage defects. The results were highly positive: the predicted shrinkage cavities were isolated entirely within the bodies of the main risers. No shrinkage porosity was predicted in the main body of the final casting part or within the blind riser. This confirms that the designed feeding system, complemented by the strategic use of chills, successfully transfers shrinkage from the critical casting part into the sacrificial risers, validating the soundness of the process for producing a dense, pressure-tight component.
Melting and Refining Process Design
The production of high-integrity ZG15Cr2Mo1 casting parts requires precise control over chemical composition, impurity levels, and melt quality to achieve the required high-temperature creep resistance and mechanical properties.
1. Process Objectives and Key Parameters
The primary goals are strict compositional control (especially for Cr, Mo, and C), low levels of impurities (P, S, H, O, N), and a homogeneous as-cast structure to minimize segregation.
| Aspect | Target / Control Parameter |
|---|---|
| Composition Control | Cr: Mid-range (~2.4%), Mo: Mid-range (~1.05%), C: 0.14-0.16% |
| Impurity Control | P, S ≤ 0.025%; Low gas content (H, O, N) |
| Melt Homogeneity | Achieved via induction melting and post-casting heat treatment |
2. Charge Make-up and Pre-Treatment
The charge consists primarily of selected, low-residual scrap steel, supplemented with high-purity pig iron and necessary master alloys (Fe-Cr, Fe-Mo). Carbon content is adjusted using recarburizers like graphite.
Charge Calculation Example (Simplified):
Aiming for 1,000 kg of final melt with Cr target of 2.4% and accounting for a 4% burn-off:
$$ \text{Required Cr mass} = 1000 \text{ kg} \times 0.024 = 24 \text{ kg} $$
$$ \text{Required Fe-Cr addition} = \frac{24 \text{ kg}}{(1 – 0.04) \times \text{%Cr in Fe-Cr}} $$
For a 60% Cr Fe-Cr alloy:
$$ \text{Fe-Cr addition} \approx \frac{24}{0.96 \times 0.60} \approx 41.7 \text{ kg} $$
Similar calculations are performed for Mo and carbon additions.
3. Melting Equipment and Temperature Control
A combined “Medium-Frequency Induction Furnace + Electric Arc Furnace (for refining)” approach is used. The induction furnace provides excellent compositional control and superheating. Key temperature parameters are:
- Tap Temperature: 1600-1650 °C
- Pouring Temperature for this thick-wall casting part: 1520-1580 °C (aiming at the lower end for better structure).
4. Deoxidation and Refining Practice
Pre-deoxidation: Before tapping, additions of Ferro-Manganese (for Mn) and Ferro-Silicon (for Si) are made to the furnace to begin reducing oxygen content.
Final Deoxidation: In the ladle, Aluminum (0.10-0.15%) is added for a strong, final kill. The residual Al is controlled to <0.025% to prevent embrittlement.
Refining Operations:
- Argon Stirring: Performed for 5-10 minutes to promote floatation and removal of non-metallic inclusions and dissolved gases.
- Slag Control: A basic slag system (CaO-SiO₂-Al₂O₃) is maintained to efficiently absorb sulfur, phosphorus, and other impurities from the molten steel for the casting part.
5. Alloying Sequence
The sequence is optimized to minimize oxidation losses:
- Add oxidation-resistant elements (Ni, Mo if not in master charge).
- Add Ferro-Chromium after deoxidation to prevent excessive Cr loss.
- Fine-tune carbon content using recarburizer or scrap additions.
6. Post-Casting Process Integration & Quality Control
Casting: The designed bottom-gated system with insulating riser sleeves is used.
Cooling: The casting part is allowed to cool slowly in the mold (cooling rate ≤ 50 °C/hour) to avoid thermal stress cracking.
Heat Treatment:
- Normalizing: 920-970 °C for 8-12 hours, air cool to refine the grain structure.
- Tempering: 720-760 °C for 8-12 hours to relieve stresses and stabilize carbides.
| Check Point | Method / Standard | Target/Requirement |
|---|---|---|
| Chemical Composition | Spectroscopic Analysis | Cr, Mo: Mid-range ±0.1%; C: 0.14-0.16% |
| Mechanical Properties (from separately cast coupons) | Tensile Test | σb ≥ 485 MPa, δ ≥ 35% |
| High-Temperature Performance | Creep-Rupture Test | 1,000 hours at 566 °C |
| Microstructure | Metallographic Examination | Uniform carbide distribution, avoiding continuous networks |
| Integrity | UT, MT, Pressure Test | 100% sound, leak-free at 8.28 MPa |
Conclusion
This study presents a holistic approach to the manufacturing of a complex high-pressure steam chamber casting part. A feasible casting process was designed, featuring a two-flask mold with chromite facing sand, a bottom-gated open pouring system, and a feeding system comprising open and blind risers supplemented with chills. Numerical simulation validated the design, confirming stable filling and directional solidification with shrinkage successfully isolated to the risers. Furthermore, a detailed melting and refining methodology was established, combining induction and arc furnace practices with precise deoxidation, slag control, and argon stirring to achieve the stringent chemical and quality standards required for the ZG15Cr2Mo1 material. The integration of robust process design, simulation-based optimization, and controlled metallurgy provides a reliable framework for producing high-integrity, heavy-section steel casting parts for critical power generation applications.