In the pursuit of precision and efficiency, our sand casting foundry has successfully developed a low allowance sand casting process for the production of long saber-shaped static blades, which are characterized by their twisted geometry resembling a saber or bow. These blades are designed for the last two stages of low-pressure steam turbines in retrofit units. Traditionally, static blades in our company were produced with generous machining allowances using sand casting foundry techniques. However, the saber blade’s complex curvature and thin sections made conventional machining impossible. Therefore, we explored low allowance sand casting foundry methods to achieve near-net shape with minimal grinding allowance, meeting stringent dimensional and surface quality standards. This paper details our experimental journey, process optimization, and the final stable production method for saber blades in our sand casting foundry.
Blade Geometry and Basic Parameters
The saber blade has a complex profile with significant twist and varying thickness. It consists of 11 cross-sections from root to tip. The basic parameters are summarized in the following table.
| Parameter | Value Range |
|---|---|
| Number of cross-sections | 11 |
| Maximum thickness $T_{\text{max}}$ | 7.5 mm |
| Minimum thickness (exit edge) | 1.0 mm |
| Maximum chord width (pressure side) | 45 mm |
| Average chord width | 38 mm |
| Blade length | 320 mm |
| Blade weight (approx.) | 1.2 kg |
Material Requirements
The blade material is a martensitic stainless steel (equivalent to 1Cr12Mo). Its chemical composition and mechanical properties must meet the standards shown in Tables 2 and 3.
| Element | C | Si | Mn | Cr | Mo | Ni | P | S |
|---|---|---|---|---|---|---|---|---|
| Specification | 0.10–0.15 | ≤0.60 | ≤0.60 | 11.0–12.5 | 0.30–0.60 | 0.30–0.80 | ≤0.030 | ≤0.030 |
| Property | Value |
|---|---|
| Tensile strength $R_m$ | ≥ 690 MPa |
| Yield strength $R_{p0.2}$ | ≥ 550 MPa |
| Elongation $A$ | ≥ 15% |
| Hardness (HBW) | 220–280 |
Acceptance Criteria
The dimensional and surface quality requirements for the finished blade are demanding:
- Maximum thickness $T_{\text{max}}$ tolerance: ±0.2 mm.
- Exit edge thickness: no knife-edge allowed, must conform to basic dimension.
- Profile-to-template light gap: for pressure side (inner arc) ≤ 0.15 mm, for suction side (back arc) ≤ 0.20 mm.
- No cracks, penetrating defects, or linear defects (length ≥ 3× width). No porosity or inclusions in the critical zone (blade root region).
Selection of Casting Method
Investment casting is another low allowance method, but for this large, thin, and twisted blade (weight ~1.2 kg), the wax pattern deformation, shell cracking, and solidification shrinkage would be severe. Moreover, the mold would be bulky and expensive. Therefore, we chose sand casting foundry as the primary route, aiming to achieve near-net shape using chemically bonded sand molds. In our sand casting foundry, we use organic ester self-hardening sodium silicate sand to obtain good dimensional accuracy.
Process Parameters and Production Conditions
Key process parameters selected for the low allowance sand casting foundry process:
- Shrinkage allowance: 1.8% (linear) based on the steel type.
- Grinding allowance on airfoil surfaces: 0.3 mm per side.
- Sand mixture: 100 parts quartz sand (70 mesh), 4–5 parts sodium silicate (modulus 2.3), 1–2 parts organic ester hardener, and 0.5 parts breakdown additive.
- Melting: 500 kg medium-frequency induction furnace. Pouring temperature controlled at 1580–1620 °C (slightly on the high side to ensure filling of thin sections and facilitate slag floatation).
Casting Process Trials
Three gating system designs were evaluated in our sand casting foundry: side-gating, top-gating, and bottom-gating. The following figure illustrates a typical sand mold used in these trials.
Side-gating system
In the side-gating system, a flat runner (like a flash) was placed along the exit edge from the small end to the large end. A spherical blind riser was located at the large end with a vent. Results from the first trial (5 blades): severe surface defects on the pressure side due to direct steel jet impact; also many inclusions. The blade warpage varied along the length, with maximum deviation of 0.8 mm. The process yield was low (~45%).
Top-gating system
The top-gating system used a shaped top riser that also served as the pouring basin. This design was simpler. First trial results: defects were more dispersed, yield was high (~70%). Warpage pattern similar to side-gating.
Bottom-gating system
Bottom-gating employed a horn-shaped ingate at the small end, with a top riser. This produced a quiet fill, but the thin exit edge suffered from concentrated inclusions. Also, the area under the riser showed shrinkage depression. This approach was abandoned after the first trial.
Based on initial results, we eliminated bottom-gating and focused on side and top systems. The comparison of the three systems is summarized in Table 4.
| Parameter | Side-gating | Top-gating | Bottom-gating |
|---|---|---|---|
| Surface defects | Severe on pressure side | Moderate, scattered | Concentrated on exit edge |
| Process yield | ~45% | ~70% | ~55% |
| Warpage magnitude | 0.5–0.8 mm | 0.4–0.7 mm | 0.3–0.6 mm |
| Shrinkage | Acceptable | Acceptable | Riser area concave |
| Cleaning effort | High (cut runner) | Low (only riser) | Medium |
Analysis of Defects and Deformation
Surface defects
Analysis of inclusions from the first trials showed they were mainly Al₂O₃ and SiO₂, originating from the corundum-based alcohol coating. The coating had poor sintering behavior and low hot strength, causing it to peel under steel flow. We replaced the coating with a zirconium silicate (ZrSiO₄) alcohol coating, which has higher refractoriness, better sintering, and lower thermal expansion. This change drastically reduced surface defects.
Deformation control
Blade warpage resulted from three factors: pattern material, blade geometry, and heat treatment. Initially we used a wood pattern, but it deformed and wore quickly. We switched to an epoxy-resin pattern, which maintained dimensional stability. The blade itself, with thickness varying from 1 mm to 7.5 mm, creates uneven cooling and contraction. The solidification shrinkage strain can be estimated by:
$$ \epsilon = \frac{L_{\text{pattern}} – L_{\text{casting}}}{L_{\text{pattern}}} \times 100\% $$
The total linear shrinkage after casting and heat treatment was measured as 1.8%–2.0%. We applied a variable compensation factor along the blade length, with larger compensation at the thin exit edge and smaller at the thick root. This adjustment brought as-cast warpage within the 0.3 mm grinding allowance.
Heat treatment distortion was the most significant. Initially, blades were placed horizontally and packed together. The uneven heating and cooling caused large bending. We designed a special fixture that holds the blade vertically (exit edge downward) with a slight tilt, allowing uniform air flow. The improvement in flatness after heat treatment can be expressed by the reduction in maximum bow height $\delta$:
$$ \delta_{\text{after}} \leq 0.15\,\text{mm} \quad \text{versus} \quad \delta_{\text{before}} \approx 0.5\,\text{mm} $$
Stable Production Results
After optimizing the top-gating system with zircon coating, epoxy pattern, and heat treatment fixture, we ran a production batch of 10 blades (5 heats of 2 blades each). All blades were inspected. The dimensional accuracy and surface quality met the acceptance criteria. Table 5 lists the measured light gap values for five representative blades.
| Blade No. | Section | Pressure side gap | Suction side gap |
|---|---|---|---|
| B1 | Root | 0.10 | 0.12 |
| Mid | 0.08 | 0.10 | |
| Tip | 0.12 | 0.14 | |
| 2nd from tip | 0.09 | 0.11 | |
| B2 | Root | 0.11 | 0.13 |
| Mid | 0.07 | 0.09 | |
| Tip | 0.10 | 0.12 | |
| 2nd from tip | 0.08 | 0.10 | |
| B3 | Root | 0.09 | 0.11 |
| Mid | 0.06 | 0.08 | |
| Tip | 0.10 | 0.13 | |
| 2nd from tip | 0.07 | 0.09 | |
| B4 | Root | 0.08 | 0.10 |
| Mid | 0.05 | 0.07 | |
| Tip | 0.09 | 0.11 | |
| 2nd from tip | 0.06 | 0.08 | |
| B5 | Root | 0.10 | 0.12 |
| Mid | 0.07 | 0.09 | |
| Tip | 0.11 | 0.14 | |
| 2nd from tip | 0.08 | 0.10 |
All gaps were within the specification (≤0.15 mm for pressure side, ≤0.20 mm for suction side). There were no cracks or linear defects. Minor surface imperfections were repaired by welding and grinding, and the final surface passed inspection.
Conclusion
The low allowance sand casting foundry process we developed has proven to be a viable, economical method for producing complex saber blades. By employing top-gating, zircon-based coating, epoxy pattern, and optimized heat treatment fixturing, we achieved dimensional tolerances and surface quality equivalent to semi-precision casting. The process is stable and repeatable. Since implementation, we have supplied saber blades for two retrofit units, saving approximately 100,000 CNY per unit in material and machining costs. As more units are retrofitted, the economic impact will grow. This sand casting foundry approach can be extended to other static blade types, further reducing production costs and lead times. Future work will focus on reducing the remaining small defects and improving the coating application uniformity.