In the demanding sectors of petroleum, chemical engineering, and nuclear power, large high-pound-class ball valve bodies serve as critical pressure-containing components. The internal density and dimensional accuracy of these castings directly determine equipment safety and service life. The traditional investment casting process, employing a top-gating system, although structurally simple, frequently introduces severe casting defects. The primary challenge is the turbulent metal flow during filling, which leads to gas entrapment and oxide slag inclusion. These issues result in a low qualification rate, often ≤ 80%, and an unsatisfactory process yield, typically ≤ 28%. To overcome these technical hurdles of low product qualification and low process yield in the investment casting of these large safety-critical components, this study employs ProCAST numerical simulation technology to optimize the casting process. The goal is to enhance the performance and production efficiency of large ball valve castings, meeting the increasing demands for precision and reliability in the industry.
Cast Part Information and Process Analysis
Basic Part Information
The casting material is CF8, a standard grade of austenitic stainless steel widely used in the investment casting industry for its corrosion resistance and mechanical properties. The chemical composition requirements for CF8 are strictly defined, with the key elements including carbon (≤0.08%), silicon (≤2.00%), manganese (≤1.50%), phosphorus (≤0.040%), sulfur (≤0.040%), chromium (18.0–21.0%), molybdenum (≤0.50%), and nickel (8.0–11.0%). The main body of the casting has an overall envelope size of 360 mm × 522 mm × 640 mm. The final cast weight is 164 kg. A critical geometric characteristic of the investment casting is its varying wall thickness; the maximum wall thickness is 16.5 mm, while the average wall thickness is 14 mm. This ratio creates a challenge for achieving consistent solidification and feeding. The internal quality requirements are exceptionally stringent. Internal defects such as shrinkage porosity, shrinkage cavities, and slag inclusions are strictly prohibited. The non-destructive testing (NDT) specification requires 100% radiographic testing (RT) of every casting, with the acceptance criteria set to ASME B16.34-2004 Class III, a high standard for pressure-containing parts in the investment casting industry.
| Element | C | Si | Mn | P | S | Cr | Mo | Ni |
|---|---|---|---|---|---|---|---|---|
| Standard | ≤ 0.08 | ≤ 2.00 | ≤ 1.50 | ≤ 0.040 | ≤ 0.040 | 18.0–21.0 | ≤ 0.50 | 8.0–11.0 |
The original process design for this investment casting utilized a conventional top-gating system. In this configuration, the molten metal is poured from the top of the mold cavity and, driven by gravity, flows rapidly downward to the bottom. This filling pattern is inherently unstable. The molten metal stream plunges directly into the cavity, causing significant splashing and turbulence. The high kinetic energy of the falling metal erodes the mold, and the chaotic flow pattern makes it impossible for the mold cavity to fill uniformly and smoothly. The original process design attempted to manage solidification shrinkage with several square-shaped risers located on the casting surface. However, because the primary problem originated from the defective filling pattern, the risers’ feeding capability was severely compromised. The original process used a pouring temperature range of 1560 °C to 1580 °C. The measured filling velocity for the molten metal was high, between 0.8 m/s and 2.4 m/s. This high speed was a major contributing factor to the severe impact of the metal stream on the bottom of the mold.
Analysis of Casting Defects
The turbulent filling within the investment casting mold was the root cause of the low pass rate. The chaotic flow of molten metal mixes excessively with the air present in the mold cavity. This mixing is the primary mechanism for two major defect types. First, it leads to severe gas entrapment (air entrainment), which manifests as gas porosity or blowholes within the solidified casting. Second, the large surface area of the turbulent metal stream is exposed to the oxidizing atmosphere, leading to the formation of oxide films and slag inclusions. These non-metallic inclusions are detrimental to the mechanical integrity and pressure tightness of the ball valve. Furthermore, the non-uniform filling caused inconsistent thermal history across the casting. Certain sections, especially thicker ones or those last to fill, experienced a concentration of heat, creating distinct hot spots. These hot spots lacked sufficient feeding from the risers due to the poor directional solidification setup, making them highly susceptible to shrinkage porosity. The actual production data confirmed these findings, showing consistent defect patterns in specific areas of the casting, as predicted by the simulation.
ProCAST Simulation Analysis of the Original Investment Casting Process
To quantify the problems with the original process for the investment casting, a detailed numerical simulation was conducted using ProCAST. The software’s parameters were carefully set to reflect the material properties of CF8 stainless steel (density 7.93 g/cm³, thermal conductivity 16.2 W/(m·K)), the pouring temperature, and the filling velocity. The simulation results provided a dynamic visualization of the metal flow and solidification, confirming the qualitative analysis. The flow pattern clearly showed the molten metal stream impinging on the bottom of the mold cavity and violently spreading outwards, creating multiple recirculation zones and vortices. This turbulent flow was the main source of the predicted air entrapment. The simulation also accurately predicted the location of hot spots, which corresponded precisely with the areas where defects were most frequent in actual production. The solidification simulation showed that these hot spots remained molten long after the surrounding areas had solidified, creating isolated liquid pockets that could not be fed by the risers. The ProCAST defect prediction module clearly identified these regions as high-risk zones for shrinkage porosity and micro-shrinkage. The high degree of correlation between the simulation and the actual manufacturing defects confirmed the model’s validity and provided a clear target for the optimization phase of this investment casting project.
Optimization of the Investment Casting Process Plan
Gating System Modification
The primary conclusion from the ProCAST analysis was that the fill pattern was the root cause of most defects. Therefore, the core optimization strategy for the investment casting process was to change the gating system from a top-gate to a bottom-gate design with stepped runners. The bottom-gate design fundamentally alters the filling mechanism. In this system, the molten metal enters the mold cavity from multiple gates located at the bottom. As the metal is poured, it fills the cavity from the bottom up, rising with a smooth, tranquil front. This eliminates the splashing and turbulent impact associated with top-gating. The stepped runner system is a critical enhancement to the bottom-gate concept. It controls the flow of metal into different sections of the casting at different times. The first and lowest step fills the bottom of the casting. As the metal level rises, subsequent gates from the runner system become active, delivering fresh, hot metal to the mid and top sections of the casting. This method ensures a progressive and uniform fill, which is essential for reducing thermal gradients and preventing hot spot formation. To further stabilize the flow, the pouring temperature was slightly adjusted to 1550–1580 °C. A key design feature was the inclusion of a “bumper” or “buffer head” in the runner system just before the metal enters the mold cavity. This flow control device absorbs the kinetic energy of the stream, significantly reducing its velocity and ensuring a laminar, non-turbulent entry into the mold. After this optimization, the simulated filling velocity dropped from 0.8–2.4 m/s to 0.3–0.8 m/s. This lower velocity is critical for a stable, defect-free filling of the investment casting mold.
Adjustments to Riser Structure and Chills
In conjunction with the gating system changes, the feeding system (risers) and cooling system (chills) were also redesigned to work in harmony with the new bottom-gating process. The original risers were poorly positioned, making it difficult for them to feed the critical seat ring area of the ball valve, which is a thick section. The distance over which they had to feed was too great. In the optimized process for this investment casting, the gating system itself was reconfigured so that the ingates were now positioned directly at the seat ring area. This allows the hot metal entering the cavity to directly feed this heavy section. The previous, large square risers on the top of the casting were found to be too high and actually increased the risk of shrinkage in the areas they were meant to feed. They were replaced with smaller, more efficient risers. The internal core of the casting was also redesigned to reduce a moderate hot spot, which also decreased the amount of material to be machined away later, thereby lowering costs. The goal was to create a clear, directional solidification path. The metal should solidify first in the thinnest sections and last in the thickest sections (the risers). By placing the ingates at the thickest part of the casting and using small, highly effective risers, the feeding efficiency was greatly improved. Furthermore, external chills were strategically placed on the mold surface. Chills are high-thermal-conductivity materials that accelerate the local solidification rate. By carefully positioning chills, we could accelerate the cooling of certain thinner sections, “squeezing” the liquid toward the riser. This combination of a bottom-gated runner system, ingates at the heavy section, optimized risers, and strategically placed chills created a much more controlled and favorable temperature field, dramatically improving the internal soundness of the investment casting.
| Parameter | Original Process | Optimized Process |
|---|---|---|
| Gating System | Top-gate | Bottom-gate with stepped runners |
| Pouring Temperature | 1560–1580 °C | 1550–1580 °C |
| Filling Velocity | 0.8–2.4 m/s | 0.3–0.8 m/s |
| Ingate Location | Top of casting | At the seat ring area (bottom) |
| Riser Design | Large, tall square risers | Smaller, efficient risers |
| Chill Application | Minimal or standard | Strategic placement for directional solidification |
The success of this investment casting optimization can be expressed in terms of key performance indicators. The reduction in defects was very significant. The calculated reduction in the hot spot area was 15%, which can be generalized with the following formula:
$$ \text{Defect Area Reduction \%} = \frac{\text{Original Hot Spot Area} – \text{Optimized Hot Spot Area}}{\text{Original Hot Spot Area}} \times 100\% = 15\% $$
The simulation predicted the final internal defect ratio (porosity) to fall below 0.5%, a dramatic improvement over the baseline process. The process yield, defined as the weight of the final cast part divided by the total weight of the metal poured, was a primary target. The original process yield was very poor.
$$ \text{Yield} = \frac{\text{Casting Weight}}{\text{Poured Weight}} \times 100\% = \frac{164 \text{ kg}}{ \approx 586 \text{ kg}} \times 100\% \leq 28\% $$
The new design, with its smaller risers and improved feeding, dramatically reduced the amount of metal in the gating and riser system. The new, high yield was a direct result of the design changes.
$$ \text{Yield} = \frac{\text{Casting Weight}}{\text{Poured Weight}} \times 100\% = \frac{164 \text{ kg}}{ \approx 328 \text{ kg}} \times 100\% = 50\% $$
This 50% yield represents a significant cost savings in both metal and energy for the investment casting operation.
Validation and Results
Simulation Validation and Trial Production Results
The first step of validation was to run a full ProCAST simulation on the new optimized investment casting design. The simulations were highly encouraging. The fluid velocity plots showed a stable, non-turbulent filling wave, rising smoothly from the bottom. The filling velocity was confirmed to be safely within the target range of 0.3 to 0.8 m/s. The solidification simulation showed a well-controlled temperature gradient. The hot spot regions that plagued the original part were no longer present. The shrinkage porosity prediction model showed a defect rate below 0.5%, which is an excellent result for a thick-section investment casting. Following this successful simulation, a small batch of castings was produced using the new process. These trial castings were subjected to rigorous X-ray inspection (RT). The radiographic images were clean, with no indications of shrinkage, gas porosity, or slag inclusions. The internal soundness of the investment casting was proven to meet the stringent requirements of ASME B16.34 Class III.
| Metric | Original Process (Simulation) | Optimized Process (Simulation) | Optimized Process (Trial Production) |
|---|---|---|---|
| Filling Velocity | 0.8–2.4 m/s | 0.3–0.8 m/s | Not measured directly |
| Hot Spot Area Reduction | Baseline | 15% reduction | Not quantified, but visually absent |
| Predicted Defect Rate (Porosity) | High (estimated >5%) | ≤ 0.5% | None detected by X-ray |
Machining Validation and Mass Production
Validation of the investment casting process continued with machining of the trial castings. The castings were machined to their final dimensions. The results showed that the parts were free of surface and subsurface defects that might be exposed during machining. Most importantly, the final dimensions were measured and confirmed to be within the tight tolerances specified by the customer. The casting held its shape accurately, proving that the new process did not introduce distortion. The chemical composition of the final castings was also verified to be within specification for CF8 stainless steel. The success in the trial phase was then replicated in full-scale mass production. Over a sustained production period, the data was collected and analyzed. The product qualification rate, which is the percentage of castings passing the 100% X-ray inspection, stabilized at over 98%. This was a monumental increase from the previous ≤ 80% rate. The process yield was also measured. Thanks to the smaller, more efficient gating and riser system, the yield for this investment casting process was confirmed to be 50%, a massive improvement from the original ≤ 28%. This double improvement—in quality and cost— validates the power of using ProCAST simulation for process optimization in the investment casting of highly demanding, large, and complex components.
| Metric | Original Process | Optimized Process |
|---|---|---|
| Machining Allowance | Large (inner core solid, reduced flow area) | 3 mm |
| Dimensional Accuracy | Poor (core and inner bore prone to distortion) | Excellent (meets all acceptance standards) |
| Product Qualification Rate (NDT) | ≤ 80% | Stable at ≥ 98% |
| Process Yield | ≤ 28% | Increased to 50% |
Conclusion
This project successfully demonstrates the immense value of using ProCAST numerical simulation for the systematic optimization of a complex investment casting process. The work successfully resolved the chronic problems of shrinkage porosity, slag inclusion, and low process yield that had plagued the production of large high-pound ball valve bodies. The primary breakthrough was achieved by abandoning the flawed top-gating system and implementing a sophisticated bottom-gating system with stepped runners. This single change, supported by a redesigned riser and chill layout, fundamentally altered the filling and solidification behavior. The optimized process leads to a smooth, controlled fill with a stable wave front, drastically reducing gas entrapment and oxidation. The improved thermal field, driven by the strategic placement of ingates and chills, ensures proper directional solidification and efficient feeding, eliminating shrinkage porosity. The results speak for themselves: a defect rate (porosity) reduced to below 0.5%, a product qualification rate of over 98% in production, and a process yield doubled from ≤ 28% to 50%. This case study provides a robust and reliable reference for the design of gating systems for similar heavy-walled, pressure-containing castings in the investment casting industry. The methodology—simulation analysis, design modification, simulation validation, and production verification—is a proven formula for achieving world-class quality and cost-effectiveness in the manufacturing of critical safety components.