Abstract
This paper presents a comprehensive process optimization study for the production of a large stainless steel volute casting used in aerospace jet engine testing equipment. The casting, made from ZG12Cr18Ni9Ti stainless steel, requires high integrity due to its exposure to extreme temperatures and pressures during operation. The research involves the design of an optimal casting process, utilizing numerical simulations to predict and mitigate potential defects. The results demonstrate the effectiveness of the optimized process in producing a defect-free casting with excellent mechanical properties.
1. Introduction
Stainless steel castings play a crucial role in various industries, particularly in aerospace, petrochemical, and nuclear applications, due to their excellent corrosion resistance and high-temperature stability. The production of large stainless steel castings, however, poses significant challenges related to defect formation during casting, such as porosity, shrinkage cavities, and hot tearing. This study focuses on the optimization of the casting process for a large stainless steel volute casting used in jet engine testing equipment.
2. Product and Material Overview
The stainless steel volute casting under study is made from ZG12Cr18Ni9Ti, a material known for its high corrosion resistance and good mechanical properties at elevated temperatures. The casting has a single weight of 3.2 tonnes and a maximum dimension of 3800 mm, making it a large and complex component. The casting features a thin-walled structure with 30 internal partitions, each 20 mm thick and 200 mm high, which create hot spots prone to defects during casting (see Figure 1).
Table 1 summarizes the chemical composition of the ZG12Cr18Ni9Ti stainless steel used in this casting.
Table 1: Chemical Composition of ZG12Cr18Ni9Ti Stainless Steel
| Element | Content (wt.%) |
|---|---|
| C | 0.05-0.12 |
| Si | ≤1.50 |
| Mn | 0.80-2.00 |
| P | ≤0.03 |
| S | ≤0.04 |
| Cr | 17.0-20.0 |
| Ni | 8.00-11.00 |
| Ti | 5[ω(C)-0.03]-0.80 |
| N | ≤0.02 |
3. Casting Process Design
The casting process design for the stainless steel volute involves several critical steps, including mold preparation, gating system design, and melting and pouring parameters.
3.1 Mold Preparation
The mold for the volute casting was designed with symmetry in mind, utilizing a divided mold approach with steel cone positioning and a parting allowance of 3 mm. The overall steel shrinkage was set at 2.5%, with a local adjustment to 1.0% for the flanges. A 15 mm machining allowance was incorporated into the mold design. To manage the large sand core required for the internal partitions, a reinforced core structure using steel bars was implemented (see Figure 2).
3.2 Gating System Design
The gating system was designed to minimize defects and ensure proper filling of the casting. Two pouring system configurations were evaluated using computer-aided engineering (CAE) simulations.
Scheme 1:
- One layer of internal gates placed on the parting plane.
- Cross and ingate diameters of ϕ120 mm and ϕ90 mm, respectively.
- Overall yield rate of 63.33%.
Scheme 2:
- Two layers of internal gates, creating a stepped pouring system.
- Improved bottom filling and reduced temperature gradients.
- Overall yield rate of 60.24%.
CAE simulations revealed that while Scheme 1 offered a higher yield rate and simpler mold configuration, it was prone to incomplete filling and shrinkage defects in the bottom flanges. In contrast, Scheme 2 demonstrated better feeding and reduced shrinkage risks but with a slightly lower yield rate (see Figure 3 and Figure 4).
Based on the simulation results, Scheme 1 was selected as the final design, with additional measures implemented to mitigate potential defects.
3.3 Melting and Pouring Parameters
The stainless steel melt was prepared using two 3-tonne electric furnaces, aiming for a pouring temperature range of 1590-1620°C. To reduce nitrogen pickup during melting, the addition of stainless steel return scrap was limited to 20% or less. Titanium was added as 70% TiFe to achieve the desired titanium content, with strict control over the timing and conditions to prevent the formation of harmful TiN inclusions.
4. Numerical Simulation and Process Optimization
CAE simulations were conducted using a commercial software package to predict the filling, solidification, and potential defect formation during the casting process. The simulations were based on the actual gating system design, material properties, and process parameters.
4.1 Simulation Setup
The simulation model considered the following parameters:
- Casting material: ZG12Cr18Ni9Ti stainless steel
- Mold material: Resin-bonded宝珠砂
- Heat transfer coefficient between the casting and mold: 700 W/(m²·K)
- Pouring temperature: 1600°C
- Initial mold temperature: 20°C
- Pouring time: 93 seconds
4.2 Simulation Results and Analysis
The simulation results for both gating schemes are presented in Figures 5-12, highlighting the gas distribution, pressure field, and shrinkage defects at various stages of the casting process.
Based on the simulation outcomes, Scheme 1 was chosen due to its higher yield rate and simpler mold configuration, despite the risk of shrinkage defects in the bottom flanges. To mitigate these risks, additional measures were implemented, including the use of larger risers, local cold irons, and chromium ore sand in critical areas.
5. Production Verification
The optimized casting process was implemented in a production environment, utilizing alkaline phenol-formaldehyde resin-bonded宝珠砂 for molding and core making. The pouring temperature was maintained within the targeted range of 1600-1610°C. Post-casting, the castings were inspected for defects and machined to final dimensions.
Mechanical testing of the machined castings revealed excellent mechanical properties, with an average ultimate tensile strength of 490 MPa, yield strength of 215 MPa, elongation of 28%, and reduction of area of 35%, all exceeding the design requirements (see Table 2).
Table 2: Mechanical Properties of the Stainless Steel Volute Casting
| Property | Value |
|---|---|
| Ultimate Tensile Strength | 490 MPa |
| Yield Strength | 215 MPa |
| Elongation | 28% |
| Reduction of Area | 35% |
Visual and radiographic inspections confirmed the absence of cracks, porosity, shrinkage cavities, and inclusions, demonstrating the effectiveness of the optimized casting process (see Figure 13).
6. Conclusion
This study successfully optimized the casting process for a large stainless steel volute used in aerospace jet engine testing equipment. By combining numerical simulations with practical process adjustments, an effective casting process was developed that produced defect-free castings with excellent mechanical properties. The results demonstrate the value of utilizing advanced simulation tools in the development and optimization of complex casting processes for stainless steel and other high-performance materials.