Abstract
Based on the structural characteristics and casting requirements of high-pressure steam chamber steel castings, a preliminary scheme of the sand casting process for steel castings was designed, and numerical simulation studies on mold filling and solidification shrinkage were conducted using ProCAST software. The simulated results show that the filling process is stable with no insufficient filling, but large shrinkage cavities appear in the thick base and top boss of the steam chamber, accompanied by dispersed porosity defects in the casting. By adding risers and chills to the defective areas based on the simulation results, the improved simulation results are satisfactory with no shrinkage cavity defects.
Keywords: high-pressure steam chamber castings; ZG15Cr2Mo1; sand casting; process simulation; ProCAST
1. Introduction
Steam turbines, also known as steam turbine engines, are rotary steam power devices. The high-pressure steam chamber is a crucial component of the steam turbine, made of ZG15Cr2Mo1 steel. The steam chamber serves as a cavity where steam enters after passing through the main steam valve and before entering the control valve to balance the steam flow. A rotor can be installed in the circular cavity of the steam chamber, with steam entering from the inlet and exiting from the outlet, driving the rotor to rotate.
Operating at a high temperature of approximately 565°C and a pressure of 8.28 MPa (approximately 82 atmospheres) for 10 minutes without leakage, the high-pressure steam chamber must maintain continuous operation and meet a 20-year service life requirement. Therefore, the high-pressure steam chamber is exposed to high temperature and pressure conditions for a long time, demanding extremely high quality of the casting body, for which cast steel is predominantly used.
Traditional casting industries often adopt a trial-and-error approach, repeatedly adjusting and improving casting schemes through experimentation, leading to significant waste of manpower and material resources and prolonged production cycles. Utilizing casting simulation software for simulation and optimization of casting processes can reduce research and development cycles and labor costs.
In this paper, the ProCAST casting simulation software is used to simulate the sand casting process of high-pressure steam chamber steel castings to obtain an optimized casting process scheme. This research topic originates from an urgent casting challenge faced by enterprises and was submitted to the competition organizing committee as part of the 14th Casting Process Design Competition of the 2023 China Undergraduate Mechanical Engineering Innovation and Creativity Competition.
2. Characteristics and Requirements of the Casting
The high-pressure steam chamber casting is made of alloy cast steel ZG15Cr2Mo1 with a density of 7.8 g/cm³. The chemical composition of ZG15Cr2Mo1 steel is presented in Table 1.
Table 1: Chemical Composition of ZG15Cr2Mo1 Steel
| Element | Content (%) |
|---|---|
| C | 0.12-0.20 |
| Si | 0.20-0.50 |
| Mn | 0.40-0.70 |
| Cr | 1.00-1.50 |
| Mo | 0.45-0.60 |
| P | ≤0.035 |
| S | ≤0.035 |
Based on the dimensional characteristics of the high-pressure steam chamber steel casting, the minimum wall thickness for sand-cast steel castings, according to the casting handbook, is 20 mm. The wall thickness analysis results of the steam chamber casting, indicating that the casting meets the requirements for minimum and critical wall thicknesses.
3. Analysis of Casting Process Plan
The design of the casting process plan includes the determination of molding (core) material, pouring position of the casting, parting surface, sand cores, and process parameters.
3.1 Determination of Pouring Position and Parting Surface
The pouring position refers to the orientation of the casting in the mold during pouring. The casting parting surface is the joint surface between the components of the mold made for easy removal of the pattern. The selection of the parting surface should align with the pouring position as much as possible, simplifying the casting process, enhancing productivity, reducing costs, and improving casting quality.
Reasonable selection of the pouring position and parting surface can directly simplify the casting process, increase productivity, reduce costs, and improve casting quality. For the studied high-pressure steam chamber casting, there are two reasonable pouring positions and parting surfaces: vertical and horizontal.
Considering the shape, structure, and dimensions of the casting, vertical pouring would result in an excessively high sandbox, causing significant impact force on the mold cavity during filling, leading to unstable filling. Meanwhile, selecting the parting surface on the upper surface of the steam chamber base satisfies the principle of placing the parting surface at the maximum cross-section but may lead to easy misalignment during mold assembly.
When pouring in the horizontal position, the maximum cross-section can be used as the parting surface, facilitating mold extraction. During the design of the feeding process, the thicker parts of the casting are positioned above, facilitating the placement of risers for feeding. Additionally, the horizontal pouring position avoids the use of core supports, simplifying the process design. Therefore, the horizontal pouring position and parting surface are adopted for the process design.
3.2 Design of the Mold
The pouring temperature of the steam chamber steel casting is high (about 1580°C), and the molten steel exerts strong thermal shock on the sand mold, requiring high refractoriness for the sand and sand cores. The SiO2 content in the selected raw sand should be ≥97%. Phenolic resin sand is used for molding and core-making, with corundum powder alcohol-based coating applied using a convenient, efficient, fast, and effective spray method.
The placement of sand cores during casting. Based on the internal cavity structure of the casting, three sand cores are required. The left and right parts of the steam chamber internal cavity are divided into two sand cores (No. 1 and No. 2), used for casting the steam chamber internal cavity. For the outer shell holes, to ensure one-time casting, a third sand core (No. 3) is designed.
3.3 Casting Process Parameters
This steel casting is produced in small batches using resin-bonded sand manual molding. The dimensional tolerance grade is CT13 (GB/T 6414—2017 “Dimensional Tolerances, Geometrical Tolerances, and Machining Allowances for Castings”), and the weight tolerance grade is MT12 (GB/T 11351—2017 “Weight Tolerances for Castings”), with a weight tolerance value of 8%. To obtain castings with high dimensional accuracy, the blocked-in linear shrinkage rate is 1.8%.
4. Casting Process Design Based on Theoretical Calculations
The pouring temperature of steel castings is high, and during pouring, the molten steel generates a large amount of radiant heat to the mold cavity, causing the cavity to heat up and expand, resulting in sand collapse. In the production of medium and large castings, the large volume of molten steel poured requires rapid pouring to mitigate the severe harm to the sand mold from the thermal radiation of the molten steel. The gating system for steel castings generally adopts an open type and is poured using a bottom-pouring ladle.
This casting belongs to a medium-sized structure within the range of large castings. Considering the pressure-bearing technical requirements of the casting, the rising velocity of the molten steel is increased by 30% to 50% compared to that of complex castings, taken as vL = 35 mm/s. The pouring time is calculated using Equation (1), considering the long metal flow path and the need for pressure verification.
5. ProCAST Numerical Simulation and Improvement
5.1 Determination of Initial Simulation Parameters
According to the preliminary casting process plan, we employed the ProCAST numerical simulation software to simulate the filling and solidification processes of the high-pressure steam chamber casting. With a known liquidus temperature of 1501°C for this casting, the pouring temperature was approximately set to 1600°C during the simulation. The mesh division of the system model for the casting process plan, which serves as the foundation for the simulation analysis.
During the simulation, specific process parameters were set to ensure the accuracy of the simulation results. The pouring time was set to 18 seconds, a choice aimed at ensuring smooth and adequate filling of the mold cavity by the molten metal. Additionally, the heat transfer coefficient between the metal and the sand mold was set to 750W/(m^2·K), reflecting the heat exchange between the molten metal and the sand mold during solidification.
Through the ProCAST software, we conducted comprehensive and in-depth analysis of metal flow, temperature distribution, and solidification behavior during the casting process. This provided valuable data support for subsequent improvements to the casting process.
In the simulation results, we focused on the filling process of the molten metal and defect prediction during solidification. The simulation results of the filling process helped us understand the flow state of the molten metal in the mold cavity, thereby judging whether there were issues such as insufficient pouring or unstable filling. The simulation results of the solidification process, on the other hand, revealed the locations and sizes of potential defects such as shrinkage cavities and porosity in the casting.
5.2 Supplementary Analysis of Simulation Results
The simulation results indicated that the molten metal exhibited stable behavior during the filling process, with no instances of insufficient pouring, suggesting that our designed casting process plan was feasible in terms of filling. However, during solidification, the simulation predicted the presence of large shrinkage cavity defects in the thick sections of the steam chamber base and at the top boss, along with dispersed porosity defects throughout the casting.
Shrinkage cavities and porosity are common defects in the casting process, typically formed due to the contraction of molten metal during solidification without effective feeding. In the high-pressure steam chamber casting, the presence of these defects can severely impact the casting’s performance and service life. Therefore, we needed to improve the casting process plan based on the simulation results to eliminate or reduce these defects.
5.3 Improvement of the Process Plan
Addressing the predicted defect locations and types in the simulation results, we made the following improvements to the casting process plan:
Firstly, risers were designed at the thick sections of the steam chamber base and at the top boss. The role of risers is to provide additional molten metal to feed the contraction that occurs during the solidification of the casting, thereby preventing the formation of shrinkage cavities and porosity defects. The size of the risers was calculated using the modulus method, ensuring that their position and quantity could meet the feeding requirements.
Secondly, chills were placed at the steam chamber partitions and in areas within the casting near the base where defects were prominent. The role of chills is to accelerate the cooling rate of localized regions of the casting, causing these areas to solidify before others. By placing chills, we could direct the solidification sequence of the molten metal, making it more likely for shrinkage cavities and porosity defects to be concentrated in the risers, thereby improving the casting quality.
After three-dimensional modeling of the improved process plan, simulations were once again conducted using the ProCAST numerical simulation software. The simulation results showed that the shrinkage cavity situation in the casting had been significantly improved, with the shrinkage cavities successfully transferred to the risers, and no obvious shrinkage cavity defects remained in the casting itself. Although the placement of chills led to an increase in the porosity depth at the boss, overall, the casting quality was significantly enhanced.
In summary, by employing the ProCAST numerical simulation software to simulate and optimize the casting process for the high-pressure steam chamber casting, we successfully designed a more reasonable casting process plan. This plan not only improved the casting quality but also provided strong technical support for subsequent actual production.