Abstract:
This comprehensive article delves into the intricate casting process of nodular cast iron components for subway split gearboxes. The focus is on introducing the structural features and technical requirements of these castings, examining three different casting process schemes, and analyzing their feasibility and effectiveness through actual production verification. Utilizing 3D printing technology for core making and MAGMA simulation software for flow rate, thermal modulus, and shrinkage criterion analyses, the study aims to achieve precise feeding in various parts of the casting, thereby preventing defects such as shrinkage cavities and porosity. The article emphasizes the advantages of the third casting scheme, which offers high process yield, fewer sand cores, simplified operation, and conformity with all technical and quality requirements.
Keywords: nodular cast iron, gearbox, casting process, 3D printing, numerical simulation, shrinkage porosity, precise feeding
1. Introduction
Nodular cast iron, also known as ductile iron or spheroidal graphite iron, is widely used in various industries due to its excellent mechanical properties, including high tensile strength, good ductility, and wear resistance. In the railway sector, nodular cast iron components for split gearboxes play a critical role in ensuring the smooth and reliable operation of subway trains. The casting process for these components is particularly challenging due to their thin-walled structures, complex geometries, and high-quality requirements.
This article presents an in-depth study of the casting process for subway split gearbox nodular cast iron components. It begins by introducing the structural features and technical requirements of these castings. Then, three different casting schemes are proposed and analyzed, followed by a discussion of the 3D printing technology used for core making and the MAGMA simulation software utilized for predictive analysis. Finally, the actual production verification results are presented and compared, highlighting the advantages of the third casting scheme.
2. Structural Features and Technical Requirements of Subway Split Gearbox Castings
Subway split gearbox castings consist of upper and lower gearbox components, which are manufactured in pairs. The upper gearbox casting has dimensions of 788 mm × 340 mm × 287 mm, while the lower gearbox casting measures 992 mm × 465 mm × 287 mm. The primary wall thickness is 12 mm, with an axial hole wall thickness of 50 mm and a flange wall thickness of 48 mm on the mating surface. The total mass of each casting is approximately 121 kg.
The material used for these castings is EN-GJS-400-15 nodular cast iron, which meets specific technical requirements, including mechanical properties and non-destructive testing (NDT) standards. The mechanical properties encompass tensile strength, yield strength, elongation, and hardness, all of which must comply with the material specifications. The NDT standards include magnetic particle inspection (MPI), penetrant testing (PT), radiographic testing (RT), and ultrasonic testing (UT), covering both critical and non-critical areas of the castings.
Table 1: Non-Destructive Testing Indicators for Subway Split Gearbox Castings
| Testing Category | Detection Area | Execution Standard | Level | Detection Quantity | Detection Frequency |
|---|---|---|---|---|---|
| MPI | Critical Areas | EN1369-2012 | <SM2, AM2 | All Samples | – |
| PT | Other Areas | DIN EN1371-1-2012 | <SM3, AM3 | – | – |
| RT | Critical Areas | EN12681-2003 | 3 or better | – | Every 10 pieces, at least 1 |
| RT | Other Areas | – | 4 or better | – | – |
| UT | Critical Areas | EN12680-3-2011 | UT2 or better | All Samples | Every 10 pieces, at least 1 |
| UT | Other Areas | – | UT3 or better | All Samples | – |
3. Casting Process Design
The casting process design for subway split gearbox nodular cast iron components involves several critical considerations, including the糊状凝固 characteristics of nodular cast iron, the complexity of the casting structures, and the need for precise feeding to prevent shrinkage defects. Three different casting schemes were proposed and analyzed in detail.
3.1 Process Analysis
In this continuation, we delve deeper into the casting process for the subway split gearbox nodular iron castings, highlighting the key advantages and applications of the chosen scheme.
4. Discussion and Comparison of Casting Process Schemes
The comparison between the three casting process schemes, as outlined in Table 3, highlights the significant advantages of Scheme Three in terms of production efficiency, process simplicity, and overall product quality. Scheme Three, which employs a horizontal middle division, bottom-side pouring, and side heat/dummy riser feeding technique, not only achieves a higher process yield but also reduces the number of sand cores required and simplifies the casting operation.
4.1 Process Efficiency and Yield
Scheme Three exhibits a significant improvement in process yield, achieving 69.6% compared to 53.3% for Scheme One and 71.6% for Scheme Two. This higher yield translates into better material utilization and cost savings during mass production. Additionally, the reduced number of sand cores and simplified casting operations contribute to a more efficient workflow, reducing production time and labor costs.
4.2 Casting Defect Prevention
Utilizing MAGMA simulation software to analyze the flow rate, thermal modulus, and shrinkage criteria of molten iron, Scheme Three ensures precise feeding to prevent casting defects such as shrinkage cavities and porosity. The low pouring velocity of 0.6-0.9 m/s through the side hot/dummy riser minimizes the oxidation of Mg in the molten iron, reducing MgO slag formation. The strategic placement of cold irons, which taper from thick to thin towards the riser, further optimizes the modulus distribution, achieving sequential solidification and minimizing shrinkage issues.
4.3 Quality Assurance
The non-destructive testing results for Scheme Three demonstrate that the castings meet or exceed the specified requirements for ultrasonic testing (UT) and radiographic testing (RT). The key areas of the castings achieve UT0-1 and RT0-3 grades, while other regions meet UT1 and RT0-3 grades, ensuring high-quality production suitable for demanding applications.
4.4 Microstructural and Mechanical Properties
Metallographic and hardness testing reveal that the castings produced under Scheme Three exhibit optimal microstructures and mechanical properties. The graphite nodule count of 160-320 per mm² promotes the formation of graphite, while the ferritic matrix structure ensures good mechanical properties. The castings’ hardness, ranging from 130 to 210 HB, meets the EN-GJS-400-15 standard, confirming their suitability for railway gearbox applications.
5. Conclusion and Future Work
The adoption of Scheme Three for the casting of subway split gearbox nodular iron castings has proven to be highly effective in terms of process efficiency, defect prevention, and product quality. The combination of horizontal middle division, bottom-side pouring, and side heat/dummy riser feeding, coupled with MAGMA simulation for optimal flow and feeding, results in castings with superior microstructural and mechanical properties.
Future research could focus on further optimizing the casting process to reduce energy consumption and material waste. Additionally, exploring the potential for automating the 3D printing and assembly processes could further streamline production and enhance reproducibility. Overall, the success of Scheme Three underscores the value of advanced simulation techniques and innovative casting methodologies.
Further Elaboration on the Casting Process
Material and Melting Process
The nodular iron casting, specifically with a material grade of EN-GJS-400-15, is designed to withstand the demanding operational conditions in the railway sector. The melting process involves precise control of the chemical composition to ensure high-quality castings. The optimal ranges for the main elements are:
- Carbon (C): 3.6% to 3.7%
- Silicon (Si): 2.55% to 2.65%
- Manganese (Mn): ≤ 0.2%
- Phosphorus (P): ≤ 0.03%
- Sulfur (S): 0.008% to 0.012%
- Magnesium (Mg): 0.04% to 0.05%
To achieve the desired nodular graphite structure, the inoculation process uses the impulsion method with 1.1% light rare earth nodularizer and 1.1% silicon-barium inoculant. This mixture is covered with silicon steel sheets to prevent premature oxidation of Mg. The pouring temperature is maintained at (1380 ± 10)°C, with in-stream inoculation using 0.15% sulfur-oxygen inoculant to ensure homogeneity of the iron melt.
Casting Techniques and Process Evaluation
As described in the document, three casting process schemes were devised and evaluated for the gearbox castings. Scheme three, featuring horizontal middle division, bottom side pouring, and side hot/dummy riser feeding, emerged as the most effective due to its high process yield, minimal sand core requirement, straightforward operational procedure, and conformity with both mechanical and non-destructive testing requirements.
- Scheme One: Although capable of satisfying the technical demands, the vertical top side pouring with multiple risers necessitated complex sand core arrangements and subsequent handling.
- Scheme Two: The inclined vertical side pouring and multiple risers scheme increased the assembly complexity, rendering it less feasible for large-scale production.
- Scheme Three: Adopting a bottom side pouring technique from the joint interface of the gearbox and using lateral hot/dummy risers for feeding, scheme three optimizes the mold filling and ensures adequate feeding throughout the casting. The simulation using MAGMA software predicted minimal porosity and shrinkage, consistent with actual production results.
Post-Casting Inspections and Tests
To verify the quality of the castings, various inspections and tests were performed. Non-destructive testing methods, including magnetic particle inspection, penetration testing, radiographic testing, and ultrasonic testing, confirmed that the castings met the stipulated standards, particularly in the critical regions where tight controls were essential.
Furthermore, metallographic examinations revealed the desired microstructure, with the graphite morphology favoring the formation of graphite nodules, which enhances the overall mechanical properties. The hardness tests indicated compliance with the 130-210 HB requirement, verifying the excellence of the casting’s metallurgical characteristics.
Conclusion
The implementation of scheme three for the subway split gearbox nodular iron castings, aided by the use of 3D printed sand cores and supported by MAGMA simulation, demonstrates the effectiveness of advanced casting techniques in ensuring high-quality output. The combined use of accurate control over melt chemistry, pouring temperature, and optimized feeding mechanisms underscores the feasibility of producing castings that not only meet but exceed industry standards.