Abstract:
This article provides a comprehensive understanding of the quality control measures for key steel castings used in electric multiple unit (EMU) trains. It highlights the technical requirements, surface quality standards, and internal quality assurance methods employed in the manufacturing process. Through detailed explanations, case studies, and tables summarizing key points, the article aims to educate steel castings manufacturer and industry professionals on the intricacies of producing high-quality steel castings for EMUs.
1. Introduction
With the rapid development of high-speed rail transportation, the quality requirements for components used in EMU trains, especially key steel castings, have become increasingly stringent. These castings, such as coupler heads, buffers, brake discs, mounting brackets, and axle boxes, play crucial roles in ensuring the safety and performance of EMUs. To meet these high standards, steel castings manufacturer must employ rigorous quality control measures throughout the manufacturing process. This article delves into the technical requirements, quality assurance methods, and case studies related to the production of key steel castings for EMUs.
2. Technical Requirements for Key Steel Castings
2.1 Material Specifications
The material specifications for key steel castings directly influence their overall performance. For instance, brake discs used in EMUs experience significant thermal stress during braking, and their ability to resist thermal shock and maintain high-temperature stability depends on the material’s properties. These properties extend beyond conventional chemical composition control, necessitating strict limits on gas content in the steel, such as:
- Oxygen content ≤ 0.010%
- Hydrogen content ≤ 0.00005%
- Nitrogen content ≤ 0.015%
Furthermore, the mechanical properties of these materials must meet rigorous standards, including:
- Tensile strength ≥ 1050 MPa
- Elongation after fracture ≥ 8%
Additionally, there are strict limits on non-metallic inclusions, with Type II and Type IV inclusions not exceeding Grade 1.
2.2 Surface Quality Standards
The technical specifications for key steel castings for EMUs place high demands on surface quality. This is not only for aesthetic reasons but also to preserve the casting’s corrosion and fatigue resistance, thereby enhancing its lifespan. For components like axle boxes, which are subjected to cyclic stress loads, surface defects can significantly compromise the casting’s structural integrity. Therefore, strict surface quality standards are enforced, such as:
- Scattered non-crack defects with diameters not exceeding Φ1.5 mm and depths not exceeding 2 mm, not exceeding 3 per 100 cm², with a minimum distance of 10 mm from edges or holes and 20 mm between defects.
- Magnetic particle inspection requirements to minimize the occurrence of thermal fatigue cracks on brake disc friction surfaces, adhering to GB/T 9444-2019 for wall thicknesses not exceeding 16 mm.
Certain critical steel castings do not permit any form of welding repairs, posing stringent quality stability requirements for manufacturers.
2.3 Internal Quality Requirements
The internal quality of key steel castings directly relates to the safety of train operations. Radiographic or ultrasonic testing is employed to ensure compliance with internal quality standards. Most critical steel castings for EMUs, such as axle boxes, undergo radiographic testing, with defect levels not exceeding ASTM E446 Class A, B, and C, Grade 2, in critical areas, and no Class D, E, F, or G defects allowed. Initial trains undergo 100% radiographic testing, and only upon full compliance can the frequency of testing be reduced. TB/T 2980-2014 specifies ultrasonic testing for brake discs, with no porosity defects within 8 mm of the friction surface and no defects larger than a 2 mm diameter equivalent flat-bottomed hole within 12 mm of the friction surface.
3. Quality Assurance Measures for Key Steel Castings
3.1 Steel Liquid Quality Control
Ensuring the quality of steel liquid is crucial in the production of key steel castings. Two significant challenges in steel melting are achieving effective final deoxidization and maintaining high purity levels. The total oxygen content in steel liquid is a key indicator of its quality, directly influencing the quantity, size, shape, and distribution of non-metallic oxide inclusions.
Implementing argon bottom-blowing (as illustrated in Figure 3) and wire feeding refining techniques significantly reduces the number and size of non-metallic inclusions in the steel liquid.
Strictly controlling raw and auxiliary materials, such as scrap steel, alloys, furnace lining materials, and stoppers, which are crucial factors affecting product quality. These materials must be purchased, stored, and used strictly according to technical requirements.
Figure 4 depicts the non-metallic inclusions in a certain type of steel casting, showing only fine grades 1.5 of type I and III inclusions.
Adhering to the principle of sequential solidification in process design and adopting measures such as placing chills in areas prone to defects to increase the temperature gradient in these regions.
By following these measures, the quality of the steel liquid is ensured, which is fundamental to producing high-quality key steel castings for EMU trains.
3.2 Surface Quality Control
The surface quality of key steel castings for EMU trains is of utmost importance, requiring both good appearance consistency and minimal surface defects. For castings that allow welding repairs, all surface defects should be addressed before the final heat treatment. For those that do not allow welding repairs, strict process design and operational control are essential.
To prevent magnetic particle inspection defects, such as those exceeding the specified standards (as shown in Figure 5a), measures like adhering to the principle of sequential solidification and increasing the temperature gradient in defect-prone areas (e.g., by placing chills) are implemented. These measures effectively eliminate excessive magnetic particle inspection defects, as evidenced by the absence of such defects in the post-processing magnetic particle inspection results (Figure 5c).
Furthermore, to prevent slag porosity defects, which are common in casting production and often appear on the upper surface of the pouring position, beneath cores, or in dead corners of castings, both process design and manufacturing processes must be strictly controlled. This includes optimizing the gating system, ensuring smooth filling, and setting up risers and slag traps to facilitate the floating of inclusions.
3.3 Internal Quality Control
Internal defects in key steel castings mainly include shrinkage and porosity. To address these defects, good feeding conditions must be established during process design to promote sequential solidification of the casting. The use of exothermic and insulating risers, combined with casting simulation results from software like MAGMA, helps select the optimal casting process plan, reducing and avoiding shrinkage and porosity.
For porosity defects, depending on their source, they can be classified as invasive porosity, reactive porosity, and precipitation porosity. With strict refining control of steel liquid and ample venting measures in molds, precipitation and reactive porosity are rarely observed in the production of key steel castings.
Invasive porosity, as illustrated in Figure 7, occurs when gases generated from the mold or core invade the metal during pouring. To mitigate this, measures such as changing the sand used for the outer mold, optimizing the formulation and structure of the coated sand core, and increasing venting measures in the mold are implemented.
In conclusion, ensuring the quality of key steel castings for EMU trains necessitates comprehensive quality assurance measures covering steel liquid quality, surface quality, and internal quality control. By leveraging advanced technologies and strict process control, the quality of these critical components can be significantly improved, thereby safeguarding the operational safety of high-speed EMU trains.