With the rapid development of high-speed railway and heavy-haul freight railway in China, the requirements for the purity, fatigue life and dimensional accuracy of wheel materials have been continuously improved. Railway wheels operate under extremely harsh service conditions: the wheel tread is constantly subjected to rolling contact stress during train operation; severe thermal shock acts on the tread in a short time during the braking process; and the wheel flange bears lateral compression when the train passes through curved tracks. Once metallurgical defects such as porosity, inclusions and internal cracks exist inside the wheel, fatigue cracks are highly likely to initiate under alternating loads, which may even lead to serious railway operation accidents. Therefore, every production procedure of wheels, from smelting, hot forming, heat treatment and mechanical processing to flaw detection, is subject to strict technical specifications.
1 Overall Production Process of Rolled Steel Wheels
1.1 Steel Smelting and Round Billet Cutting
Smelting is the core source of wheel quality. Wheel steel is initially smelted via electric arc furnace or converter, followed by LF furnace refining and RH vacuum degassing treatment to achieve deep purification of molten steel. The contents of harmful impurities including hydrogen, oxygen, sulfur and phosphorus in steel are strictly controlled. For high-speed railway wheels, the hydrogen content is limited to ≤1 ppm, and the total oxygen content is maintained at a low level, which reduces the size and quantity of non-metallic inclusions and eliminates fatigue crack sources from the metallurgical source.
The molten steel that meets the refining standards is processed into large-diameter cylindrical round billets through continuous casting. Subsequently, the continuous cast round billets are cut to fixed lengths by CNC sawing machines to obtain steel blanks for individual wheels. The weight and dimensional accuracy of the blanks ensure reasonable metal distribution in the hub, web and rim during subsequent forging and rolling processes.
1.2 Steel Billet Heating and High-Pressure Water Descaling
The cut steel blanks are sent to a ring heating furnace and heated to 1250‑1300℃ to ensure excellent high-temperature plasticity of the steel. The heating rate is strictly controlled during the heating process to avoid thermal stress caused by internal temperature differences. Immediately after discharging from the furnace, the steel blanks are descaled by high-pressure water spraying to remove high-temperature oxide scales on the surface. Residual oxide scales will be pressed into the surface of the workpiece during subsequent forging and rolling, forming surface indentation defects and significantly reducing the fatigue life of wheels.
1.3 Hot Forging Preforming
The high-temperature steel blanks are transported to a 10,000-ton hydraulic press unit. Firstly, the cylindrical steel blanks are upset into thick round discs. The upsetting process compacts the internal casting porosity of the cast billets and improves the density of the steel matrix. Afterwards, indentation preforming is carried out with dies to form the preliminary contours of the hub, web and rim on the round blanks, completing the initial metal distribution and preparing the blanks for the subsequent wheel rolling process.
1.4 Wheel Rolling (Core Forming Process)
The pre-forged blanks are sent to a special wheel rolling mill, where multi-group radial and axial rollers perform rolling and diameter expansion on the rotating blanks. Under the action of the rollers, the outer diameter of the blanks expands continuously, and the contours of wheel flanges, treads and curved webs are gradually formed. The core advantage of the rolling process is that the metal fibers at the rim extend continuously along the circumferential direction of the wheel, forming a closed annular metal streamline. Compared with the disordered metallographic structure of cast steel wheels, the continuous annular metal streamline can significantly improve the rolling contact fatigue resistance of the rim, which is the fundamental reason why rolled steel wheels outperform cast steel wheels in service performance.
1.5 Punching and Web Bending and Shaping
After rolling, the blanks are transported to the third hydraulic press for center punching to remove the excess core material in the middle of the hub. The web is then bent and shaped by dies to form the final hot-state contour of S-shaped or flat webs, completing the full hot forming of wheel blanks. The formed hot blanks are transferred to a roller conveyor line for slow cooling to release part of the residual stress generated by forging and rolling and prevent cracking during the cooling process.
1.6 Heat Treatment: Tread Local Spray Quenching + Tempering
Heat treatment is a critical process that determines the service performance of wheels. Instead of integral quenching, rolled steel wheels adopt a local quenching process for the rim tread to achieve differentiated performance matching at different wheel positions. Firstly, all wheels are re-heated to 860‑900℃ and held for a period to complete austenitization. After furnace discharging, only the rim tread and outer circle of the flange are rapidly cooled by high-pressure water spraying, while the web and hub are not cooled. After quenching, the rim tread obtains high hardness to ensure excellent wear resistance; the web and hub maintain high toughness to resist impact loads during operation and prevent brittle fracture of the web.
After quenching, all wheels are sent to a tempering furnace and tempered at 480‑520℃ for more than 3 hours to eliminate internal stress generated by quenching, stabilize the metallographic structure and reduce the risk of later cracking caused by residual quenching stress.
1.7 CNC Precision Machining
Oxide scales remain on the wheel surface after heat treatment, which requires CNC machining. Firstly, rough turning is performed to remove the oxide layer. Then finish turning is conducted on the tread and flange to strictly machine the tread taper contour specified by railway standards. The inner hole of the hub is finish-bored to achieve micron-level dimensional accuracy, as it serves as the matching surface for interference oil-pressure assembly with the axle. Meanwhile, finish machining of wheel end faces and all chamfers is completed to ensure that all geometric dimensions of the wheels comply with railway technical standards.
1.8 Non-Destructive Testing and Finished Product Performance Inspection
Railway wheels are subject to mandatory factory inspection, and defective wheels are not allowed to be reused after repair.
Ultrasonic Testing (UT): Detects internal buried defects such as pores, inclusions and internal cracks in wheels;
Magnetic Particle Inspection (MPI): Screens tiny surface and near-surface cracks at the tread fillet and web transition fillet;
Multi-point Hardness Testing: Measures the hardness of the rim, web and hub respectively to verify that the heat treatment hardness zoning meets the design requirements;
Geometric Dimension Re-inspection; High-speed EMU wheels require additional static balance testing;
Laser marking of product traceability marks, including furnace number, production batch and serial number, to realize full-life cycle quality traceability.
1.9 Surface Coating and Warehousing
After passing all inspections, the wheel surface is sprayed with anti-rust protective coatings. The finished wheels are then warehoused, waiting for subsequent oil-pressure cold press interference assembly with axles to form wheel sets.
2 Brief Introduction to Cast Steel Wheel Technology
Cast steel wheels are mainly applied to ordinary freight vehicles with a speed below 100 km/h. Adopting the graphite sand-lined mold casting process, molten steel is poured into wheel molds, and directional solidification is realized through riser feeding. After shakeout, the pouring and riser heads are removed, and the wheels are integrally annealed to eliminate casting stress. Finally, finished products are obtained through mechanical processing and flaw detection. The casting process features a short forming flow, but it has prominent limitations: the metal fibers of the cast structure are disordered, and defects such as residual porosity and shrinkage cavities are prone to exist inside the wheels, resulting in limited rolling contact fatigue performance. Therefore, cast steel wheels cannot meet the service requirements of high-speed and high-power locomotives.
3 Summary of Key Process Points for Rolled Steel Wheels
First, in the smelting stage, the focus is on controlling molten steel purity, reducing hydrogen and oxygen contents and inclusions to restrain defects from the metallurgical source. Second, the forging-rolling forming process relies on large plastic deformation to compact internal porosity and form continuous circumferential metal streamlines at the rim, improving the fatigue resistance of wheels. Third, the differentiated local quenching heat treatment realizes the performance matching of 'hard and wear-resistant rim, tough and impact-resistant web and hub', which is the core concept of wheel design and manufacturing. Fourth, multiple non-destructive testing processes serve as the final factory quality barrier to strictly screen internal and surface crack defects and ensure railway operation safety.
4 Conclusion
Railway rolled steel wheels are typical high-end forged and rolled metallurgical products. Their quality depends on the collaborative control of multiple processes including smelting, forging and rolling, heat treatment, precision machining and non-destructive testing. With the continuous development of China’s rail transit industry towards high speed and heavy haul, wheel manufacturing technology will continue to iterate and upgrade focusing on higher steel purity, optimized tread heat treatment processes and new lightweight web structures, so as to further improve the fatigue life of wheels and provide a solid guarantee for the safe operation of rail transit.
