How Are Seamless Rails Welded?

At the Wuhan Welding Base of the Wuhan Maintenance Depot, Wuhan Railway Bureau Group Co., Ltd., every steel rail undergoes a remarkable transformation. Hundred-metre rails transported from steel mills are subjected to 17 meticulous processes-including welding, rough milling, normalising, precision milling, and flaw detection-before ultimately being forged into 500-metre-long seamless rails.

 

Welding seamless rails, though seemingly straightforward, represents an extreme pursuit of precision, strength, and safety.

Since the start of this year, the Wuhan Rail Welding Base of the Wuhan Engineering Maintenance Depot has completed over 600 kilometres of seamless rail welding for projects including the Wuhan-Yiyang section of the Yangtze River High-Speed Railway and the Xiangyang-Jingzhou High-Speed Railway. Annual welding capacity has increased from 1,500 to 1,800 kilometres, with the deep integration of intelligent equipment and digital systems driving simultaneous leaps in production efficiency and product quality.

 

Welding precision reaches 'millimetre level'

Just as every person possesses a unique identity card, each rail carries its own distinctive 'identity barcode'. This QR code, imprinted at the rail's end, chronicles its entire history. Prior to welding, staff scan the barcode to input details, completing the rail's 'registration process'. Specialised inspection tools then meticulously examine the rail's profile shape and surface quality, ensuring all specifications meet standards before welding commences.

Prior to welding, the rail ends must be thoroughly cleaned. An automatic descaling machine meticulously treats the rail head, rail foot, and end faces, with the descaling depth strictly controlled within 0.2 millimetres – approximately the thickness of two sheets of standard printer paper. Too shallow a depth compromises weld quality, while excessive depth damages the rail body.

Welding constitutes the most critical stage of the entire process. Before positioning the rails on the fixed flash butt welding machine, the ends of both rails must be precisely aligned, with vertical and horizontal deviations not exceeding 0.2 millimetres. Xu Tengfei, foreman of the rail welding section, explains that seamless rails must withstand immense centrifugal forces during high-speed cornering and endure hundreds of tonnes of thermal stress from seasonal temperature fluctuations. Any minute flaw at a weld point could be amplified indefinitely during prolonged operation. After end alignment, tens of thousands of amperes of current pass through the joint, instantly heating the rail ends to over 1000 degrees Celsius. Enormous pressure then forcibly compresses the molten metal. Though the entire welding process takes only about two minutes, it involves seven precisely controlled stages including preheating, burning through, and forging.

Human-machine collaboration boosts efficiency by 15.4%

Following welding, protruding weld beads remain at the rail joint, necessitating further processing. Within the precision grinding zone, fully automated grinding robots meticulously refine the weld heads. Guided by laser systems, robotic arms execute precise movements, generating cascades of golden sparks as grinding wheels contact the weld beads. 'Our objective is to achieve zero excess weld material, ensuring the weld head's geometric profile perfectly matches the original rail,' explains on-site technician Yu Runze. The intelligent grinding system employs laser 3D scanning to automatically identify weld contours, dynamically adjusting grinding paths to prevent over-grinding while ensuring no areas are missed. Previously, this task relied entirely on manual grinding wheel operation, characterised by high labour intensity, poor working conditions, and inconsistent quality control. Today, the integration of 'CNC milling machines and fully automated grinding robots' enables human-machine collaboration, boosting efficiency by 15.4%.

Three Strict Tests Before Approval

Welded rails undergo rigorous inspection. At the flaw detection station, technician Tu Aibing meticulously scans the weld head surface with an ultrasonic probe, eyes fixed on the screen's waveform fluctuations. 'We enforce an uncompromising standard: any waveform indicates rejection,' Tu states. Ultrasonic testing reveals internal flaws invisible to the naked eye. Combining automated inspection with manual verification ensures zero internal damage or hidden defects in every welded joint. Passing this test, joints then endure the more rigorous drop hammer test. A one-tonne iron hammer strikes the welded joint from a height of 5.2 metres. 'This simulates the most extreme stress conditions during train passage,' explains quality engineer Li Wei. 'Weld heads must withstand three consecutive powerful impacts without fracture to earn their "factory qualification certificate",' explained quality engineer Li Wei.

In the finishing section, quality inspectors conduct final checks using electronic straightedge gauges. Here, each weld head must pass three critical stages: visual inspection, specialised flaw detection, and finished product verification. 'We've established a three-tiered control system-spanning self-inspection and mutual checks at workstations, sectional re-inspection, and workshop sampling-to institutionally guarantee 100% welded joint compliance,' stated Zhang Hanfang, Director of the Rail Welding Workshop at this facility.