Welding process and quality control of rail joints
- What is the application scenario difference between flash welding and gas pressure welding?
Flash welding (heated to 1,300℃ + upsetting) achieves joint strength of 95% of the base metal, suitable for high-speed railways (speed ≥300km/h), with straightness ≤0.3mm/m. Gas pressure welding (oxygen-acetylene heating to 1,250℃ + pressurization) reaches 90% of base metal strength, suitable for conventional railways, with straightness ≤0.5mm/m. Flash welding is efficient (3 minutes per joint) but requires expensive equipment, while gas pressure welding is portable for field repairs. A high-speed rail project using gas pressure welding mistakenly exceeded straightness standards, causing severe train (bumping) during passage.
- How does the "preheating temperature" in thermite welding affect welding quality?
Thermite welding preheating must reach 900-1,000℃ (rail head appears cherry red). Inadequate preheating leads to: ① poor weld fusion (slag inclusion rate increases by 20%); ② joint tensile strength <800MPa (standard ≥900MPa); ③ "incomplete penetration" defects in ultrasonic testing. At a construction site, insufficient oxygen pressure caused preheating temperature to only reach 800℃, resulting in 40% of joints failing inspection and requiring full rework. It is recommended to use an infrared thermometer (accuracy ±20℃) to monitor preheating temperature.
- What are the key process points for "heat treatment" of welded joints?
After flash welding, induction quenching (heated to 900℃ + water cooling) is required to make the weld hardness reach HB330-350, consistent with the base metal. Inadequate quenching temperature (e.g., 800℃) causes soft spots in the weld, with wear rate 3 times faster than the base metal; excessive temperature (1,000℃) leads to quenching cracks. Due to a quenching coil failure in a factory, the temperature reached 1,050℃, causing longitudinal cracks in 20% of joints, resulting in batch scrapping.
- How is the "scanning sensitivity" set for ultrasonic flaw detection?
Flaw detection requires setting sensitivity ≥Φ3mm flat-bottom hole, with scanning speed ≤100mm/s. A batch of welded joints set sensitivity at Φ4mm, missing Φ3.5mm pore defects, leading to joint fracture during train passage and causing an operational shutdown. The correct approach is to calibrate with a contrast test block to ensure Φ3mm defect detection, with signal-to-noise ratio ≥6dB.
- What are the operation steps for "flatness adjustment" of welded joints?
① Measure height difference with a 1m straightedge; grind if >0.3mm. ② Use a rail grinder (2000rpm) to grind longitudinally, feed ≤0.1mm/time. ③ Check roughness (Ra≤6.3μm) with a profilometer. ④ Final check with a 3m chord: flatness ≤0.2mm. A construction site skipped roughness testing, causing wheel wear and ¥500,000 in rework costs after 6 months.