Veritasium - Thermite Welding and Railroads

Introduction to Thermite Reactions

• Hans Goldschmidt's Discovery:
  • Thermite reactions involve metal oxides reacting with aluminum powder to produce pure liquid metal.
  • Initially used for repairing heavy machinery in remote areas.

Application in Railroads

• Early Railroads:

  • Rails were around 12 meters, joined by fish plates creating gaps.
  • Gaps caused the characteristic "ta-tak" sound and required frequent maintenance.

• Challenges with Welded Rails:

  • Concerns about thermal expansion causing rail buckling and derailments.
  • Tram rails were the first to adopt welded rails to avoid noise.

Thermite Welding Process

• Steps in Welding Rails:
  1. Temperature Considerations:
     • Weld at neutral temperature to avoid stress.
  2. Creating a Gap:
     • Cut 2.5 cm from rail end with a saw or torch.
  3. Alignment of Rails:
     • Align rails vertically and horizontally to ensure seamless connection.
     • Correct any twisting.
  4. Preheating:
     • Preheat rail ends to avoid rapid cooling and brittleness.
     • Demonstrated effects of rapid cooling vs. slow cooling on hardness.
  5. Molding and Sealing:
     • Attach molds and seal gaps with sand to prevent leaks.
  6. Igniting Thermite:
     • Proper timing after preheating.
     • Thermite creates molten steel and aluminum oxide.
  7. Cooling and Cleanup:
     • Let the steel solidify and then remove excess material.
     • Grind welds to smoothen rail surface.

Structural Insights

• Crystal Structure:
  • Thermite weld creates different zones: normal rail, thermite steel, heat-affected zone (HAZ).
  • HAZ is typically the weakest.
  • Tested weld strength by bending until failure.

Thermite Welding in Practice

• Global Use:
  • 2 million thermite welds annually; 50 km of rail created from liquid steel.
• Thermal Expansion Compensation:
  • Rail stress managed by sleepers and ballast.
  • High neutral temperature preferred to avoid summer buckling.

Conclusion

• Benefits:
  • Reduces rail maintenance and allows faster trains.
• Future Content:
  • Next episode to explore thermite in challenging conditions.
  • Encouraged to subscribe for updates.