RAI/Track Safety/TBT-RAI-014

Rail Welding (Thermite) Safety

Rail Works › Track Safety › Rail Welding (Thermite) Safety

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Thermite (aluminothermic) welding joins railway rails using a chemical reaction between iron oxide and aluminium powder.
The reaction produces molten steel at approximately 2,500°C which fills a mould clamped around the rail gap.
The intense heat, molten metal, and violent chemical reaction create extreme burn and fire hazards.
Crucibles containing the thermite charge are positioned above the mould and ignited with a sparkler igniter.
Moisture in the mould, crucible, or rail can cause a violent explosion of molten steel on ignition.
All equipment must be completely dry — even condensation can trigger a dangerous steam explosion.
A safe exclusion zone of at least 5 metres around the weld is required during the pour.
Post-weld grinding and profiling generates noise, sparks, and metal dust requiring additional PPE.
Thermite welding requires specific competence certification under Network Rail standards.
The weld must cool before the rail stress is released or the track is returned to service.

Extreme heat	Molten steel at 2,500°C causes catastrophic burns — any contact with skin, clothing, or eyes is devastating.
Explosion risk	Moisture trapped in the mould or crucible vaporises instantly on contact with molten steel, causing violent explosions.
Rail integrity	A defective thermite weld on an operational railway can cause rail fracture and train derailment.

Don't

Ensure all thermite welding operatives hold current Network Rail certification
Check that the mould, crucible, rail ends, and all equipment are completely dry
Establish an exclusion zone of at least 5 metres around the weld during the pour
Wear full flame-resistant clothing, face shield, and heat-resistant gloves throughout
Preheat the rail ends to the specified temperature before positioning the mould
Inspect the crucible and mould for cracks, damage, or moisture before loading
Keep a fire extinguisher and first aid burn kit within immediate reach
Allow the weld to cool to the specified temperature before grinding or stressing
Use hearing protection and RPE during post-weld grinding and profiling
Record the weld location, date, operative, and batch numbers for quality traceability

DON'T ignite the thermite charge if any moisture is present on equipment or rail
DON'T stand within the exclusion zone during the reaction and pouring phase
DON'T look directly at the thermite reaction without appropriate eye protection
DON'T attempt thermite welding without the required Network Rail certification
DON'T use damaged or cracked crucibles or moulds — they can fail catastrophically
DON'T store thermite portions in damp conditions — moisture causes explosive reactions
DON'T rush the cooling period before grinding or releasing rail stress
DON'T allow combustible materials within the exclusion zone during the weld
DON'T touch the mould, crucible, or weld area until confirmed cool by temperature check
DON'T carry out thermite welding in rain without weatherproofing the entire work area

See also: Rail Safety Awareness | Welding Safety Awareness (Comprehensive)

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