Thermite welding defects are the leading cause of premature rail joint failure, unplanned line closures, and costly re-welding work. Most of these defects are preventable when crews understand the root cause behind each one.
This guide follows TB/T 1632.3-2019 and railway research published by the China Academy of Railway Sciences. Where a consumable supplier specifies different values, the supplier instruction takes precedence.
Here are the 7 most common thermite welding defects, why they form, and how to prevent them.
1. Thermite Welding Defects Explained: Shrinkage Cavities and Porosity
Shrinkage cavities have an irregular inner surface and typically form at the rail web centre and the triangular zone at the rail base centre of the weld.
Causes
- Localised overheating with uneven cooling rates
- Excessive preheat time or incorrect preheat pressure
- Preheat torch or sand mold positioned off-centre, causing one-side overheating
- Excessive rail gap
- Poor riser feeding, so solidification shrinkage is not compensated
Prevention
- Control preheat time; never burn the rail ends
- Keep preheat flame length moderate
- Centre the preheater over the rail gap
- Keep the rail gap within specification
- Match riser design and weld metal volume to ensure full feeding
2. Thermite Welding Defects Explained: Gas Porosity
Gas porosity appears as smooth, round holes trapped below the weld surface, most often at the rail base and its side slopes. It is the most frequently seen thermite welding defect in field work.
Causes
- Dirty rail ends with water or oil residue
- Moist flux
- Moist sand mold
- Excessive or wet mould sealing compound
- Preheat temperature too low
- Long delay between preheating and pouring, cooling the joint too much
Prevention
- Clean both rail end faces thoroughly
- Protect all welding consumables from moisture
- Use sealing compound sparingly and keep it dry
- Preheat rail ends to 700–1000°C per the consumable supplier specification
- Prepare the crucible in advance and pour immediately after preheating
3. Thermite Welding Defects Explained: Slag Inclusion
Slag inclusion is a low-magnification inclusion trapped inside the weld or attached to the weld profile. It forms when molten slag does not float out before solidification.
Causes
- Slag ejected from the crucible enters the mould cavity and sticks to the rail surface
- Insufficient settling time: pouring before the reaction is complete (e.g. plug released too early)
- Excessive rail gap, insufficient weld metal, or mould leakage
- Poor sand mold-to-rail fit, allowing sealing compound into the cavity
Prevention
- Clean both rail end faces thoroughly
- Control the rail gap
- Inspect the crucible for damage; centre the pouring spout above the mould
- Check mold-to-rail contact; apply sealing compound evenly without forcing it into the cavity
- Control settling time; do not release the plug early
4. Thermite Welding Defects Explained: Sand Inclusion
Sand inclusion forms when refractory material from the crucible or sand mold breaks away and becomes trapped in the solidifying weld metal.
Causes
- Loose sand not fully removed after mold grinding
- Foreign material entering the mould cavity
- Mold halves misaligned during assembly, breaking off debris
- Excessive preheating burning the sand mold surface
Prevention
- Thoroughly clean loose sand from the mould cavity after grinding
- Never allow foreign objects into the cavity
- Re-open and clean the mold if halves are misaligned
- Never knock or vibrate the assembled mold, which dislodges refractory debris
5. Thermite Welding Defects Explained: Hot Cracking
Hot cracks develop in the cast structure before solidification is complete, when the weld is under tensile stress. They usually appear as transverse cracks in the middle of the weld head, with a bluish oxidised fracture surface.
Causes
- Weld movement or tensile loading during welding
- Premature mold stripping
- Premature riser removal, loading the weld head too early
- Impact loading before solidification is complete
- Rail contraction during cooling
- Rapid cooling in cold or windy conditions, increasing shrinkage stress
Prevention
- Never step on, strike, or pry the rails during welding
- Strip molds and remove risers only at the time specified in the procedure
- Keep rail alignment equipment in place until required release time
- Control the rail gap when temperature contraction is significant
- Extend insulation time in cold or windy weather to slow cooling
6. Thermite Welding Defects Explained: Incomplete Fusion
Incomplete fusion occurs when the rail end faces are not fully melted, so the weld metal does not form a metallurgical bond with the parent rail.
Causes
- Rail end faces not fully cleaned, leaving oxide scale
- Preheat temperature too low for the molten steel to melt the rail ends
- Rail gap too small, reducing the heat capacity of the weld pool
Prevention
- Clean both rail end faces thoroughly
- Preheat to the required range (typically 700–1000°C per supplier specification) with uniform distribution
- Set the rail gap within specification
7. Thermite Welding Defects Explained: Undercut
Undercut appears as grooves or depressions along the weld toe. It creates stress concentration, which can trigger early fatigue fracture under dynamic wheel loading.
Note: aluminothermic welding is a casting process, so its undercut mechanism differs from arc welding. In thermite welding, undercut is mainly caused by mold profile mismatch, sealing compound intrusion, localised overheating, and insufficient weld metal feeding.
Causes
- Excessive rail gap causing insufficient feeding at the weld toe
- New and old rail head profiles do not match, leaving gaps where sealing compound intrudes
- Excessive rail head preheating, over-melting the parent metal
- Poor sand mold-to-rail contact, shifting the mould profile
Prevention
- Control the rail gap within procedure limits
- Grind and dress old rail head profiles before welding to match the new rail and mold
- Follow preheat temperature and time strictly; avoid localised overheating
- Check mold-to-rail contact before assembly; keep sealing compound out of the cavity
- Grind out minor undercut; cut out and re-weld joints where undercut is too deep to repair
Conclusion
Thermite welding defects are avoidable. Clean preparation, dry consumables, correct preheating, controlled rail gap, and disciplined mold handling eliminate most of the 7 defects described above.
After welding, inspect every joint visually and with ultrasonic testing. Record the weld number, rail temperature, weather, and consumable batch for full traceability. When a defect exceeds repair limits, cut out and re-weld rather than risk an in-service failure.
ForgeSail supplies certified thermite welding kits and consumables for crane rails, mine tracks, and industrial railways. Contact our engineering team for application-specific welding guidance.
References
- TB/T 1632.3-2019 — Rail Welding, Part 3: Aluminothermic Welding (Chinese railway industry standard)
- China Academy of Railway Sciences — Analysis of Rail Thermite Welding Defects, 2008.10
- Consumable supplier welding instruction manuals (take precedence over general guidance)
