Why thermit welds fail — and why you can't see it coming
22 July 2026 · MPIA Services
The thermit (aluminothermic) weld is the workhorse joint of continuously welded rail: molten steel cast around two rail ends, in the field, hundreds of times per renewal campaign. It is also, statistically, where rail breaks concentrate. Published research on one Australian heavy-haul mainline found that over an 18-month period, roughly 75% of all broken-rail reports occurred at aluminothermic welds — and the majority of those failures were in welds that had been in track for less than six months. The lesson travels: young welds fail first, and they fail where you cannot see.
Little warning, rapid fracture
Failure analysis of aluminothermic welds under high axle loads found fractures that were predominantly rapid, with little evidence of progressive fatigue growth beforehand — including horizontal split-web failures on straight track where loading was assumed benign. A weld that gives no visible warning before breaking is precisely the case where periodic visual inspection under-delivers: by the time anything is visible at the surface, the useful intervention window has often closed.
The defects are internal by nature
The defect catalogue for thermit welding — gas porosity, shrinkage cavities, slag and sand inclusions from the mould, lack of fusion at the rail-end interface — consists almost entirely of internal features. Industry defect analyses note that spherical voids within the weld metal are commonly concealed and undetectable by visual examination. European and British practice accordingly requires ultrasonic examination of rail welds across three zones — head, web and foot — and radiography renders the weld's internal condition directly onto film or a digital detector. Neither is optional decoration; they are the only ways to know what is inside the casting before traffic runs over it.
Where MPIA Services fits
MPIA Services radiographs thermit welds at the point of installation, classifying results to C1/C2/C3 against PRASA and Transnet specifications, and inspects rail heads and running surfaces ultrasonically where sub-surface fatigue is the concern. The combination covers both the new-weld and in-service failure modes the research describes.
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