What is aluminothermic welding in rail replacement?

The reaction, in one line

Iron oxide is reduced by aluminium: the aluminium takes the oxygen, the iron comes out as liquid metal, and the alumina that is left over floats as slag on top of the pool. Because the reaction is self-sustaining once ignited, the whole pour depends on preparation rather than on plant: the rail ends are cut square and cleaned, the gap is set to the mould design, the mould is sealed against molten steel leaking out, the rail ends are preheated with a propane/oxygen flame, and the crucible is charged and ignited.

The published performance figures for the Molaton TWS rail thermite welding system give the working envelope: reaction temperature ≥2500 °C, weld cooling time of about 5–8 minutes depending on ambient temperature, and a total cycle of ≤25 minutes including preheating, pouring and mould removal, producing a weld strength of ≥95 % of the base rail tensile strength. Preheating is held in the 350–500 °C range and regulated with an oxygen flow meter, because both a cold joint and an overheated rail end are defects that only appear after the track is back in service.

Where thermite welding belongs — and where it does not

The rules are explicit rather than a matter of taste. Under TB 10082—2017 §8.0.6, rail should be welded by factory flash-butt welding with factory strings of not less than 500 m, on-site welding should also use flash-butt welding, and thermite welding may be used for the locking weld inside a turnout and at the two ends of a turnout where the rail is short, constrained or already part of an assembled crossing.

Since 2025 the direction of travel has been stated even more plainly. A management opinion issued with the notification on fractured weld seams in turnout curved rails (工电线路电〔2025〕5号) requires new and upgraded railways to replace thermite welding with flash-butt and gas-pressure welding. The stated reasons are technical and economic: a thermite weld has lower strength than the parent rail and is prone to surface depression and low spots, and its non-destructive testing frequency is far higher than for flash-butt or gas-pressure welds, so the inspection and maintenance workload keeps climbing over the life of the joint.

Read together, the two documents define the honest role of aluminothermic welding: it is the closure and connection method for positions that cannot be reached with a welding machine — turnout locking welds, short closure rails, confined locations, mixed sections — not the default for building long strings on a new line.

What the kit contains

The coverage is wider than plain line: 50 kg/m and 60 kg/m rail, 59R2/60R2 grooved rail, QU70–QU150 crane rail, and the German A/S, Japanese and Korean CR, British BS and UIC series, under TB/T 1632.3-2014 — which is how the same process is used for port crane rails and for cathode steel bars in aluminium smelters, where the environment is hot, dusty and magnetically noisy.

Grinding is half of the job

A finished thermite weld is not a finished joint. The poured metal leaves a collar and flash that stand proud of the running surface, and that collar is exactly the kind of short-wave irregularity that raises dynamic wheel load. The acceptance limits applied to the dressed joint are the same ones used for profile work: surface roughness Ra ≤ 10 μm, flatness within 0.3 mm under a 1 m straightedge, no continuous blue burning, grinding depth ≤ 0.5 mm per pass and blend-out better than 1‰.

Because a thermite weld is a heat-affected zone with a different hardness from the parent rail, the cut has to stay controlled. Running wheel speed from 1,000 to 3,000 rpm raises roughness from 6.6 to 8.4 μm and white-layer thickness from about 45 μm to 320 μm, and that white layer measures roughly 870 HV0.3 against 340 HV0.3 for the parent metal and fractures early in service. A shallow, repeated pass with the right wheel does the joint no harm; one aggressive pass does.

Case and data

A Liuzhou GMC-96X campaign removed 18.72 pass-kilometres in a two-hour possession, and a 2026 trial on a Loram DM01 grinding train finished with no blue burning and surface roughness of Ra 1.05–9.0 μm, mostly 2–4 μm — all inside the ≤10 μm acceptance band. RailwayCare has produced rail grinding wheels since 2004 as the first dedicated manufacturer in China, drafted the industry standard JB/T 11431, and holds ISO 9001, ISO 45001 and CRCC certification, with independent testing at the China Academy of Railway Sciences measuring wheel wear 1.4–2.6× better than specification.