In this animated video we can see the process, and some technical data, for rail joint by thermite welding. (Thanks Thermitaustralia)
Procedure compendium:
1. The rails are aligned, separated correctly and leveled with the aid of wedges.
2. Molds are placed and they are fixed by the press of the welding equipment.
3. Trays are placed to receive the surplus of the casting (corundum).
4. Torch is introduced and preheated rail's ends at a temperature of 900 ° C, by combustion of oxygen propane, for 5 minutes.
5. Thermite load is a powder consisting of (typically) 37% aluminum, 13% steel, 37% iron oxide and 13% of any material which ensures alloy correctly.
6. This powder is deposited into the crucible and cover it.
7. Initiate the reaction when ignite the dust by a flare.
8. The reaction reaches a temperature of 3500 ° C in, approximately, 30 seconds, after which time the powder melts and poured into molds automatically. Melting excess is deposited in the corundum trays.
9. 3 minutes after melting completion the trays are removed, and 30 seconds then remove the crucible and molds.
10. 5 minutes after the molds are removed, the upper remaining part of the melt is cutted out. 9 minutes after, the "pipes" (side remaining part of the melt) and wedges that level the rail will retreat.
11. It grinds the weld to remove burrs and ensuring that the rail´s head has a uniform surface.
The following video shows this process in reality. (Thanks Wolfgand Lendner)
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