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What are the areas of application for Thermite welding?

Thermite welding or aluminothermic welding is mainly used in rail construction. For this, an aluminum or iron oxide powder is ignited in a melting pot above the joint between two rails. This burns at a temperature of 2450 °C, which liquefies and permanently joins the two rail ends. Thermite welding is also used in shipbuilding, in the construction of large ship components and their repair without having to separate them from the hull.

No other external energy sources are required for thermite welding processes. The granulate that is ignited has a strong, exothermic reaction, which in itself provides sufficient heat. The resulting joint is safe and can withstand high loads, for example from heavy freight trains or very fast high-speed passenger trains. They also enable a comfortable riding experience, which is also important to the railroads.

How does the thermite welding process work?

The mixture of iron oxide and fine aluminum, which is ignited, provides the heat to liquefy the iron. A slag of molten aluminum is also produced. Both have a temperature of around 2400 °C. The slag is lighter than the thermite-steel mixture. It therefore floats on the molten bath. After the melting time, the crucible is tapped and the result can be processed.

The crucible

The weld pack is prepared for the reaction in the crucible. For the very first use of the day, the crucible must first be dried out with a burner and heated to a temperature of approx. 100 °C.

After each pass, i.e. after each pour, the loose slag particles that are still present must be removed. The inside of the crucible must also be cleaned. It is therefore advisable to always use a dry crucible.

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The pan has a holding ring with 3 legs. This allows it to be set up and prepared. For the welding process, it is then placed with its tubular leg on the holding or clamping device.

The ends of the rail

Thermite welding is used to join rail sections to form an endless overall rail. For this to succeed, the rail ends are first laid together and roughly aligned by eye. A ruler with a length of one meter is then used to align them precisely. Correct alignment will have a major impact on the riding surface later on. It must therefore be very precise.

In addition, the rail feet must also be aligned with each other. This increases the area that can be welded. This ensures sufficient strength in the connection.

The half forms

The welding process takes place in a mold. So that this can be attached as required, it must first be adapted to the profile of the rails. The mold also consists of two halves so that it can be repeatedly attached to the new welding points. Holding plates and the clamping device are used for this.

The clamping device

The clamping device must be prepared in such a way that it corresponds to the heads of the rails and the half molds. This will enable it to hold them reliably over the gap later.

The quartz sand

The quartz sand is used to fill the gaps between the half molds, the clamping device and the rail ends that are welded. This is also to prevent the hot metal compound from leaking into the finished mold.

Align the pan and the mold

The half molds are placed with the holding plates and the clamping device around the rail ends to form a single mold. The crucible is now aligned over the mold so that it does not touch it and there is a distance of less than 4 cm between the two. In this way, there is sufficient space for the jar to close so that it can open.

The thermite mixture in the crucible is ignited. Immediately afterwards, the crucible is covered with its cap. Casting then takes place by pouring the thermite steel into the mold. The steel now mixes with the molten metal that forms at the ends of the rails. When both solidify, the rails are welded together. This takes about 3 minutes.

How is the weld processed?

As soon as the molten bath has solidified, the mold can be removed slowly and carefully. To do this, the holding plates are removed. The head bead is cut off with a shearing device. The molded parts and the foot riser remain at the welding point until the steel has cooled completely.

The half molds are removed and cleaned. The weld is now ground. No impact tools may be used.