Principles of Aluminothermic Welding

Rail welding in 1900	Rail Welding Today

History:

There is more than 100 years between those two pictures regarding the same Aluminothermic welding process. In 1893, experimentations on Aluminothermic reactions for the production of high purity Chromium and Manganese started. By the end of the 19th century this technology had been applied to the rail industry to weld two pieces of rail together. It is when all started. Since then the principle is identical but technology improved dramatically the process.

• The reusable crucible disappeared and Railtech developed a disposable One Shot Crucible which brought many advantages
• The mould technology improved and Railtech brought the last hybrid mould technology

Both technologies contributed to improve the reliability and efficiency of the Aluminothermic welding.

How It Works

The reaction is based on the reduction of heavy metal oxides by aluminium which, as part of the reaction, gives off heat (hence, exothermic). It was found that once the reaction had been started the aluminothermic reacts with metal oxide to produce Al₂ O₃ thus liberating the metal from the oxide and generating heat sufficient to raise the temperature to approx 2000°C so that both metal and Al₂ O₃ are in liquid form. With the reaction complete the heavier metal separates from the less dense Al₂ O₃, which floats to the top of the moulds as slag.

A general description of the reaction is: - 3MeO + heat. (Me = metal)

With iron oxide the following reaction can occur: -

Fe₂O₃ + 2Al = Fe + Al₂O₃ + 760kJ.

Because this exothermic reaction can produce both high heat energy and a large volume of filler metal without the requirement for an outside power source it is ideal for the site welding of heavy sections i.e. railway lines.

Whatever the section being welded the procedure is fundamentally the same. In most cases aluminothermic welding is used as a butt weld to join heavy sections. A gap is prepared between the sections, refractory moulds are placed around the gap and the moulds are sealed by a suitable luting medium. A crucible holding the charge is prepared and the moulds are preheated. A high heat source fuse, such as a barium peroxide ignitor strip starts the aluminothermic reaction. Once the reaction is complete the crucible automatically releases the superheated liquid into the moulds. The moulds are designed to give sufficient internal volume for the metal only, the slag flows out into heavy receptacles and the weld is allowed to set. When the weld has cooled to approximately 1100°C the runners and risers can be sheared and the weld left to cool to a permissible grinding temperature.

PLA Process

There are different types of processes for Alumonthermic welding depending on the type of preheating used. In the UK we use the PLA process which is based on an Oxy Propane preheating.