Pressure Thermit Welding:
Pressure thermit welding is a process in which the heat produced by the thermit reaction is utilized only for heating the ends of the pieces to be joined, while the actual joint formation occurs by solid pressure welding.
History of Thermit welding:
In 1893 Hans Goldschmidt of Germany began to experiment with Alumino-thermic reactions. They are highly exothermic processes involving reactions of metallic oxides with Aluminium powders. This work led to a patent application for the Thermit process in 1895. due to the large amount of heat released by exothermic chemical reactions and the versatility of the thermit process, other applications were quickly found. By the end of the 19th Century, the Thermit process had been successfully used to make repairs to large cast and forged steel parts and in thermit welding of railways.
Two variations of the process are in use.
When welding pieces of small cross-section area like the ends of a pipe, the ends are first aligned properly. A mould is built around the section and heated. When the slag is poured into mould, the slag enters and forming the thin film around the ends. The metal follows next and displaces the slag. The film formed on the parts to be welded does not permit the thermit steel to fuse with the metal of the work but heats it to welding temperature. When the joint reaches the welding temperature the parts are drawn together forming a pressure butt joint. Specially designed clamps are used to apply pressure to bring the butting faces together for welding.
In the other version used for thicker work pieces the ends are spaced to a small distance apart in a removable mould. The space between the ends of the work pieces is filled with thermit metal and the ignition started. The reactions products are allowed to stay in place till the parts to be joined reach the required temperature for solid welding. The mold and the reaction products are then removed and pressure applied to complete the solid weld.
The quality of weld produced by either method is, however, not very satisfactory and the cost is very high. The process is therefore not very commonly used in practice.