Resistance heating
In recent years, resistance heating has been accepted in the industry and used successfully at home and abroad for manufacturing:
- steel suction pipes
- boiler water pipes
- carbide paper knives
- solid carbide tipped drill bits
The principle of brazing by electric resistance heating or diffusion welding
Since the 19th century, the resistance heating process has been used for welding, brazing and soldering or forming various materials and devices. Inventor of the process was Elihu Thomson (b. 29 March 1853 in Manchester, England; d. 13 March 1937 in Swampscott, Massachusetts, USA), who got 700 inventions patented during his working life. In 1877, he presented the first resistance spot welding machine to the public.
In Germany, three of his inventions related to resistance heating (Numbers 50388, 54709 and 57097) were patented in 1889.
In electric resistance heating, it is distinguished between direct and indirect heating. In indirect heating, the work piece, which is located between the electrodes, is heated, according to its resistance, directly by the heat of the current. This process is employed to advantage for braze welding all kinds of steels. The principle of direct heating is based on using high-resistance electrodes, such as carbon electrodes. These electrodes are placed near the braze joint and annealed by the heat of the current. The heating of the braze point is caused by the heat conduction between carbon electrode and braze joint. The heating of well conducting materials, such as copper, is well applicable with indirect heating for soft soldering.
The advantage of resistance heating consists in the exact dosing of the input energy. It is possible to provide only as much energy as is necessary for reaching the working temperature of the filler metal. The thermal influence on the areas adjacent to the braze joints is insignificant, so that possible damage to the material, e.g. by coarse grain formation, is largely excluded.
A special development of the above-mentioned process is brazing under shielding gas by using electric resistance heating.
The principle of brazing under shielding gas can be explained as follows:
A sufficient amount of current is generated in a transformer and heats the joint located in a special shielding gas chamber in the direct current flow. Brazing joints are achieved with a filler metal whose liquidus temperature is much lower than the solidus temperature of the base material. After it has molten, the filler metal flows through capillary action into the brazing gap.
Initially, the introduction of the heat is point-shaped at the transition point of the brazing partners and then area-shaped across the whole cross-section. In direct heating the heat comes from inside.
Diffusion welding joints are made without using a filler metal. As in brazing, the component parts are heated in a special gas shielding chamber in the direct current flow. A temperature shall be chosen as a joining temperature, however, which is only little below the solidus temperature of the base material.