Atomic hydrogen welding is an arc welding process that makes use of hydrogen gas to surround and transfer heat from an arc to the base metal. An atomic hydrogen welding torch contains two tungsten metal electrodes that form the electric arc. Hydrogen gas (H2) is used as a shielding gas, which dissociates into its elementary form (H) as it passes through the arc.

The process was invented by American Chemists and Physicists, Alexander and Langmuir in the 1920s. At the time of discovery, they had been conducting experiments on the use of gases for shielding of arcs due to the limitations of bare carbon electrodes. Irving Langmuir filed a patent for an atomic hydrogen welder in 1926.

This welding process is nowadays obsolete and very rarely used. The difference between atomic hydrogen welding and MMA welding or TIG welding is that in the latter cases, the arcs are formed between the electrode and the base metal. In this case, however, the workpiece does not form part of the electrical circuit that causes the arc, which makes for a more manageable arc.

The tungsten electrodes are non-consumable, and the distance between them can be altered to control the heat produced at the arc. An a.c transformer typically supplies the required current. External filler metal may or may not be required depending on the thickness of base metal.

The amount of heat absorbed by the hydrogen gas ($$H_2$$) is equal to the bond energy of the two hydrogen molecules ($$H$$).

$$H_2 \rightleftharpoons H + H – 436 kJ/mol$$

Once the hydrogen molecules hit the surface of the base metal, the reverse reaction occurs and the heat previously absorbed to break the bonds is released to the base metal. The chemical reaction involved provides for a very efficient transfer of heat from the arc to the base metal.

In addition to its involvement in heat transfer, hydrogen also serves as a shielding gas that protects the weld pool from atmospheric contamination. For example, any oxygen near the weld pool quickly combines with hydrogen to form water that escapes as steam. If oxygen is not kept away from the weld pool, the formation of metal oxide is inevitable.

The temperatures reached by this process are much higher than acetylene gas welding process nearing 4200 degrees Celcius. The flow of hydrogen around the electrode also serves to cool and prevent evaporation of the tungsten. During its operation, the atomic hydrogen torch produces a characteristic buzzing sound.

The atomic hydrogen welding process is very versatile and can be applied to almost any metal. It is especially useful in welding light gauge metals, ferrous alloys, and most non-ferrous alloys. However, it was very rarely used but it was still found useful in special applications. It is especially useful for any process requiring precision welds, such as correcting imperfections on machined parts and welding of tool steels.