ATOMIC WELDING WITH HYDROGEN
Atomic hydrogen welding (AHW) is a combination of electric and gas welding techniques. It is a thermochemical arc welding process in which parts are joined by the heat obtained by passing a current of hydrogen through an electrical electrode between two tungsten electrodes.
The arc provides energy for a chemical reaction to occur. During the process, more heat is released due to the exothermic reaction. The electric arc efficiently breaks the hydrogen molecules that recombine with tremendous release of heat at a temperature of 3,400 to 4,000°C. Without the arc, an oxyhydrogen torch can only reach 2,800 degrees C. It is the third hottest flame after di-cyanoacetylene at 4,987°C and cyanogen at 4,525 degrees C. An acetylene torch only reaches 3,300°C. This device is called a hydrogen atomic torch or
nascent hydrogen torch or Langmuir torch. The process was also known as atom arc welding. The filler rod may or may not be used during the welding process.
Atomic hydrogen welding principleThe heat produced by this torch is sufficient to weld 3422°C tungsten and most refractory metal. Hydrogen gas acts as a heating element and also as a shielded gas to protect the molten liquid metal from oxidation and contamination by carbon, nitrogen or oxygen, which can seriously damage the properties of many metals. Eliminates the need for flux for this purpose.
The arc is maintained independently of the part or parts to be welded. Hydrogen gas is normally diatomic (H2), but where temperatures are greater than 6,000°C near the arc. When hydrogen hits a relatively cool surface, it will recombine into its diatomic form, releasing the energy associated with bond formation. The energy in AHW can be easily varied by changing the distance between the arc flow and the part surface. This process is being replaced by gas metal arc welding, mainly due to the availability of cheap inert gases.
In this process, the arc is maintained completely independent of the work or parts being welded. The work is part of the electrical circuit only to the extent that a portion of the arc comes into contact with the work, at which time a voltage exists between the work and each electrode.
It differs from shielded metal arc welding, in which the arc is independent of the base metal, making the electrode holder movable without the arc being extinguished. Thus, the heat input to the weld could be controlled manually to control the properties of the weld metal.
The process has the following special features.
- High heat concentration is obtained.
- Hydrogen acts as a shield against oxidation.
- Base composition filler metal could be used.
- Most of your applications can be met by the MIG process. Therefore, it is not commonly used.
Atomic hydrogen welding work
The equipment consists of a welding torch with two tungsten electrodes inclined and adjusted to maintain a stable arc as shown in Figure 4.4. Annular nozzles around the tungsten electrodes transport the hydrogen gas supplied by the gas cylinders. AC power source is suitable compared to DC because equal amount of heat will be available at both electrodes. A transformer with an open circuit voltage of 300 Pis required to start and maintain the arc.
atomic hydrogen welding setupParts are cleaned to remove dirt, oxides and other impurities to obtain a solid weld. The hydrogen gas supply and the welding device are turned ON. An arc is formed by placing two tungsten electrodes in contact with each other and instantly separated by a small distance of 1.5 mm. Therefore, the arc still remains between two electrodes.
As the jet of hydrogen gas passes through the electric arc, it disassociates into atomic hydrogen by absorbing large amounts of heat provided by the electric arc.
H2aBH + H = 422 kJ (endothermic reaction)
Advantages, Limitations and Applications of Atomic Hydrogen Welding
Advantages of atomic hydrogen welding:
1. The welding process is faster.
2. During the process an intense flame is obtained that can be concentrated on the joint. Consequently, less distortion occurs.
3. There is no requirement for separate flow and shielding gas or flow. The hydrogen envelope itself prevents the oxidation of the metal and tungsten electrode. It also reduces the risk of nitrogen capture.
4. The workpiece is not part of the electrical circuit. Consequently, problems such as arch opening and arch column maintenance are eliminated.
5. It is also possible to weld thin materials that may not be successfully accomplished by metal arc welding.
6. The workpiece is not part of the electrical circuit. The arc remains between two tungsten electrodes and can be easily moved to other locations without extinguishing.
Limitations of atomic hydrogen welding:
1. Post-welding is high when compared to other processes.
2. The welding process is limited to flat positions only.
3. The process cannot be used to deposit large quantities of metals.
4. The welding speed is lower when compared to metal arc or MIG welding.
Applications of Atomic Hydrogen Welding:
1. These welding processes are used in welding tool steels containing tungsten, nickel and molybdenum. .
2.They are used in joining parts, hardfacing and repairing dies and tools.
3. Atomic hydrogen welding is used where rapid welding is required on stainless steels, non-ferrous metals and other special alloys.
