Atomic Hydrogen Welding ( AHW ) thermo-chemical is a welding process in which welding is done using the arc produced between two tungsten electrodes and hydrogen gas provided by a hydrogen gas cylinder.
It is named atomic hydrogen welding because the arc disassociates the hydrogen into atomic form. This process is also known as atomic arc welding.
This welding process was invented by an Irving Langmuir, an American Chemist and Physicist at the beginning of the 20th century.
A filler rod is optional in this welding process, it may or may not be used.
In this welding process, a high temperature of 3400 to 4000°C is obtained by using electric arc and hydrogen gas.
Tungsten and most of the refractory metal can be weld using the high temperature of this welding process.
Hydrogen gas has two functions in this welding process, the first one is to provide a heating effect.
Also, the molten metal produced while welding is surrounded by a shield produced by hydrogen gas, and in this way, hydrogen gas acts as a shielding gas for the molten metal. Hydrogen gas has a very vital role in this welding process.
The main difference between the atomic hydrogen process welding and welding processes like MMA welding and TIG welding is that in the MMA and TIG welding process the arc is formed between the electrode and base metal while in the AHW the arc is formed between two electrodes.
Main parts of the tungsten electrode:
1) Tungsten Electrode:
In this welding process, the welding torch has two tungsten electrodes which are inclined and adjusted to produce and maintain a stable arc.
2) Electrode Holder:
Electrode Holders are used to hold the tungsten electrodes.
Annular nozzles are present around the tungsten electrodes through which the hydrogen gas comes out supplied from the hydrogen gas cylinder.
4) Hydrogen Gas Cylinder:
A cylinder filled with hydrogen gas is used in this process to supply hydrogen gas at the required place in this welding process.
5) AC Power Supply:
300 V AC power supply machine with a controller is required in this process. This power supply will be used to create an arc between the tungsten electrodes.
AC is preferred over DC because equal heat is needed at both the electrodes.
A transformer is required to produce arc and maintain it. The transformer should have an open-circuit voltage of 300 volts.
8) Filler Rod:
The filler rod is optional. It is used if it is needed otherwise this process can be done without a filler rod.
9) Pressure Regulator Valve:
A pressure regulator valve is present over the hydrogen gas cylinder. It is used to measure the pressure in the cylinder and display it. It is also connected to the electrode handle so that hydrogen gas reaches the required place.
10) Workpiece Plates:
The welding joint is created over these workpiece plates and they are joined together.
Dirt, oxides, and other impurities are removed from the workpiece to obtain a good quality weld.
At first, when the AC power supply is powered and the two electrodes are brought near to each other, an arc will be created between these two electrodes, and these electrodes are instantaneously separated by a small distance of nearly 1.5 mm. The electrodes are separated slightly to maintain a stable arc between the electrodes. The heat produced by the arc can be controlled by varying the distance between the electrodes.
Also, the hydrogen gas will be supplied by the hydrogen gas cylinder to the electrodes. This hydrogen gas will provide shielding which will surround the arc produced by the electrodes.
Hydrogen will be supplied from the cylinder and will come out from the nozzle present around the electrodes. This hydrogen gas will act as a shielding gas which means it will protect the welding joint from the atmospheric gases and will ensure the quality of the welding joint to be good.
If atmospheric gas like oxygen comes near the welding process it combines with hydrogen to form water and quickly evaporates due to the heat of the welding process and prevents the formation of metal oxide. Hydrogen shielding will also protect the welding joint from contamination by atmospheric gases like carbon, nitrogen or oxygen.
Also, when the arc will come in contact with the jet of hydrogen gas, the hydrogen gas which is in the diatomic form will deionize into H+
single ions and absorbs heat from the arc. Hydrogen gas gets deionized when it is placed at more than 6000o C near the arc. The amount of heat absorbed by the hydrogen atoms will be equal to the bond energy between the two hydrogen atoms when in diatomic form. The reaction shown below takes place when hydrogen deionizes.
H2 = H + H – 422 KJ ( Endothermic Reaction )
When this deionized H+ ions will come near the workpiece and hit the surface of the base metal, they will cool down and will combine with other H+ ions. But when two H+ ions combine, suddenly a large amount of heat will be liberated which is denoted by the following reaction:
H + H = H2 + 422 KJ ( Exothermic Reaction )
The heat liberated during the recombination of hydrogen atoms will be absorbed by the workpiece plates. Hence, metal will fuse due to which fusion joint is created between the metal plates or workpieces. Hence, a molten pool will be created between the workpieces. In this process, heat can be produced ranging from 3500oC to 4000oC.
The workpiece in this welding process is not completely part of the electrical circuit. The workpiece becomes part of the electrical circuit only when the portion of the arc comes in contact with the workpiece and hence voltage is created between the workpiece and each electrode.
Advantages Of Atomic Hydrogen Welding:
1) Welding is faster in this welding process.
2) Since hydrogen itself act as shielding gas, separate shielding gas is not required.
3) There is very little distortion of the flame as the intense flame is obtained which can be concentrated at the joints.
4) The electrodes remain cool as the hydrogen gas flow by the electrodes in the holder which also increases the electrodes life.
5) The flow of hydrogen gas and the arc can be easily controlled by the operator and hence the heat produced can also be controlled. So, heat can be adjusted for welding different materials using this welding process.
6) Alloys can be melted without fluxes and without surface oxidation due to the powerful reducing action of the atomic hydrogen.
Disadvantages Of Atomic Hydrogen Welding:
1) This process is more costly as compared to other welding processes.
2) A skilled operator is required to operate this welding process.
3) Large quantities of metal cannot be deposited using this welding process.
4) This welding can be done on flat positions only.
5) This welding process is riskier as hydrogen is a highly inflammable gas.
Applications Of Atomic Hydrogen Welding:
1) It is mainly used where rapid welding is required like in the case of stainless steel and other special alloys.
2) It can be used for welding most of the ferrous and non-ferrous metals.
3) It is also used for welding thin sheets of metal and small diameter alloys.
4) This process is also used in repairing dies and tools, hard surfacing, and joining parts.
5) It can also be used for very precision welding like correcting machining errors.