Absorption of gases in weld

 

Absorption of gases in welds

Absorption of gases can cause a significant metallurgical change in the formation of molten weld pools.

The gases may react with the molten welds and with each other. Gases such as hydrogen, nitrogen, and oxygen are the most frequently absorbed in the weld pool.

Typical faults include 

Defects such as Porosity, Cracking, and Oxide inclusions are produced as a result of the gases absorbed in the weld metal.

Source of gas

There are three major sources that cause Absorption in almost all metal arc welds.

• Electrodes
• Atmosphere and 
• Parent metal

Electrodes

Electrode core wire is a source that can contain significant amounts of dissolved oxygen as well as some hydrogen and nitrogen.

Electrode coatings release gases such as hydrogen, CO, and CO2 during welding. Coatings cause more problems if they are moist.

Atmosphere

In the absence of adequate shielding, the molten weld metal comes in contact with atmospheric oxygen, nitrogen, and water vapor. The gas content of the base material frequently depends on the method of manufacture of the metal.

Parent Metal

The gas content of the parent metal will depend to a very great extent on the method of manufacture of the plate. Usually, the hydrogen content of plate materials is much lower, which is of the order of 1ml H2 per 100 grams. The oxygen content may range from 0.001%to 0.004% or higher if a Bessemer-quality plate is considered.

Hydrogen and its effects

As discussed above, the main source of hydrogen is electrode coating or the atmosphere. Hydrogen may often be present in the coating, as cellulose or moisture, and as moisture in the atmosphere.

Immediately after the welding process, the hydrogen weld starts to come out of the metal and continues to do so for a long time.

Defects arising due to the presence of hydrogen

The presence of hydrogen may cause of reduction in the tensile strength of the weld, make the arc erratic, and contributes to the formation of colling cracks or fisheyes in welds. Underbead cracking is often attributed to the effect of hydrogen dissolved in retained austenite in combination with other factors.

Nitrogen and its effects

The source of nitrogen is also the base metal, electrode core wire, and atmosphere.

Compared to the acetylene welding process, the absorption of nitrogen is much higher in arc welding.

Nitrogen is found in weld metal trapped in a blowhole (although nitrogen itself does not form a blowhole)and in the form of crystals or iron nitride (Fe4N).

 Defects arising due to the presence of Nitrogen

Nitrogen tends to

• Lower the corrosion resistance of weld metal
• Increase hardness, yield, and tensile strength
• Decrease ductility and impact resistance.

Oxygen and its effects

Oxygen is a major factor in gas contamination of the weld metal as it is present in significant quantities in the electrode core wire and may also be present in large amounts in the parent metal and on the parent metal as oxide or scale.

Oxygen may be absorbed into the weld, forming iron oxide(Fe3O4)and other oxides such as silicon. The iron oxide may react with carbon in the steel to form CO resulting in blowholes.

                C + O  ⇾ CO ( gas bubbles)

If the iron oxide is present, oxidation of the weld will occur and this will produce a great increase in the grain size.

Absorption of oxygen by weld:

Reduces its tensile strength and ductility and

Decrease its resistance to corrosion.

Deoxidizers such as Mn, Si, Al, Ti, etc. may be added to electrodes to prevent the formation of iron oxide

Prevention of Gaseous Contamination

In metal arc welding, the method employed to prevent contamination of the weld metal by gases is to coat the electrode wire with non-metallic materials. This coating serves two purposes

By decomposition in the heat of the arc, it provides a protective atmosphere of the gas, which resists contamination of the weld metal by the air.

It forms a low melting point slag which protects the molten weld metal from contamination during solidification.

Conclusion

Finally, in some coatings, additions of deoxidants are made, which reduces the available oxygen in the weld metal.