9 primary reasons for using CO2 shielding in GMAW of Carbon Steels

Carbon dioxide is often used as shielding gas for GMA welding of carbon steels. In case of other metals, it may provoke weldment oxidation, impairing the metallurgical attributes but in carbon steels, oxygen content assists in achieving some useful weld characteristics rather than vitiating your weld. Using carbon dioxide shielding in carbon steels, may not produce elegant welds but the use of some other gases (like argon) in conjunction with carbon dioxide, renders improvement of some other factors like arc stability, weld pool fluidity etc. to enhance the soundness and quality of welds.
  • Why Carbon dioxide is used in shielding as it may cause oxidation in welding.
  • What makes Carbon dioxide exceptional for using as a shielding gas?
Here are some main reason for using carbon dioxide shielding in gas metal arc welding of carbon steels.

Improved Penetration

Carbon dioxide shielding provides better joint penetration as it promotes high arc voltage during welding. In this way you can achieve good results for sidewall and root penetration.
Cost Benefit
The advantage of low cost enhances its worth among other shielding gases. Using carbon dioxide shielding instead of oxygen, will not allow oxidation in weld metal, as oxygen do. Being heavier it provides better shielding characteristics. Though it is cheaper than argon and helium but comparatively less quality welds are obtained.
Add-on Oxidizing
Due to high-temperature arc, carbon dioxide dissociates into carbon mono oxide and oxygen that encourages oxidation. In this case a little bit oxidizing may prove an accompaniment to GMA welding of carbon steels, by reducing the polar spots during the process. As the polar spots formation may cause unstable arc and spatter during welding. During spray transfer mode electrode is connected to positive terminal (anode) of power source and work piece with negative (cathode), this type of settings constitutes to reverse polarity.
Oxidation may reduce the deoxidizers for consumables, for example, silicon content can be lessened and as a result glassy slag (black in color) is developed in weldment. So a controlled oxidation is a key to achieve good penetration, weld bead definition. On the other hand carbon dioxide may help in providing fluxing and prevent porosity by eliminating any impurity present on a joint.  
Combination with other gases
In spray transfer mode, Carbon dioxide singly not gives better results and may cause severe spatter. By developing an association with other gases, mutual benefits can be achieved. For example in combination with inert gases (like argon), smooth spray transfer is achieved with low voltage settings, eliminating the problem of spattering and arc instability.
Prevention of Undercut
As it is stated that carbon dioxide is a denser gas and capable of sound shielding. Having the ability to prevent severe weld imperfections like undercut and as a result, good profile weld beads are made.
Well safety is another concern with carbon dioxide shielding. Less hazardous in workplace, however, the threatening behavior of released carbon mono oxide may prove dangerous. Proper ventilation at the workplace is the recommended step in order to make procedure safer.
Removal of Rust
This gas assists in removal of rust present on the joint. It eliminates rusting by reacting with rust oxides and in this way other impurities are also removed. Apart from atmospheric protection, it also helps to prevent weld defects like porosity, lack of fusion, lack of penetration in the weld metal.
Improvement in Toughness
In a welding procedure, suitable composition of gases and suitable consumables are the primary concerns, to produce required toughness in weld metals. Carbon dioxide in combination with other gases, also helps to enhance the toughness of weldment.
Reduction of Surface tension
Surface tension is another issue in carbon steels that causes less penetration. The molten weld gains high surface tension which cannot be lessened by the use of inert gases like helium, argon etc. Only in that case carbon dioxide is the only shielding gas that reduces the intensity of surface tension and provides better penetration results. This makes carbon dioxide more exceptional in carbon steels.

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