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When performing mechanized welding, the question often arises: how to choose a protective gas. Unfortunately, it is difficult to give an unambiguous answer to this question. In each particular case, the choice of protective gas is determined by the type of material, its thickness, the type of welding process and the requirements for welded joints.
The correct choice of type of protective gas is important for several reasons. First, it must protect the welding bath from penetration of atmospheric air components, which lead to the formation of porosity, cracks, nitriding of the metal seam, reducing the strength of the welded joint. Secondly, the protective gas in varying degrees influences the ionization of the arc gap, which ultimately determines the nature of the burning arc and the stability of the welding process.
The most commonly used carbon dioxide (C02) and argon (Ag) as a protective gas for mechanized welding of carbon steels is solid wire. Let's consider the influence on the process of welding each gas individually, as well as their joint action when used in a mixture of gas mixtures :
С02 (carbon dioxide, carbon dioxide)
The use of C02 as a monocomponent protective gas provides a large roller of welded metal with a sufficiently deep melting. Most often it is used when welding with powder wire. The use of C02 involves an increase in the arc voltage, compared with other gases, due to the low purchasing power of the molecular structure of the gas prior to ionization. When welding steel of a small thickness, high voltage can create certain problems.
The main useful property of CO2 is its ability to increase the depth of melting, so this gas is added to practically all welding mixtures of gases.
Carbon dioxide is an active gas, which dissolves into O2 oxygen and carbon monoxide CO in the arc. Therefore, when using СО2, the saturation of the metal of the seam with oxygen increases and there is intense oxidation of the alloying components in the admixture. The use of a welded wire with high content of silicon and manganese can reduce the oxygen content of the metal seam, however, it should be borne in mind that both elements form oxides that are involved in the formation of slag on the surface of the seam. Therefore, one should not forget about the need for a thorough seam stripping after each passage and weld joint in general before painting or galvanizing the metalwork.
The use of CO2 as a protective gas is always characterized by high spatter of electrode metal, due to the need for welding at high voltage. For the same reasons, when welding in CO2, a large amount of welding aerosol is released.
Argon is an inert gas with an atomic mass of 39.9, under normal conditions, colorless, odorless and tasteful, about 1.38 times heavier than air. Argon is considered to be the most affordable and relatively cheap among inert gases. The car is ranked third in content in the air (after nitrogen and oxygen), it accounts for approximately 1.3% of the mass and 0.9% of the Earth's atmosphere.
In the industry, the main method of obtaining argon is the method of low temperature air rectification with the production of oxygen and nitrogen and the associated extraction of argon. Argon is also obtained as a by-product when ammonia is produced.
Gaseous argon is stored and transported in steel cylinders (according to GOST 949-73). A pure argon cylinder is painted gray, with the inscription "pure argon" green.
According to GOST 10157-79, gaseous and liquid argon comes in two types: a higher grade (with a volume fraction of argon of at least 99.993%, a volumetric fraction of water vapor not more than 0.0009%) and a first grade (with a volume fraction of argon not less than 99,987%, volume fraction of water vapor not more than 0,001%).
Argon is used as an inert protective gas with argon-arc welding, including as a base of a protective gas mixture (with oxygen, carbon dioxide). It is the main protective medium for welding aluminum, titanium, rare and active metals.
Argon is also used for plasma welding as plasma gas, with laser welding as a plasma-repelling and protective gas.
Argon as a protective gas allows for a very concave, concentrated arc, providing a narrow and convex form of a roll of welded metal with very deep melting . When welding with argon, the profile of the roller has a higher median and better formation at the edges than when welding in a medium of CO2.
Since argon is an inert gas, it virtually eliminates the saturation of the metal with oxidation and oxidation products. Therefore, when welding in an argon medium, you can use solid wire with a minimum content of silicon and manganese, provided the conditions of uniformity of the welded joint does not require the introduction of additional alloying elements in the seam. Argon also does not require high energy for ionization, and when it is used, the voltage can be much less than with the use of CO2. Due to light ionization, argon provides more stable burning of the arc and its easy burn, small spattering of the electrode metal and small separation of the welding aerosol
In mechanized welding argon is used as a pure form, mainly during welding of high-alloy steels and nickel alloys, as well as in the composition of gas mixtures.
Gas mixtures Аr + СО2
Argon and carbon dioxide when used for welding in pure form have a number of positive and negative andproperties. At the same time, mixing argon and CO2 reduce the negative effects of each other and mutually reinforce the beneficial properties. For example, pure argon is undesirable for welding carbon and low-alloy steels, as it contributes to the formation of a seam with a narrow profile and a high depth of sintering. At the same time to achieve the formation of a profile of the seam, providing the required strength of the connection, it is difficult enough. The high melting depth is also not always desirable, especially when welding a thin metal. In a mixture with carbon dioxide argon provides good quality and the formation of welds.
Only gas mixtures containing not more than 25% CO2 can be used for storage and transport in cylinders. Carbon dioxide is pumped into a liquid cylinder and argon is in a gaseous state. Due to the fact that evaporating carbon dioxide can exhaust argon, with a carbon dioxide content in the mixture of more than 25%, its quality can not be guaranteed. In connection with this, welding mixtures containing from 75 to 95% argon and from 5 to 25% of carbon dioxide are most commonly used for welding.
Mix 75% Аr + 25% СО2. The welding compound is most commonly used in the composition of 75% argon and 25% carbon dioxide. This mixture provides a large-grained transfer of electrode metal, while the CO2 enhances the melting effect of argon, thus forming a smooth profile of the seam. On the other hand, argon can stabilize burning of the arc, reduce spraying and improve the formation of the seam. However, since it is a large-duplex metal transfer mode, a small spray will be present in any case, even with optimal welding parameters settings.
Mix 85% Аr + 15% С02. If high welding speed and minimum spatter are required, then it is best to use a mixture containing 85% argon and 15% carbon dioxide as a protective gas (some manufacturers of gas mixtures offer it in the variant 82% Аr and 18% СО2).
In combination with higher voltage, this mixture provides an inkjet type of transfer of electrode metal. In this case, the transfer of metal occurs in the form of a continuous flow of small droplets, which gives a good seam formation with a minimum spatter of the electrode metal.
At the same time, due to the high speed of metal transfer into a weld bath and large heat supply, this mode is recommended only for welding in the lower, angular position of welding. For other spatial weld positions requiring high performance on a weld metal, it is recommended to use pulsed weld modes, welding modes that provide short-circuit welding, or a large-scale metal transfer type. In order to obtain high-quality connections at pulsed welding modes or when coarse-grained metal transfer, it is recommended to use gas mixtures containing 85% argon or more.
Mix 90% Аr + 10% СО2. This is a very common mixture, which provides a stable inkjet mode for metal transfer in a wide range of voltages. It is often used in robotics because the mixture provides the highest welding speed. This mixture is also successfully used when welding metal powder pulp.