Classification of welding gases
Welding gas mainly refers to the protective gas used in gas shielded welding (carbon dioxide gas shielded welding, inert gas shielded welding) and the gas used in gas welding and cutting, including carbon dioxide (CO2), argon (Ar), helium (He), oxygen (O2), combustible gas, mixed gas, etc.
During welding, shielding gas is not only the protective medium in the welding area but also the gas medium generating arc;
Gas welding and cutting mainly rely on the high-temperature flame with concentrated heat generated during gas combustion.
Therefore, the characteristics of gas (such as physical and chemical characteristics) not only affect the protection effect, but also affect the ignition of arc and the stability of welding and cutting process.
1. Types of welding gases
Welding gas mainly refers to various gases used in welding or cutting.
According to the role of gas in the working process, welding gas can be divided into shielding gas and gas welding and cutting gas.
(1) Shielding gas:
Shielding gas refers to the protective gas used in gas shielded welding, mainly including carbon dioxide (CO2), argon (Ar), helium (He), oxygen (O2), nitrogen (N2), hydrogen (H2) and its mixture (such as Ar + He, Ar + CO2, Ar + CO2 + O2, etc.).
According to the international welding society, shielding gases are uniformly classified according to oxidation potential: inert gas or reducing gas (class I), weak oxidizing gas (class M1), medium oxidizing gas (class M2) and strong oxidizing gas (classes M3 and C).
(2) Gas for gas welding and cutting:
According to the nature of gas, gas for gas welding and cutting can be divided into combustion supporting gas (O2) and combustible gas.
However, when the gas and oxygen are mixed and burned, a large amount of heat is released to form a high-temperature flame with concentrated heat, which can heat and melt the metal.
The natural gas commonly used in gas welding and cutting is mainly acetylene (C2H2), and other widely used combustible gases include propane (C3H8), propylene (C3H6), natural gas (mainly methane CH4), liquefied petroleum gas (mainly propane), etc.
Characteristics of welding gas
The role of gas in different welding or cutting processes is also different, and the selection of gas is also related to the welded material, which requires the selection of gas with a specific physical or chemical property or even the mixture of multiple gases in different occasions.
The main properties and uses of gases commonly used in welding and cutting are shown in Table 1, and the characteristics of different gases in the welding process are shown in Table 2.
Table 1 Main characteristics and uses of common welding gases
|Gas||Symbol||Main properties||Application in welding|
|Carbon dioxide||CO2||Stable chemical properties;|
Non combustion and non combustion supporting;
It can decompose into CO and o at high temperature and has certain oxidation to metals;
When liquid CO2 evaporates, it absorbs a lot of heat and can solidify into solid carbon dioxide, commonly known as dry ice
|Welding wire can be used as shielding gas during welding, such as CO2 gas shielded welding and CO2 + O2, CO2 + a mixed gas shielded welding|
The chemical property is not active, and it does not play a chemical role with other elements at room temperature and high temperature.
|It is used as shielding gas for mechanical protection during arc welding, plasma welding and cutting.|
It is very active at high temperature and directly combines with many elements;
During welding, oxygen entering the molten pool will oxidize metal elements and play a harmful role.
|Mixed combustion of oxygen and combustible gas can obtain extremely high temperature for welding and cutting, such as oxygen acetylene flame and hydrogen oxygen flame. It can be mixed with argon and carbon dioxide in proportion for mixed gas shielded welding|
|Acetylene||C2H2||Commonly known as calcium carbide gas;|
Less soluble in water, soluble in alcohol, a large amount soluble in acetone;
Mixed with air and oxygen to form explosive gas mixture;
It burns in oxygen and emits 3500 ℃ high temperature and strong light
|For oxyacetylene flame welding and cutting|
It is not active at room temperature and very active at high temperature;
It can be used as reducing agent for metal ore and metal oxide;
During welding, a large amount of molten metal can be precipitated during cooling, which is easy to form pores.
|Hydrogen is used as reducing shielding gas during welding. Mixed combustion with oxygen can be used as the heat source of gas welding|
|Nitrogen||N2||Inactive chemical properties;|
It can combine directly with hydrogen and oxygen at high temperature;
Entering the molten pool during welding is harmful;
It basically does not react with copper and can be used as protective gas
|When nitrogen arc welding, copper and stainless steel can be welded with nitrogen as shielding gas. Nitrogen is also commonly used in plasma arc cutting as an outer protective gas.|
Table 2 characteristics of different gases during welding
|Gas||pure||Column position gradient||Arc stability||Metal transition characteristics||Chemical properties||Weld penetration shape||Heating characteristics|
|CO2||99.90%||high||satisfied||Satisfied, but some splashes||Strong oxidation||Flat shape, large penetration|
|He||99.99%||high||satisfied||satisfied||Flat shape||The heat input of butt weldment is higher than that of pure ar|
|N2||99.90%||high||bad||bad||Pores and nitrides are produced in steel||Flat shape|
Selection of welding gas
Corresponding gases shall be used for CO2 gas shielded welding, inert gas shielded welding, mixed gas shielded welding, plasma arc welding, brazing in protective atmosphere, oxygen acetylene gas welding and cutting.
The selection of welding gas mainly depends on the welding and cutting methods. In addition, it is also related to the properties of the welded metal, the quality requirements of the welded joint, the thickness of the weldment, the welding position and the process method.
3.1. Select gas according to the welding method
According to the different welding methods used in the welding process, the gases used for welding, cutting or gas shielded welding are also different.
See Table 3 for the selection of welding methods and welding gases.
The selection of common brazing gases in protective atmosphere is shown in Table 4. The applicability of various gases in plasma arc cutting is shown in Table 5.
Table 3 selection of welding methods and welding gases
|Welding method||Welding gas|
|Gas cutting||C2H2+O2||Liquefied petroleum gas+O2||coal gas+O2||natural gas+O2|
|Plasma arc cutting||air||N2||Ar+N2||Ar+H2||N2+H2|
|Tungsten inert gas welding (TIG)||Ar||He||Ar+He|
|Solid wire||Inert gas metal arc welding (MIG)||Ar||He||Ar+He|
|Active gas metal arc welding (MAG)||Ar+O2||Ar+CO2||Ar+CO2+O2|
|CO2 gas shielded welding||CO2||CO2+O2|
|Flux cored wire||CO2||Ar+O2||Ar+CO2|
Table 4 Selection of common brazing gases in protective atmosphere
|Gas||Nature||Chemical composition and purity requirements||Purpose|
|Argon||inertia||Argon > 99.99%||Alloy steel, thermal strength alloy, copper and copper alloy|
|Hydrogen||Reducibility||Hydrogen 100%||Alloy steel, thermal strength alloy and oxygen free copper|
|Decomposing ammonia||Reducibility||Hydrogen 75% nitrogen 25%||Carbon steel, low alloy steel and hydrogen free copper|
|Decomposing ammonia with insufficient compression||Reducibility||Hydrogen 7% ~ 20% and other nitrogen||mild steel|
|Nitrogen||It is inert relative to copper||Nitrogen 100%||Copper and copper alloys|
Table 5 Applicability of various gases in plasma arc cutting
|Ar,Ar+H2， Ar+N2， Ar+H2+N2||Cutting stainless steel, non-ferrous metals or alloys||Ar is only used for cutting thin metals|
|N2,N2+H2||N2, as the working gas of water recompression plasma arc, can also be used to cut carbon steel|
|O2, Air||Cutting carbon steel and low alloy steel, also used for cutting stainless steel and aluminum||Important aluminum alloy structural parts are generally not used|
|Carbon dioxide||Decomposing ammonia||Ar+CO2|
|Argon||LPG + O2||Ar+O2|