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Things About Welding Gas You Should Know

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.

welding gases

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

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;

Can liquefy;

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
Argon Ar Inert gas;

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.
Oxygen O2 Colorless gas;

combustion-supporting;

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
Hydrogen H2 Can burn;

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  
Ar 99.995% low good satisfied   Mushroom shaped  
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 welding C2H2+O2 H2
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

Gas Main purpose remarks
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

brazing gases

Carbon dioxide Decomposing ammonia Ar+CO2
Nitrogen C2H2+O2 CO2+02
Argon LPG + O2 Ar+O2
Oxygen Ar+N2 Ar+H2+N2
Acetylene N2+H2 Welding mixture
Hydrogen Ar+H2 Ar+He

 

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