Exploring Welding Gas Characteristics and Applications

The role of gases varies in different welding or cutting processes, and the selection of gases is also related to the material being welded. This requires the use of gases with specific physical or chemical properties, and sometimes even a mixture of multiple gases, in different situations. The characteristics of different gases in the welding process can be found in Table 6-12, while the main properties and uses of commonly used gases in welding are listed in Table 6-13.

Table 6-12: Characteristics of Different Gases in the Welding Process

GasPurity (%)Arc column potential gradientArc StabilityMetal Transition CharacteristicsChemical PropertiesWeld Pool Penetration ShapeHeating Characteristics
CO299.9HighSatisfactorySatisfactory, but some spatteringStrong oxidizingFlat shape with greater penetration
Ar 99.995LowGoodSatisfactoryMushroom shape
He 99.99HighSatisfactorySatisfactoryFlat shapeHigher heat input to the workpiece compared to pure Ar
Nz99.9HighPoorPoorResults in porosity and nitrides in steelFlat shape

Table 6-13: Properties and Applications of Gases for Welding

NamePurity not less than (%)Main PropertiesApplications in Welding
OxygenGrade 1: 99.2                                                                      Grade 2: 98.5Colorless, odorless, oxidizing, highly reactive at high temperatures, capable of combining with various elements. During welding, oxygen entering the molten pool can oxidize metal elements, causing harmful effects.When mixed with a combustible gas, extremely high temperatures can be achieved for welding or cutting, as with an oxygen-acetylene flame. The mixture of oxygen with argon or carbon dioxide can be used for mixed gas shielded welding.
ArgonWelding Steel: 99.7                                                                             Welding Aluminum: 99.9                                                                     Welding Titanium: 99.99Colorless, inert gas, chemically unreactive, does not undergo chemical reactions with other elements at normal or high temperatures.It serves as a shielding gas for processes like gas tungsten arc welding, plasma arc welding, and plasma arc cutting, providing mechanical protection.
Krypton99.6Inert gas, similar properties to argon, higher arc heat compared to hydrogen arc.It is used as a shielding gas for nitrogen arc welding, suitable for automatic and semi-automatic welding.
Carbon Dioxide (CO2)Grade I: 99.8                                                                Grade II: 99.5                                                                      Grade IChemically stable, non-flammable, non-oxidizing, decomposes into CO and O2 at high temperatures, exhibits certain oxidizing properties towards metals.When welding, it is used in combination with deoxidized welding wire as a shielding gas. It can also be mixed with oxygen or argon for mixed gas shielded welding.
Hydrogen99.5Flammable, highly reactive at room temperature, extremely reactive at high temperatures, can serve as a reducing agent for metal ores and metal oxides, hydrogen can dissolve in liquid metal in large quantities, precipitates during cooling, forming pores.When mixed with oxygen, it serves as the heat source for gas welding.
Nitrogen99.7Chemically unreactive, can combine with lithium, magnesium, titanium, and other elements when heated, can directly combine with hydrogen and oxygen at high temperatures, harmful when dissolved in the molten pool during welding, does not react with copper, provides protective properties.It is commonly used in plasma arc cutting and serves as the outer protective gas during gas shielded welding.
Acetylene98.0 Silver nitrate paper does not change color or turns pale yellowCommonly known as carbide gas, slightly soluble in water, soluble in alcohol, highly soluble in acetone, forms explosive mixtures when mixed with air or oxygen, highly reactive, emits a high temperature of 3500°C and intense light when burned in oxygen.When combined with oxygen, it forms an oxygen-acetylene flame used for welding, cutting, or heating.

Carbon Dioxide Gas (CO2)

CO2 gas is an oxidizing protective gas and exists in three states: solid, liquid, and gas. Pure CO2 gas is colorless and odorless. At 0°C and 1 atm (1 atm = 101325 Pa), the density of CO2 is 1.978 g/L, which is 1.5 times that of air. CO2 is readily soluble in water and imparts a slightly acidic taste when dissolved.

At high temperatures, CO2 undergoes decomposition (CO2 → CO + O, -283.24 kJ), producing atomic oxygen and thereby creating a highly oxidizing arc atmosphere. In the high-temperature arc zone, the decomposition of CO2 results in the simultaneous presence of three gases (CO2, CO, and O2). The extent of CO2 decomposition is related to the arc temperature during the welding process.

With increasing temperature, the decomposition reaction of CO2 becomes more intense. When the temperature exceeds 5000 K, almost all of the CO2 gas decomposes. The relationship between the degree of CO2 decomposition and temperature is illustrated in Figure 6-1. Liquid CO2 is a colorless liquid, and its density changes with temperature.

Below -11°C, its density is greater than that of water, while above -11°C, its density is lower than that of water. The properties of saturated CO2 gas are detailed in Table 6-14. Due to its low boiling point (-78°C), industrial CO2 is generally used in its liquid state, as it vaporizes at room temperature. Under conditions of 0°C and 1 atm, 1 kg of liquid CO2 can vaporize into 509 L of CO2 gas.

Figure 6-1   The relationship between the decomposition of CO2 gas and temperature
Figure 6-1 The relationship between the decomposition of CO2 gas and temperature

Table 6-14 Properties of Saturated CO2 Gas

Temperature/℃Pressure/MPaDensity
/(kg/L)
Specific heat capacity
/(10^5J/(kg·K))
liquidgasliquidgas
-500.670.86755.43.146.5
-4010.89738.23.336.54
-301.420.931273.526.55
-201.960.97119.53.726.56
-102.581.0214.23.946.56
03.481.0810.44.196.54
104.41.177.524.466.47
205.721.35.294.776.3
307.181.6335.275.9
317.322.162.165.595.59

The CO2 gas used for welding is typically stored in liquid form inside steel cylinders. A standard CO2 steel cylinder usually has a capacity of 40kg, and it can be filled with 25kg of liquid CO2. The 25kg of liquid CO2 occupies approximately 80% of the cylinder’s volume, while the remaining 20% is filled with vaporized CO2.

The pressure indicated on the cylinder’s pressure gauge represents the saturation pressure of this portion of gas. This pressure is dependent on the environmental temperature; as the temperature rises, the saturation pressure increases, and as it falls, the saturation pressure decreases. Only when all the liquid CO2 inside the cylinder has vaporized into gas will the pressure of the gas inside the cylinder gradually decrease as the CO2 is consumed.

The liquid CO2 contained in a standard steel cylinder can vaporize into 12,725 liters of CO2 gas. Based on the selection of CO2 gas flow rate during welding (refer to Table 6-15), if the average consumption of CO2 gas during welding is 10 liters per minute, then one cylinder of liquid CO2 can be continuously used for approximately 24 hours.

Table 6-15 Selection of CO2 Gas Flow Rate for Welding

Welding Method  MIG Welding with CO2  Flux-Cored Arc Welding with CO2  Heavy-Duty Flux-Cored Arc Welding with CO2  
CO2 Gas Flow Rate/(L/min)5~1515~2525~50

The pressure in a standard CO2 steel cylinder when full ranges from 5.0 to 7.0 MPa. As the pressure inside the cylinder decreases, the amount of water vaporized in the liquid CO2 also increases. The relationship between the water content in the CO2 gas and the pressure inside the cylinder is illustrated in Figure 6-2.

Empirical evidence suggests that when the gas pressure inside the cylinder falls below 0.98 MPa (at a temperature of 20°C), the CO2 in the steel cylinder should not be further used. This is because at this point, the liquid CO2 has essentially evaporated, and if continued to be used, welding defects such as porosity in the weld metal will occur, necessitating the need for refilling the CO2 gas.

The technical requirements for using liquid CO2 for welding are detailed in Table 6-6.

To reduce the moisture content in commercially available CO2 gas used in production sites, the following measures can be taken:

1.Invert the newly filled gas cylinder for 2 hours, open the valve, and discharge the water that has settled at the bottom (typically repeating this process 2-3 times with an interval of approximately 30 minutes each time). After draining the water, keep the gas cylinder inverted.

2.Before use, release the gas for 2-3 minutes, as the gas at the top generally contains more air and moisture.

3.Install a high-pressure dryer and a low-pressure dryer in the gas system to further reduce the moisture in the CO2. Typically, silica gel or anhydrous copper sulfate is used as a desiccant, which can be reused after drying out the water multiple times.

4.When the gas pressure inside the cylinder drops below 980 MPa, it should no longer be used. At this point, the liquid CO2 has evaporated, the gas pressure decreases as the gas is consumed, the partial pressure of water vapor relative to the total pressure increases, and the evaporation rate increases (approximately threefold). If used intermittently at this stage, the weld metal will develop porosity.

Figure 6-2 The Relationship between Moisture Content in CO2 Gas and Pressure in the Cylinder
  • 1-Cylinder not inverted with water, no desiccant
  • 2-Cylinder inverted with water, no desiccant
  • 3-Cylinder not inverted with water, with desiccant
  • 4-Cylinder inverted with water, with desiccant

Argon Gas (Ar)

Argon gas is a colorless and odorless gas, approximately 25% heavier than air, with a volume fraction in the air of about 0.935% (by volume). It is a rare gas with a boiling point of -186°C, which falls between the boiling points of O2 (-183°C) and N2 (-196°C), and is a byproduct of the fractional distillation of liquid air for the production of oxygen.

Argon gas is an inert gas, meaning it does not chemically react with metals nor dissolve in them. This property helps to prevent alloy element burn-off in the weld seam (though some evaporative loss of alloy elements still occurs, albeit to a lesser extent) and associated welding defects, making the metallurgical reactions during welding simpler and easier to control, thus providing favorable conditions for achieving high-quality welds.

The relationship between the thermal conductivity of Ar, He, H2, and N2 with temperature is shown in Figure 6-3. It can be observed that argon gas has the lowest thermal conductivity and being a monatomic gas, it does not decompose endothermically at high temperatures. When an arc burns in argon gas, there is minimal heat loss, making argon gas shielded welding the most stable in terms of arc combustion among various gas shielded welding methods.

Due to its high density, argon gas is less prone to floating and dispersing during shielding, thus providing effective protection. In gas tungsten arc welding (GTAW), the molten tungsten arc exhibits a stable axial jet transition, resulting in minimal spatter.

TIG welding is suitable for welding high-strength steel, aluminum, magnesium, copper and their alloys, as well as dissimilar metal welding. TIG welding is also suitable for repair welding, positional welding, and backside forming welding.

Argon gas can be stored and transported in liquid form at temperatures below -184°C. However, during welding, argon gas is commonly used in steel cylinders. In China, the commonly used volumes for argon gas steel cylinders are 33L, 40L, and 44L, with a pressure of 15MPa when the cylinder is full at temperatures below 20°C. It is strictly prohibited to knock or collide argon gas cylinders during use.

When the valve of the cylinder is frozen, it should not be heated with fire. Argon gas cylinders should not be moved using electromagnetic lifting equipment. In the summer, they should be protected from direct sunlight. The gas in the cylinder should not be completely used up, and the argon gas cylinder should generally be kept upright.

As a shielding gas for welding, argon gas typically requires a purity (by volume fraction) of 99.99% to 99.999%, which should be selected based on the properties of the metal being welded and the quality requirements of the weld seam.

Technical requirements for argon gas are detailed in Table 6-4, and the purity of argon gas used for welding different materials is specified in Table 6-16.

If the impurity content of argon gas exceeds the specified standard during welding, it not only affects the protection of the molten metal but also easily leads to defects such as porosity and slag in the weld seam, affecting the quality of the welded joint and increasing the burn-off of the tungsten electrode.

Figure 6-3     The Relationship between Thermal Conductivity of Ar, He, H2, N2 and Temperature
Figure 6-3 The Relationship between Thermal Conductivity of Ar, He, H2, N2 and Temperature

Table 6-16 Purity of Argon Gas Used for Welding Different Materials

Materials to be weldedPercentage of Each Gas/%
ArN2O2H2O
Titanium, zirconium, molybdenum, niobium, and their alloys≥99.98≤0.01≤0.005≤0.07
Aluminum, magnesium and their alloys, chromium-nickel heat-resistant alloys≥99.9≤0.04≤0.05≤0.07
Copper and copper alloys, chromium-nickel stainless steel≥99.7≤0.08≤0.015≤0.07

Helium (He)

Helium is also a colorless, odorless inert gas, similar to argon, and does not form compounds with other elements. It is not easily soluble in other metals and is a monatomic gas with a boiling point of -269°C. It has a higher ionization potential, making arc initiation difficult during welding. Compared to argon, it has higher thermal conductivity, resulting in higher arc voltage and temperature at the same welding current and arc intensity.

Consequently, the base metal receives more heat input, leading to faster welding speeds and a narrower, more concentrated arc column, resulting in greater weld penetration. These are the primary advantages of using helium for arc welding, although its arc stability is slightly inferior to that of argon arc welding.

Helium has a light atomic mass and low density, requiring a much larger flow rate than argon to effectively shield the welding area. Due to its high cost, helium is only used in specific applications with special requirements, such as welding critical components like cooling rods for nuclear reactors and thick aluminum alloys. A comparison of the characteristics of argon and helium in the welding process is shown in Table 6-17.

Table 6-17: A Comparison of the Characteristics of Argon and Helium in the Welding Process

GasesSymbolsCharacteristics
ArgonAr1. Low arc voltage: Generates low heat, suitable for tungsten inert gas welding of thin metals                                                                                                                                                                       2. Good cleaning action: Suitable for welding metals that form refractory oxide, such as aluminum, aluminum alloys, and high-aluminum-content iron-based alloys                                                                                                                                                   3. Easy arc initiation: Particularly important when welding thin metals                                                                                                                                                                                       4. Low gas flow rate: Argon has a higher density than air, providing good protection with less susceptibility to air movement                                                                                                                             5. Suitable for vertical and overhead welding: Argon can effectively control the weld pool during vertical and overhead welding, although its protective effect is inferior to nitrogen                                                                                          6. Welding dissimilar metals: Generally, argon is superior to helium
AmmoniaHe1. High arc voltage: Generates high heat, suitable for welding thick metals and those with high thermal conductivity                                                                                                                                       2. Small heat-affected zone: Results in minimal welding deformation and higher mechanical properties                                                                                                                                                                   3. High gas flow rate: Helium has a lower density than air, requiring 0.2 to 2 times more gas flow than argon. Helium is sensitive to air movement, but provides good protection for overhead and vertical welding                                                           4. High automatic welding speed: When the welding speed exceeds 66mm/s, smaller weld porosity and undercut can be achieved

Due to the unstable arc and less pronounced cathode cleaning effect of helium, direct current positive polarity is generally used for tungsten inert gas (TIG) welding with helium, even for welding aluminum, magnesium, and their alloys. The high heat concentration of the helium arc results in strong arc penetration. When the arc is very short, the positive polarity also has a certain effect in removing oxide films.

With direct current positive polarity TIG welding, single-pass weld thickness can reach 12mm for aluminum alloys, and the thickness of welds on both sides can reach 20mm. Compared to alternating current argon arc welding, the weld with helium has greater depth, a narrower weld bead, less deformation, a smaller softened zone, and the metal is less prone to overheating.

For heat-treated reinforced aluminum alloys, the mechanical properties at room temperature and low temperatures are superior to those obtained with alternating current argon arc welding.

As a shielding gas for welding, helium gas generally requires a purity of 99.9% to 99.999%. This requirement is also related to the type, composition, properties of the base metal, and the quality requirements for the welded joint.

In general, when welding reactive metals, high-purity argon should be used to prevent oxidation and nitriding during the welding process, thereby improving the quality of the welded joint. Technical requirements for welding with nitrogen gas are detailed in Table 6-5.

Nitrogen (N2)

Nitrogen constitutes approximately 78% (by volume) of air and has a boiling point of -196°C. Nitrogen has a lower ionization heat compared to argon, and its relative atomic mass is smaller. When decomposed, it absorbs a large amount of heat. Nitrogen can be used as a shielding gas during welding.

Due to its good thermal conductivity and heat-carrying properties, nitrogen is commonly used as the working gas for plasma arc cutting, producing a long arc column and molecular composite thermal energy, allowing for cutting of thicker metals.

The purity requirements for nitrogen gas used in welding or plasma arc cutting should comply with the technical requirements of Class I or Class II Level 1 as specified in GB/T3684 “Industrial Nitrogen,” as shown in Table 6-8.

Oxygen (O2)

At room temperature and in the atmosphere, oxygen is a colorless, odorless gas. Under standard conditions (0°C and 101.325 kPa pressure), 1m3 of oxygen has a mass of 1.43kg, making it heavier than air. Oxygen itself does not burn but is an active oxidizer.

It is an essential assist gas in gas welding and cutting. The purity of oxygen has a significant impact on the efficiency and quality of gas welding and cutting. For high-quality gas welding and cutting, oxygen with a purity of ≥99.5% should be used.

Oxygen is also commonly used as an additional gas for shielding gas welding to refine droplet formation, overcome cathode spot drift in the arc, increase heat input into the base metal, and improve welding speed. Technical requirements for industrial liquid oxygen are detailed in Table 6-7.

As oxygen is a highly reactive assist gas, when the cylinder is full, the pressure can reach up to 150 atm, posing a risk of explosion if not handled carefully. Therefore, special attention should be paid during the use and transportation of oxygen, including:

Prevention of oil

It is prohibited to touch oxygen cylinders and their accessories with gloves contaminated with oil. During transportation, they should not be placed together with flammable materials and oils.

Prevention of vibration

Oxygen cylinders must be securely placed to prevent vibration that could lead to an explosion. When stored upright, they should be securely fastened with iron hoops or chains. When stored horizontally, they should be supported with wooden blocks to prevent rolling, and it’s best to use two rubber shock-absorbing rings on the cylinder body. Special vehicles should be used for transportation.

Prevention of high temperature

Oxygen cylinders should be kept at least 10m away from a source of fire and at least 1m away from a heat source. When working outdoors in the summer, they must be covered with canvas to prevent explosions.

Prevention of freezing

If the valve of an oxygen cylinder freezes during winter use, it should be covered with a cloth soaked in hot water to thaw. Under no circumstances should heat be applied directly to thaw the valve to avoid causing an explosion.

Before opening the valve of an oxygen cylinder, check if the gland nut is tightened.

When turning the handwheel, it should be done steadily without excessive force, and the person should stand on the side of the oxygen outlet. When using oxygen, the cylinder should not be completely emptied, leaving at least 1-3 atm of oxygen.

When not in use, the oxygen cylinder must be covered with a protective cap to prevent damage to the valve.

When repairing the valve of an oxygen cylinder, special attention should be paid to safety to prevent an explosion.

Flammable Gases (C2H2, C3H8, C3H, CH4, H2)

There are many types of flammable gases, and acetylene gas (C2H2) is currently the most widely used in gas welding and cutting, followed by liquefied petroleum gas. Depending on the local conditions or the material being welded or cut, hydrogen, natural gas, or coal gas may also be used as flammable gases. The physical and chemical properties of several commonly used flammable gases are listed in Table 6-3.

When choosing a flammable gas, the following factors should be considered:

1.It should have high heat release, meaning the heat released per unit volume of flammable gas when completely burned should be high.

2.The flame temperature should be high, generally referring to the highest temperature of the flame when burned in oxygen.

3.The flammable gas should require less oxygen for combustion to improve its economy.

4.The explosive limit range should be small.

5.Relatively easy to transport.

Acetylene (C2H2)

Acetylene is an unsaturated hydrocarbon (C2H2) and is a colorless gas at normal temperature and pressure. Acetylene used for welding generally has a distinctive odor due to impurities such as H2S and PH3.

The flame temperature of acetylene burning in pure oxygen can reach around 3150°C, and its heat is concentrated, making it the most widely used flammable gas in gas welding and cutting.

The density of acetylene is 1.17 kg/m3. Its boiling point is -82.4°C, and it becomes a liquid at -83.6°C, solidifying at temperatures below -85°C. Acetylene gas can dissolve into water, acetone, and other liquids.

At 15°C and 1 atm, 1 liter of acetone can dissolve 23 liters of acetylene, and as pressure increases, the solubility of acetylene in acetone also increases. When the pressure increases to 1.42 MPa, approximately 400 liters of acetylene can dissolve in 1 liter of acetone.

Acetylene is a highly explosive gas, and its explosive characteristics are as follows:

1.Pure acetylene will explode when the pressure reaches 0.15 MPa and the temperature reaches 580-600°C when exposed to fire. The pressure of acetylene in generators and pipelines must not exceed 0.13 MPa.

2.When mixed with air or oxygen, its explosiveness significantly increases. When acetylene is mixed with air, by volume, if acetylene constitutes 2.2% to 81%, and when mixed with oxygen, if acetylene constitutes 2.8% to 93%, the mixed gases will reach their autoignition temperatures (305°C for acetylene-air mixture and 300°C for acetylene-oxygen mixture) or will explode when exposed to sparks, even at normal pressure.

Mixing acetylene with chlorine gas, hypochlorite, etc., and exposing it to sunlight or heat will also cause explosions. Mixing acetylene with nitrogen, carbon monoxide, or water vapor will reduce the risk of explosion.

3.Long-term contact with copper, silver, and other materials can lead to the formation of explosive substances such as acetylene copper and acetylene silver.

4.Dissolving acetylene in a liquid significantly reduces its explosiveness.

5.The explosiveness of acetylene is related to the shape and size of the container in which it is stored. The smaller the diameter of the container, the less likely an explosion will occur. Storing acetylene in containers with capillary-like materials, even if the pressure increases to 2.65 MPa, will not cause an explosion.

Due to the risk of explosion when pressurized, acetylene cannot be stored by directly pressurizing cylinders. In industry, it is commonly stored in containers filled with acetone or porous materials due to its high solubility in acetone, a method commonly known as dissolved acetylene or acetylene cylinder filling. Due to its safety, convenience, and economic advantages, acetylene cylinder filling is currently being widely promoted and used.

For high-quality gas welding, purified and dried acetylene should be used. Generally, acetylene purity greater than 98% is required for welding.

Petroleum Gas

Petroleum gas is a product or by-product of the petroleum refining process. The petroleum gases used in cutting include elemental gases such as propane and ethylene, as well as by-products of refining—multi-component mixed gases, typically composed of propane, butane, pentane, and butene mixtures.

(1) Propane (C3H8)

Propane is a commonly used fuel gas in cutting, with a relative molecular mass of 44.094. Its calorific value is higher than that of acetylene, but the combustion heat per unit mass molecule is lower than that of acetylene.

As a result, the flame temperature is lower, and the flame heat is relatively dispersed when propane undergoes complete combustion in pure oxygen. The chemical reaction equation for the complete combustion of propane in pure oxygen is as follows:

C3H8+5O2 → 3CO2+4H2O (6-1)

From the above equation, it can be deduced that the theoretical oxygen consumption for complete combustion of one volume of propane is 5 volumes. When propane burns in the air, the actual oxygen consumption of 3.5 volumes results in the formation of a neutral flame, with a temperature of 2520℃. The maximum temperature of an oxidizing flame is approximately 2700℃.

The combustion speed of an oxygen-propane neutral flame is 3.9m/s, with a relatively low risk of flashback. Its explosive range is narrow, between 23% and 95% in oxygen. However, its oxygen consumption is higher than that of acetylene, and due to its high ignition point, it is not easily ignited.

(2) Propylene (C3H6)

The relative molecular mass of propylene is 42.078. Its calorific value is lower than that of propane, but its flame temperature is higher. The chemical reaction equation for the complete combustion of propylene in pure oxygen is:

C3H6+4.5O2→3CO2+3H2O (6-2)

The theoretical oxygen consumption for complete combustion of one volume of propylene is 4.5 volumes; the actual oxygen consumption for forming a neutral flame during combustion in air is 2.6 volumes. The temperature of a neutral flame is 2870°C. When the propylene-to-oxygen ratio is 1:3.6, an oxidizing flame can be formed, resulting in a higher flame temperature.

Due to propylene’s lower oxygen consumption compared to propane and its higher flame temperature, it was once used as a cutting gas abroad.

(3) Butane (C4H10)

Butane, with a relative molecular mass of 58.12, has a higher calorific value than propane. The chemical reaction equation for complete combustion of butane in pure oxygen is:

C4H10+6. 5O2→4CO2+5H2O (6-3)

The theoretical oxygen consumption for complete combustion of one volume of butane is 6.5 volumes; its actual oxygen consumption for forming a neutral flame during combustion in air is only 4.5 volumes, which is higher than that of propane. The explosive range of the butane-oxygen or butane-air mixture is narrow (volume fraction of 1.5% to 8.5%) and is not prone to flashback. However, due to its low flame temperature, it cannot be used alone as a cutting gas.

(4) Liquefied Petroleum Gas

Liquefied petroleum gas is a by-product of the petroleum industry, mainly composed of propane (C3H8), butane (C4H10), propylene (C3H6), butene (C4H8), and small amounts of acetylene (C2H2), ethylene (C2H4), pentane (C5H12), and other hydrocarbons.

At normal temperatures and atmospheric pressure, these hydrocarbons exist in a gaseous state, but with a pressure of about 0.8 to 1.5 MPa, they become liquid, facilitating storage and transportation in cylinders.

In industry, gaseous petroleum gas is generally used. Gaseous petroleum gas is a slightly odorous, colorless gas. Under standard conditions, the density of petroleum gas is greater than that of air, ranging from 1.8 to 2.5 kg/m3. The main components of liquefied petroleum gas can form explosive mixtures with air or oxygen, but the explosive range is small.

Compared to acetylene, it is cheaper, relatively safe, and not prone to flashback. The amount of oxygen required for the complete combustion of liquefied petroleum gas is greater than that for acetylene, its flame temperature is lower, and its combustion speed is slower.

Therefore, the cutting torch for liquefied petroleum gas should be modified accordingly, requiring a larger cross-sectional area for the gas mixture outlet to reduce the flow rate and ensure proper combustion.

When using liquefied petroleum gas for cutting, attention must be paid to adjusting the gas supply pressure, which is generally regulated through the gas supply equipment for liquefied petroleum gas. The gas supply equipment for liquefied petroleum gas mainly includes gas cylinders, vaporizers, and regulators.

Natural Gas

Natural gas is a product of oil and gas fields, and its composition varies with the place of production. Its main component is methane (CH4), which also belongs to hydrocarbons. Methane is a colorless gas with a slight odor at room temperature. Its liquefaction temperature is -162°C, and it can explode when mixed with air or oxygen. The explosive range of methane mixed with oxygen is 5.4% to 59.2% by volume.

The burning rate of methane in oxygen is 5.5% per second. The chemical reaction formula for the complete combustion of methane in pure oxygen is:

CH4+2O2→CO2+2H2O (6-4)

From the above equation, it is known that the theoretical consumption is 1:2. When burning in the air, the actual consumption to form a neutral flame is 1:1.5. Its flame temperature is about 2540°C, much lower than that of acetylene, so it requires a longer preheating time for cutting. It is commonly used as a cutting gas in areas abundant in natural gas.

Hydrogen Gas (H2)

Hydrogen gas is a colorless, odorless flammable gas. Hydrogen has the smallest relative atomic mass and is soluble in water. Hydrogen gas has the highest diffusion rate and very high thermal conductivity, with a thermal conductivity 7 times greater than that of air. It is highly prone to leakage and has low ignition energy, making it one of the most dangerous flammable and explosive gases.

Its autoignition point in air is 560°C, and in oxygen, it is 450°C. The flame temperature of the hydrogen-oxygen reaction can reach 2660°C (neutral flame). Hydrogen gas has strong reducibility and can reduce metal oxides to their respective metals at high temperatures.

Hydrogen gas is commonly used in plasma arc cutting and welding. Sometimes it is also used for lead soldering. When added to Ar in appropriate amounts during consumable electrode gas shielded arc welding, it can increase the input heat of the base metal, improve welding speed, and efficiency. The technical requirements for the use of hydrogen gas in gas welding or cutting are listed in Table 6-18.

Table 6-18 Technical Requirements for the Use of Hydrogen in Gas Welding or Cutting

Name of Indicator (Volume Fraction)Ultra-pure hydrogenHigh-purity hydrogenPure hydrogen
Hydrogen Content (≥) / %99.999999.99999.99
Oxygen Content (≤) / 10-60.215
Nitrogen Content (≤) / 10-60.4560
CO Content (≤) / 10-60.115
CO2 Content (≤) / 10-60.115
Methane Content (≤) / 10-60.2110
Water Content (Mass Fraction ≤) / 10-61330
Note: In ultra-pure hydrogen and high-purity hydrogen, the oxygen content refers to the total amount of oxygen and argon. Ultra-pure hydrogen refers to pipeline hydrogen, excluding bottled hydrogen.

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