Introduction to the concepts and characteristics of gas welding

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1. Gas welding concept

A welding method that uses the flame generated by the mixed combustion of combustible gas and combustion-supporting gas as a heat source to melt the weldment and welding materials to achieve interatomic bonding.

The combustion-supporting gas is mainly oxygen, and the combustible gas mainly uses acetylene, liquefied petroleum gas, etc. The welding materials used mainly include combustible gas, combustion-supporting gas, welding wire, welding gas flux, etc. Features: The equipment is simple and does not require electricity. Equipment mainly includes oxygen bottles, acetylene bottles (if acetylene is used as combustible gas), pressure reducers, welding guns, hoses, etc. Since the gas cylinder used to store the gas is a pressure vessel and the gas is flammable and explosive, this method is one of the most dangerous of all welding methods.

2. Introduction to gas welding

Gas welding is also called air welding. PMA plasma brazing machine completely replaces traditional brazing equipment. It is a new generation of brazing equipment that does not require oxygen, acetylene, liquefied gas, alcohol, and gasoline combustible gases. Adopting the IGBT inverter control principle, the flame is very stable during welding. The entire equipment is lightweight and convenient, suitable for outdoor welding. The welding machine is simple to operate and requires only those who have some basic knowledge of welding. No special training is required. This product does not require flammable and explosive gases such as acetylene, and its safety performance is greatly improved. It only requires electricity, and the advantages of using plasma brazing machines in some underdeveloped areas (without oxygen, acetylene, gasoline, etc.) are very obvious. During the welding process, gas flux can be used directly instead of the traditional manual addition of borax to improve the wettability and fluidity of the brazing and reduce the formation of pores. Improve the tensile strength of the weld, and there will be no oxidation or blackening on the surface during the welding process. No pickling is required, which greatly improves welding efficiency.

3. Advantages of gas welding

Gas welding as a welding method has many advantages:

(1) Wide range of applications: Gas welding can be used to weld various metals, including steel, iron, aluminum, copper, etc., so it has wide applicability in different industries and applications.

(2) Low cost: Compared with some high-end welding methods, welding gas equipment is relatively cheap, and the cost of gas and materials required is also low.

(3) Easy to learn and operate: Compared with other complex welding methods, such as TIG (argon arc welding) and MIG (metal inert welding gas), welding gas technology is relatively simple and easy to learn and master.

(4) Suitable for outdoor environments: Gas welding is not affected by the environment and can be operated in outdoor environments. It is suitable for field repairs, construction sites and other scenes.

(5) Portability: Gas welding equipment is usually relatively lightweight and easy to carry, so it is more convenient to use in scenes that need to be moved.

(6) High-temperature welding capability: Gas welding can provide high-temperature welding capabilities and is suitable for some applications that require high temperatures, such as welding thick metal materials.

(7) Flexibility: Gas welding equipment is usually flexible and can be operated relatively freely, and can operate in various welding forms and welding angles.

(8) Controllable welding depth: Gas welding can control the welding depth by controlling the gas flame size and welding speed, making the welding process more controllable.

4. Defects of gas welding

Although welding gas has many advantages, there are also some disadvantages:

(1) Melting point limitation: The welding temperature of welding gas is usually low, so it may not be suitable for the welding of some high melting point metals or special alloys.

(2) The quality of welds is unstable: Since welding gas is greatly affected by environmental factors, such as wind and welding position, the quality of welds may be unstable, and more operating skills and experience are needed to ensure welding quality.

(3) Gas requirements: welding gas requires the use of oxygen and gas, such as acetylene or propane. The supply and storage of these gases requires special equipment and attention, adding to operational complexity and cost.

(4) Safety risks: fuel welding involves the use of oxygen and gas, so there are safety risks such as fire, explosion and toxic gas leakage. Operators need to strictly follow safety procedures and use appropriate protective equipment.

(5) Environmental limitations: welding gas is generally not suitable for humid or highly corrosive environments because these conditions may affect welding quality and safety.

(6) Not suitable for automation: Compared with some modern welding methods, such as MIG (metal inert welding gas) and TIG (argon arc welding), welding gas is not suitable for large-scale production on automated production lines because it requires more manual operations and monitoring.

(7) Slower welding speed: Compared with some other modern welding methods, the welding speed of welding gas is usually slower, which may affect production efficiency.

5. Flame classification of gas welding

(1)Neutral flame: The flame formed by combustion when O2/C2H2 = 1~1.2. The flame structure can be divided into three parts: flame core, inner flame, and outer flame. The core of the flame is composed of unburned oxygen and acetylene. A layer of carbon particles generated by the decomposition of acetylene is distributed on the surface of the flame core. The temperature is relatively high (about 900°C). The hot carbon particles emit bright white light and are in the shape of a sharp cone with a clear outline. The inner flame is mainly composed of acetylene and the products of incomplete combustion (H2 and CO). It is reducing, blue-white in color, has an unclear outline, and has no clear boundary with the outer flame. The temperature of the inner flame is very high, up to 3150℃. The outer flame is composed of CO2 and water vapor produced after complete combustion of CO and H2 with O2 in the air, and is oxidizing. The temperature of the outer flame is in the range of 1200 to 2500°C, gradually changing from lavender to orange from the inside to the outside.

(2) Oxidation flame: The flame formed by combustion when O2/C2H2 > 1.2. The flame structure can be divided into two parts: the flame core and the outer flame. There is excess oxygen in the flame, forming an oxidizing oxygen-rich zone outside the flame core. The flame core is short and pointed, bluish white in color. Outside the flame center is a slightly purple outer flame, which is shorter than the normal outer flame and the flame is straight.

(3) Carbonizing flame: When O2/C2H2 < 1, the flame formed by combustion. There is not enough oxygen to completely burn acetylene, and the excess acetylene breaks down into carbon and hydrogen. Carbon will penetrate into the molten pool and cause carbonization of the weld, so it is called carbonizing flame. Carbonizing flame has a strong reducing effect. The flame structure is also divided into three parts: flame core, inner flame, and outer flame. The center of the flame is white and the periphery is slightly blue; the inner flame is light white; the outer flame is orange-yellow. When the amount of acetylene is large, there is black smoke and the flame is long and soft.

6. Gas welding equipment

Gas welding wire

During welding gas, the welding wire is continuously fed into the molten pool and fused with the molten base metal to form a weld. The quality of the weld is largely related to the chemical composition and quality of the welding gas wire. The types and uses of commonly used fuel welding wires are as follows:

1) Structural steel welding wire: Generally, the welding wire used for low carbon steel welding parts is H08A; important low carbon steel welding parts use H08Mn and H08MnA; medium strength welding parts use H15A; and higher strength welding parts use H15Mn.

When welding ordinary carbon steel with a strength level of 300 to 350MPa, welding wires such as H08A, H08Mn and H08MnA are used.

When welding high-quality carbon steel and low-alloy structural steel, carbon structural steel welding wire or alloy structural steel welding wire can be used, such as H08Mn, H08MnA, H10Mn2, and H10Mn2MoA.

2)Welding wire for cast iron Cast iron welding wire is divided into gray cast iron welding wire and alloy cast iron welding wire. Please refer to relevant national standards for its model and chemical composition.

Solder flux

1) The role of fuel welding flux. During the fuel welding process, the heated molten metal easily combines with oxygen in the surrounding air or oxygen in the flame to form oxides, causing defects such as pores and slag inclusions in the weld. In order to prevent the oxidation of metal and eliminate the oxides that have been formed, fuel welding flux must be used when welding non-ferrous metals, cast iron, stainless steel and other materials.

2) Commonly used welding gas fluxes and their selection Gas welding fluxes should be selected based on the types of oxides produced by the base metal during the fuel welding process. The selected flux should be able to neutralize or dissolve these oxides.

7. Gas welding operation technology

When operating fuel welding, generally hold the welding torch with your right hand, place your thumb at the acetylene switch, and your index finger at the oxygen switch to adjust the gas flow at any time. Hold the welding torch handle with the other three fingers and hold the welding wire in your right hand. The basic operations of welding gas include: ignition, flame adjustment, welding and flameout.

(1) Ignition, flame adjustment and flameout

When igniting, first open the oxygen valve slightly, then open the acetylene valve, and use an open flame (available electron gun or low-voltage electric spark, etc.) to ignite the flame. The flame at this time is a carbonizing flame, and then gradually open the oxygen valve to adjust the carbonizing flame to a neutral flame. If you continue to increase oxygen (or reduce acetylene), you can obtain an oxidizing flame. When the ignition returns, there may be a continuous “shooting” sound. The reason is that the acetylene is impure. The impure acetylene should be released and re-ignited. Sometimes it is difficult to ignite because the amount of oxygen is too large. At this time, the oxygen valve should be slightly closed again. When igniting, the hand holding the fire source should not face the welding tip, and do not point it at others to prevent burns. When the flame needs to be turned off after welding, the acetylene valve should be closed first, and then the oxygen valve to avoid backfire and reduce smoke.

(2) Stacking solder wave

① Weldment preparation: Clean the oxide scale, rust, oil and dirt on the surface of the weldment with a wire brush, emery cloth, etc., so that the metal surface of the weldment is exposed.

② At the beginning of the weld, low carbon steel generally uses a neutral flame and a left-hand welding method. That is, the welding torch is welded from right to left, so that the flame points to the part to be welded, and the filled wire end is located in front and below the flame. When welding together, due to the initial heating, the inclination angle of the welding torch should be larger (50~70), which is beneficial to The workpiece is preheated, and the axis projection of the welding tip coincides with the welding seam. At the same time, the flame should be reciprocated at the welding point to ensure uniform heating of the welding area. When the piece to be welded melts from red to a bright white and clear molten pool, the welding wire can be melted, and then the welding wire is immediately lifted, and the flame moves forward evenly to form a new molten pool.

③ In normal welding, in order to obtain high-quality and beautiful welds and control the heat of the molten pool, the welding torch and welding wire should make uniform and coordinated movements; that is, longitudinal movement along the seam of the weldment; the welding torch should swing laterally along the weld; the welding wire should move horizontally along the weld. Feed in vertical direction of weld seam and move up and down.

④Weld end When welding to the end of the weld, due to poor heat dissipation conditions at the end, the angle between the welding torch and the weldment should be reduced. (20~30°). At the same time, increase the welding speed and add more welding wire to prevent the molten pool from expanding and causing burn-through.

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