Gas welding of pipes
Installation of piping systems does not do without welding work. The use of gas is widely used for pipes up to 150 mm in diameter, making triangles, elbows and other elements.
The technology of work
The work with the use of gas is based on heating up of pipe edges with a flame and filling of the gap with molten metal filler. The choice of flammable mixture is carried out according to the melting point of the processed material. The thickness of the butt weld depends on the wall thickness, and the bulge should not exceed 3 mm.
Do not melt the pipe walls in the pipeline system, as the resulting overlaps create additional resistance to the movement of gases and liquids.
To improve the quality of the connection of pipes with a thickness of more than 3.5 mm is made beveled edges at an angle of 35. 45 °. The weld is formed by making right and left hand welds. To avoid the ingress of molten metal inside, the sharp edge part is blunted.
Joining of blanks with thickness less than 3 mm the joint is made with a gap up to ½ of the pipe wall without bevel. Depending on the purpose of the pipeline system, the following welding options may be used:
Welding of pipes designed for transportation of gases and liquids is not allowed with the use of remaining trim rings.
Workpieces with more than 5 mm thickness are joined by the right hand method, thin-walled. by the left hand method. Pipes are welded in the lower position and can be rotated. A non-swivel joint is created by a vertical or overhead joint. To do this, the lower half is welded first, and then the upper half in the opposite direction.
If it is not possible to approach the area to be welded with a torch, welding with a cut-out of a visor is used. After completing the work on the hard-to-reach area on the inside, it is returned to its original position, and the remaining joints are welded.
Manual gas welding is performed in one layer. Width of weld should not exceed the pipe wall thickness by more than 2.5 times. The machined surface is not allowed to have laps and rough scales.
The described method of welding allows to keep the metal from burning through, gives a durable quality weld. It is suitable for welding thin-walled pipes.
Gas flame treatment of metals is a series of technological processes involving the treatment of metals with a high-temperature gas flame.
These processes include gas fusion welding. During such welding, edges of joined parts are heated by a flame of gases, which are burned at the outlet of a gas-welding torch.
The gas flame is most often formed as a result of combustion (oxidation) of combustible gases with technically pure oxygen (purity of at least 98.5%). Acetylene, hydrogen, methane, propane, propane-butane mixture, gasoline, lighting kerosene are used as combustible gases.
Gas welding flame zones
Gas welding acetylene-oxygen normal flame is similar in shape to the diagram from figure 1.
The gas mixture arriving from the nozzle is heated to ignition temperature in the inner part of the flame core 1. Partial decomposition of acetylene occurs in the outer shell of the core. The emitted carbon particles are red-hot, brightly glowing and clearly distinguish the outlines of the core shell The temperature of the gases in the core is low and does not exceed 1500 C.
Zone 2 or welding zone is the most important part of the welding flame. Here, the first combustion stage of acetylene takes place, fed by oxygen from the cylinder into the nozzle. That is why the maximum temperature is reached here. Gases in the welding zone have reducing properties in respect to oxides of many metals, including iron oxides. That is why it can be called a reducing zone. the carbon in the weld metal changes insignificantly.
In Zone 3 or the plume of flame, gases are afterburning due to oxygen in the air. The gases contained in the flare and their dissociation products oxidise the metals, t.е. the zone is an oxidizing zone. The type of acetylene-oxygen flame depends on the ratio of oxygen to acetylene in the gas mixture fed into the burner. This ratio is called the coefficient β.
At β = 1.1 1.2 flame normal (cm. fig. 1). If this ratio t.е. As the relative amount of oxygen (oxidizing flame) increases, the shape and structure of the flame changes Oxidizing reactions are accelerated, and the core of the flame becomes paler, shorter, and conically pointed. Then the welding zone loses its reducing properties and becomes oxidizing, the carbon in the weld metal is reduced and burned out.
As the coefficient decreases β т.е. Increasing the acetylene content in the gas mixture slows down oxidation reactions. The nucleus elongates, and its outline becomes blurred. The amount of free carbon increases, its particles appear in the welding zone. With a strong acetylene surplus, carbon particles also appear in the flame plume. In this case, the welding zone becomes carburized carbon in the weld metal increases.
The flame of acetylene substitutes is fundamentally similar to the acetylene-oxygen flame and has three zones. Unlike hydrocarbon gases, a hydrogen-oxygen flame does not have a luminous core, because it does not contain luminous carbon particles.
It includes several essential steps: preparation of parts, connection of electrodes, start-up of the torch and its heating, making the weld at the correct temperature and moving the torch to a new location, checking the readiness of the torch.
The technology of plasma welding
Parts are prepared in such a way that they are sorted beforehand or supplied to the workplace already sorted. If the parts are obtained by heat cutting or rough machining, the edges are machined to a clean metal finish and degreased to produce a quality seam.
After that, the parts are brought into contact with the weld’s beveling line. In manufacturing, it is not done by hand, as in repairs, but with the help of fixtures.
Hot seam from plasma welding
If required, fluxes are applied to the weld line. These are usually strong reducing agents for high temperature applications (welding fluxes) mixed with fusible binders, which are themselves reducing agents, or give a minimum of hard-to-remove fouling (slag). The molten slag protects the bath from the action of oxygen, and the reducing agent takes it away from the oxides that have had time to form. Fluxes are not required for all metals or metal pairs.
The torch is started by a high voltage pulse or a contact between nozzle and cathode for fractions of a second. The arc is ignited, the working and shielding gases are fed to the torch, and cooling water is supplied to the anode body (for high-powered long-term torches). The torch is heated up to plasma stabilization and the welding operation starts.
The welding melts the joined edges of the part and a band or rod-shaped filler material is introduced into the molten material. In case of automatic welding, the feeding is mechanized. Welding is considered as a continuous process of melting and solidification of metal in the weld and must ensure the monolithicity of the weld, the same mechanical properties along the entire length, equal thickness of the weld, the complete absence of cavities, foreign inclusions and impurities.
The molten weld is quite defenseless in relation to many factors, so you have to create special conditions for quality: before the bath, in it, and after, in the area of crystallization of the melt. These conditions are strongly dependent on the metals that are to be welded.
Plasma cutter welding process
After completing the weld check the readiness of the torch for the next operation, so that the weld did not have to stop in the process of welding without bringing it to an end. Any such interruption, if forced, creates unnecessary mechanical stress, which then will be difficult or impossible to remove. For this reason, welding of critical welds: vessels (tanks) for rocket equipment, hulls of ships, especially submarine vessels, vessels for nuclear equipment, etc.п. welding with continuous feeding of cathodes at torches with powerful nozzle cooling.
Such technology is used in repair and construction works for dismantling metal structures, disassembly of old pipelines, rough cut of scrap metal. It is used when it is necessary to pierce holes or cut steel, cast iron products, base metal workpieces.
Electric arc cutting is used when gas cutting is not possible, and when there is no necessary equipment for gas cutting metal.
In addition to welding, thermal techniques are used for separating operations. metal cutting.
Gas flame, plasma arc, electron beam and laser beams are used as heat sources.
Oxygen cutting is used for cutting carbon steel blanks. The principle of oxygen cutting is burning of the metal in oxygen medium. In the first stage, the metal is heated with a heated oxyacetylene flame to its ignition temperature in oxygen (for steel 1000. 1200 °С).
Then the metal itself starts burning through an exothermic reaction with release of a considerable quantity of heat (Q):
This is realized with the help of a special cutting torch, which has separate channels for feeding the heating mixture and oxygen (Fig. 21.13).
Metal 3 is heated at the starting point of the cut 2, then a jet of cutting oxygen is sent there 7.
The heat from the combustion of iron, along with the heating flame, heats the underlying layers and extends to the entire thickness of the metal. The smaller is the thickness of the cut metal, the greater is the role of heating flame (at thickness of 5 mm. up to 80% of the total amount of heat emitted when cutting, at thickness over 50 mm. only 10%). Formed oxides 5, as well as partially molten metal, are removed from the cutting zone 4 with a stream of oxygen. A continuous supply of heat and cutting oxygen ensures a continuous process.
A number of conditions must be met for the oxygen cutting process to work properly.
- 1. The heat source must have the necessary power to heat the metal to the required temperature of the metal combustion reaction, and the amount of heat released by the combustion of metal in the oxygen jet must be sufficient to maintain a continuous cutting process.
- 2. The melting temperature of the metal must be above the temperature of its oxidation (combustion) in oxygen, otherwise the metal when heated will melt, not burn. In this case there will be no additional heat generation due to combustion (oxidation), which is the main heat source.
- 3. Metal melting point should be above the melting point of the oxides formed during cutting, otherwise refractory oxides isolate the metal from contact with oxygen and make the cutting process more difficult.
- 4. The formed oxides and slag must be liquid and easily blown out by the cutting oxygen jet, otherwise the contact of oxygen with the liquid metal will be delayed or even impossible.
Carbon steel satisfies all the above conditions and can be cut with oxygen.
Copper does not meet the first condition for gas cutting because of its high thermal conductivity, which greatly complicates the beginning of the cutting process, and low heat release during oxidation. Therefore, gas torches are not powerful enough to cut copper, and copper can be cut using a more powerful thermal source. electric arc.
Cast iron does not meet the second and fourth conditions. As the carbon content increases, the cutting gets much worse as the melting point decreases and the ignition temperature rises. Cast iron cannot be cut with oxygen. In addition, the viscosity of the slag increases significantly with increasing silicon content, which is necessarily contained in cast iron, which is also one of the reasons why it is impossible to oxyfuel cut cast iron.
The third condition is not met when cutting aluminum, magnesium, alloys thereof and steels with large amounts of chromium and nickel. When these materials are heated, refractory oxide films are formed on the surface preventing oxygen from reaching the metal.
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A variation of oxyfuel cutting is oxyfuel flux cutting. This technology is used to cut chromium and chromium-nickel stainless steels, cast iron and non-ferrous metals. A special powdered flux, the main component of which is iron powder, is fed into the cutting zone together with oxygen. Combustion of iron generates additional heat and increases the temperature in the zone of the cut. In addition, the combustion products of flux, interacting with refractory oxides, produce a liquid slag that is easily removed from the cutting zone.
Plasma cutting Based on melting of the metal and removal of molten metal from the cutting zone.
Electron beam и laser cutting. Energy sources that provide powerful concentrated heating, in which the metal is vaporized. The result is high accuracy and purity of cut, a narrow asphalt zone. The limitations of these methods are complex equipment and high costs.
Why welding with heated metal is needed
The advantages of using preheated metal welding are:
- Eliminates or reduces cracking of material with high surface humidity. Heating the product removes moisture, which reduces the likelihood of cracks.
- Improving the processes of melting of the weld metals and their deposition, which occurs during basic welding.
- Stress reduction of materials. Preheating helps the metals of the welded joint and workpiece expand and contract evenly.
- Improved weld structure quality. Preheating of the metal slows down its subsequent cooling. Consequently, the joint hardens more uniformly, improving the mechanical properties of the material microstructure.
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There are several ways of heat treatment of products, which are determined by their further application:
- Preheating. even before welding, the craftsman sets the minimum temperature of the joint. This information can be obtained from the WPS (welding data sheet), which contains data on the temperature range.
- Warming up between passes. when carrying out multipass welding, the welder must warm up the material as much as possible before starting a new stage. The heating temperature must not drop below the minimum machining temperature.
- maintaining the welding temperature, below which the welding area should not cool down before the end of work. If the joining process is stopped, the heat must be maintained at a specified level.
What electrodes are used for metal welding
The use of these materials improves the quality of the cut. The composition of the coating makes the welding process more comfortable:
- the transition of the arc to the side surfaces of the cut is prevented;
- provides stability of arc combustion and excludes the possibility of arc extinction;
- the gas pressure in the place of processing is created and the metal in the place of cutting is oxidized.
Laser metal cutting Metal bending Metal powder coating Metal welding
Keep in mind that the cutting process requires a lot of current power, the type of voltage is influenced by the brand of electrodes used.
Consumables with special coating differ from conventional welding electrodes by higher thermal capacity of the arc, high heat resistance of the cladding, intense oxidation of liquid metal.
Metal consumables can be used to effectively remove defective welds, tacking, rivets, bolts, repair cracks.
In the absence of information on the packaging must be ignited welding consumables for one hour at a temperature of 170 ° C.
Handheld metal welding allows the use of conventional welding electrodes, it is enough to increase the current intensity by 30-40%. The type of voltage is affected by the brand of electrodes used.
However, conventional electrodes have certain disadvantages:
- the consumption of electrodes and electricity increases;
- coating of some rods does not allow to work in such modes, because the coating melts and flows into the working area, which makes it difficult to obtain a quality cut.
That is why it is better to use special consumables for metal welding.
Use of carbon (graphite) electrodes is virtually identical to cutting with metal rods. Metal is completely molten by the arc and flows down. The difference is that carbon consumables do not melt, but burn over time. Therefore, less molten metal and slag are formed in the process, which results in a cleaner cut.
The advantage of carbon electrodes is also the ability to heat them to a high temperature, while the current intensity will be low. The melting point of the rods is over 3,800 °C, which makes them more durable and economical.
Carbon (graphite) electrodes are suitable for manual arc and oxyfuel cutting.
Direct polarity DC current is required for operation, cutting is performed “top-down”. However, it is possible to perform processing using alternating current.
Oxygen-arc welding of metals is performed using tubular electrodes. They differ in that the fusing element is not a welding wire but a hollow, thick-walled tube. The process consists of several steps:
- the arc occurs between the electrode and the workpiece;
- Under the influence of the arc the metal melts;
- The oxygen from the tube oxidizes the metal over the entire thickness and blows it out.
The disadvantage of this cutting technique is that the oxygen has a negative effect on the stability of the arc.
Shielded arc cutting and plasma arc cutting are performed using tungsten nonconsumable electrodes.
In the first case, the metal is cut at a higher value of current (20-30% higher than necessary for welding), and it melts throughout the thickness.
Plasma arc cutting produces an arc between the metal to be cut and the tungsten electrode.
The peculiarity of this type of metal welding is the need for mastery of welding. This skill will help to perform the work easily. Knowing how to properly excite the arc, guide the weld and create quality joints will help in cutting metal competently.
Once again, note that this technology will not achieve a neat cutting edge. It helps you quickly cut blanks that do not require high accuracy.
Materials suitable for gas welding
Gas welding is indispensable in industry, construction, agriculture. It allows a large number of metals to be bonded.
Welding of cast iron is necessary to eliminate defects, cracks, disintegrated parts of the product. Gas torch in this case should be with a small flame to avoid graining of the weld.
Bronze brazing involves the use of a reducing flame. The work uses a wire that is identical to the material to be welded.
Processing of copper does not provide a gap between the edges. This is due to the fluidity of the material, which can hinder the gas-welding process.
Carbon steels can be joined using various welding methods. Welds become coarse-grained due to the use of low-carbon steel wire.
Gas welding technology
Gas welding is used for all kinds of welds and most welds in various positions.
penetration ability of gas flame is low, so joints with small thickness of edges (3-4 mm) are welded without edge cutting.
The main parameters of gas welding are: thermal output of the flame, the ratio between oxygen and acetylene, and welding speed.
Output of a welding torch is determined by the hourly consumption of acetylene.
The filler wire used for gas welding is the same as used for arc welding electrodes. Sv-08, Sv-08A wires are used for gas welding of low-carbon steel. For welding cast iron, special cast iron rods with increased m carbon and silicon.
When gas welding cast iron, non-ferrous metals and some special steels fluxes are used that are added to the weld pool to dissolve oxides and formation of fusible slag, floating to the surface. Fluxes are used in the form of powders and pastes applied to the base or filler metal. For welding copper and its alloys acid fluxes are used (borax, borax with boric acid) for welding aluminum alloys. oxygen-free fluxes based on fluorine, chloride salts of lithium, potassium, sodium and calcium.
When welding brass they provide metered supply of gas flux (boric acid ether) into the welding pool through the acetylene channel of the torch. When flux burns in the flame boric anhydride is produced, which binds zinc oxides, resulting in slag build-up on the surface, preventing zinc burn-up.
One of the main technological differences between gas welding and arc welding is a smoother and slower heating of the metal. In some cases, this is a disadvantage of gas welding, in others it is an advantage and determines the field of application.
- steels of small thickness (0,2-5 mm);
- non-ferrous metals;
- metals that require gradual soft heating (e.g., tool steels);
- metals that require heating during welding (cast iron and special steels).
On the other hand, the relatively slow heating of the metal by the gas flame reduces the welding productivity with increasing metal thickness. In addition, during slow heating a large volume of the base metal surrounding the weld pool is heated, which causes significant deformation (warping) of the products. Delayed heating also causes a prolonged stay of the metal in a zone of high temperatures, which leads to overheating, grain enlargement and reduced mechanical properties of the metal. This limits the use of gas welding and makes it technically impractical for large building structures.