In recent years, with the development of the oil pipeline project, large-diameter pipe welding process advances from the previous single electrode arc welding (acidic, basic welding rod), the development of the pipeline automatic welding, automatic welding process. Arc welding electrode 507 is also a single species by the end all-position cellulosic electrode welding development of efficient welding vertical down welding and other construction technology. Stand up to change the common electrode welding traditional craft methods, the original location of the level of the fixed tubular docking operation of the whole operation of law should be changed to a similar level of welding operation to weld under the law, that is, from 12 o'clock position from the arc to the lower nozzle the end of the 6 o'clock position, increase the efficiency of melting 1 to 3 times the number, the unit received by pipeline construction. Large-diameter natural gas pipeline project (Φ1016) roots backing part of the pipeline welding and multi-layer welding are welding using cellulosic electrodes.
Vertical down welding cellulosic electrodes, arc with deep penetration, penetration, weld root side welding backing good shape, stomatal sensitivity to small, difficult to operate a small, high quality internal and external weld, welder easy to master, technological features and short training period. Widely used in petrochemical, gas, electricity, civil industry, oil, gas, water laying large diameter pipe installation construction.
1, cellulose electrode composition and process characteristics of drug skin
1. A cellulose electrode drug skin main components:
25 ~ 40% cellulose (wood flour, starch, phenolic resin powder, trace fiber, powder form);
8 ~ 16% carbonate (potassium carbonate, calcium);
8 to 20% of the ferroalloy (SiO2, TiO2, MnO, FeO, MgO, Al2O3, etc.);
10 ~ 15% of the metal oxide;
20% of the other ingredients.
1. 2 skins in a large number of drug decomposition of organic matter, the formation of large amounts of gas (CO, CO2, H2, H2O, etc.), the weld has a strong protective effect, and Arc force, the transition was sprayed droplet.
1. 3 In the DC arc welding machine power source is connected and the reverse, its different forms droplet transition.
       DC is connected, the Tungsten Electrode tip to form the molten metal, small size, Arc force, air enough to melt metal fly rod ends, the transition of small particles. Arc stability, degree, weld penetration, generally applied to the roots backing welding, forming the back side welding good shape.
As the arc is then a large splash, deep penetration, is not easy to obtain a satisfactory surface forming the heat welding, filling and cover pass welding is not commonly used.
       DC reverse polarity, the electrode end of the molten metal is almost entirely block the molten metal, the surface wind force, although such a large gas-making agent can hardly be blown into small particles of transition droplet shape. Therefore, the DC reverse, its transition to a droplet contact short transition. Short time after the transition, due to the volume of molten metal electrode tip smaller, leather sleeve and the drug under the influence of air, particles of transitional forms has emerged. Therefore, the DC fiber electrode short-circuit reverse transition when mixed with particles of transition transitional form. This is different from other forms of special form of the transition electrode, the DC reverse generally applies to heat welding, welding filling, capping layer welding.
2, cellulose electrode arc welding power source characteristics of the process
2. An ordinary dc arc welding power source, used for cellulose electrodes, the arc will be broken, extinction, "breathing" arc, sticker, arc instability, the transition process and other undesirable phenomena coarse particles. Therefore, cellulose electrode arc voltage is higher than normal electrode 8 ~ 12V (such as E6010, Uf = 26 ~ 35V). Elongated arc, the arc voltage is higher than the external characteristics of the constant current power supply section, find a stable operating point, the inevitable extinction. The arc voltage is low, the arc voltage is low, the DC reverse, the melt drops short transition, a large volume of molten metal, it is easy to stick bar, in particular the small current (If = 70A). Next to the welding, sticking tendency of serious disruption to normal welding, and welding joints and more layers, the internal depression, porosity and other serious defects.
2.2 cellulose added thrust rod special welding current, the external characteristic curve shown in Figure 1.
In the short arc, the arc voltage is low, the original instantaneous welding current value plus 50 ~ 80A on the thrust current. Thus, gas evolution and current drug skin is proportional to the square, arc force is proportional to the square of the current surge, short circuit instantaneous droplet will blow open, increasing the number of small particles of transition, to effectively prevent the adhesive electrodes and molten pool phenomenon.
Special welding electrodes cellulose 2.3 second output circuit in series with high-capacity output reactor, slowing the rate of increase of short-circuit current, a small current when the current solution stability of the dynamic characteristics of continuous problems effectively solve the extinction, interrupter, sticker phenomenon. Φ3.2 cellulose electrode (E6010) in DC is connected, the minimum current If = 30A. DC reverse If = 40A, the arc is still very stable, it is difficult to achieve other DC arc welding machine process parameters.
2.4 As to fiber electrode dedicated welder control circuit on the output inductor and electrode welding machine is different from the normal power features, so the output characteristics of cellulose electrodes, welding electrode alkaline (such as E5015) reduce the stability of the arc , brings the basic electrode extinction, such as poor process of sticky issues. Compatible with two types of electrodes to the melting characteristics of DC arc welding machine power supply design should consider the best of both properties on the coexistence of arc welding power source, that is satisfied with the melting characteristics of cellulose electrodes, but also for ordinary alkaline electrode melting characteristics that this type of arc welding power source on the characteristics of the process are to achieve the best results.
2. 5 According to this line of thinking, Tangshan Matsushita introduced, developed a YD-400AT2HGF cellulosic electrode welding process performance and general compatibility of the DC inverter welding machine. The welding machine has three output terminals, one cellulose electrodes, the second is common electrode and the third is the base metal workpiece. Positive and negative then converted by the push-in fittings quickly. Current, arc current regulation, applicable to the welding current range (20 ~ 400A), and with electric shock function and protection circuit. For two years, by our users, stable performance, high reliability, for all kinds of welding arc welding process, especially cellulosic Welding Electrode process to the next pipe.
3, cellulose type electrode machine process parameters
3.1 cellulose electrodes are commonly used:
E6010 (equivalent to E4310, J425G) for backing welding, heat welding, filling welding;
E8010 (equivalent to E5511, J555) for heat welding, welding filling, capping layer, usually under a low hydrogen to the electrode;
E7048 (equivalent J506X) weld appearance neat and beautiful.
4 Summary
4,1 vertical down welding with welding quality, welding speed, high efficiency, easy to learn, master, etc., are widely used in oil, gas, water in large diameter pipeline construction. Cellulose electrodes is, when the reverse transition droplet shape is different from the DC arc welding machine in general will have a broken arc welding, extinction, straps and other undesirable phenomena. Increase the thrust output inductor current and increase cellulose electrodes can improve arc stability.
4.2 Tangshan Matsushita to introduce the development of advanced inverter control technology developed by YD-400AT2HGF DC arc welding machine, take special circuit design, compatible with the basic welding rod and cellulose rod process characteristics. Coexistence of two best process specifications that meet in the melting characteristics of cellulose electrodes, and electrodes for alkaline melting properties by users.

A groove in the form of welded joints

Pipeline construction welding groove type commonly used as an API standard 30 ° V-groove, suitable for manual welding. But for large-diameter, thick-walled steel pipe, this type of weld filler metal type groove joints large, labor-intensive and not suitable for automatic welding of the weld, usually semi-automatic welding with a 23 ° V-groove, automatic welding with narrow gap compound groove pattern.

2 Preheat and interpass temperature

The purpose of preheating the base metal is slow and reduce the stress state of the root weld root of the cooling rate to prevent cold cracking. Construction of the pipeline welding preheat temperature range should consider the strength of base metal, and properties changes of diameter and wall thickness, and welding materials, hydrogen and other factors. For multi-layer thick-walled pipe welding, the weld must also consider the control layer to control the temperature of the cooling rate near seam zone. Interpass temperature and preheat temperature is generally similar. To avoid overheating in the near seam zone under the premise of a higher temperature to prevent multi-layer cold Welding Electrode.

3 welding wire energy

Welding heat input to be taken into account to determine the composition of the base material, welding material type of drug skin (cored, flux) of the metallurgical response and protective effect of shielding gas, welding preheat condition, temperature control layer case, the tube specifications , wire (bar) diameter, heat affected zone of embrittlement and softening tendencies, the desired and mechanical properties, as well as welding position, welding pass, each solder layer thickness, weld and droplet forms of the transition factors.

4 after the heat treatment and heat treatment

Pipeline construction welding welding is generally not carried out after the heat treatment and heat treatment. But in the alpine region of construction, when welding preheat and interpass temperature control is difficult to play a greater role, it is necessary to take some of welded joints after the heat and heat treatment measures to ensure the organization and performance of welded joints.

5 Other

Welding in pipeline construction process should take into account the high-intensity, large wall thickness of steel are exposed to the impact of external stress. Taking into account the ambient temperature, humidity and wind speed on the environmental impact of different methods of welding, to take the necessary measures to ensure the welding quality.

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