(2) Flux cored wire
In recent years, with the development of long-distance pipelines toward high strength, large diameter and thick wall, the traditional manual welding method has been gradually replaced by semi-automatic welding and automatic welding methods, among which semi-automatic welding applications are the fastest growing. What comes with it is the rapid development of flux-cored wire. The reason why the flux-cored wire can get such attention and development is inseparable from its many characteristics. It is characterized by fast deposition speed and high welding productivity. Compared with solid wire, the flux-cored wire arc is soft. The spatter is small, the welding process performance is good; the penetration depth is large, the molding is beautiful; the comprehensive cost is low.
The flux-cored wire can be divided into gas-protected flux cored wire and self-shielded flux-cored wire according to the different protection methods during welding. The self-shielded flux-cored wire is widely used in long-distance pipelines with its unique superiority. The implementation standard is GB/ T 17493-1998 and AWS A. 29-98. Typical examples include T8-Ni1 type (such as Tianjin Jinqiao JC-29Ni1 Φ2.0mm welding wire, American Haobot HOBART 81N1Φ2.0mm welding wire), T8-Ni2, T8-K6 type (Lincoln NR207 Φ2.0mm welding wire), etc. The welding wire has good operation performance at all positions, and the welding speed is fast, and the weld metal toughness is good, but the appearance of the coarse columnar crystal structure of the weld metal in the welded state makes the weld metal impact toughness in the welded state and the heat treatment. There is a big difference between multi-layer welding and single-pass welding. Therefore, when welding with T8 self-shielded welding wire, the welding specification parameters, heat input amount, welding pass and thickness of each weld layer should be strictly controlled.
2.3 Protective gas
The automatic welding of long-distance pipelines mostly uses carbon dioxide gas shielded welding and oxidative mixed gas shielded welding, that is, the gas used is CO 2 , CO 2 +Ar or CO 2 +Ar+O 2 . The role of an inert gas (such as Ar) in the gas metal arc welding is to discharge the arc and the air around the molten metal to prevent harmful components in the air from affecting the stability of the arc and contamination of the liquid metal. Other non-inert gases (such as CO 2 , O 2 ) can also be used as shielding gas for gas metal arc welding. The premise is that although these gases can undergo some metallurgical reaction with the protected liquid metal, conditions can be created during the welding process so that the consequences of these reactions do not cause damage to the welded joint. If CO 2 is used as the shielding gas, although CO 2 decomposes O 2 and CO at the high temperature of the arc during the welding process, and then Fe is oxidized to form FeO and may cause pores, this adverse effect can be obtained by adding appropriate amount to the welding wire. The deoxidizing elements such as Si and Mn are solved. The study found that the protective gas composition and flow rate have a certain influence on the weld formation, the composition and flow rate are different, the oxygen content in the weld is different, the weld formation is different, and the defect probability is also different. For example, when pure CO 2 is used as the shielding gas for the STT gas shielded root welding, the weld condensation is accelerated due to the decomposition and heat absorption of CO 2 , and the fluidity of the molten iron is deteriorated, so that the front weld is easy to form a ridge shape. In the subsequent welding process, the depressions are likely to cause defects such as unmelting and slag inclusion, and the back welds are liable to cause false melting. This problem is particularly prominent when the welding environment temperature is low and the line energy is low. In addition, welds are prone to venting in the weld due to rapid condensation. If a CO 2 +Ar mixed gas such as CO 2 (15-20%) + Ar (85-80%) is used to improve the flowability of molten iron, good weld formation is obtained, the transition between the base metal and the weld is good, and the weld contains oxygen. The amount is low and the weld impact toughness is good. This should be taken into account when choosing the protective gas composition and flow rate.
2.4 flux
The choice of flux mainly considers the type of flux, the matching characteristics of the flux and the wire, the metallurgical properties of the flux and the process performance. In addition, the particle size, water content, mechanical inclusions, and sulfur content of the flux should also be considered. From the viewpoint of improving the toughness of the weld metal, a high alkalinity flux can be selected. However, it should be noted that when the alkalinity exceeds a certain critical value, increasing the alkalinity will result in a decrease in weld toughness. This is mainly because the welding speed is required for the pipeline steel welding, especially when the thick plate is not opened. Under the condition of no groove and no gap, the process performance deteriorates, and there are pores and pitting on the surface of the weld. The oxide inclusions in the weld are obviously increased, resulting in a decrease in toughness. Therefore, the reasonable choice of flux is of great significance to improve the weld toughness.
3. Welding power supply
With the development of electronic technology and modern control technology, digital inverter welding power supply is the main direction of arc welding power supply development. It is small in size, light in weight, energy-saving and material-saving, and has good control performance, fast dynamic response, easy real-time control of the welding process, and great advantages in performance. At the same time, the digital inverter welding power source integrating intelligent control methods such as expert system, fuzzy control and neural network technology can realize unified adjustment and real-time processing of uncertain factors in the welding process to ensure stable welding process and welding quality. . The welding machine manufacturers such as the domestic era and Aotai have already successfully introduced the inverter welding machine with soft switch control. The technology of double wire double arc, double wire single arc, multi wire and multiple arc has also been applied abroad.
Now, China's inverter welding machine power supply has formed 4 generations of products: the first generation is the inverter with thyristor SCR as the main power device; the second generation is the transistor inverter; the third generation is the FET inverter The fourth generation is an IGBT inverter with high inverter frequency, reduced saturation voltage, low power consumption, high efficiency, and no noise. Compared with the previous 3 generation inverters, the advantages are more obvious.
However, IGBT inverter welders are rarely used at construction sites. The cellulose-type electrode is welded with a general DC welding machine, which is prone to arcing, sticking, and arc instability at low currents. Low-hydrogen welding rods have lower requirements for arc welding equipment, and general DC arc welding equipment can meet the requirements. The welding machines used for manual arc welding in pipeline construction include DC-400 of American LINCOLN Company, XMT-304 of American MILLER Company, ZX7-400B of Beijing Times Group Company, ZX7-400ST of Jinan Aotai Company, and so on.
For self-shielded flux cored wire welding, DC-400 DC power supply + LN-23P wire feeder from LINCOLN, USA, and XMT304 DC power supply + S-32P wire feeder from MILLER, Tang Wang DC-400+LINCOLN LN The -23P wire feeder is also available.
For CO 2 gas shielded root welding, the optional welder is LINCOLN's STT inverter + LN-742 wire feeder, and the company's ULTRA FLEX PULSE 350 welder + ULTRAFEED 1000 wire feeder.
4. Welding method and equipment
At present, the common welding methods for long-distance pipelines at home and abroad mainly include: (1) manual welding, including electrode welding arc welding (SMAW), manual tungsten argon arc welding (TIG); (2) semi-automatic welding, including semi-automatic welding of molten gas protection [Including reactive gas protection STT (Surface Tension TransferTM) semi-automatic welding, semi-automatic argon-shield welding (MIG), semi-automatic reactive gas shielded welding (MAG), self-shielded flux-cored arc welding (FCAW); (3) melting pole activity Gas protection automatic welding (AW). In addition, there are submerged arc automatic welding (SAW), electric resistance welding - flash butt welding (FBW). After the West-East Gas Pipeline Project, China mainly focuses on semi-automatic welding of self-shielded flux-cored wire and automatic welding of molten active gas protection.
4.1 manual welding
Manual welding mainly refers to electrode arc welding and manual tungsten argon arc welding.
(1) Welding rod arc welding
It has the characteristics of flexibility, simplicity and adaptability. At the same time, due to the continuous improvement of the process performance of the electrode, its deposition efficiency and mechanical properties can still meet the needs of today's pipeline construction, especially when welding repair welding is more widely used. The electrode is a cellulose type electrode and a low hydrogen type electrode. The organic combination of the two methods of the down welding and the up welding and the good root welding adaptability of the cellulose electrode are still not replaceable by other welding methods in many cases.
(2) Manual tungsten argon arc welding
The welding quality is good, and there is no welding slag on the back. It is generally used for equipment import and export of compressors, ball valves and other equipment, as well as process pipes with small pipe diameter and thin wall thickness, and installation and welding of placed fillet welds. The tungsten argon arc welding method requires strict groove cleaning before welding, and windproof measures must be taken during the welding process.
4.2 semi-automatic welding
That is, semi-automatic welding of self-shielded flux-cored wire and semi-automatic welding of CO 2 gas protection, they are all down-welding methods.
(1) Self-protecting flux cored wire semi-automatic welding
This technology was first applied in the Kusong pipeline project in 1996, and was subsequently applied in pipeline projects such as Sudan, Lanchengyu and Suininglan. The welding method is flexible in operation, strong in environmental adaptability, high in welding deposition efficiency, good in welding quality, easy to grasp by welders, and high in welding pass rate. It is an important filling and cap welding method in domestic pipeline engineering.
(2) CO 2 gas protection semi-automatic welding
With the improvement of welding power supply characteristics, by controlling the droplets and arc shape, the splash problem of CO 2 gas shielded welding has been basically solved, and it has begun to play an important role in pipeline welding, such as the application of STT type CO 2 inverter welding machine. . The welding method is flexible in operation, the welder is easy to grasp, has strong adaptability to different groove, good welding quality, high welding efficiency and smooth weld bead, but the welding process is greatly affected by the ambient wind speed. STT semi-automatic root welding requires that the nozzle group keeps the matching gap uniform during the process. Otherwise, defects such as unmelted and slag inclusions will be generated in the subsequent filling and cap welding.
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