API 5L pipelines are usually connected using various welding techniques, each with its own advantages and limitations. Commonly used methods include metal arc welding (SMAW), submerged arc welding (SAW), gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), and flux-cored arc welding (FCAW)
(1) The advantages of metal arc welding are simple, lightweight equipment and flexible operation. It can be used for welding short seams in maintenance and assembly, especially for welding in hard-to-reach areas. The disadvantages are high requirements for welder operation skills, high welder training costs, poor working conditions, low production efficiency, and unsuitable for welding special metals and thin plates. Metal arc welding with corresponding electrodes can be used for welding most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and their alloys.
(2) Submerged arc welding can use a larger current. Under the action of arc heat, part of the flux melts into slag and reacts with liquid metal in liquid metallurgy. The other part of the slag floats on the surface of the metal pool. On the one hand, it can protect the weld metal, prevent air pollution, and produce physical and chemical reactions with the molten metal to improve the composition and properties of the weld metal; on the other hand, it can also slowly cool the weld metal to prevent defects such as cracks and pores. Compared with arc welding, its biggest advantage is high weld quality, fast welding speed and good working conditions. Therefore, it is particularly suitable for welding straight seams and circumferential seams of large workpieces, and mechanized welding is mostly used. The disadvantage is that it is generally only suitable for welding flat seams and angle seams. Welding in other positions requires special devices to ensure that the flux covers the weld area and prevents the molten pool metal from leaking out; the relative position of the arc and the groove cannot be directly observed during welding, and an automatic weld tracking system is required to ensure that the welding torch is aligned with the weld without welding deviation; the current is large, the electric field strength of the arc is high, and when the current is less than 100A, the arc stability is poor, and it is not suitable for welding thin parts with a thickness of less than 1mm. Submerged arc welding has been widely used in the welding of carbon steel, low-alloy structural steel and stainless steel. Since slag can reduce the cooling rate of the weld joint, some high-strength structural steels and high-carbon steels can also be welded by submerged arc welding.
(3) Gas tungsten arc welding is an excellent method for connecting thin sheet metal and base welding because it can well control heat input. This method can be used for welding almost all metals, especially for dry welding of metals that can form refractory oxides such as aluminum and magnesium, as well as active metals such as titanium and berkelium: this welding method has high welding quality, but compared with other arc welding, its welding speed is slower, the production cost is higher, and it is more affected by the surrounding airflow, making it unsuitable for outdoor operation.
(4) Gas tungsten arc welding usually uses argon, helium, carbon dioxide or a mixture of these gases. When argon or nitrogen is used as the shielding gas, it is called metal inert gas shielded welding (internationally referred to as MIG welding); when a mixture of inert gas and oxidizing gas (O2, CO2) is used as the shielding gas, or a mixture of C02 and C02+02 is used as the shielding gas, it is collectively referred to as metal active gas shielded welding (internationally referred to as MAG welding). The main advantage of metal active gas shielded welding is that it can be easily welded in various positions, and it also has the advantages of fast welding speed and high deposition rate. Metal active gas shielded welding can be applied to the welding of most major metals, including carbon steel and alloy steel. Metal inert gas shielded welding is suitable for stainless steel, aluminum, magnesium, copper, titanium, zirconium and nickel alloys. This method can be used for arc spot welding.
(5) Flux-cored arc welding can be considered as a type of metal active gas shielded welding. The welding wire used is a flux-cored wire, and the core of the welding wire is filled with powder of various components. During welding, an additional shielding gas, mainly CO2 gas, is added. The powder is decomposed or melted by heat, playing the role of gasification and slag formation to protect the molten pool, alloy infiltration and arc stabilization. When flux-cored arc welding does not add additional shielding gas, it is called self-shielded flux-cored arc welding. It uses the gas generated by the decomposition of powder as the shielding gas. The change in the dry extension length of the welding wire in this welding method will not affect the protection effect, and its range of variation can be large. Flux-cored arc welding has the following advantages: good welding process performance, beautiful weld bead forming; fast deposition speed, high productivity, continuous automatic and semi-automatic welding; convenient alloy system adjustment, the chemical composition of the deposited metal can be adjusted through two ways: metal sheath and flux core; low energy consumption; low overall cost. The disadvantages are complex manufacturing equipment, high requirements for manufacturing process technology, high requirements for the storage of flux-cored wire, and the wire is easily affected by moisture. Flux-cored arc welding can be applied to the welding of most ferrous metals of various thicknesses and various joints.
