OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 a b Fig. 4. Laboratory version of laser-arc hybrid technology (a) and field version for pipe welding (b) [33] of API 5L X80 steel with a root thickness of 14 mm were welded with MF 940 M welding wire. It is shown that a decrease in the cooling rate of welds from 588 °C/s to 152 °C/s reduces the hardness of the weld metal from 343 ± 12 HV to 276 ± 6 HV and the tensile strength from 1,019.5 ± 14 MPa to 828 ± 10 MPa, as well as an increase in the bainitic phase of the weld metal is revealed when increasing the preheating temperature to 180 °C and the maximum running energy. The work [31] notes that to develop oil and gas resources in deep-sea areas, it is necessary to lay a large number of underwater pipelines. J-lay is the primary method for laying deep undersea pipelines. Welding the circumferential seam in a horizontal-vertical position is a mandatory part of the J-lay method. Currently, the following sequence is usually used: hot pass welding of the root, filling and facing layers of the welded joints [31]. Due to problems with welding efficiency and quality, traditional welding methods could not meet the requirements of industrial pipelines with thicker pipe wall and larger pipe diameter, so there was an urgent need to develop a welding method with high efficiency and productivity, as well as a high degree of automation. The heat source characteristics of laser-MAG hybrid welding, which combines deep laser penetration and wide arc adaptability, make it very suitable for welding pipes with thicker walls [29– 34]. Compared to conventional welding in a horizontal-vertical position, it has the following advantages: deep penetration, high welding speed and high welding quality. The level of penetration with single-sided welding is the same as with other root welding methods + one fill pass. At the same time, it reduces spatter and welding distortion, reduces the need for back gouging, and improves production efficiency [29–32]. A lot of work has been carried out in the country and abroad to study the technology of hybrid laserMAG welding in the field of pipeline laying (for welding in a horizontal-vertical position) [34, 35]. The use of hybrid laser-MAG welding not only increases the speed and quality of welding, but also gives great advantages in reducing the sensitivity of butt joints and welding defects [34, 35]. Despite the significant progress of LAHW in technical implementation, research work on the structure and properties of metals, and taking into account the indisputable fact that this technology has a high penetrating power and efficiency; at this stage of development it is considered an industrial innovation. The technology and equipment need constant improvement in the process to meet the requirements of field welding. The transfer controlled MAG (TC) welding process is a derivative of the MAG process for root pass welding in pipelines. There are various patents for short circuit switching control [35]. Among them there is a control developed and patented by The Lincoln Electric Company under the trade name “STT® (Surface Tension Transfer) [35]. One of the variants of the MAG-TC welding process is to control the current without changing the electrode feed rate, using a special welding source for this, which ensures low welding energy, smoke and spatter. Reducing the spatter rate reduces the time required for cleaning both the burner and the welded joint [35]. The metal transfer obtained by this process is carried out by short-circuiting using pure CO2 or Ar/CO2 mixtures as a protective gas [35]. Fig. 5 shows the waveform used in the MAG-TC process.
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