OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 and pipe was below -30 °C. Bainitic ferrite, exhibiting a lath and granular morphology, was the main phase, and M/A existed as the second phase. Works [2, 11–18] note that when welding pipes made of steel X80, X100 and X120 grades in field conditions, difficulties arise in ensuring an optimal structure in the HAZ and a decrease in the mechanical properties of the weld metal. Welding technologies In the standard GOST 29273-92, a definition of weldability is given for all metal materials, taking into account all processes, various types of structures and whatever properties it should satisfy: “Definition of weldability. A metal material is considered to be weldable to a certain extent in these processes and for this purpose, when metal integrity is achieved by welding with an appropriate technological process so that the parts to be welded meet technical requirements, both in terms of its own qualities and in terms of its influence on the structure it forms.” According to AWS (American Welding Society), weldability is defined “the capacity of a material to be welded under the imposed fabrication conditions into a specific suitably designed structure and to perform satisfactorily in the intended service.” This concept, although unique, can be divided into three: operation weldability, metallurgical weldability and weldability during operation. Operation weldability is related to the operational conditions of welding, such as: the combination of the process and the nature of the base metal; welding position; welder skills; co-assembly methods, etc. Metallurgical weldability is associated with thermal and chemical conditions that can create defects or undesirable mechanical properties in the welded joint associated with metallurgical phenomena such as phase transformation, microsegregation, etc. Weldability during operation is more related to the service life of the component being welded. At this point, the main focus will be on metallurgical weldability. Metallurgical issues of steel pipe production are widely covered in the literature; however, the subsequent welding of pipes in the field makes its own adjustments to the operational efficiency of the entire pipeline. The main methods of pipe welding are: arc welding with a low hydrogen electrode, submerged metal automatic welding (SMAW), gas metal arc welding (GMAW), flux-cored arc welding (FCAW-S). The technological features of these methods and equipment are well covered in the literature. Let’s consider promising technologies [29–39]. Laser-arc hybrid welding (LAHW) and automatic welding equipment have been in the research, development and design stages since 2,000 [29–33]. In the laser-arc hybrid welding (LAHW) process, the laser beam and the electric arc interact in the welding bath, and its synergetic effect is used to perform deeper and narrower welds (fig. 3), increasing productivity [30–33]. This method has been successfully implemented in the laboratory when welding the root in all positions of linear pipes with a tip diameter of 8 mm, and the laser source and cooling system are under investigation for its in-situ applicability [29, 30]. In the review paper [32], data on the thickness of the materials being welded are given in table 5. The paper [33] presents industrial options for welding pipelines (fig. 4). In [30], the influence of the parameters of hybrid laser-arc welding: heat input and preheating on the cooling rate, microstructure and mechanical properties of the welded joint is investigated. Specimens made Fig. 3. Cross-section of welds joined by different welding methods: GMAW, LBW and LAHW [31]
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