OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 The most common materials naturally chosen by pipe manufacturers are steel alloys due to its sufficient mechanical reliability and economic feasibility. Specifications concerning the chemical composition, mechanical properties and other important aspects such as welding, cutting, production, etc. of materials for oil and gas pipelines are determined by the American Petroleum Institute (API) [3], the International Organization for Standardization (ISO) and other national agencies [3-5]. API standards are commonly used by many national agencies as a reference to establish its own specifications for these materials. API specifications are accepted and widely used all over the world. In accordance with API requirements, pipeline materials are manufactured or supplied with product specification requirements: PSL 1 and PSL 2. The PSL 1 document contains only recommendations for the carbon equivalent; there are no restrictions on the impact strength, yield strength and ultimate strength. The PSL 2 document already prescribes mandatory values in a certain range for carbon equivalent, impact strength, yield strength and ultimate strength. Another significant difference is based on the type of pipe ends [1–3]. Knowledge of the chemical composition and mechanical properties of these pipes is necessary to understand the weldability and other aspects of welding these pipes. Pipe steels from different manufacturers that meet the requirements for strength and ductility [1–5] may have different microstructures [1–3, 10–34]. The most common steels are those with ferrite-perlite or ferrite-bainite microstructure [10–33]. Pipes can be made in two traditional ways: cold stamping (UOE: Upressing, O-pressing, and expanding) and seamless [3]. The production of pipes by cold stamping (UOE) tends to introduce intense deformation gradients into the sheet in different directions relative to a fixed orthogonal coordinate system during forming, with more serious gradients occur in the transverse direction [1, 2]. This affects not only the yield strength, but also the deformation hardening and subsequent instability (neck formation), which, finally, are the driving forces of the initiation and propagation of fracture. On the other hand, the production process of seamless pipes makes it possible to obtain a product with improved mechanical properties due to heat treatment, which removes residual stresses and reduces the out-of-roundness of the final shape. Consequently, it is expected that the mechanical properties of the final product will be uniform in space and direction [1, 2, 10]. Regardless of the method of pipes production, later during the construction of the pipeline pipes are connected to each other by welding. In recent decades, many studies of annular welds of onshore and offshore pipelines with cracks under operational load have been carried out [11, 12]. Cracks in the cup welds of pipelines made of high-quality steel are mainly located on the fusion line of the root material and in the heat-affected zone [13]. At the same time, cup welds have zones of material with different properties, such as base metal (BM), weld material (WM), root material (RM) and heat affected zone (HAZ). The heterogeneity of welded joints in geometry and material properties leads to a significant concentration of stresses and deformations in defective parts, which significantly reduces the deformation bearing capacity of welded pipe joints [13, 14]. During the welding process, the metal being welded heats is heated, the filler wire melts and a weld with a cast structure is formed, which has a transition zone to the base metal structure (HAZ). It is in this zone that the impact strength values decrease [14–20]. Due to the fast-flowing process of heating and melting of metal in the weld zone and the adjacent area of the base metal, HAZ structure with different sizes of austenitic grains is formed, with metal sections heated above and below the points Aс1 and Aс3. All this leads to a decrease in the mechanical properties of the metal. Consequently, considerable efforts to study high-strength steels for pipelines have been focused on increasing the impact strength in the heat-affected zone. The relationship between microstructure and impact strength for metals of multiple passes is very complex, since various factors can have beneficial and adverse effects depending on the material under study and its microstructural state. In addition to microstructural components, the influence of reheating, the presence of microphases and inclusions are recognized as critical factors affecting the microstructure and, consequently, the impact strength. Although little research has been conducted on the microstructure characterization of weld metals due to the aforementioned complexity, knowledge of the microstructure characteristics is critical for predicting impact strength. Thus, a more systematic study is fundamental to uncover this relationship between microstructure and strength.
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