OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 1 2024 Introduction In the review [1], the features of the chemical composition of pipe steels, welding methods, and regulatory documents regulating mechanical properties are considered. In this paper we will consider the characteristics of the microstructure of welded joints. Increasing the yield strength is known to increase the loading capacity and reduce the transportation costs. Thus, high strength combined with high toughness and formability are the main requirements in the steel industry for pipelines [2–10]. The addition of micro-alloying elements such as Nb, V, Ti and Mo, coupled with advanced thermo-mechanical control process (TMCP) technology, can provide an excellent combination of strength and toughness [2, 3]. Microalloying elements such as Ti and Nb form fi nely dispersed carbide and carbonitride precipitates during TMCP of high-quality pipeline steels, which increase the strength of the steel. It has been established that fairly homogeneous dispersed particles containing Nb, Ti and V eff ectively inhibit the growth of austenite grains [11–15]. Besides, the additions of Mo, Nb and Cu contributed to the formation of a bainite microstructure [11–16]. The eff ect of carbide size on fracture may be indirectly related to grain size. The authors of [3, 11, 12] noted that the largest size of carbides in the microstructure is proportional to the size of the ferrite grain in annealed or normalized steels. Grain size is important even when cracks are initiated by pearlite particles or colonies, [11, 12] because the grains around the fracture source can control crack propagation [1–3]. Larger grains, if present around the source of the spall, encourage the nucleated crack to grow beyond the critical size required for unstable propagation before it can be blocked by the grain boundary. As a result, failure occurs at a lower stress than required when smaller grains are present around the start of the fracture. Observations such as the presence of non-propagating ferrite grain-sized cracks on the fracture surface [11], large cleavage facets at the crack nucleation (larger than the average facet size) [12–15], and better correlation between fracture stress and largest grain size (and not the average grain size) in fractured ferritic-pearlite steel specimens [17–25] is important in the initiation and propagation of cleavage cracks. At the same time, it should be understood that within the volume of a structural material, spatial inhomogeneities can arise in various forms, such as a non-uniform distribution of non-metallic inclusions and precipitates, a spatial distribution of pearlite and ferrite, a mixed (fi ne- and coarse-grained) granular structure (or crystallographic texture) [1–3]. The authors [3, 11, 12, 24, 25] concluded that spatial heterogeneity in any form can lead to a wider than usual scatter of fracture toughness results, depending on the local microstructure sampled at the “critical distance” (at where the local tensile stress exceeds the cleavage stress). Fracture stress [25] in front of the notch root. The grain size in steels can be irregular, and in some Nb-V steel plates subjected to thermo-mechanical control process (TMCP), a bimodal ferrite grain size distribution has been reported (coarse grains present in a matrix of fi ne grains) [11]. Therefore, depending on whether the grains are large or small at the root of the notch, the fracture stress values for a bimodal ferrite structure may diff er. Understanding the spread of Charpy energy values for steels after TMCP is very important from an industrial point of view. However, it is scientifi cally diffi cult to study the eff ect of particle size distribution on impact strength using Charpy tests. Charpy tests often produce complex fracture surfaces that make it diffi cult to identify the original location of the onset of cleavage [11, 25–28]. For example, works [11, 12] have shown that in a blunt notch test, if a coarse grain band is present in the active area just before the root of the notch, the coarse grains initiate spalling, which results in low shear failure stress. However, if large grains are absent at the root of the notch, small grains initiate spalling and fracture stress values are higher. Similarly, in the Charpy impact transition (IT) region, the magnitude of the plastic fracture area depends on the location of the coarse grain band relative to the root of the notch. If the coarse grain band is located close to the base of the cut, cleavage failure begins at that location, resulting in low impact energy. However, if the coarse grain strip is located far from the base of the notch, a ductile crack will propagate fi rst, absorbing higher impact energy. In addition to the above, the works [3, 11, 12, 15–19] show that the addition of a large amount of micro-alloying elements poses a serious problem for the weldability of pipeline steel due to the increased equivalent carbon content (Ceq according to the Russian standard), especially such elements as Ni, V, Cr, Mo and Cu [2, 4, 11–28].
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