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O. M. Berdnikova, A. V. Bernatskyi, V. D. Pozniakov, T. O. Alekseienko, V. M. Sydorets, O. I. Bushma
«Nanoscale Structures of Laser–Arc Welded Joints of High-Strength Low-Alloy Steels»
PACS numbers: 06.60.Vz, 42.62.Cf, 81.07.-b, 81.20.Vj, 81.40.Np, 81.70.Bt, 89.20.Bb
Low-alloyed high-strength steels with a yield strength of above 600 MPa are widely used for the manufacture of various types of critical-purpose constructions (freight cars, bridges, pressure vessels, truck bodies of lorries, parts of load-lifting cranes, pipelines, ship hulls, etc.). Arc welding of these steels cannot satisfy the industry due to both the low productivity and the need for a heat treatment before or after welding. Without heat treatment, cold cracks form in these steels in the overheating area or in the weld where the metal is quenched during welding, and softening zones appear in the heat-affected zone outside the quenching sites. The hybrid laser–arc welding is becoming more common in the industry. This is due to the prospect of introducing hybrid laser–arc welding instead of arc processes, since such a replacement does not require a relatively large expenditure on the re-training of production and provides a noticeable increase in productivity. At the same time, a significant part of the thermal power required to melt the metal in hybrid laser–arc welding is provided by the use of cheap arc power sources. Previously, the authors determined the optimal speed of hybrid laser–arc welding from the point of view of the phase composition of the structural components, dispersion of the grain structure, the proportion of brittle fracture, etc. However, it was not clear, what effect of the dislocation structure is on the crack resistance measure—fracture toughness. The aim of this paper is to study the effect of external bending load on the dislocation structure and on fracture toughness of low-alloyed high-strength steel welded joints produced by hybrid laser–arc welding by the mode at optimum welding rate. The structural factors, which guarantee a high level of strength and crack resistance of welded joints of high-strength steel, are identified. As shown, the complex of properties of welded joints under external loading in a wide range of temperature conditions ensures the formation of fragmented lower bainite structure with a uniform distribution of the dislocation density and nanoparticles of carbide phases.
Keywords: low-alloyed high-strength steel, hybrid laser–arc welding, structure–phase composition, nanoscale structures, dislocation density, crack growth resistance
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