PSI - Issue 16

Olena Berdnikova et al. / Procedia Structural Integrity 16 (2019) 89–96 Olena Berdnikova et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Keywords: High strength steel; arc welding; laser welding; hybrid laser-arc welding; welded joints; structural phase composition; structural parameters; dislocation density; mechanical properties; local internal stresses; crack resistance.

1. Introduction

Low-alloyed high-strength steels are widely used in various branches of modern industry, including construction, agricultural, transport, engineering and defense, for the manufacture of welded metal structures (Sharma and Maheshwari (2017); Garcia (2017); Lan and Chan (2019)). Machinery and constructions of critical purpose require the use of steels of a wide class of strength in a fairly wide range of mechanical properties and, accordingly, of different structural and phase composition (Bright et al. (2011); Dubovska et al. (2014)). So, in the building and transport industry for agricultural purposes, structural steels with a yield strength of 350...740 MPa are used. These steels have a ferrite-pearlite and bainite-ferrite structure. The tensile strength of such steels reaches 490...940 MPa. For steels of special purpose of martensitic type, the strength reaches 1500...1700 MPa. Many structures of high strength steels are structures of long-term use under external loading conditions. Therefore, the study of the influence of structural factors on the mechanical properties and crack resistance of welded joints of these steels becomes important (Berdnikova et al. (2016); Doncheva et al. (2018); Markashova et al. (2019a); Saadati et al. (2019); Zhang et al. (2011)). Most often, in the manufacture of these metal structures, mechanized or automatic welding in shielding gases is used (Alipooramirabad et al. (2017); Nathan et al. (2015); Surian et al. (2010)). As well, such progressive technologies as laser and hybrid laser-arc welding have recently been implemented, which allows to obtain welded joints at increased speed with much smaller dimensions of welds and heat-affected zones (HAZ), to improve the quality of high-strength steel joints and the to raise productivity of their manufacture (Afkhami et al. (2019); Bunaziv et al. (2018); Krivtsun et al. (2016); Markashova et al. (2017); Oyyaravelu et al. (2016); Reisgen et al. (2015); Shelyagin et al. (2018); Sokolov et al. (2015); Turichin et al. (2018)). Nowadays the processes of structure formation in the metal of welded joints of high-strength steels under the influence of thermal welding cycles are well studied (Bhole and Fox (1996); Jorge et al. (2018); Shi and Han (2008)). Numerous experimental data were obtained on how the structure and phase composition of the metal of welded joints depends on the cooling rate (Gianetto et al. (2012); Gomes et al. (2013) ; Gutiérrez (2014); Kim et al. (2016). A significant contribution to the development of welding technologies based on the above results was made by the authors of Refs Bunaziv et al. (2015), Garcia (2017), Kitagawa and Kawasaki (2013), Zhang et al. (2014), Turichin et al. (2015). In Refs Afkhami et al. (2019), Alipooramirabad et al. (2016), Markashova et al. (2019b), Turichin et al. (2017) the estimates of welding stresses and strains in the welded joint zone during welding of structural steels were made, and also regressive relations were obtained that connects the relative portion of phase components of austenitic, ferritic, pearlitic, martensitic and bainitic structures, chemical composition of steels with mechanical properties. However, the question of the influence of the structural-phase composition and specific parameters of the structure forming in the weld metal and heat-affected zone on the strength and crack resistance of welded joints of high-strength steels has not been studied enough. It requires research at all structural levels (from grain to dislocation) using modern electron microscopy. It is necessary to expand the general understanding of the structure and phase composition of the metal of welded joints of low-alloyed high-strength steels. Now, there is not enough information on how the fine structure, namely, the substructure, the distribution of the density of dislocations, the size of carbides in the structural components (bainite, martensite, etc.), effects on the physical and mechanical properties of welded joints. Studies at the dislocation level will also obtain more accurate information about the phase composition of the metal. Only by the method of transmission electron microscopy (TEM), specific information on such structural components as the lower or upper bainite, tempered and quenched martensite, the parameters of their fine structure (size of sub-grains, laths, fragments, carbides, dislocation distribution) can be obtained. Obtaining reliable and high-quality welded joints of low-alloyed high-strength steels is an actual problem. For its solution, the detailed and comprehensive study of the physical and mechanical properties of the metal of welded joints and their dependence on the structure and phase composition is important. 2. Current state of the problem