Zhukov V. Features of structure and mechanical properties of 14KhGNDC welded joints modified with dispersed particles of carbides, oxides and titanium-based compounds.

The dissertation is devoted to the establishment of regularities of influence of modification by dispersed particles of oxides, carbides and compounds on the basis of titanium (SiC, VC, NbC, TiC, ZrO2, Al2O3, MgO, TiO2, FeTi, TiN) on features of structure and mechanical properties of metal of welded joints of high strength low alloy steel 14KhGNDC. The structure, distribution and composition of non-metallic inclusions and phase precipitates of modified metal of welded joints of high strength low alloy steel 14KhGNDC were studied. A new method for estimating the kinetic parameters of the structural-phase transformation has been developed. The kinetic parameters are determined: the value of the maximum intensity and the volume effect of the structural-phase transformation, which characterize the degree of change in the volume of the metal during the reconstruction of the crystal lattice during the structural-phase transformation. The temperatures of the maximum intensity of structural-phase transformation for the metal of modified welded joints are determined. An analysis of the relationship between the kinetic parameters of the structural-phase transformation, chemical composition, transformation temperatures of austenite, structural-phase composition and mechanical characteristics of the metal of modified welded joints of steel 14KhGNDC. The mechanism of influence of different types of modifiers (carbide, oxide and titanium-based compounds) on the kinetics of structure formation of weld metal is determined. It is established that carbide modifiers affect the kinetics of transformation and formation of the secondary crystal structure due to the dissolution and change of the composition of the solid solution; oxide modifiers and modifiers based on titanium compounds are dissolved and released on the surface of non-metallic inclusions, as well as in the form of new non-metallic inclusions, which affect the structure and mechanical properties of the metal of modified welded joints. It is established that modifiers based on titanium compounds lead to the formation of non-metallic inclusions formed inside the metal grain with a dislocation density of 1010–1011 cm-2 around the inclusion, increase the strength of the metal and reduce resistance to cracking; oxide non-metallic inclusions are formed near the grain boundaries of the metal with a dislocation density of 108–109 cm-2 around the inclusions and increase the values of ductility and toughness of the metal. Experimental and industrial verification of the possibility of applying the technology of modification of the welded joints of high strength low alloy steels was carried out at PJSC NKMZ (Kramatorsk), for steels grades A514 and 16KhGMFTR. When comparing the mechanical properties of the metal of the seams obtained by welding wires of foreign (BÖHLER NiCrMo 2.5 - IG, BOHLER X 70 - IG) brands with the metal obtained using experimental welding wires with dispersed modifiers TiO2 and Al2O3 found that cheaper use of domestic materials allows the level of mechanical properties of the welded joints of high strength low alloy steels.