Crack Resistance of 14KhGN2MDAFB High-Strength Steel Joints Manufactured by Laser Welding

Today welded joints of high-strength steels, which are used in machine building and manufacture of structure, operating under difficult service conditions (static, dynamic, cyclic loads etc.) require a high level of mechanical properties of welded structures that mainly depend on a series of factors. Taking into account significant changes of technological modes in laser welding in comparison with traditional arc welding the investigation of structure change in welding zone and effect of these changes on service properties of welded joints is a relevant problem. The aim of the present work in investigation of effect of structure transformation and metal phase content on change of the most significant service properties of welded joints, namely indices of strength and crack resistance in zone of laser welding in butt joints of 14KhGN2MDAFB high strength steel. Structural-phase state and chemical elements composition in the weld metal and in the heat-affected zone were studied by optical microscopes, analytical scanning microscope, and transmission micro diffraction electron microscope. The dependencies of effect of laser welding speed on properties of welded joint of 14KhGN2MDAFB steel were determined. The most favorable structures (formation of fine grain lower bainite structures, absence of extended dislocation accumulations, uniform dislocation distribution) in the investigated joints is obtained at welding speed 13.9 mm/s. This mode guarantees their crack resistance. (paper)