Crack resistance of welded joints is one of the most important indices of service reliability of critical designation structures. The purpose of the research is the establishment of the influence of technological modes, hybrid laser-arc welding of high strength steel NA-XTRA-70 on the formation of structure in the metal welded joints, as well as analysis of the influence of this structure on their crack resistances. For the experiments the laboratory setup has been developed on which the experiments were carried out in the technological scheme that provides the location of the consumable electrode arc at a certain distance in front of the laser beam in the welding direction. Nd: YAG-laser DY 044 of company ROFIN (Germany) was a laser radiation source. Power for consumable electrode arc was carried out by welding generator PSG-500. Investigations of structural-phase changes in weld metal and HAZ were carried out by means of optical microscopy (optical microscope Versamet-2 and Neophot-32), microhardness was measured using M-400 gage of Leco company at 0.98N loading. Fractographic investigations were carried out by scanning electron microscopy (scanning electron microscope SEM-515 of PHILIPS company, Netherlands) on fractured specimens of welded joints, made as a result of impact bend tests. An analysis of crack resistance of the welded joints was carried out under different conditions of test temperatures. It was based on experimental data, received by a quantitative fractography method. It is shown that metal of a welded joint produced by hybrid laser-arc welding at Vw = 72m/h rate has mainly failure of ductile nature after external loading. This indicates sufficient level of crack resistance and further working capacity under operation conditions.
The advantages of laser and hybrid laser-arc welding in production of welded structures from high strength steels are increase of efficiency due to rise of the welding rate by order; significant (3 – 5 times) decrease of heat input; reduction of weld size; receiving of fine grain structures in the welds and the heat affected zone increasing strength of the joint and reducing susceptibility to crack formation. Research of the peculiarities of laser and hybrid processes, their effect on weld formation, revealing the advantages and eliminating the disadvantages of laser and arc constituents of the heat sources are currently the main problems for the production engineers and researchers. Therefore, the aim of the investigation lies in determination of the effect of process mode parameters of the hybrid laser-arc and laser welding in high-strength steel welding on formation of the structure of the welded joint metal as well as the analysis of effect of this structure on their mechanical properties. The investigation was focused on the high-strength steel N-A-XTRA-70 which is widely used in European Union countries for manufacture of critical structures of different designations. A laboratory bench was developed for experiments. It was used for carrying out investigation on the process flow sheet which provided location of a consumable electrode arc at some distance ahead of a laser beam during welding proceeding. The laser irradiation source was Nd:YAG-laser DY 044 of ROFIN company (Germany). For the first time the results of carried investigations allowed determining the process modes of laser and hybrid laser-arc welding for producing quality and sound welded joints of the high-strength steel from the point of view of the structure formation and metal phase composition providing high indices of mechanical properties and crack resistance of the welded joints.
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