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At modern mechanical facilities, the friction-welded joints are getting widespread as the most advanced production technique characterized by high efficiency, processability, cost-effectiveness, and safety. Moreover, it allows producing high-quality joints of a large number of different analogous and opposite metals and alloys. Despite all these advantages, one should consider that metal, in the process of welded joint formation, suffers a local thermo-deformational effect, which causes the gradient nature of the structure and residual strains of a welded joint. These factors directly influence the structure’s working ability and durability under fatigue loads, which are the most common cause for parts failure. The paper contains the assessment of the post-weld tempering influence on the cyclic life of welded joints of 32G2 and 40HN steels produced using the rotational friction welding technique. The authors tested laboratory specimens with welded joints under the high-cycle fatigue using the simulation machine with the two-point fastening of a revolving specimen under the action of even twisting moment. The study involved the statistical processing of the obtained results of cyclic life. Based on the metallographic analysis, the authors identified the weak points in welded points where the fatigue cracks initiation and progress occurred in the initial state and after tempering. The paper presents the fractographs illustrating the fracture mechanism of specimens under the study. The authors identified the influence of different tempering temperature modes on the cyclic life of the studied welded joints and the nature of their fracture. The study shows that tempering at the temperature over 400 °C promotes fracture acceleration under the effect of fatigue loads due to the development of return and polygonization processes in the vulnerable area of the thermomechanical action zone.

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This study investigates the influence of friction pressure at a given  burn-off length on the microhardness, tensile properties and microstructure of the welded joints from steel 32-2-Mn and 40-Cr-Ni. Phase transformations occurring in the materials to be welded as a result of thermal deformation effects are analyzed. The change in the length of the thermomechanical affected zone (TMAZ) depending on the friction pressure is shown. The results of the distribution of microhardness in the weld, clearly illustrating the formation of hardened and weakened areas. The results of tensile tests of welded joints are given. Analyzed the place of fracture at various welding parameters. The necessity of studying the distribution of internal residual stresses to explain the mechanism of fracture of welded joints is shown.