| With the continuous development of the country’s comprehensive national strength, railway, as the critical link of stimulating domestic demand and stimulate the economy, with low energy consumption, high speed, light pollution, transport and many other economic advantages, more and more national attention is increasingly being attract to it. Bogie as an important part of the train, its resistance to fatigue is facing greater challenges with the train speed-increasing; weld, even more so, as a basic connection of bogies.16MnR as a low alloy steel, which is common used in bogies, is being the test material in the article. Study the factors which has been greatly improved after ultrasonic impact treatment such as residual stress, grain refinement and stress concentration, can play a role in the process to what extent in increasing fatigue life of16MnR butt joint. Comparative analysis of experimental data, the conclusions are as following:Firstly, it is the analysis of residual stress. After heat treatment at600℃, its fatigue life of16MnR butt joints welded is reduced by37.8%than samples of welded style. It seems that this treatment softens the butt joint organization. And higher cyclic stress ratio makes the residual tensile stress of the vicinity of the weld release rapidly; by the way it reduces the influence of residual tensile stress in the fatigue fracture process. After Ultrasonic Impact and200℃heat treatment of low temperature, the fatigue life of16MnR butt joint reduces9.0%than the samples of heat treatment. That seems there is severe plastic deformation in the area of weld toe, and introducing the fatigue fracture beneficial compressive residual stress. At the same time, the grains can be refined, but the tissue of the surface strengthened is improved.Secondly, it is the analysis of grain refinement, if there is smooth transition in the weld toe area, the fatigue life of16MnR butt joint is improved at least179%higher than the line cutting with a given transition radius and samples under500℃heat treatment, after15min Ultrasonic Impact and200℃heat treatment of low temperature. But with obvious defects in the weld toe area, it can only be increased by111.9%. This seems that there is region grain refinement phenomenon in the weld toe after Ultrasonic Impact Treatment. Meanwhile, as severe plastic deformation of the weld toe area, it also improves to a certain extent of its surface morphology. At the same time, after ultrasonic impact and200℃heat treatment of low temperature, as there are such as defects of depression and gap in the area of weld toe defects of16MnR Butt Joints Smooth weld seams, its fatigue life is increased by49.7%under6min Ultrasonic Impact treatment samples than the samples3min after UIT. That seems it improves depression of the weld toe defects with Ultrasonic Impact treatment. In a certain period of time, the longer the time of Ultrasonic Impact treatment is, more intense plastic deformation of the weld toe, the higher the degree of grain refinement, and the better the weld toe morphology improved.Finally, in the analysis of the concentration of stress:16MnR Butt Joints Warp eliminates defects after Line cutting, and its fatigue life of welded style is improved more than seven times. After the tread on weld, the fatigue life of the weld and near-smooth style can be improved even more than10times. The fatigue life of the sample is23.9%lower than the welded style with defects still exist after line cutting. The fatigue life of the sample was reduced by10.4%with defects still exist after weld seams being polished. This experiment seems that there may be lead to eliminate fatigue cracks with the elimination of high stress concentration, while reducing the probability of fatigue crack nucleation. And it also seems the geometric morphology of the weld toe area smoother, the fatigue life higher. At the same time,16MnR butt joints with a wide range of ultrasonic of10min and200℃heat treatment of low temperature, compared with ultrasonic impact small-scale samples, its fatigue life is increased by247%. This experiment seems that within a certain time, the time of the force acting in the same position is longer, the plastic deformation is higher in the the ultrasound impact region within the vicinity of the weld toe, which further reduces the stress concentration resulting from defects of the region of the weld toe.In summary, it comes to the conclude that, compare with16MnR welded butt joint, improvement of stress concentration increased fatigue life of16MnR butt joint by about50%, residual compressive stress about15%, grain refinement about35%within a certain time after UIT. Therefore the level of influence factors for being improved the fatigue life of16MnR butt joints followed that the improvement of the stress concentration, the residual compressive stress and the grain refinement after UIT. |
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