Table 3 Effect of fatigue strength improvement of different types of welded joints
Since the low phase transition spot welding rod is the residual compressive stress obtained by the martensitic transformation volume expansion at a lower temperature, the residual compressive stress has a greater relationship with the weld joint restraint degree, and the greater the restraint degree, the residual The greater the compressive stress, the greater the effect of improving the fatigue strength.
(2) Low phase change point welding rod toe melting repair (LTTE-dressing) method
However, in order to cause the martensite transformation of the weld metal at a normal cooling rate and at a lower temperature, more alloying elements are added to the welding material, thereby increasing the cost of the low phase change point welding material. . If the entire weld of a welded structure is welded with a low phase change welding material, the cost of the welded structure will also increase significantly, which is uneconomical.
It is well known that the fatigue fracture of the welded joint is mainly cracked from the weld toe. If the residual compressive stress is generated at the weld toe of the welded joint, the fatigue strength of the welded joint can be improved, and it is not necessary to use the low phase change spot electrode, which can reduce The cost. Based on this consideration, Tianjin University proposed a method to improve the fatigue strength of welded joints by LTTE-dressing on the basis of experiments. The two joint types of non-bearing cross joint and longitudinal circumferential fillet joint are used to compare the fatigue strength of LTTE-dressing and common electrode welded joints of low phase change point welding rod. The fatigue strength of the former is better than that of the latter. The increase of 19.9% ​​and 41.7% proves the feasibility and practicability of this idea. For the low phase change point welding rod LTTE, it is more reasonable to apply the preliminary test in the engineering practice, and the low phase change point welding rod toe melting LTTE-dressing joints can also reflect the application of low phase change spot electrodes in the face weld and near weld toe weld bead.
4.2.3 Advantages and Disadvantages of Low Phase Change Point Welding Rods
advantage:
(1) The welding method of the low phase change point welding rod is carried out simultaneously with the welding process, which avoids the inconvenience of post-weld processing;
(2) The low phase change point electrode method requires no special operation requirements, so the operation is simple and convenient;
(3) Low phase change point welding material is used to improve the fatigue strength of welded joints. It is more suitable for concealed welds, covered welds, back welds of single-sided welds, etc. due to the influence of subsequent weld bead heat. The fatigue strength of the weld after the post-weld processing is improved;
(4) LTTE electrodes can also be used to repair fatigue cracks in welded structures.
Disadvantages: More alloying elements are added to the welding material, which increases the cost of the low phase change point welding material. But it can be compensated by methods such as LTTE-dressing.
5 Conclusion
In summary, it can be seen that in recent years, due to the trend of the development of high-speed and heavy-duty welding structures, the requirements for dynamic load capacity are becoming higher and higher. Therefore, the development and application of new technologies to improve the fatigue performance of welded joints have promoted welding. The application of the structure is of great significance. Relatively speaking, the latest development of ultrasonic impact technology at home and abroad and the use of low phase transition point welding materials to improve the fatigue strength of welded joints is an important research direction of welding structure fatigue performance improvement technology and process.
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