3.3 .2 shot peening
Shot blasting is another form of hammering and is also a method of impact processing. The effect of the spray nine depends on the diameter of the shot, and the shot size should not be too large to handle small defects. At the same time, the size of the shot peening should not be too small to ensure a certain degree of cold work hardening performance, and shot peening generally works at a depth of a few thousandths of a millimeter on the surface. The research results show that shot peening can significantly improve the fatigue strength of high-strength steel joints. Shot peening has outstanding effects on argon-arc welded high-strength steel materials, and the degree is even higher than TIG welding. At the same time, TIG melting repair with shot peening hammer, the effect is more significant.
4 The latest technology to improve the fatigue strength of welded joints
4.1 Ultrasonic impact treatment method
The ultrasonic shock developed in recent years improves the fatigue strength of welded joints and structures, and its mechanism is basically consistent with hammering and shot peening. However, the method has the advantages of light weight, good controllability, flexible and convenient use, low noise, high efficiency, limited application time, low cost and energy saving, and is suitable for various joints, and is an ideal post-weld improvement welding. The method of joint fatigue performance. Ultrasonic impact treatment was carried out on the butt joints and non-loaded longitudinal angle joints of several typical welded structural steels. Then the comparative fatigue test of welding state and impact treatment was carried out. The practical effect of ultrasonic impact method to improve the fatigue strength of welded joints was studied. See Table 2. It can be seen that after the ultrasonic joint treatment of the welded joint, the fatigue strength is increased by 50%~170%, and the effect is very remarkable.
4.2 Low phase change point electrode method
4.2.1 Principles and development of improving the fatigue strength of welded joints
Compressive stress can increase the fatigue strength of welded joints. There has been a lot of literature discussion, but the problem is how to introduce compressive stress conveniently in welded joints.
Table 2 Comparison of fatigue strength before and after ultrasonic shock treatment
It is well known that due to differences in chemical composition, alloy content and cooling rate, steel materials undergo different structural transformations or multiple structural transformations during the cooling process. This structural transformation is accompanied by volume expansion, which will cause phase transformation under restraint conditions. Stress is a compressive stress. For weld metal, this will facilitate the reduction of residual tensile stress and even residual compressive stress, thereby improving the mechanical properties of the welded joint. Low Transformation Temperature Welding Electrode (LTTE) is a new type of welding material that uses the phase transformation stress to generate compressive stress in the welded joint to improve the fatigue strength of the welded joint.
As early as the 1960s, the former Soviet Union welding experts proposed that the low phase change point electrode method can improve the fatigue strength of the welded structure, but at the time did not put forward the concept of "low phase change point electrode", only called a special electrode The surfacing metal composition mainly relies on 3%-4% Mn content to reduce the phase transition point and achieve metallurgical phase transformation. According to the literature, when these special welding rods are used, the fatigue strength after surfacing with these electrodes is higher than that of the non-surfacing test when fatigue testing is performed on small specimens.
In recent years, relying on Cr and Ni to reduce the martensitic transformation point of the deposited material of the welding material, and due to the development of ultra-low carbon steel, the low phase transition point electrode has been rapidly developed, and Japan and China have carried out this aspect. A lot of research, but still in the laboratory stage.
4.2.2 Effect of LTTE welding rod on improving fatigue strength
The School of Materials Science and Technology of Tianjin University designed and optimized the low phase change point electrode and carried out a large number of fatigue tests and process performance tests on various welded joints.
(1) LTTE method
The low-phase-change point welding rod LTTE and the ordinary welding rod E5015 were respectively used to weld the transverse butt joint, the non-bearing cross joint, the longitudinal circumferential fillet joint, the longitudinal parallel fillet joint and the longitudinal butt joint, and the fatigue comparison test was performed. The results show that the fatigue strength of the LTTE joint of the phase change point electrode is 11%, 23%, 42%, 46% and 59% higher than that of the common electrode E5015, and the fatigue life is increased from several times to several hundred times.
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