| In order to optimize the comprehensive mechanical properties of weld joints and to study the problem of crack propagation near the weld joints, 9 kinds of welding process parameters were designed in this paper, and a study on the mechanical properties of such 9 welding process parameters was carried out. With the help of metallographic microscope and scanning electron microscope and other means, the rules of tissue changes in weld seam zones and welding heat-affected zone were researched in different welding processes; the mechanical properties in terms of the welding process parameters were comprehensively evaluated through room temperature stretching, impact toughness, bending and micro hardness tests, and micro morphology of the fracture was analyzed and evaluated using a scanning electron microscope; through a hot simulation test, the tissues and performance of the heat-affected zone with multiple weld joints were comprehensively evaluated; the impact of the welding process parameters on the axial fatigue performance of weld joints was researched, and an S-N curve under different processes was drawn; the propagation modes of fatigue cracks in the weld seam zones, heat-affected zone and base zones were evaluated through the method of SE(B) sample bending fatigue, and the threshold Kth of fatigue crack propagation was obtained with different welding heat inputs on this basis.The research results show that the impact of the welding process parameters on the tissues of weld seam zones is as follows: with an increase in welding heat input, a gradual reduction in the proportion of columnar crystal, a slight increase in the proportion of isometric crystal, an obvious increase in the quantity of proeutectoid ferrite precipitated along austenite grain boundary, a reduction in the quantity of plate ferrite growing within the crystal, and first increase and then reduction in the content of disorderly needle-like tissues within austenite grains; with the changes in the welding process parameter, the rough crystal of the heat-affected zone is subject to a maximum impact, and the i mpact of the welding process parameters is not very large on the phase-change recrystallization zone and incomplete phase-change recrystallization zone of the heat-affected zone.Mechanical performance testing results show that if different welding process parameters were used to weld high strength bridge steel Q345 q D, the welded joint strength of high strength bridge steel was mostly above the strength of base material; excellent impact toughness was reflected in the weld zone, fusion line and heat-affected zone at-40? to 20?; the micro hardness of welded joints is minimum in the base material zone, and the hardness of welded joints in the heat-affected zone gradually increased from a side near the base material side to near weld seams, of which the hardn ess of the heat-affected zone near the weld seams was the maximum in the entire welded joints; the high strength bridge steel Q345 q D was uneasily subject to cold cracks in the welding process, and the process welding performance was good.The hot simulation test results demonstrate that in the heating process when the highest temperature was 800?, ferrite and pearlite tissues in varying sizes were reflected in the samples; in the heating process when the highest temperature was 900?, there were relatively small and uniform ferrite and pearlite tissues; again with a rise in peak temperature, the grains became constantly rough, and in the heating process when the highest temperature was 1200?, the coarse original austenite grain boundary was retained, while fe rrite and pearlite tissues growing within the crystal precipitated along the crystal boundary; at 800?~ 900?, the comprehensive mechanical properties showed a good trend, but after 900?, the precipitation mechanical properties deteriorated gradually and a fragile state was fully rendered at 1200?.Based on the axial fatigue test on welded joints, under the condition of the heat inputs 16 k J·cm-1 and 20 k J·cm-1, the fatigue ultimate strength is 376 MPa, and the value is close to the yield strength; the fatigu e ultimate strength under 36 k J·cm-1 is 355 MPa, a certain degree of decline compared with the yield strength. After a study on the propagation of fatigue crack by use of SE(B) samples, the da/d N-? K and lgda/d N-lg? K curve and equation were derived. The stu dy results showed that the propagation threshold for the fatigue crack in the base material zone is Kth=35.24MP·mm1/2, that in the heat-affected zone is Kth=42.85MPa·mm1/2 and that in the weld seam zone is Kth=48.08 MPa·mm1/2, and the propagation rate of fatigue crack in the weld seam zone is lower than that in the heat-affected zone and base material zone. |
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