| The flange joint used in the pipe-structure can be divided into welding flange joint and forging flange joint. The welding flange joint is a kind of more common and more mature pipe-connection form, including flange with stiffening ribs and flat flange. Forging high-neck flange contains inserting-welding forging high-neck flange and butt-welding forging high-neck flange. However, some problems of forging high-neck flange used in the tower structures of overhead transmission line can not be ignored:1. The circumferential weld of the two type of forging high-neck flange joint located in the same cross section of the pipe, which leads to the reducing of the strength in the cross section by40%, for the defects in the circumferential weld. So, the strength of the forging high-neck flange joint would decided by the circumferential weld, and the40%of the strength of the pipe would not be made full use of. Thus, in the forging high-neck flange joint, a waste of steel would be caused or a reducing of reliability would be leaded to.2. As a result of the existing of welding residual stress, the stress in a certain area around the weld would exceed the yield strength of the steel under a relatively small load. Then the joint would be damaged from a cross section of the pipe. Few researches about welding residual stress could be found at present. The welding residual stress was also ignored in the formula for calculating the bearing capacity of welding components in Technical Regulation of design for tower and pole structures of overhead transmission line (DL/T5154-2002).3. The welding heat affected zone of the circumferential weld, located in the same cross section, would increase the risk of crispy failure of the pipe, which was not taken into account in the designing method at present.4. In the calculating of connecting-strength of inserting-welding forging high-neck flange joint, only the weld at the neck of the flange was treated as force-bear-member, while the weld at the bottom of the flange was just treated as construction-member. So, the connecting-strength of inserting-welding forging high-neck flange joint would be underestimated.In order to solve the above problems in the forging high-neck flange joint, a new type of forging high-neck flange, called wave-neck flange joint was invented. Then, the mechanical behavior and failure mechanism of the flange joints, were studied by theoretical calculation? finite element analysis and experimental investigation. The flanges of the studied joints contain4forms as inserting-welding forging high-neck flange?butt-welding forging high-neck flange?tooth-neck flange and wave-neck flange;3types of steel as Q345?Q420and Q460;2types of pipe diameter as ?219mm and ?273. At the same, the welding residual stress was also studied.It turned out that:1. The bearing capacity of wave-neck flange could be increased by40%than inserting-welding forging high-neck flange. Then the bearing capacity of wave-neck flange could reach100%of the bearing capacity of pipe. Therefore, the steel consumption of towers using wave-neck flange could be greatly reduced for the bearing capacity of pipe was made full use of.According to the results of tests, the degree of bearing capacity of Q345wave-neck flange increased than inserting-welding forging high-neck flange is54.0%, while the value of theoretical calculation is39.7%, and the value of FEM calculation is42.1%. In Q420wave-neck flange, the value was34.0%. So the results of tests were quite similar with the results of theoretical calculation.Then, the degree of bearing capacity of Q460wave-neck flange increased than inserting-welding forging high-neck flange is1.3%, which was completely different from the values of theoretical calculation and FEM calculation. Reason leading to such results was found as the unqualified material strength of the pipe used in Q460wave-neck flange specimens. So, the using of Q460steel should be treated strictly.2. The stress concentration in the area near the weld was greatly reduced in wave-neck flange. Amoung different typies of flange joints, the failure of inserting-welding forging high-neck flange joint was mainly caused by the damange of the weld at the top fo the flange.3. The bearing capacity of the weld at the bottom of the flange could not be ignored. By tests and FEM analysis, it was proved that the ratio of the bearing capacity between the weld at the bottom and at the top of the flange was about3.5:1. So the bearing capacity of the weld at the bottom of the flange should be made full use of in project designing.4. The bearing capacity of different forms of flange joints was different. The order of the bearing capacity from the highest to the lowest should be wave-neck flange joints? tooth-neck flange joints?inserting-welding forging high-neck flange and butt-welding forging high-neck flange. The bearing capacity of wave-neck flange could reach100%of the bearing capacity of pipe.5. The bearing capacity under horizontal load and seismic energy dissipation performance were greatly improved in wave-neck flange joint than in inserting-welding forging high-neck flange joint.6. In the project designing, the stress ratio of the flange was always controlled to be less than0.8, so in this circumstance, the flange could be treated as rigid flange. The impact of pry force was bigger than the value in the standard, so the formula in the standard was suggest to modified as ftb=0.75f.Besides, According to the results of investigate and survey in the flange factories and steel tower factories, the weld in the wave-neck flange was easier to implement and more reliable. The requirement for the quality of the pipe was lower in the wave-neck flange joint. Compared to inserting-welding forging high-neck flange joint, the detection of welds in wave-neck flange joint was easier in implement and lower in cost. Therefore, the manufacturing worker prefer wave-neck flange joint to inserting-welding forging high-neck flange joint. At the same time, it was the automatic welding of wave-neck flange joint could be achieved by the combination of the Numerical control programming technology and Automatic welding machine.The wave-neck flange would be the prefer choose in the joint of pipes for its good in mechanical property, reliable in security, reasonable in economy and feasible in manufacture. |
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