Is the higher the weld reinforcement, the better?

Reinforcement is not only the need of welding process, but also has an impact on the safe operation of the vessel, especially the fatigue life. The welding of pressure vessel is mostly multi pass welding (such as manual welding and automatic welding). The post weld bead is equivalent to the role of heat preservation, slow cooling and tempering for the previous weld bead. The grain of the previous weld bead is refined, and the reinforcement plays a role of heat preservation and tempering for the outer strength weld bead Slow cooling and tempering, so the reinforcement is the need of welding process

Harm

After the joint is welded, the process function of reinforcement is completed, and the existence of reinforcement will have some adverse effects on the operation safety of the vessel. Someone once made two groups of test vessels. One group polished off the reinforcement after welding to make the weld flush with the base metal surface, which is called removing reinforcement group, and the other group did not do treatment after welding, which is called retaining reinforcement group, The pressure fatigue tests of two groups of vessels are carried out under the same working conditions to compare their fatigue life. The test results show that the vessels in the remaining reinforcement group break first, and the average fatigue life difference between the two groups is 2.1-2.5 times.

In China, the pressure fatigue test of three-layer hot jacket simulation vessel has been carried out. The inner diameter of the vessel is 350 mm, and each layer is made of 15MnVR steel plate with a thickness of 6 mm. Because the inner diameter of the vessel is small, the surface of the weld is not treated after welding, After the pressure fatigue test, the vessel leaked. Through dissection and magnetic particle testing, it was found that there were magnetic marks on the fusion line of the longitudinal weld on the inner and outer surface of the inner cylinder, indicating that the longitudinal weld of the inner cylinder had been cracked through, but only the grinding part was intact, and there was no accumulation of magnetic particles on the inner and outer surfaces.

Fatigue failure generally goes through three processes: nucleation (i.e. fatigue crack source), fatigue crack propagation and fracture. Inevitably, some defects will appear on the weld surface, such as undercut, crack, inclusion, incomplete welding and so on. These welding defects may become the fatigue Crack source.

In addition, the existence of reinforcement damages the continuity of weld surface geometry, resulting in additional bending moment due to shape mutation, resulting in higher local stress. These crack sources caused by welding defects will accelerate the propagation until fracture under the action of local stress. If the reinforcement is removed by grinding after welding, the appearance of weld will not only be consistent with that of base metal, Therefore, only after a long time of nucleation and slow propagation process, can fatigue fracture occur.

Requirement

Because the existence of excess height will shorten the fatigue life of the vessel, domestic and international standards stipulate that the residual height shall be removed for the vessels requiring fatigue analysis and design, so that the weld surface and the base metal surface are in line, and for vessels that do not need fatigue analysis and design, Most domestic and foreign standards allow for a certain excess (this allowable amount is generally subject to non-destructive testing operation).

Basic knowledge about welded pipe