The functions and hazards of weld excess height

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Definition: The parts ei1 and e2 in the figure below represent the remaining height. The excess height is necessary for welding processes, and it also has a certain impact on the safe operation of the container, particularly on its fatigue life. Function of images: Welding of pressure vessels usually involves multiple passes (such as manual welding or automatic welding). The subsequent weld passes serve to insulate, slow down the cooling process, and carry out tempering for the previous weld pass, thereby refining the grain structure of that pass. The excess height serves to insulate, slow down the cooling rate, and provide tempering for the outermost layer of the weld bead; therefore, it is necessary for the welding process. Hazard: Once the joint welding is completed, the function of removing excess material is finished; thereafter, the presence of such excess material can have some adverse effects on the safe operation of the container. The Japanese once created two sets of test containers: in one set, the excess height on the weld was ground off so that the weld surface was level with the base material surface; this set was called the group with excess height removed ； The other group, after welding, was left untreated and is referred to as the retained burr group. These two sets of containers were subjected to pressure fatigue testing under the same operating conditions in order to compare their fatigue life. The test results show that the containers in the retained height group failed first, and the average fatigue life of the two groups differed by 2.1 to 2.5 times. Our country has conducted pressure fatigue tests on three-layer heat-shielded simulated containers. The inner diameter of this test vessel is 350 mm, with each layer being formed by roll welding 15MnVR steel plates that are 6 mm thick. Due to its small inner diameter, the weld surface was not treated after welding; only a section of the longitudinal weld on the inner surface was ground down for the purpose of attaching resistance strain gauges to measure stress. The container leaked after undergoing pressure fatigue testing; upon dissection and magnetic particle inspection, magnetic traces were found on the fusion lines on the inner and outer surfaces, adjacent to the longitudinal welds, indicating that these longitudinal welds had cracked. However, the area that had been ground was intact, with no magnetic particle accumulation on either the inner or outer surfaces. Fatigue failure generally goes through three processes: nucleation (i.e., the formation of a fatigue crack source), the propagation of the fatigue crack, and fracture. The weld surface is inevitably subject to certain defects, such as undercutting, cracks, inclusions, and incomplete welding; all of these welding defects can serve as sources of fatigue cracks. Furthermore, the presence of protrusions disrupts the continuity of the weld surface geometry; the sudden changes in shape generate additional bending moments, leading to high local stresses. These crack initiation sites caused by welding defects will expand more rapidly under the influence of these local stresses until they eventually cause failure. If the excess height is removed after welding by methods such as grinding, not only is the appearance of the weld made consistent with that of the base material and is the source of localized stress eliminated, but various welding defects that may exist on the weld surface are also removed; as a result, fatigue fracture can only occur after a long nucleation process and slow growth. Requirement: Since the presence of a rim height reduces the fatigue life of containers, both domestic and international standards stipulate that for any container subject to fatigue analysis and design, this rim height must be removed so that the weld surface is level with the base metal surface. For containers that do not require fatigue analysis in their design, the vast majority of domestic and international standards permit a certain amount of excess height to be present (this allowance is generally based on the principle that it should not interfere with non-destructive testing procedures). Only Japan’s Ministry of Labor standard, the \"Code for Construction of Pressure Vessels,\" requires that this excess height be removed so that the weld surface is level with the base metal surface; otherwise, the welding joint coefficient must be reduced.