Single-pass welding and multi-pass welding
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This post was last edited by megaherz_IGGE on 2023-2-25 09:29. 1) Dear teachers, what is the definition of multi-pass welding? \"When the number of weld passes in each groove side is ≥2, it is considered multi-pass welding.\" Does \"number of weld passes ≥2\" refer to the total number of weld passes in each groove side, or is it calculated based on whether the number of weld passes for each welding method is ≥2 (if a welding method has ≥2 weld passes, it is considered multi-pass; otherwise, it is single-pass, with each welding method being evaluated separately)? For example, in Figure 1: Are all four methods—GTAW, PAW, SMAW, and SAW—considered multi-pass welding, or is only GTAW considered multi-pass welding while PAW, SMAW, and SAW are considered single-pass welding? 2) In my personal view, the differences between single-pass and multi-pass welding generally lie in the mechanical effects of heat transfer between passes: preheating in the previous pass and interpass annealing; as well as in the welding procedures and evaluation criteria adopted to address defects such as slag inclusions and lack of fusion between multiple weld passes ； Based on this premise, as shown in Figure 2, can the first layer of GTAW welding be considered a single pass of welding? If considered in terms of the total number of passes per groove, as shown in Figure 2 for GTAW+SMAW, there are 3 passes in total, which would suggest multiple weld passes. However, when the root pass is carried out using GTAW, it is not subject to the thermal effects of subsequent passes; there is also no preheating from preceding passes, nor are there issues such as interpass slag or lack of fusion from subsequent passes. In this case, can GTAW be regarded as a single weld pass? 3) Building on Question 2, as shown in Figure 3 for the plasma P+T project, if the welder codes are PAW-1G-06/08/19 and GTAW-1G(K)-05/06/09/19, is it reasonable to write 08 for PAW and 09 for GTAW?
Comparison: Single-pass welding: 1. Single-pass welding features simple installation, low equipment costs, and high efficiency, making it suitable for situations with relatively low workloads. 2. Single-pass welding can only be used for welding smaller models, as it has a limited welding range and is not suitable for large sheets of metal. Multi-pass welding: 1. Multi-pass welding features a compact structure, excellent welding quality, and a large welding range, making it suitable for tasks with high workloads. 2. Multi-pass welding can be used to weld large sheets, covering a wide welding area; however, it requires high standards for the quality of the weld, is more complex to operate, and involves higher costs for the equipment. -
In the poster’s image, it’s not multiple welds; it’s multi-layer welding.
In the pressure vessel industry, regardless of whether multi-layer or multi-pass welding is used, any weld pass that is 2 or more layers thick is classified as multi-pass welding; even in the case of multi-layer welding, where only one pass is applied per layer, it is still considered multi-pass welding
This post was last edited by ftxswb on 2023-2-27 at 14:05. Could someone tell me where the source is? The assessment rules for welders of special equipment do not specify multiple passes of welding. Appendix A states the following: \"(7) For the test pieces made of Fe Class I steel in Table A-2, except for the first pass of welding on pipe fillet welds, butt welds, and tube sheet corner joints, where it is allowed to grind the joint area when changing electrodes, no grinding or rework is permitted for other passes. For other materials (except those using nickel-based electrodes), except for the first and middle layers of welding where grinding of the joint area is allowed when changing electrodes, no grinding or rework is permitted for other passes.\" ”Can it be considered that these “assessment rules” separate the layers from the paths? The detailed assessment criteria state that in A9: (7) For the S290 steel pipe, the outer diameter is 320 mm and the wall thickness is 12 mm; it is fixed horizontally. Welding is carried out manually in a downward direction using EXX10 electrodes, with no backing on the back side. The thickness of the weld metal is 4 mm. Afterwards, GMAW is used for welding in an upward direction, without an automatic tracking system – the welding process is controlled remotely. Multiple passes and layers of welding are performed to fill the groove. The project codes are SMAW-Fe II-5GX-4/320-Fef2 and FCAW-5G(K)-07/09/20; (8) For the 06Cr19Ni10 steel plate with a thickness of 16 mm, submerged arc welding is used for flat welding, with a flux pad applied on the back side. The welding machine does not have an automatic tracking system; the welding wire used is HO8Cr21Ni10Ti, and the flux used is HJ260. Welding is carried out visually, with 2 layers of welding on one side to fill the groove. The project code is SAW-1G(K)-0O7/09//19. “Are there any contradictions in these two regulations regarding layers and tracks? NB/T47015 states the following: “3.6.10 In the case of multi-pass or multi-layer welding, attention should be paid to cleaning between passes and layers; slag, harmful oxides, grease, rust, and other contaminants on the weld surface must be removed before continuing with welding.” ”Is it possible to have differences at this standard level, among the paths? Section 7.4 of HG/T20580 states the following: \"For containers made by multi-layer, multi-pass welding of thick plates or those that undergo stress-relief heat treatment after welding, the strength of the welded joints may decrease as a result of such heat treatment; this factor should be taken into consideration when selecting materials.\" ”It should be understood that the layers are distinct.
Question 1: The original poster has a misunderstanding regarding multi-layer and multi-pass welding. Multi-pass welding is usually related to the width of the weld; generally, when the weld width is more than 3 times the diameter of the wire/electrode, multi-pass welding is required. Multi-layer welding is related to the thickness of the weld; generally, multi-layer welding is used when the thickness is greater than the diameter of the wire/electrode. The welded joint in Figure 1 is definitely multi-layer welding, but it is not necessarily multi-pass welding. The same welding method can also be used for multi-layer, multi-pass welding. Question two: The original poster has a misunderstanding of the qualifications required for welders. For the understanding of multiple layers or channels, see the previous question. The pre-welding heating + post-welding tempering process refers to a welding procedure that is developed based on the welding of certain special materials. A preliminary welding procedure plan is prepared in advance, specifying whether heating is required, the type of groove, the welding method, the appropriate welding materials, the number of layers and passes, whether heat treatment is needed, and whether any performance tests are required, among other things. The welder then uses this preliminary welding procedure plan to fabricate test pieces in order to determine whether the process meets the required performance standards; this process has nothing to do with the welder’s qualifications. A welder’s competence refers to their proficiency in welding certain types of materials; it is determined based on the category of metal involved. Once a welder passes the examination for a particular metal category and obtains the necessary certification, they are able to weld several or even dozens of metals within that category. However, whether they are qualified to weld depends on the outer diameter and thickness of the welding specimens, as assessed according to the requirements of the examination they have taken. Question three: The original poster has a misunderstanding regarding the qualified items listed on the welding qualification certificate. The sequence of codes on the qualification certificate each has its own meaning; a picture is attached for reference. The poster used manual welding for everything.
This post was last edited by niat8888 on 2023-2-28 08:32. Pass: A weld layer in which the molten metal travels one full path along the weld joint. Layer: Refers to the layer as we imagine it. . The first layer can consist of a single weld pass or multiple weld passes. When conducting process evaluation, it is important to note an additional factor: changing from multiple passes per side to a single pass per side. . Pay attention to the word “each side”. . . A weld joint is divided into 2 surfaces. . During process qualification, if you weld once on the front side and once on the back side, it counts as one pass per side. If you weld twice on the front side and once on the back side, it means multiple passes on one side and one pass on the other. If you weld twice on both the front and back sides, then there are multiple passes on each side. For skill assessment: 1. Any welding method may be used; the specified thickness of the deposited metal is 12 mm or more (or 13 mm according to ASME standards), with no less than 3 layers – ASME uses the term “layers”. In other words, the welding method must be used to achieve this thickness, and it’s not sufficient for just that single joint to meet the requirements. 2. For manual welding, there is one factor. Weld beads of various welding methods within each groove. . . Note that this refers to the weld bead. So, during regular exams, choose code 09:lol
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