Lumped pass effects for thick wall section

When pipe or vessel wall thickness becomes large, such as 4” or up to 10”, the number of weld passes involved can be in the order of hundreds. One question for residual stress analysis is: can the actual number of passes be effectively lumped for the purpose of numerical modeling

efficiency without losing important residual stress information? If the answer is yes, in addition to its obvious benefit to residual stress modeling, reduced number of weld passes can also result in significant increase of productivity in fabrication of thick pipes/vessels. Because of this consideration, it was suggested at the beginning of the project that lumped pass effect be first examined before performing a large number of parametric analyses.

Narrow groove joint preparation was selected to perform the parametric analyses for the lumped pass effect, owing to its nature of weld profile without introducing additional localized residual stress features. Cases studied are summarized in Table 4-1. As we can see, each thickness of narrow groove girth welds has 5 or 6 models and each model is associated with a difference number of passes. The number of passes varies from the largest number in Model 1 to the smallest one in Model 6. The pipe inner radius to thickness ratio (r/t) is also varied from 2 to 10 for each thickness. The total number of cases performed in Table 3-1 is 69. The pass size is maintained to be the same for each lumped model in order to obtain an idealized residual stress distribution without other effects. It is interesting to note that with this particular weld profile, the characteristics of through-thickness residual stress distributions at the weld centerline and weld toe are very similar. As an example, the cases for thickness of 4” are discussed in detail in the section below.

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Table 4-1 Cases analyzed for lumped pass effects

Figure 4-5 shows a 4” thick narrow groove axisymmetric finite element model with 4024 nodes and 3880 quadrilateral linear elements. Symmetric boundary condition is applied at the weld centerline in the stress analysis and rigid body motion is eliminated. The area marked in red is the half weld zone with a width of 11mm. Figure 4-6 shows the detailed information of the weld pass size and welding sequence for each lump model with decreasing pass number from 52 to 2. It can also be seen from Figure 4-6 that the pass size retains almost the same for each lumped model. The welding sequence is from ID to OD.

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Figure 4-6 Six lumped models with pass number from 52 to 2 for NG 4” thickness welds: detailed pass size and welding sequence

Perpendicular residual stress distributions at the weld centerline (marked in Figure 4-5) are exhibited in Figure 4-7. r/t ratio for these cases is 10. It is can be seen that the overall

characteristics of perpendicular residual stress distribution alters significantly with the decrease of the number of passes, especially when the number of passes is smaller than 4 (Lump Model 5 and 6 in Figure 4-6). With the decrease of pass number, the degree of stress oscillation is

increased through the thickness. This is primary due to the fact that the larger a weld pass becomes, the more heat input is required. A larger heat input results in a larger area surrounding the weld pass being affected. Despite the difference, the overall perpendicular stress distribution of Lump Model 3 (13-pass) is very similar to the ones of Model 1 and 2 which have 52-pass and 26-pass, respectively. This suggests that 52-pass/26-pass for 4” thickness Narrow Groove joint preparation may not be necessary compared with 13-pass case as far as residual stress is concerned. The result of Lump Model 4 (7-pass) has a similar overall trend, but the stress oscillation is clearly overwhelming. From Figure 4-7, it suggests that there exists a minimum number of passes with which the overall characteristics of residual stress distribution can be well captured.

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Figure 4-7 Perpendicular residual stress distributions along weld centerline for Narrow Groove joint preparation with thickness of 4” and r/t ratio of 10

Decomposed residual stress results for the above narrow groove weld of 4” thick pipe is shown in Figure 4-8 as a function of both number of passes modeled and r/t ratio. Through-wall perpendicular (axial) and parallel (hoop) residual stress distributions are decomposed into membrane and bending components according to Eq. (3.1). Figure 4-8(a) shows the bending component of the axial residual stresses at the weld centerline (membrane stress is negligible for all cases). Indeed, there is a clear indication of the existence of a minimum number of passes, e.g. about 5-10, beyond which the bending component of the axial residual stress no longer changes in any significant manner. The slight reduction in all residual stress components with more passes than the minimum number of passes suggests that the use of the minimum number of passes will result in a conservative residual stress estimate. Hoop residual stresses in terms of membrane and bending components show an identical behavior (Figure 4-8 b and c). In

addition, r/t ratio seems not have any significant effects on the minimum number of passes and can be characterized as a simple shifting in stress magnitude in all cases.

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Figure 4-8 Decomposed residual stress components vs. Number of passes modeled for 4” thickness Narrow Groove joint preparation – (a) axial bending residual

stresses (b) hoop bending residual stresses and (c) hoop membrane residual stresses

For completeness, the self-equilibrating parts calculated based on Eq. (3.1) are plotted in Figure 4-9 corresponding to residual stress distributions shown in Figure 4-7. Same trends as discussed for Figure 4-7 can be found. Stress distribution of self-equilibrating part alters drastically with the decrease of the number of passes, especially when the number of passes is smaller than 4.

Figure 4-9 Decomposed self-equilibrating parts of perpendicular residual stress distributions along weld centerline for Narrow Groove joint preparation with thickness of

4” and r/t ratio of 10

Other cases shown in Table 3-1 are summarized in the same manner. Detailed pass lumping information for the wall thickness of 1”, 2”, 10” are represented in Figure 4-10, Figure 4-12, and Figure 4-14 respectively. Decomposed residual stress components with respect to a function of

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pass number and r/t ratio are shown in Figure 4-11, Figure 4-13 and Figure 4-15 for wall thickness of 1”, 2” and 10”, respectively. By examining all of the results, it is observed that a minimum number of passes is existent for each wall thickness.

Figure 4-10 Six lumped models with pass number from 20 to 2 for NG 1” thickness welds: detailed pass size and welding sequence

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Figure 4-11 Decomposed residual stress components vs. Number of passes modeled for 1” thickness Narrow Groove joint preparation – (a) axial bending residual stresses (b) hoop

bending residual stresses and (c) hoop membrane residual stresses

Figure 4-12 Six lumped models with pass number from 20 to 2 for NG 2” thickness welds: detailed pass size and welding sequence

Figure 4-13 Decomposed residual stress components vs. Number of passes modeled for 2” thickness Narrow Groove joint preparation – (a) axial bending residual stresses (b) hoop

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Figure 4-14 Five lumped models with pass number from 25 to 2 for NG 10” thickness welds: detailed pass size and welding sequence

Figure 4-15 Decomposed residual stress components vs. Number of passes modeled for 10” thickness Narrow Groove joint preparation – (a) axial bending residual stresses (b) hoop

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With a further analysis, it is found that instead of the number of weld passes, the number of weld pass layers (a layer may contain more than one pass) is a better parameter for characterizing the through-thickness residual stress distributions such as those shown in Figure 4-8 to Figure 4-12. An approximate minimum number of weld pass layers as a function of wall thickness is shown in Fig. 3.14 based on the results from the present study. Note that the vertical axis should be

identified as the number of through-thickness layers which often consists of more than one weld pass. On the basis of this finding, FE models (shown in Figure 4-2) are generated and used with confidence for all of the parametric studies in this investigation.

Figure 4-16 Minimum number of weld pass layers versus pipe wall thickness