NOMENCLATURE
CHAPTER 5 Results and discussions
A. Numerical thermo-mechanical analysis
5.6 Effect of welding sequences on thermal history, residual stresses and welding distortions
Large stiffened structures are generally joined by several welding passes which generates thermal stresses and angular deformation and finally premature failure of the structure. The different welding sequences lead to non-uniform heating and cooling which creates complex welding residual stress and angular deformation in the structure. In this present work the effect of four different welding sequences on residual stress, angular deformation of submerged arc welded fillet joint was studied. Thus the developed model presents the convenient welding sequence for enhancing the quality of fabrication process.
5.6.1 Experimental details
The welding parameters used in this study are shown in Table 5.13. In this analysis fillet welding joint was considered which is shown in Figure 5.50. The dimension of mild steel base plate was 200 mm × 200 mm × 8 mm with a stiffener of a web height and thickness of 50 mm
& 8 mm respectively.
Table 5.13 Welding parameters used in numerical analysis
Current (A)
Voltage (V)
Welding speed (mm/s)
490 25 5
Figure 5.50 Schematic diagram of joint geometry of the fillet joint
To study the effect of welding sequences on residual stress and deformation, four different welding sequences were considered which is given in Table 5.14 as per the Figure 5.50.
Table 5.14 Welding sequences and their nomenclatures
Welding Sequences Nomenclatures
Point E and J to G and H WS-I
Point J to H and G to E Point WS-II Point F and I to G,H and E,J WS-III
G,H and E,J to F and I WS-IV
5.6.2 Thermal history of welding sequences
Numerical and experimental transient thermal and mechanical analysis was performed. Figures 5.51 (a)-(d) show the numerical and experimental comparison of transient thermal history for four different welding sequences i.e. WS-I, WS-II, WS-III and WS-IV respectively. Transient
temperature distribution of two different location for each welding sequences were compared.
The experimental thermal history of the welded plate for all four welding sequences was captured by K-type thermocouples. From Figures 5.51 (a)-(d) it can be observed that the trends of the predicted thermal history are similar and well matched with the experimental ones.
0 500 1000 1500 2000 2500 3000
0 100 200 300 400 500 600 700
Temperature ( °C )
Time (s)
Pred.1 Pred.2 Exp.1 Exp. 2 Thermocouple locations
Figure 5.51 (a) Comparison of predicted and experimental time temperature history for WS-I
0 500 1000 1500 2000 2500
0 100 200 300 400 500 600 700 800 900
Temperature (°C)
Time (s)
Pred.1 Pred.2 Exp.1 Exp.2 Thermocouple locations
Figure 5.51 (b) Comparison of predicted & experimental time temperature history for WS-II
0 750 1500 2250 3000 3750 4500 0
200 400 600 800 1000
Pred.1 Pred.2 Exp.1 Exp.2
Temperature (°C)
Time (s) Thermocouple locations
Figure 5.51 (c) Comparison of predicted & experimental time temperature history for WS-III
0 900 1800 2700 3600 4500 5400
0 200 400 600 800
1000 Numerical 1
Numerical 2 Experimental 1 Experimental 2
Temperature (°C)
Time (s) Thermocouple
locations
Figure 5.51 (d) Comparison of predicted & experimental time temperature history for WS- IV
Four different welding passes went through the same sample, therefore the temperature raises
& cools for four times which depicts the variation of the thermal gradients involves during the welding of a multi-pass sample.
5.6.3 Comparison of the residual stresses
After validation of transient thermal history, the same model was used in structural analysis where the transient thermal history was considered as input load to predict the residual stresses and angular deformation for four different welding sequences. Figures 5.52 and 5.53 show the
longitudinal and von-Misses residual stress distribution respectively in the direction perpendicular to the weld line for all cases of welding sequences.
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90 -100
0 100 200 300 400 500 600
Longitudional residual stress (MPa)
Distance (mm)
Case-I Case-II Case-III Case-IV
Figure 5.52 Longitudinal residual stress perpendicular to the welding direction for different welding sequences
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90 0
50 100 150 200 250 300 350 400
von-Misser Residual Stress (MPa)
Distance (mm)
Case-I Case-II Case-III Case-IV
Figure 5.53 von-Misses residual stress distribution
From the above results, it is observed that the welding sequences have significant effect on magnitude and distribution pattern of residual stresses. It can be seen from the Figures 5.52 to 5.53 that all the residual stresses are tensile in nature near the weld line and compressive away from the weld line in WS-I to WS-IV. It is seen that highest magnitude of peak value of von- Misses residual stress found in WS-III and lowest magnitude of peak value of von-Misses residual stress found in WS-IV. Highest magnitude of longitudinal residual stress found in WS-
III and lowest magnitude of peak value of longitudinal residual stress found in WS-IV. It is also observed that by incorporating suitable welding sequence the von-Misses and longitudinal residual stress can be reduced by approximately 5% and 20 % respectively.
5.6.4 Comparison of the angular distortion
The welding sequences induced angular distortion are shown in Figures 5.54 & 5.55. Figure 5.54 shows the angular distortion patterns perpendicular to the weld line at the mid length of the plate for four different welding sequences. The effect of welding sequences is well prominent. Figure 5.55 represents the experimental angular deformation perpendicular to the weld line.
-80 -60 -40 -20 0 20 40 60 80
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Opposite Side Points for measurement
of distortion
Angular deformation (mm)
Distance transverse to weld line (mm)
Case-I Case-II Case-III Case-IV Y
X
Weld Start Side
Figure 5.54 Plot of angular deformation perpendicular to the weld line
It can be seen from the above Figure 5.54 that the maximum and minimum value of angular deformations are observed in case of WS-II (1.359 mm) and WS-III (0.805 mm) respectively.
Figure 5.55 represents the experimental angular deformations of four different welding sequences.
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90 0.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Opposite Side Points for measurement of distortion
Angular deformation (mm)
Distance transverse to weld line (mm)
Case-I Case-II Case-III Case-IV Y
X
Weld Start Side
Figure 5.55 Experimental angular deformation perpendicular to the weld line
It can be seen from Figure 5.55 that WS-II (1.342 mm) and WS-III (0.7555 mm) gives the maximum and minimum value of the angular deformation respectively. The trends of the plots are also well matching with the predicted ones as shown in the Figure 5.54. Thus, it can be concluded that the angular deformation in a welded sample can be well controlled and reduced by introducing proper welding sequences.
The comparison among the numerical and experimental maximum value of angular deformation obtained from the four different welding sequences are shown in the Table 5.15.
It is observed that the maximum error between the predicted and experimental value of angular deformation is 13.14%.
Table 5.15 Comparison between experimental and predicted maximum angular deformation
Maximum angular deformation (mm)
Welding start side (left) Opposite side (right) Experimental Predicted Error
(%)
Experimental Predicted Error (%)
WS-I /DS-I 1.254 1.35 7.11 1.047 1.11 6.01
WS-II /DS-II 1.342 1.359 1.25 1.135 1.04 8.37
WS-III /DS-III 0.901 0.851 5.87 0.755 0.8056 6.70
WS-IV/DS-IV 0.813 0.936 13.14 0.825 0.927 12.36
5.6.5 Summary
Based on the observations the following conclusions can be derived from the present investigation:
A feasible 3-D finite element model for studying the effect of welding sequence in fillet welding was developed utilizing the nonlinear transient elasto-plastic thermo- mechanical analysis.
Experiments were conducted to measure the thermal history and angular deformation of the different welding sequences. The predicted thermal history and angular deformation profiles are well matching with the experimental ones.
The residual stress is tensile in nature near and within the weld region and it is compressive in nature away from the weld region. As the distance from the weld line increases the stress reaches towards zero. The magnitude of longitudinal residual stress is much higher than that of transverse residual stress.
Four different welding sequences were considered to perform the FE analysis of double sided fillet joint. It was observed that welding sequences have prominent effects on both of distortions & residual stresses. The angular deformation can be reduced if the welding is done by proper welding sequences.