Figure 5.13 (a) shows that without the SSPT phenomenon, the quasi-static region of the weld joint model expectedly developed tensile residual stress. Though, the HAZ region unexpectedly shows tensile peak value. The steel (like austenitic steel) with no solid state phase transformation phenomenon also exhibits similar residual stress distribution [150], [151]. In contrast, the contour plot in Figure 5.13 (b) shows that the whole quasi- static weld region developed compressive residual stress in the P91 steel weld. However, the start and end regions show tensile residual stress. Consequently, the HAZ region shows tensile residual stress distribution. Figure 5.14 presents the residual stress distribution across the weld centreline to understand and differentiate the SSPT effect.
0 40 80 120 160
-450 -300 -150 0 150 300 450 600 750 900
Residual stress (MPa)
Length across the weld (mm)
Transverse stress (SSPT effects) Longitudinal stress (SSPT effects) Transverse stress (No SSPT effects) Longitudinal stress (No SSPT effects)
Observation Line z
x
Figure 5.14 Residual stress distribution across the weld in square butt single side single pass welded joint models
P91 steel weld shows compressive residual stress in the weld region and tensile stress in the nearby region between weld and base metal. Maduraimuthu et al. [44] also observed a similar residual stress distribution (i.e. compressive peak of about -275 MPa in the FZ region) across the weld on the top surface for square butt single pass activated TIG welding of P91 steel plate.
5.4.2 Residual stresses in square butt single and double-side welded joint models Figure 5.15 presents the longitudinal residual stress along the weld centreline in the FZ and along an observation line in HAZ of square butt single-side welded joint similar to DHD measurement. The predicted results in the plot are the average value of longitudinal residual stress within the trepanned core region (i.e. 12 mm diametral region of the trepanned core
around the reference hole). Figure 5.15 shows that the predicted results are quite comparable and reasonably matches the measured results. However, the weld bead top crown region shows compressive peak values while bottom bead crown shows lower tensile or compressive residual stress. The possible reasons for lower tensile or lower compressive residual stress in bottom bead crown may be less volumetric expansion under martensitic transformation and dominance of the volumetric shrinkage effect during cooling. The predicted results vary with a slight difference in the maximum value of compressive residual stress at thickness 4 mm (from the weld bead top surface) is observed as -169.1 MPa. The corresponding measured value is -118 MPa.
0 2 4 6 8 10 12 14 16
-450 -300 -150 0 150 300 450
Longitudinal residual stress (MPa)
Length along the weld centreline in FZ (mm) DHD
FE (Average values)
Weld centreline (a)
0 2 4 6 8 10
0 50 100 150 200 250 300 350
(b)
Longitudinal residual stress (MPa)
Plate thickness in HAZ (mm) DHD
FE (Average values)
Observation line
Figure 5.15 Longitudinal residual stress distribution in (a) FZ taken at 25 mm away from the starting end of the weld bead and (b) HAZ of square butt single-side single pass welded joint However, the predicted results are reasonably matching the measured results for the bottom side points. The weld bead top and bottom crowns region show higher peaks of compressive residual stresses. The comparatively faster cooling rate in top and bottom bead regions induces the martensitic transformation while cooling, resulting in volumetric expansion and compressive residual stress. The stress distribution in HAZ also matches the residual stress distribution pattern similar to the measured results. However, the point values show some significant deviation within the thickness range of 3-5 mm. The minimum prediction error for longitudinal residual stress in FZ of square butt single side single pass weld joint is 9.5% at the 9 mm thickness.
Figure 5.16 shows the cross-sectional view of the longitudinal residual stress contour in the FZ. The section is taken within the DHD measurement region in FZ (explained in section 4.5.1)
after removing weld bead top and bottom crowns, i.e., within the plate thickness range from 1 mm to 9 mm. It also reveals compressive residual stress throughout the thickness in the FZ of single-pass single side SAW welded butt joint, which is similar to low tensile or compressive residual stress distribution in single side single-pass laser-welded 10 mm thick P91 steel plate as observed by Kumar et al. [36].
Figure 5.16 Cross-sectional view of longitudinal residual stress contour in the FZ of square butt single-side welded joint of the section taken parallel to reference hole perpendicular to
weld centreline similar to DHD measurement
The residual stress values in the FZ region are close enough to the measured results. Though, it does not match precisely because it is not equivalent to the average value of nodal residual stress values. Figure 5.17 shows the predicted and measured longitudinal residual stress in the FZ of the square butt double-side single pass welded joint.
[Pa]
0 2 4 6 8 10 12 14 16 -300
-250 -200 -150 -100 -50 0 50 100 150
Length along the weld centreline in FZ (mm)
Longitudinal residual stress (MPa)
FZ (Contour) FZ (FEM)
Weld centreline
Figure 5.17 Longitudinal residual stress along the weld centreline in the FZ of square butt double-side single pass welded joint
It reveals tensile residual stress in the mid thickness region of the weld fusion zone, similar to measured results. However, the lower half of the mid thickness range between 5- 11 mm of the square butt double-side single pass welded joint model shows compressive residual stress of lower magnitude, which slightly deviates from the experimental results. Both top and bottom regions observe compressive residual stress with maximum predicted values of -122 MPa and -214.2 MPa, respectively. However, the measured residual stress values for the same points in the top and bottom regions are -58.9 MPa and -193.1 MPa. However, the residual stress values within the thickness region 2- 9 mm, fairly matched the experimental results. The minimum prediction error for longitudinal residual stress in FZ of square butt double side single pass weld joint is 4.2% at the thickness of 4.7 mm. Hence, the overall distribution of predicted longitudinal residual stress along the weld centreline in FZ of square butt double side welded joint replicates the measured spatial distribution of longitudinal residual stress in the fusion region on the cut surface.