4.5 Welding induced residual stresses
4.5.2 Square butt double side single pass weld joint
Contour measurement is performed on square butt double-side single pass welded joint to measure spatial residual stress distribution on the cut surface across the weld centreline. The whole experiment involves four significant steps, as explained in methodology section 3.4.2.
First, both the surface deformations are precisely measured for targeted points (Total number of targeted points= 900) on the cut surface. The surface deformation at a point represents the vertical position (z- coordinate) of that point. Whereas x and y coordinates of the cut surface define the location of targeted points. The surface deformation measurement on CMM is shown in Figure 4.9.
Figure 4.9 Surface deformation measurement on CMM
The precision of deformation data is subjected to various factors like; cutting precision, surface roughness, positioning of sample on CMM bed and probe surface, etc. Hence, both contours of the cut surfaces are averaged at each point (x, y) to remove the effect of shear stresses and imperfection in cutting.
However, despite proper levelling and alignment of the sample on the CMM platform, the surface plot obtained for the exposed surface remains distorted. Consequently, the third most important step, ‘data conditioning’, takes place. The surface plot's orientation and smoothness are rectified, as explained in methodology section 3.4.2.1. The surface plot of average surface deformation values after the data conditioning is shown in Figure 4.10 below.
(a)
Figure 4.10 Surface plot for average surface distortion of both the cut surfaces after the data conditioning
Smoothening of the surface plot is accomplished through the nonlinear surface fitting, using the Levenberg- Marquardt (L-M) algorithm [117], as explained in methodology section 3.4.2.
Hence, the best-fitted surface for the measured surface deformation plot is obtained, as shown in Figure 4.11 (a). At last, static structural FE analysis is performed with nodal displacement load matching the surface deformation plot. The deformation values are applied as displacement load in the negative direction to the corresponding nodes on the surface equivalent to the cut surface to obtain the analogous stress distribution same as welding induced residual stresses. The six degrees of freedom are constrained as initial boundary conditions to prevent rigid body motion of the plate. The cut section's FE model with appropriate meshing and constraints is also shown in Figure 4.11 (b).
Figure 4.11 (a) Best fitting surface deformation plot (Adjusted R2 value= 0.9105), (b) FE model of one of the halves of weld sample for surface deformation load structural FE analysis Hence, the contour plot of longitudinal residual stress distribution on the cut surface of the double-side single pass welded joint is shown in Figure 4.12.
Figure 4.12 Contour plot of longitudinal residual stress distribution on the cut surface of square butt double-side single pass welded joint (contour measurement technique)
(b) (a)
[Pa]
The tensile residual stress distribution is observed in the mid-region of the weld plate.
However, compressive stresses are observed in the top and bottom sides of the fusion zone.
The possible reason for compressive residual stress is fast cooling through exposed areas of top and bottom regions compared to mid-region. The slow cooling rate affects the martensite transformation of the P91 steel weld during cooling. It leads to less volumetric expansion and originates tensile residual stress in a double-side butt weld's mid thickness region. The maximum compressive residual stress in the double-sided weld's contour plot is observed as - 194 MPa. On the other hand, the peak value of tensile residual stress is observed as 38 MPa.
Figure 4.13 presents longitudinal residual stress values in the FZ of both square butt single- and double-side single pass welded joints. It presents a comparative explanation of residual stress distribution in the FZ of both the weld joints. For the square butt single side single pass weld joint, the observation points are within 4 mm (top) to 11 mm (bottom) thickness in the FZ, signifying the removal of the top and bottom weld bead crowns DHD measurement.
However, the whole thickness, including weld bead crowns, is considered for square butt double-side single pass weld joint.
0 2 4 6 8 10 12 14 16
-250 -200 -150 -100 -50 0 50 100 150
Length along the weld centreline in FZ (mm) Long. residual stress (MPa) Single side welded (DHD)
Double side welded (Contour)
Line for residual stress plot
Figure 4.13 Longitudinal residual stress across the thickness in the fusion zone of square butt single and double side single pass welded joints
Compressive residual stress is observed in the FZ of square butt single-sided single pass weld, decreasing from top to bottom. However, the square butt double-sided single pass weld shows tensile residual stress in the mid-thick region, as shown in Figure 4.13. The residual stresses
on top and bottom points are observed as –58.9 MPa and -193.1 MPa, respectively. It is also observed that the tensile residual stress reaches its peak in the lower half of the plate thickness at 9 mm from the top surface with a value of 32.4 MPa. The top and bottom weld bead crown regions of square butt single side single pass joint may also show compressive residual stress, which is assessed from FE results explained in the next chapter.