CHAPTER 7
7 INFLUENCE OF PREHEATING AND PWHT ON RESIDUAL
STRESSES AND DISTORTION OF SAW WELDED BUTT JOINTS
OF P91 STEEL
(as welded) to ~ 0.03 mm (preheated), i.e., by 57%. Angular distortion angle also reduced from
~ 0.23o to ~ 0.143o (i.e., 37.8%) for the preheated single-side welded butt joint. The mitigation effects of preheating are observed in Figure 7.1.
1
0 19 38 57 76 95 114 133 152
-0.4 -0.2 0.0 0.2 0.4 0.6
Vertical distortion (mm)
Length across the weld (mm) As-welded ( Experiment ) Preheated ( Experiment )
z
x Observation Line
(a)
0 30 60 90 120 150
-0.4 -0.2 0.0 0.2 0.4 0.6
Vertical distortion (mm)
Length along the edge (mm) As-welded (Experiment) Preheated (Experiment)
z x Observation Line
(b)
Figure 7.1 (a) Angular distortion, (b) edge deflection in square butt single-side single pass welded joints with and without preheating
The residual stress is measured through the thickness of the welded plate in the HAZ and FZ by performing the DHD technique. Residual stress distributions in the HAZ and FZ throughout the plate’s thickness (neglecting weld bead top and bottom crowns) for the preheated and as- welded joints are shown in Figure 7.2.
0 1 2 3 4 5 6 7 8 9 10 -150
-100 -50 0 50 100
Residual stress (MPa)
Length along the weldcentreline in FZ (mm)
Longitudinal (Preheated) Transverse (preheated) Longitudinal (As-welded) Transverse (As-welded)
(a)
Weld centreline
0 1 2 3 4 5 6 7 8 9 10
-50 0 50 100 150 200 250
Residual Stress (MPa)
Plate thickness in HAZ region (mm)
Longitudinal (Preheated) Transverse (Preheated) Longitudinal (As-welded) Transverse (As-welded)
(b)
Observation line
Figure 7.2 Residual stresses in (a) FZ and (b) HAZ of preheated and as-welded square butt single side single pass welded joints
In FZ, longitudinal residual stress shows the same pattern throughout the thickness for both as- welded and preheated weld joints. However, the residual stress values decreased to -75 MPa from -118 MPa at 1 mm of thickness, i.e., 36%. The longitudinal residual stress transformed to tensile stress of 22.5 MPa (preheated) from compressive stress of -4.2 MPa (as-welded) in the bottom region of the welded plate. The transverse residual stress values in the FZ dramatically changed from top to bottom of the plate thickness after preheating. The top side shows the highest variation or transformation of compressive residual stress values from -38.1 MPa to a tensile residual stress of 41.2 MPa. In HAZ, the maximum value of longitudinal stress effectively reduced to just 26 MPa (preheated) from 200 MPa (as welded weld) at 1 mm thickness of the plate, i.e. by 87%. After preheating, transverse residual stress within the HAZ
varies similar to as-welded case throughout the plate thickness with lower values. However, the transverse residual stress value remains the same for the bottom side region of HAZ and FZ, as shown in Figures (a) and (b).
The same comparative study is carried out to analyse the effect of PWHT on residual stress distribution. The residual stress distribution (neglecting weld bead top and bottom crowns) in HAZ and FZ of post-weld heat-treated and as-welded cases are shown in Figure 7.3 (a) and (b) below.
0 1 2 3 4 5 6 7 8 9 10
-150 -100 -50 0 50 100 150
Length along the weldcentreline in FZ (mm)
Residual stress (MPa)
Longitudinal (As-welded) Transverse (As-welded) Longitudinal (PWHT) Transverse (PWHT) (a)
Weld centreline
0 1 2 3 4 5 6 7 8 9 10
-100 -50 0 50 100 150 200 250 300 350
Residual stress (MPa)
Plate thickness in HAZ region (mm)
Longitudinal (As-welded) Transverse (As-welded) Longitudinal (PWHT) Transverse (PWHT)
(b)
Observation line
Figure 7.3 Residual stress in (a) FZ and (b) HAZ of as-welded and post-weld heat-treated square butt single side single pass welded joints
On performing PWHT, both longitudinal and transverse residual stresses values are reduced in both FZ and HAZ. The FZ's longitudinal residual stress at 1 mm thickness transformed from compressive stress of -118 MPa (as-welded) to tensile residual stress of 26 MPa (PWHT).
Similarly, transverse residual stress also transformed from compressive residual stress of –50 MPa to tensile residual stress of around 50 MPa at 1 mm thickness. HAZ also experiences a similar mitigation effect for longitudinal residual stress, which reduced from 202 MPa to around 49.1 MPa, i.e., by almost 76%. Transverse residual stress was also reduced in FZ and HAZ and developed a linear distribution throughout the thickness.
Finally, both preheating and PWHT processes are applied to assess the combined effect of both processes. The residual stress distributions for this case are shown in Figure 7.4 (a) and (b) for both transverse and longitudinal residual stress in HAZ and FZ.
0 1 2 3 4 5 6 7 8 9 10
-150 -100 -50 0 50 100 150
Length along the weldcentreline in FZ (mm)
Residual stress (MPa)
Longitudinal (Preheating+PWHT) Transverse (Preheating+PWHT) Longitudinal (As- welded) Transverse (As-welded)
(a)
Weld centreline
0 1 2 3 4 5 6 7 8 9 10
-50 0 50 100 150 200 250
(b)
Residual stress (MPa)
Plate thickness in HAZ region (mm)
Longitudinal (Preheating+PWHT) Transverse (Preheating+PWHT) Longitudinal (Welding) Transverse (Welding)
Observation line
Figure 7.4 Residual stress in (a) FZ and (b) HAZ of combined preheated and post-weld heat- treated weld and as-welded square butt single side single pass welded joints
It is observed that the combined preheating and PWHT processes efficiently mitigates the welding induced residual stress distribution in both longitudinal and transverse directions for both HAZ and FZ.
Figure 7.4 (a) shows the longitudinal and transverse stress distribution in the weld fusion zone.
Here, it is observed that combined preheating and PWHT efficaciously mitigates the stress distribution. The longitudinal stress value at 1 mm thickness is observed as 15 MPa, transformed from compressive to tensile value from as-welded condition. Similarly, the transverse stress value at 1 mm thickness is observed as -16 MPa, compared to the corresponding value (-40 MPa) of as-welded welds. It is also observed that for both the regions, i.e. HAZ and FZ, the variation in residual stress distribution is not as sharp as for as-welded conditions followed by preheating and PWHT. The longitudinal stress value in HAZ at 1 mm thickness transformed from tensile stress to compressive stress (i.e.-15 MPa), as shown in Figure 7.4 (b). The longitudinal stress values remain tensile in the mid thickness region but with a much lower value of 5-12 MPa than the as-welded case. The transverse stress distribution is observed to be compressive throughout the thickness. The mid thickness region shows the significantly small tensile residual stress in the transverse direction.
To clearly understand the effect of preheating, PWHT and combined pre and post-weld heat treatments on residual stress distribution, a bar diagram of the extreme value of longitudinal and transverse residual stresses are presented here, as shown in Figure 7.5.
Figure 7.5 Extreme values of longitudinal and transverse residual stress in the square butt single side single pass joints of P91 steel under four different cases
-150 -100 -50 0 50 100 150 200 250
Max Min Max Min Max Min Max Min
As welded Preheating PWHT Preheating + PWHT
Longitudinal (FZ) Longitudinal (HAZ) Transverse (FZ) Transverse (HAZ)
Both longitudinal and transverse maximum residual stress values (tensile or compressive) are observed in the as-welded sample. As expected, the minimum residual stress values (tensile or compressive) are observed for the combined preheated & PWHT weld sample. It signifies that preheating combined with post-weld heat treatment on submerged arc square butt single side single pass joint of P91 steel shows maximum and better mitigation effect than PWHT without preheating.