6.3 Effect of in-process mitigation heat treatments
6.3.3 Welding induced residual stress
The residual stress distribution is compared for each in-process mitigation heat treatment with an as-welded condition. The longitudinal residual stress across the weld region on a top surface of the weld plate is considered for comparison as follows-
6.3.3.1 Transient side heating
In side heating, the localized secondary heating source produces compressive stress during welding (−σz). The magnitude of developed compressive stress is proportional to the temperature gradient (∂T/∂x). Subsequently, the compressive stress would induce the thermal
tensioning stress (+σtension) in the weld bead region [155], [157]. It unveils that an increase in the heating temperature at a distance ‘x’ from the weld zone produces sharp thermal gradients in the heating region, which produces a thermal tensioning effect.
Figure 6.15 show the longitudinal residual stress distribution on the weld plate's top surface across the weld centreline for square butt single side single pass welding of P91 steel plate with side heating. It considers heating intensity and heating torch location as two deciding factors for side heating's mitigation effect.
0 30 60 90 120 150
-300 -150 0 150 300 450 600
Long. Residual stress (MPa)
Length across the weld centerline (mm)
(SH_350oC) (SH_500oC) (SH_650oC) (Welding)
Observation line Z
X
0 30 60 90 120 150
-600 -400 -200 0 200 400 600 800
Long. Residual stress (MPa)
Length across the weld centerline (mm)
(SH_350oC) (SH_500oC) (SH_650oC) (Welding)
Observation line Z
X
0 30 60 90 120 150
-400 -200 0 200 400 600
Long. residual stress (MPa)
Length across the weld centerline (mm)
(SH_30 mm) (SH_40 mm) (Welding)
Observation line Z
X SH_500 oC
0 30 60 90 120 150
-600 -400 -200 0 200 400 600
Long. Residual stress (MPa)
Length across the weld centreline (mm) (Welding) (SH_30 mm) (SH_40 mm)
Observation line
Z X
SH_500 oC
Figure 6.15 Longitudinal residual stress distribution across the weld of square butt single side single pass weld joint with side heating considering (a) different side heating temperatures without SSPT, (b) different side heating temperatures with SSPT, (c) different heating locations without SSPT and (d) different heating locations with SSPT
Figure 6.15 (a) shows the longitudinal residual stress distribution for three SH temperatures, 30 mm away from the weld bead, without considering the SSPT phenomenon. The tensile residual stress value at the weld centreline dropped from 362 MPa (as-welded) to 264 MPa
(a) (b)
(c) (d)
(SH_350 oC), i.e., by 38%. The heating area displays changes like compressive stress (welded) to tensile (SH_350 oC). A similar result is observed for higher heating temperatures, i.e., higher tensile residual stress is developed for higher heating temperatures. However, the weld region's residual stress values did not vary much for higher heating temperatures of 500 oC and 650 oC.
Figure 6.15 (b) presents the longitudinal residual stress distribution considering the SSPT effect. As side heating slows down the cooling rate, it affects the martensitic transformation, contributing to less volumetric contraction and reduces the weld region's compressive residual stress value. As the SH temperatures are below the transformation temperature, the P91 steel plate experiences the usual volumetric expansion in the heating region during side heating. As a result, it induces compressive plastic strain during welding. Hence, it developed tensile residual stress in side heating areas.
Figure 6.15 (c) and (d) show the longitudinal residual stress distribution for different heating torch locations away from the weld bead with and without considering the SSPT phenomenon.
It is observed that heating torch location affects the tensile longitudinal residual stress value in side heating areas. The more we increase the heating torch's distance from the weld bead, the less it affects the thermal gradient near the weld region. Hence, the change in the heating location does not significantly affect longitudinal residual stress distribution in the weld region.
However, the heating regions experience tensile peaks for both with and without SSPT phenomenon cases.
6.3.3.2 Heat sinking
The localized secondary cooling source in heat sinking produces a quenching effect in the weld and nearby regions. It induced compressive stress (-σcompressive) in the vicinity of the weld bead region [97], [158]. Figure 6.16 (a) and (b) show the longitudinal residual stress across the weld centreline on the weld plate's top surface for P91 steel with and without SSPT, respectively.
0 30 60 90 120 150 -400
-200 0 200 400 600
Long. residual stress (MPa)
Length across the weld centerline (mm)
(HS_12000 W/m2k)
(HS_15000 W/m2k)
(HS_20000 W/m2k)
(HS_25000 W/m2k)
(Welding)
zx
Observation line
0 30 60 90 120 150
-600 -400 -200 0 200 400 600
Long. residual stress (MPa)
Length across the weld centerline (mm)
(HS_12000 W/m2.K) (HS_15000 W/m2.K) (HS_20000 W/m2.K) (Welding))
Observation line
Z X
Figure 6.16 Longitudinal residual stress across the weld in square butt single side single pass welded joint models with heat sinking (a) without SSPT effect, (b) with SSPT effect For a higher convective heat transfer coefficient (h= 20000 W/m2k), the tensile residual stress on the fusion zone centreline reduced from 362 MPa to 227 MPa, i.e. by 37.3%. The plate’s edge region shows a massive compressive residual stress value change from -200 MPa to -23 MPa. The possible reason for this alteration is that parent base metal balances the state of residual stress across the weld centreline in the transverse direction against the change in the residual stress in the weld region [97].
However, on considering the SSPT effect, Figure 6.16 (b) shows that compressive longitudinal residual stress distribution shrinks within the weld bead region due to contraction in the thermal profile (observed in Figure 6.10 ). The rapid cooling or quenching effect of heat sinking reduces longitudinal residual stress values throughout the weld centreline. The maximum tensile longitudinal residual stress value in the HAZ region dropped from 235 MPa to 133 MPa, i.e.
by 43.4%. Similarly, the longitudinal residual stress in and near the weld centreline expectedly diminished to a much lower compressive value of -388 MPa (h= 20000 W/m2k) from -339 MPa (as-welded), i.e., by 12.1%.
6.3.3.3 Combined side heating and heat sinking
Both transient side heating and static heat sinking effectively mitigate the welding induced distortions and residual stresses in P91 steel welds with and without the SSPT phenomenon.
However, side heating and heat sinking exhibited a different kinds of mitigation results.
Therefore, the study is further extended to assess the effect of combining transient side heating
(a) (b)
and static heat sinking processes. Figure 6.17 (a) and (b) show the longitudinal residual stress across the weld for combined TSH and HS during welding.
0 30 60 90 120 150
-300 -150 0 150 300 450 600 750
SH_30 mm
Long. residual stress (MPa)
Length across the weld centerline (MPa) (Welding)
(HS+ SH_350 oC) (HS+ SH_500 oC) (HS+ SH_650 oC)
HS_(h= 25000 W/m2k)
z x
Observation line
0 30 60 90 120 150
-450 -300 -150 0 150 300 450 600
SH_ 30 mm HS_(h= 25000 W/m2k)
Long. residual stress (MPa)
Length across the weld centerline (mm) (Welding)
(HS+ SH_350oC) (HS+ SH_500oC)
(HS+ SH_600oC) z
x
Observation line
Figure 6.17 Residual stress distribution for combined side heating and heat sink (a) without considering SSPT, (b) considering SSPT
The combined mitigation heat treatment develops a crown shape longitudinal residual stress distribution across the weld in the transverse direction. The residual stress is expectedly reduced in the weld bead region more efficiently than TSH and HS processes for no SSPT phenomenon case, as shown in Figure 6.17 (a). The longitudinal residual stress value on weld centreline diminished from 362 MPa (as- welded) to 184.1 MPa (combined TSH_600 oC+
HS_h= 25000 W/m2k) i.e., by 49.1%. The heating regions also developed tensile peaks of higher values (> 200 MPa) under transient side heating effects. However, the region between side heating and heat sinking areas shows compressive residual stress and cause an abrupt change in residual stress distribution across the weld in the transverse direction.
(a)
(b)
On considering the SSPT phenomenon, it is observed that residual stress value on weld centreline decreased in magnitude but remained compressive. The reason behind this alteration is that transient side heating weakens the overall cooling effect of heat sinking. Therefore, the compressive residual stress value decreased for the weld centreline by increasing the side heating temperature. The crown shape distribution of residual stress across the weld centreline develops a sharp gradient of residual stress value with multiple compressive and tensile residual stress peaks.