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Residual stress measurements in coil, linepipe and girth welded pipe
Materials Science and Engineering A 437 (2006) 60–63
Residual stress measurements in coil, linepipe and girth welded pipe M. Law a,∗ , H. Prask b , V. Luzin b,c , T. Gnaeupel-Herold b,d a
Australian Nuclear Science and Technology Organisation (ANSTO), Institute of Materials Science and Engineering, PMB1, Menai, NSW 2234, Australia b NIST Center for Neutron Research (NCNR), Maryland, United States c State University of New York at Stony Brook, New York, United States d University of Maryland, Department of Materials Science and Engineering, Maryland, United States Accepted 15 April 2006
Abstract Residual stresses in gas pipelines come from forming operations in producing the coil and pipe, seam welding the pipe, and girth welding pipes together to form a gas pipeline. Welding is used extensively in gas pipelines, the welds are made without post weld heat treatment. The three normal stresses were measured by neutron diffraction for three types of sample: coil, unwelded rings cut from the pipe made from this coil, and girth welded rings cut from linepipe. All three specimens came from three thicknesses of manufacture (5.4, 6.4, and 7.1 mm). The welds are manual metal arc cellulosic electrode welds made in X70 linepipe, these were measured at 5 through-thickness positions at 19 locations (from the center of the weld up to 35 mm away from the weld) with a spatial resolution of 1 mm3 . The coil and unwelded rings were measured at the same five through-thickness positions. © 2006 Elsevier B.V. All rights reserved. Keywords: Residual stress; Welding; Neutron; Diffraction
1. Introduction Residual stresses are of special significance in gas pipelines as they are designed with smaller safety factors (in ASME B31.8, the ratio between the specified minimum yield strength, and the operating stress is 1.25) than other major engineering components. Residual stresses may contribute to premature failure by fatigue, stress corrosion cracking, fracture, or lead to unacceptable deformation. Residual stresses can alter fatigue life by changes in the mean stress level, particularly at the pipe surfaces [1]. Stress corrosion cracking (SCC) occurs where there is a combination of unfavorable microstructure, stress, and corrosive environment [2]. Residual stress may alter the critical flaw size for crack propagation, in both pipelines and test specimens made from them. In the manufacturing process, after a series of controlled temperature rolling operations, the resulting strip is wound onto a coil, and unwound from this to be fed into the pipe mill. Elastic and plastic deformations occur in this operation. This pipe
∗
Corresponding author. Tel.: +61 2 9717 9102; fax: +61 2 9543 7179. E-mail address: [email protected] (M. Law).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.04.062
is formed from coil, bent, and longitudinally seam welded. The stages of manufacture are bending (A) from the coil into the pipe form, followed by seam welding, then compressive sizing (B), and finally expansion (C) in the mill hydrotest. This process introduces different strain cycles in the inner and outer walls of the pipe [3]. The inner wall strain cycle is A, compression; B, compression; C, tension, while the outer wall strain cycle is A, tension; B, compression; C, tension. The Bauschinger effect plays a significant role in the magnitude of the resulting plastic strains and residual stresses, particularly in higher grade pipe. The rolling direction (longitudinal) yield strength is generally lower than that of the transverse (hoop) direction. There is little data on residual stresses in as-manufactured pipe. In welding, local heating leads to residual stresses through the combined effects of thermal strains, phase transformations, and variation of material properties with temperature. Welding may produce high tensile stresses (up to the yield stress) balanced by lower compressive residual stresses elsewhere in the component. Post weld heat treatment is not common in constructing gas pipelines. There is little published data on residual stresses in shielded and manual metal arc welds (SMAW & MMAW) which are common in gas pipelines. Previous studies have shown that residual hoop stresses in girth welded gas pipelines may be
M. Law et al. / Materials Science and Engineering A 437 (2006) 60–63
61
Table 1 Pipe and weld composition (%)
X70 pipe E6010 weld
C
P
Mn
Si
Ti
Ni
Cr
Mo
Cu
0.08 0.13
0.15 0.16
1.38 1.12
0.33 0.21
0.020 0.013
0.021 0.049
0.018 0.032
0.10 0.03
0.012 0.035
equal or greater than the yield strength [4,5], particularly for thin walled pipe. 2. Experimental 2.1. Sample description The coil and pipe are in three thicknesses (5.4, 6.4 and 7.1 mm) of X70 grade steel with a specified minimum yield strength of 483 MPa and an actual yield strength of between 543 and 575 MPa. The pipe which is welded to form the pipeline is a 274 mm diameter electric resistance (seam) welded (ERW) pipe. The chemical composition of the pipe and weld metal are similar (Table 1). 2.2. Method of measurement The residual stress measurements were performed on the BT8 instrument at the NIST Center for Neutron Research (NCNR). Because the basic principals of this technique are well known [6–8] only details specific to this measurement will be reported. ˚ and diffraction from A monochromatic beam with λ = 1.459 A {2 1 1} planes were used in this analysis. This combination of wavelength and scattering planes resulted in a scattering angle of 76.6◦ . Measurements were made without cutting the full ring of pipe so as to fully preserve the stress field. A nominal gauge volume of 1 mm × 1 mm × 1 mm was used to perform through-thickness scans at five depths. To improve the statistical reliability of the data the pipe specimens were sampled at three different locations opposite the seam weld and the results averaged. The girth welded rings were measured in 19 areas (from 10 mm one side of the weld to 35 mm on the other side of the weld) at five positions through-thickness. A section through the weld and parent in a girth welded ring was cut into a 3 mm wide comb, with teeth 2.5 mm wide to derive d0 values across the weld, heat affected zone (HAZ) and parent. The stress free d0 values were equivalent within a measurement accuracy (d/d = 7E−5) across all three zones, this may be due to the similar composition of the parent and weld material (Table 1). Stresses were calculated with this d0 value and the additional constraint of setting the normal component to equal zero.
Fig. 1. Typical residual stresses in coil.
Much of the residual stress field comes from coiling and uncoiling the steel strip. The coiling process compresses the inner face of the coil (this becomes the outer wall of the pipe) and stretches the outer face. In both cases the deformation involves considerable plastic deformation (Fig. 2). The stresses seen in Fig. 1 correspond to the predictions in Fig. 2. 3.2. Unwelded pipe The unwelded pipe samples were analyzed at five points through wall, the axial stress component was the greatest in magnitude (Fig. 3). The hoop stresses are strongly affected by bending the coil into a circular shape, this will result in compression on the inner wall and tension on the outer wall. The axial stresses should be largely un-altered by the subsequent pipe forming operations, however a significant change is seen. This is due to the fact that the coil is still curved in the axial plane when it was measured. The first stage of the pipe forming involves straightening the
3. Results 3.1. Coil The coil samples were analyzed at five points through wall, the axial stress component was the greatest in magnitude (Fig. 1).
Fig. 2. Predicted stress pattern after uncoiling.
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M. Law et al. / Materials Science and Engineering A 437 (2006) 60–63
Fig. 3. Typical residual stresses in unwelded pipe.
Fig. 6. Evolution of hoop stresses.
coil, this results in compression on the inner wall and tension on the outer wall, the same strain pattern that is seen in the hoop stress. Both changes are visualized in Fig. 4 and correspond to those seen in Fig. 3. 3.3. Welded pipe The welded pipe had the following maximum residual stresses (Table 2 and Fig. 5).
Fig. 4. Predicted stresses in pipe.
4. Discussion 4.1. Weld results Table 2 Maximum tensile stresses in girth welded pipe (MPa)
5.4 mm girth weld 6.4 mm girth weld 7.1 mm girth weld
Hoop
Axial
447 ± 25 594 ± 27 589 ± 27
486 ± 28 269 ± 27 394 ± 26
Fig. 5. Inner wall hoop stresses adjacent to girth welds.
4.1.1. Hoop stresses The cooling weld bead shrinks around the pipe to impose a hoop direction force, similar to a tourniquet around the cylinder [9]. The highest hoop stresses are in the parent metal adjacent to the weld (+10 mm) while the stresses in the weld are much lower. An explanation for this is that as the cooling, shrinking weld bead is restrained by the parent material adjacent to it, the parent material goes into compression (Fig. 6). At the same time it is also being heated which causes this material to expand, this increase the compressive stresses on it as it is restrained by parent material further away from the weld. The compressive stress exceeds the yield strength (which is reduced by the increased temperature) and the material yields compressively. As the weld pool and adjacent material continue cooling, the parent material adjacent to the weld is restrained from shrinking. The magnitude of these tensile stresses is in accord with published data [1] showing hoop residual stresses to be primarily between 0.2σ y and 1.0σ y , but sometimes outside these bounds. The low hoop stresses (compared to those seen in the parent material) in the weld are a result of the weld contraction being restrained by the adjacent parent material, which is also heated by the weld. The weld metal yields under tensile stress. When
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5. Summary
Fig. 7. Axial stresses from “tourniquet” of weld metal around the pipe.
Residual stresses were measured by the neutron diffraction method at a number of positions in girth welded ERW gas linepipe. The residual stresses from manufacture and from welding were measured, and models presented to account for the stress evolution. Residual stress measurements are important for reasons of pipeline integrity such as SCC and cracking, and the measurements also allow numerical models of residual stress evolution to be calibrated and verified. Acknowledgements
the adjacent parent metal cools and contracts, low tensile hoop stresses are the result. 4.1.2. Axial stresses Due to bending from the compressive “tourniquet” of weld metal around the pipe (Fig. 7), the axial stresses are compressive on the outer surface and tensile on the inner surface. The highest stresses were found [10] to be in thin wall pipes with a low number of welding passes. The residual stresses in the axial direction were between 60 and 80% of the yield strength. As stresses in this direction have the greatest effect on flaws in the girth weld, assuming that residual stresses to be of YS magnitude would be unnecessarily conservative, and would lead to an overly small allowable defect size, thus sentencing many adequate welds to repair and adding considerably to construction time and costs. 4.1.3. Maximum residual stresses The maximum value of residual stress in the 5.4 mm pipe in the hoop direction was 594 MPa, well above the actual yield strength (AYS) of 575 MPa and the nominal yield strength (SMYS) of 483 MPa. If an integrity analysis had assumed nominal yield strength magnitude residual stresses, the actual stresses are much higher and the analysis would be inaccurate and nonconservative. The acceptable defect size in the girth weld is a function of the stress perpendicular to the weld, i.e. the axial stress. It is customary to assume the magnitude of residual stress to be equal to the yield strength, but in this case the axial stresses are approximately two-third of this.
The authors would like to acknowledge Bluescope steel for supplying the material and the NIST Center for Neutron Research (NCNR) for beamtime and support. Certain commercial firms and trade names are identified in this report in order to specify aspects of the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. References [1] P. Withers, H. Bhadeshia, Mater. Sci. Technol. 17 (2001) 355–365. [2] M. Meyers, et al., Proceedings of the Fourth Int. Conf. on Pipeline technology, Ostend, Belgium, May 2004, pp. 1789–1812. [3] N. Millwood, et al., Proceedings of the Fourth Int. Conf. on Pipeline technology, Ostend, Belgium, May 2004, pp. 1857–1879. [4] W. Mohr, Proceedings of the ASME PVP Conf., PVP, vol. 327, Residual Stresses in Design Assessment and Repair, 1996. [5] L. Clapham, et al., Characterization of Texture and Residual Stress in a Section of 610 mm Pipeline Steel, NDT & E V28, 1995, pp. 73–82. [6] A. Allen, M.T. Hutchings, C.G. Windsor, C. Andreani, Adv. Phys. 34 (1985) 445–473. [7] M.T. Hutchings, A.D. Krawitz (Eds.), NATO ASI Series 216E, Kluwer Academic Publishers, Dordrecht, 1992. [8] H.J. Prask, P.C. Brand, in: G. Vourvopoulos (Ed.), Neutrons in Research and Industry, Proceedings of the Int. Conf. on Neutrons in Research and Industry, SPIE Proceedings Series, vol. 2867, 1997, pp. 106–115. [9] A. Scaramangas, G. Porter, Proceedings of the 17th Offshore Technology Conference OTC5024, May 1985, pp. 25–30. [10] P. Michaleris, Incorporation of residual stresses into fracture mechanics models, EWI Report 7412, 1996.
Residual stress measurements in coil, linepipe and girth welded pipe M. Law a,∗ , H. Prask b , V. Luzin b,c , T. Gnaeupel-Herold b,d a
Australian Nuclear Science and Technology Organisation (ANSTO), Institute of Materials Science and Engineering, PMB1, Menai, NSW 2234, Australia b NIST Center for Neutron Research (NCNR), Maryland, United States c State University of New York at Stony Brook, New York, United States d University of Maryland, Department of Materials Science and Engineering, Maryland, United States Accepted 15 April 2006
Abstract Residual stresses in gas pipelines come from forming operations in producing the coil and pipe, seam welding the pipe, and girth welding pipes together to form a gas pipeline. Welding is used extensively in gas pipelines, the welds are made without post weld heat treatment. The three normal stresses were measured by neutron diffraction for three types of sample: coil, unwelded rings cut from the pipe made from this coil, and girth welded rings cut from linepipe. All three specimens came from three thicknesses of manufacture (5.4, 6.4, and 7.1 mm). The welds are manual metal arc cellulosic electrode welds made in X70 linepipe, these were measured at 5 through-thickness positions at 19 locations (from the center of the weld up to 35 mm away from the weld) with a spatial resolution of 1 mm3 . The coil and unwelded rings were measured at the same five through-thickness positions. © 2006 Elsevier B.V. All rights reserved. Keywords: Residual stress; Welding; Neutron; Diffraction
1. Introduction Residual stresses are of special significance in gas pipelines as they are designed with smaller safety factors (in ASME B31.8, the ratio between the specified minimum yield strength, and the operating stress is 1.25) than other major engineering components. Residual stresses may contribute to premature failure by fatigue, stress corrosion cracking, fracture, or lead to unacceptable deformation. Residual stresses can alter fatigue life by changes in the mean stress level, particularly at the pipe surfaces [1]. Stress corrosion cracking (SCC) occurs where there is a combination of unfavorable microstructure, stress, and corrosive environment [2]. Residual stress may alter the critical flaw size for crack propagation, in both pipelines and test specimens made from them. In the manufacturing process, after a series of controlled temperature rolling operations, the resulting strip is wound onto a coil, and unwound from this to be fed into the pipe mill. Elastic and plastic deformations occur in this operation. This pipe
∗
Corresponding author. Tel.: +61 2 9717 9102; fax: +61 2 9543 7179. E-mail address: [email protected] (M. Law).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.04.062
is formed from coil, bent, and longitudinally seam welded. The stages of manufacture are bending (A) from the coil into the pipe form, followed by seam welding, then compressive sizing (B), and finally expansion (C) in the mill hydrotest. This process introduces different strain cycles in the inner and outer walls of the pipe [3]. The inner wall strain cycle is A, compression; B, compression; C, tension, while the outer wall strain cycle is A, tension; B, compression; C, tension. The Bauschinger effect plays a significant role in the magnitude of the resulting plastic strains and residual stresses, particularly in higher grade pipe. The rolling direction (longitudinal) yield strength is generally lower than that of the transverse (hoop) direction. There is little data on residual stresses in as-manufactured pipe. In welding, local heating leads to residual stresses through the combined effects of thermal strains, phase transformations, and variation of material properties with temperature. Welding may produce high tensile stresses (up to the yield stress) balanced by lower compressive residual stresses elsewhere in the component. Post weld heat treatment is not common in constructing gas pipelines. There is little published data on residual stresses in shielded and manual metal arc welds (SMAW & MMAW) which are common in gas pipelines. Previous studies have shown that residual hoop stresses in girth welded gas pipelines may be
M. Law et al. / Materials Science and Engineering A 437 (2006) 60–63
61
Table 1 Pipe and weld composition (%)
X70 pipe E6010 weld
C
P
Mn
Si
Ti
Ni
Cr
Mo
Cu
0.08 0.13
0.15 0.16
1.38 1.12
0.33 0.21
0.020 0.013
0.021 0.049
0.018 0.032
0.10 0.03
0.012 0.035
equal or greater than the yield strength [4,5], particularly for thin walled pipe. 2. Experimental 2.1. Sample description The coil and pipe are in three thicknesses (5.4, 6.4 and 7.1 mm) of X70 grade steel with a specified minimum yield strength of 483 MPa and an actual yield strength of between 543 and 575 MPa. The pipe which is welded to form the pipeline is a 274 mm diameter electric resistance (seam) welded (ERW) pipe. The chemical composition of the pipe and weld metal are similar (Table 1). 2.2. Method of measurement The residual stress measurements were performed on the BT8 instrument at the NIST Center for Neutron Research (NCNR). Because the basic principals of this technique are well known [6–8] only details specific to this measurement will be reported. ˚ and diffraction from A monochromatic beam with λ = 1.459 A {2 1 1} planes were used in this analysis. This combination of wavelength and scattering planes resulted in a scattering angle of 76.6◦ . Measurements were made without cutting the full ring of pipe so as to fully preserve the stress field. A nominal gauge volume of 1 mm × 1 mm × 1 mm was used to perform through-thickness scans at five depths. To improve the statistical reliability of the data the pipe specimens were sampled at three different locations opposite the seam weld and the results averaged. The girth welded rings were measured in 19 areas (from 10 mm one side of the weld to 35 mm on the other side of the weld) at five positions through-thickness. A section through the weld and parent in a girth welded ring was cut into a 3 mm wide comb, with teeth 2.5 mm wide to derive d0 values across the weld, heat affected zone (HAZ) and parent. The stress free d0 values were equivalent within a measurement accuracy (d/d = 7E−5) across all three zones, this may be due to the similar composition of the parent and weld material (Table 1). Stresses were calculated with this d0 value and the additional constraint of setting the normal component to equal zero.
Fig. 1. Typical residual stresses in coil.
Much of the residual stress field comes from coiling and uncoiling the steel strip. The coiling process compresses the inner face of the coil (this becomes the outer wall of the pipe) and stretches the outer face. In both cases the deformation involves considerable plastic deformation (Fig. 2). The stresses seen in Fig. 1 correspond to the predictions in Fig. 2. 3.2. Unwelded pipe The unwelded pipe samples were analyzed at five points through wall, the axial stress component was the greatest in magnitude (Fig. 3). The hoop stresses are strongly affected by bending the coil into a circular shape, this will result in compression on the inner wall and tension on the outer wall. The axial stresses should be largely un-altered by the subsequent pipe forming operations, however a significant change is seen. This is due to the fact that the coil is still curved in the axial plane when it was measured. The first stage of the pipe forming involves straightening the
3. Results 3.1. Coil The coil samples were analyzed at five points through wall, the axial stress component was the greatest in magnitude (Fig. 1).
Fig. 2. Predicted stress pattern after uncoiling.
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Fig. 3. Typical residual stresses in unwelded pipe.
Fig. 6. Evolution of hoop stresses.
coil, this results in compression on the inner wall and tension on the outer wall, the same strain pattern that is seen in the hoop stress. Both changes are visualized in Fig. 4 and correspond to those seen in Fig. 3. 3.3. Welded pipe The welded pipe had the following maximum residual stresses (Table 2 and Fig. 5).
Fig. 4. Predicted stresses in pipe.
4. Discussion 4.1. Weld results Table 2 Maximum tensile stresses in girth welded pipe (MPa)
5.4 mm girth weld 6.4 mm girth weld 7.1 mm girth weld
Hoop
Axial
447 ± 25 594 ± 27 589 ± 27
486 ± 28 269 ± 27 394 ± 26
Fig. 5. Inner wall hoop stresses adjacent to girth welds.
4.1.1. Hoop stresses The cooling weld bead shrinks around the pipe to impose a hoop direction force, similar to a tourniquet around the cylinder [9]. The highest hoop stresses are in the parent metal adjacent to the weld (+10 mm) while the stresses in the weld are much lower. An explanation for this is that as the cooling, shrinking weld bead is restrained by the parent material adjacent to it, the parent material goes into compression (Fig. 6). At the same time it is also being heated which causes this material to expand, this increase the compressive stresses on it as it is restrained by parent material further away from the weld. The compressive stress exceeds the yield strength (which is reduced by the increased temperature) and the material yields compressively. As the weld pool and adjacent material continue cooling, the parent material adjacent to the weld is restrained from shrinking. The magnitude of these tensile stresses is in accord with published data [1] showing hoop residual stresses to be primarily between 0.2σ y and 1.0σ y , but sometimes outside these bounds. The low hoop stresses (compared to those seen in the parent material) in the weld are a result of the weld contraction being restrained by the adjacent parent material, which is also heated by the weld. The weld metal yields under tensile stress. When
M. Law et al. / Materials Science and Engineering A 437 (2006) 60–63
63
5. Summary
Fig. 7. Axial stresses from “tourniquet” of weld metal around the pipe.
Residual stresses were measured by the neutron diffraction method at a number of positions in girth welded ERW gas linepipe. The residual stresses from manufacture and from welding were measured, and models presented to account for the stress evolution. Residual stress measurements are important for reasons of pipeline integrity such as SCC and cracking, and the measurements also allow numerical models of residual stress evolution to be calibrated and verified. Acknowledgements
the adjacent parent metal cools and contracts, low tensile hoop stresses are the result. 4.1.2. Axial stresses Due to bending from the compressive “tourniquet” of weld metal around the pipe (Fig. 7), the axial stresses are compressive on the outer surface and tensile on the inner surface. The highest stresses were found [10] to be in thin wall pipes with a low number of welding passes. The residual stresses in the axial direction were between 60 and 80% of the yield strength. As stresses in this direction have the greatest effect on flaws in the girth weld, assuming that residual stresses to be of YS magnitude would be unnecessarily conservative, and would lead to an overly small allowable defect size, thus sentencing many adequate welds to repair and adding considerably to construction time and costs. 4.1.3. Maximum residual stresses The maximum value of residual stress in the 5.4 mm pipe in the hoop direction was 594 MPa, well above the actual yield strength (AYS) of 575 MPa and the nominal yield strength (SMYS) of 483 MPa. If an integrity analysis had assumed nominal yield strength magnitude residual stresses, the actual stresses are much higher and the analysis would be inaccurate and nonconservative. The acceptable defect size in the girth weld is a function of the stress perpendicular to the weld, i.e. the axial stress. It is customary to assume the magnitude of residual stress to be equal to the yield strength, but in this case the axial stresses are approximately two-third of this.
The authors would like to acknowledge Bluescope steel for supplying the material and the NIST Center for Neutron Research (NCNR) for beamtime and support. Certain commercial firms and trade names are identified in this report in order to specify aspects of the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose. References [1] P. Withers, H. Bhadeshia, Mater. Sci. Technol. 17 (2001) 355–365. [2] M. Meyers, et al., Proceedings of the Fourth Int. Conf. on Pipeline technology, Ostend, Belgium, May 2004, pp. 1789–1812. [3] N. Millwood, et al., Proceedings of the Fourth Int. Conf. on Pipeline technology, Ostend, Belgium, May 2004, pp. 1857–1879. [4] W. Mohr, Proceedings of the ASME PVP Conf., PVP, vol. 327, Residual Stresses in Design Assessment and Repair, 1996. [5] L. Clapham, et al., Characterization of Texture and Residual Stress in a Section of 610 mm Pipeline Steel, NDT & E V28, 1995, pp. 73–82. [6] A. Allen, M.T. Hutchings, C.G. Windsor, C. Andreani, Adv. Phys. 34 (1985) 445–473. [7] M.T. Hutchings, A.D. Krawitz (Eds.), NATO ASI Series 216E, Kluwer Academic Publishers, Dordrecht, 1992. [8] H.J. Prask, P.C. Brand, in: G. Vourvopoulos (Ed.), Neutrons in Research and Industry, Proceedings of the Int. Conf. on Neutrons in Research and Industry, SPIE Proceedings Series, vol. 2867, 1997, pp. 106–115. [9] A. Scaramangas, G. Porter, Proceedings of the 17th Offshore Technology Conference OTC5024, May 1985, pp. 25–30. [10] P. Michaleris, Incorporation of residual stresses into fracture mechanics models, EWI Report 7412, 1996.