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Numerical simulation to study the effect of repair width on residual stresses of a stainless steel clad plate
International Journal of Pressure Vessels and Piping 87 (2010) 457e463
Contents lists available at ScienceDirect
International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp
Numerical simulation to study the effect of repair width on residual stresses of a stainless steel clad plate Wenchun Jiang a, *, Zibai Liu b, J.M. Gong c, S.T. Tu c a
College of Mechanical and Electronic Engineering, China University of Petroleum, Dongying 257061, PR China School of Material Science and Engineering, Jilin University, Jilin 130025, PR China c School of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing 210009, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2010 Received in revised form 31 May 2010 Accepted 17 June 2010
Clad plates are widely used in the construction of corrosion resistant equipment. During the repair of clad plates, residual stresses are generated and influence the structure integrity. This paper uses the finite element method (FEM) to predict the residual stresses in a repair weld of a stainless steel clad plate. The effect of repair width on residual stresses has also been investigated by numerical simulation. Due to the material mismatching between clad metal and base metal, a discontinuous stress distribution has been generated across the interface between clad and base metals. The peak residual stress occurs in the heat affected zone (HAZ) of the base metal, because the yield strength of the base metal is larger than that of the clad metal. With an increase in repair width, the residual stresses are decreased. When the repair width is increased to 24 mm, the residual stresses in the weld have been decreased greatly and the peak residual stresses have been reduced to less than the yield strength. Therefore, the recommended repair width should not be less than 24 mm, which provides a reference for optimizing repair welding technology for this stainless steel clad pate. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Clad plate Repair weld Residual stress Repair width
1. Introduction Clad plate consisting of a thin layer of reactive (or refractory) metal integrally bonded onto a lower cost, thicker base metal, has been widely used in construction of corrosion resistant equipment. During the manufacture or service, cracks or crack-like defects are often formed in clad layer and penetrate into base metal [1]. The best method for repairing the cracked region is to remove and then refill that part by weld layers (repair weld technique). The repair weld of clad plate belongs to dissimilar steel weld, which causes difficulties in quality control [2,3]. For the repair of clad plate, a very important aspect is to control the welding residual stresses. This is because the tensile residual stress has an adverse effect on life [4e6] and corrosion [7,8]. When combined with stress generated by service load, a tensile residual stress reduces crack initiation life, accelerates growth rates of pre-existing or service-induced defects, and increases the susceptibility of structures to catastrophic failure by fracture [9]. Therefore, an accurate knowledge of residual stress in weld repairs is essential for structural integrity assessment [10,11].
* Corresponding author. Tel.: þ86 546 8391776; fax: þ86 546 8393620. E-mail address: [email protected] (W. Jiang). 0308-0161/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2010.06.003
Finite element method (FEM) has been widely used to predict the residual stress in the repair welding [12e16]. From the open literatures, it can be concluded that repair length [17], heat input [18], groove shape and pass number [19] have great effects on residual stress. Dong [13] found weld repairs typically increase the transverse stress compared with the initial weld, and the shorter the repair length the greater the increase in transverse stress. Welding procedure related parameters (pass lumping, heat input and inter-pass temperature) are more important in analyzing weld repairs than in initial fabrication welds [20e22]. Soanes [14] modeled residual stresses at a repair in a steam header to tubeplate, which supports the development and optimization of a successful repair and heat treatment strategy and thus underwrite the safety case for return to service. With respect to the repair weld of clad plate, Motarjemi [1] performed a fracture assessment of weld-repaired clad plate by European flaw assessment procedure. Rybicki [23] investigated the residual stress in a heat treated weld clad plate by FEM and experiment. Hein [24] researched the residual stress in repair welds in a clad plate by FEM and numerical simulation method, which supported the repair weld optimization by minimizing the residual stress. In this paper, a FEM model is developed to estimate the residual stress at repair weld in a stainless steel clad
458
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463 Table 1 Chemical composition of 304 and Q345R.
unit: mm 304
Composition
C
Si
Mn
S
P
Cr
Ni
304 Q345R
0.06 0.15
1.00 0.32
2.0 1.38
0.03 0.014
0.035 0.016
17.0e19.0 e
8.0e11.0 e
400
600
800
166 108 0.28 7840 18.2
150 82 0.28 7755 18.6
125 69 0.28 7667 19.5
20.2
22.8
25.4
Y Q345R
X(S11) Z(S33)
Table 2 Material properties for 304 and Q345R steel. Temperature( C)
Fig. 1. The geometrical model of repair weld in a clad plate.
plate. The effect of repair width on residual stress has been studied, which provides a reference for optimizing the repair welding technology. 2. FE analysis of repair welding residual stress A sequential coupling FE program is developed to calculate residual stress by general purpose FE software code ABAQUS. Firstly, a thermal analysis is carried out to determine temperature history field, and then the temperature results are applied incrementally to mechanical model to simulate residual stress. 2.1. Geometrical model and meshing Fig. 1 shows the geometrical model of the repair weld joint in a clad plate. The flaws generated on clad layer are removed and then overlay welded. Clad thickness and base metal thickness are 3 and 17 mm, respectively. The groove angle is 30 and the repair width is 8 mm. A 2-D plane strain FE model is built and the FE meshing is shown in Fig. 2. In total, 2144 nodes and 2022 elements are meshed.
20
200
a. Material properties for 304 stainless steel Young’s Modulus (GPa) 199 180 Yield strength (MPa) 206 153 Poisson’s ratio 0.28 0.28 3 8010 7931 Density (kg/m ) 16.0 17.2 Thermal expansion (1/ C 106) Thermal conductivity 15.26 17.6 (W/m C) Specific Heat (J/kg C) 500 544.3 b. Material properties for Q345R steel Young’s Modulus (GPa) 200 183 Yield strength (MPa) 345 310 Poisson’s ratio 0.3 0.3 7850 7840 Density (kg/m3) 14.0 14.2 Thermal expansion (1/ C 106) Thermal conductivity 53.17 47.73 (W/m C) Specific Heat (J/kg C) 461 523
582
634
686
160 280 0.3 7830 16.0
150 210 0.3 7820 16.6
125 160 0.3 7810 18
39.57 607
36.01 678
33 700
and mechanical properties are incorporated. The temperature dependency of physical and mechanical properties of 304 [25] and Q345 [26] are listed in Table 2. 2.3. Welding temperature analysis In thermal analysis, the welding process is primarily simulated by applying a distributed heat flux to weld elements. The distributed heat flux, DFLUX, is calculated by
2.2. Material properties The material of base metal is Q345R, which is a Chinese grade and meets the requirements of strength and stiffness. The clad metal is 304 stainless steel, which is used for corrosion resistance. The weld metal is assumed to have the same material properties as 304. Their chemical compositions are listed in Table 1. For thermal and mechanical analyses, temperature-dependent thermo-physical
DFLUX ¼
U$I$h V
(1)
where U is the voltage, I is current, h is the arc efficiency (0.6) and V is the weld pass volume. The net line energy is calculated by,
Q ¼
U$I$h v
(2)
where v is the welding speed. From Eqs. (1) and (2), the distributed heat flux can be calculate by,
DFLUX ¼
Q $v V
(3)
The voltage, current and speed used in the calculation are 24 V, 150I and 2 mm/s. The simulation of weld metal deposition is achieved by Element Birth and Death. Before welding, the weld metal elements are removed. Once the welding starts, the welded pass is added and heated, and then it is cooled down until the next pass cycle begins. The temperature history of all nodes is stored in a file for the subsequent residual stress calculation. The material properties relevant to thermal analysis are density, specific heat capacity, latent heat capacity, and solidus/liquidus temperatures. 2.4. Residual stress analysis
Fig. 2. FE meshing.
The residual stress is calculated by using the temperature distribution obtained from thermal analysis as input data. Birth and
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463
Death technology used in section 2.3 is also used in stress analysis. The material properties relevant to residual stress are elastic modulus, yield stress, Poisson’s ratio, and coefficient of thermal expansion. For 304 and Q345R steel, solid-state phase transformation does not occur. Therefore, the total strain rate can be decomposed into three components as follows:
3 ¼ 3e þ 3p þ 3ts
(4)
where 3 , 3 and 3 stand for elastic strain, plastic strain and thermal strain, respectively. Elastic strain is modeled using the isotropic Hooke’s law with temperature-dependent Young’s modulus and Poisson’s ratio. The thermal strain is calculated using temperaturedependent coefficient of thermal expansion. For the plastic strain, a rate-independent plastic model is employed with a von Mises yield surface, temperature-dependent mechanical properties and linear kinematic hardening model. Kinematic hardening is considered because material points undergo both loading and unloading in the welding process. The same FE mesh used in the temperature simulation is applied in residual stress analysis. e
p
ts
459
calculated the residual stresses in the butt-welded joints according to the same parameters in Ref [27]. Fig. 4 presents a comparison of the residual stresses computed by us and Chang. It is shown that our results are consistent with Chang’s experiment and FE results [27]. Therefore, the FE program developed here is suitable to be used for residual stress analysis in the repair weld of a clad plate. 4. Results and discussion 4.1. Residual stress distribution contour Fig. 5 shows the contours of transverse (S11) and longitudinal stress (S33). The peak S11 is 202 MPa, which is near the yield strength and shown in heat affected zone (HAZ) of clad metal. The peak S33 is 384 MPa, which is shown at base metal. The yield strength of base metal is larger than that of clad metal; therefore the peak stress is shown at the side of base metal.
2.5. Boundary and initial conditions The initial ambient temperature is 20 C. During thermal analysis, convection and radiation are both taken into consideration. During stress analysis, nodes of A and B at the end of bottom surface shown in Fig. 2 are constrained in X and Y-direction. Thus, the rigid body motion is avoided. 3. Verification of the FE simulation method A butt-welded joints the same as Chang’s [27], is developed to verify the present FE method as shown in Fig. 3. Chang measured the residual stresses along the surface by X-ray method. Here we
unit: mm
Fig. 5. The contours of transverse (a) and longitudinal stress (b).
Y X
Z
In order to discuss the results fully, five reference paths named P1, P2, P3, P4 and P5 have been selected to investigate the stress distribution, as shown in Fig. 2. P1 is the center line of the repair weld. P2 and P3 are located in the right and left of HAZ, respectively. P4 is along the HAZ beneath the weld. P5 is on the surface of the weld.
Fig. 3. A butt-weld joint model to verify the FE method.
b
60
400
FEM by us [27] Experiment by Chang
40
Longitudinal stress (MPa)
Transverse stress (MPa)
a
20
0
-20
0
10
20
30 X (mm)
40
50
FEM by us [27] Experiment by Chang
300 200 100 0 -100 -200 0
10
20
30 X (mm)
Fig. 4. A comparison of the transverse (a) and longitudinal residual stress (b) by us and Chang [27].
40
50
460
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463
250
400
S11 S33
200
Residual stress (MPa)
Residual stress (MPa)
350
150
100
300 250
S11 S33
200 150 100 50
50 0
1
2
3
4
0
2
4
Distance (mm)
6
8
Distance (mm)
Fig. 6. Residual stress distribution along P1.
Fig. 8. Residual Stress distribution along P4.
4.4. Effect of repair width on residual stress 4.2. Residual stress distribution in weld metal Fig. 6 shows the residual stress distribution along P1 in the weld center. Along the weld thickness, the transverse stress shows a gradual increase while longitudinal stress remains almost unchanged. The longitudinal stress in weld metal is 215 MPa, which has reached the yield strength of clad metal. 4.3. Residual stress distribution in HAZ Fig. 7 shows the residual stress distribution along P2 and P3 in HAZ. It is obviously observed that the stresses in the right and left HAZ are different. This is because the heat input is not symmetric along the weld center line P1. The subsequent heating of weld pass has effect on the previous pass, which leads to an asymmetric stress distribution. Fig. 8 shows the residual stress distribution along P4. Along the HAZ beneath the weld metal, the stresses in the right are larger than those in the left. S11 and S33 are about 55 MPa and 290 MPa in the left, respectively, but they jump to 90 MPa and 350 MPa in the right.
Keeping the rest of the welding parameters constant, five models with a repair width of 8.0, 12.0, 16.0, 20.0 and 24.0 mm were developed to discuss the influence of repair width. 4.4.1. Effect of repair width on peak stresses Fig. 9 shows the effect of repair width on peak residual stresses. It is shown that the peak stresses are decreased with repair width increase. When repair width increases from 8 to 24 mm, both the transverse and longitudinal stresses are decreased about 23%. 4.4.2. Effect of repair width on residual stress in the weld metal Fig. 10 shows the transverse and longitudinal stress with different repair length, with distance along the weld surface (P5), normalized by their length. It is shown that the stresses are decreased with repair width increase. When the repair width is 8 mm, the maximum S11 is 185 MPa, and S33 along the surface is about 210 MPa. When the repair width increases to 24 mm, the average of transverse stress in 70% of the width is about 120 MPa while stress in the rest 30% is very small. S33 in the half length has been less than 100 MPa and the maximum has been decreased to
400
360
Residual stress (MPa)
300
Peak residual stress (MPa)
S11 along P2 S11 along P3 S33 along P2 S33 along P3
240
180
120
60
350
300
S11 S33
250
200
150 0
1
2
3
Distance (mm) Fig. 7. Residual stress distribution along P2 and P3.
4
6
8
10
12
14
16
18
20
Repair width (mm) Fig. 9. Effect of repair width on peak stresses.
22
24
200 8mm 12mm 16mm 20mm 24mm
150
100
50
0.0
0.2
0.4
0.6
0.8
Longitudinal stress (MPa)
140
120
8mm 12mm 16mm 20mm 24mm
80
60
0
1
120 8mm 12mm 16mm 20mm 24mm
80 40 0 -40
360 320
0.0
0.2
0.4
0.6
0.8
3
4
8mm 12mm 16mm 20mm 24mm
280 240 200 160
-80
2 Distance (mm)
b
200 160
461
100
1.0
Normalized distance
b
a
Longitudinal stress (MPa)
Transverse stress (MPa)
a
Transverse stress (MPa)
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463
0
1
1.0
2
3
4
Distance (mm)
Normalized distance
Fig. 11. Effect of repair width on residual stress along P2.
Fig. 10. Effect of repair width on residual stress along P5.
165 MPa. Based on this, it can be shown that the repair width on residual stress is notable. 4.4.3. Effect of repair width on residual stress in HAZ Fig. 11 shows the effect of repair width on residual stress along path P2. It is shown that the stresses are decreased with an increase
b
200 8mm 12mm 16mm 20mm 24mm
160 120 80 40 0
0
1
2
3
4
350
8mm 12mm 16mm 20mm 24mm
280 Longitudinal stress (MPa)
Transverse stress (MPa)
a
in repair width. When the repair width is increased from 8 to 24 mm, the transverse stress has been less than 100 MPa and the longitudinal stress has been decreased about 20%. Fig.12 shows the effect of repair width on residual stress along P3. It can be observed that the stresses are greatly decreased when the repair width increase from 8 to 24 mm. When the repair width is
210 140 70 0 -70 -140
0
Distance (mm) Fig. 12. Effect of repair width on residual stress along P3.
1
2 Distance (mm)
3
4
462
Transverse stress (MPa)
a
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463
60
(1) During the repair weld of clad plate, the peak residual stresses are shown in HAZ of base metal with larger yield strength. The mismatching of yield strength between clad metal and base metal causes an unbalance stress distribution. (2) The repair width has great effect on residual stresses. With repair width increase, the residual stresses are decreased. In order to decrease the residual stress, the recommended repair width should not be less than 24 mm.
40
Acknowledgments
20
The authors gratefully acknowledge the support provided by Research Fund of Jiangsu Key Laboratory of Digital Manufacture for Industry Equipment and Control Technology (2010) and Young Teachers Fund of China University of Petroleum (2009).
100 8mm 12mm 16mm 20mm 24mm
80
0.0
Longitudinal stress (MPa)
b
0.2
0.4
0.6
0.8
1.0
Normalized Distance
References
350 300 250 200 8mm 12mm 16mm 20mm 24mm
150 100 50 0
0.0
0.2
0.4
0.6
0.8
1.0
Normalized distance Fig. 13. Effect of repair width on residual stress along P4.
24 mm, 80% of the transverse stress has been less than 80 MPa. The longitudinal stress in clad metal has been reduced to compressive stress, and the longitudinal stress in base metal is only about 50 MPa. Fig. 13 shows the effect of repair width on residual stress along path P4, with distance normalized by their length. It can be found that the stresses are also greatly decreased when the repair width increases to 24 mm. When the repair length is 24 mm, all the transverse stress has been less than 50 MPa, and half of the longitudinal stress has been less than 100 MPa. Based on the above analysis, it is found that the residual stresses are decreased with the repair width increasing. When the repair width is increased, the weld passes are increased. And the corresponding increase of heat input makes the cooling time increase, which leads to a decrease in residual stress. When the repair width is increased to 24 mm, the residual stress has been decreased greatly, and the peak residual stress has been reduced to below the yield strength. Therefore, it is proposed that the repair width is best not less than 24 mm. 5. Conclusion This study developed a sequential coupling finite element procedure to predict residual stresses in repair weld for a stainless steel clad plate. And the effect of repair width on residual stress has been discussed. Based on the obtained results, the following conclusions may be drawn:
[1] Motarjemi AK, Kocak M. Fracture assessment of weld-repaired cladded steel wide plates by SINTAP defect assessment procedure. Weld Res Abroad 2002;48(6e7):27e34. [2] Joseph A, Sanjai K, Rai B, Jayakumar T, Murugan N. Evaluation of residual stresses in dissimilar weld joints. Int J Pres Ves Pip 2005;82(9):700e5. [3] Lee Hyeong-Yeon, Lee Se-Hwan, Kim Jong-Bum, Lee Jae-Han. Creepefatigue damage for a structure with dissimilar metal welds of modified 9Cr-1Mo steel and 316L stainless steel. Int J Fatig 2007;29(9e11):1868e79. [4] Lee Song-In, Koh Seung-Kee. Residual stress effects on the fatigue life of an externally grooved thick-walled pressure vessel. Int J Pres Ves Pip 2002;79 (2):119e26. [5] Teng Tso-Liang, Chang Peng-Hsiang. Effect of residual stresses on fatigue crack initiation life for butt-welded joints. J Mater Process Tech 2004;145(3):325e35. [6] Webster GA, Ezeilo AN. Residual stress distributions and their influence on fatigue lifetimes. Int J Fatig 2001;23(1):375e83. [7] Amirat A, Mohamed-Chateauneuf A, Chaoui K. Reliability assessment of underground pipelines under the combined effect of active corrosion and residual stress. Int J Pres Ves Pip 2006;83(2):107e17. [8] Mochizuki Masahito. Control of welding residual stress for ensuring integrity against fatigue and stressecorrosion cracking. Nucl Eng Des 2007;237 (2):107e23. [9] Barsoum Z, Barsoum I. Residual stress effects on fatigue life of welded structures using LEFM. Eng Fail Anal 2009;16(1):449e67. [10] Bouchard PJ. Special issue on residual stresses at repair welds. Int J Pres Ves Pip 2005;82(4):243. [11] Revision 4 R6. Assessment of integrity of structures containing defects. Gloucester, UK: British Energy Generation Ltd; 2003. [12] Brown TB, Dauda TA, Truman CE, Smith DJ, Memhard D, Pfeiffer W. Predictions and measurements of residual stress in repair welds in plates. Int J Pres Ves Pip 2006;83(11e12):809e18. [13] Dong P, Hong JK, Bouchard PJ. Analysis of residual stresses at weld repairs. Int J Pres Ves Pip 2005;82(4):258e69. [14] Soanes TPT, Bell W, Vibert AJ. Optimising residual stresses at a repair in a steam header to tubeplate weld. Int J Pres Ves Pip 2005;82(4):311e8. [15] Mirzaee-Sisan A, Fookes AJ, Truman CE, Smith DJ, Brown TB, Dauda TA. Residual stress measurement in a repair welded header in the as-welded condition and after advanced post weld treatment. Int J Pres Ves Pip 2007;84 (5):265e73. [16] Mcdonald EJ, Hallam KR, Flewitt PEJ. A strategy for accommodating residual stresses in the assessment of repair weldments based upon measurement of near surface stresses. Int J Pres Ves Pip 2005;82(4):339e46. [17] Bouchard PJ, George D, Santisteban JR, Bruno G, Dutta M, Edwards L, et al. Measurement of the residual stresses in a stainless steel pipe girth weld containing long and short repairs. Int J Pres Ves Pip 2005;82(4):299e310. [18] Akbari D, Sattari-Far I. Effect of the welding heat input on residual stresses in butt-welds of dissimilar pipe joints. Int J Pres Ves Pip 2009;86(11): 769e76. [19] Sattari-Far I, Farahani MR. Effect of the weld groove shape and pass number on residual stresses in butt-welded pipes. Int J Pres Ves Pip 2009;86(11): 723e31. [20] Rudland D, Chen Y, Zhang T, Wilkowski G, Broussard J, White G. Comparison of welding residual stress solutions for control rod drive mechanism nozzles. In: 2007 Proceedings of the ASME pressure vessels and piping conference e materials and fabrication, vol. 6. American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP; 2008. p. 997e1011. [21] Zhang T, Wilkowski G, Rudland D, Brust F, Mehta HS, Sommerville DV. Weldoverlay analyses-an investigation of the effect of weld sequencing. 2008 ASME Pressure Vessels and Piping Division Conference, July 27e31, 2008, Chicago, IL, USA.
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463 [22] Rudland D, Csontos A, Zhang T, Wilkowski G. Welding residual stress solutions for dissimilar metal surge line nozzle welds. J Press Vessel Technol 2010;132. pp. 021208.1e021208.7. [23] Rybicki EF, Shadley JR, Sandhu AS, Stonesifer RB. Experimental and computational residual stress evaluation of a weld clad plate and machined test specimens. J Eng Mater Tech 1988;110(4):297e304. [24] Hieronymus H, Bruce B, Didier L, Carsten O, Chris T, Wimpory Robert C. Enpower e repair welds and residual stresses in clad plates. Proc ASME PVP Conf 2005;6:361e9.
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[25] Jiang W, Gong J, Chen H, Tu ST. The effect of filler metal thickness on residual stress and creep for stainless-steel plateefin structure. Int J Pres Ves Pip 2008;85(8):569e74. [26] Wen-chun J, Jian-ming G, Jian-qun T. Numerical simulation of hydrogen diffusion under welding residual stress. Trans China Weld Inst 2006;27 (11):57e60. [27] Chang PH, Teng TL. Numerical and experimental investigations on the residual stresses of the butt-welded joints. Comp Mater Sci 2004;29(4): 511e22.
Contents lists available at ScienceDirect
International Journal of Pressure Vessels and Piping journal homepage: www.elsevier.com/locate/ijpvp
Numerical simulation to study the effect of repair width on residual stresses of a stainless steel clad plate Wenchun Jiang a, *, Zibai Liu b, J.M. Gong c, S.T. Tu c a
College of Mechanical and Electronic Engineering, China University of Petroleum, Dongying 257061, PR China School of Material Science and Engineering, Jilin University, Jilin 130025, PR China c School of Mechanical and Power Engineering, Nanjing University of Technology, Nanjing 210009, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2010 Received in revised form 31 May 2010 Accepted 17 June 2010
Clad plates are widely used in the construction of corrosion resistant equipment. During the repair of clad plates, residual stresses are generated and influence the structure integrity. This paper uses the finite element method (FEM) to predict the residual stresses in a repair weld of a stainless steel clad plate. The effect of repair width on residual stresses has also been investigated by numerical simulation. Due to the material mismatching between clad metal and base metal, a discontinuous stress distribution has been generated across the interface between clad and base metals. The peak residual stress occurs in the heat affected zone (HAZ) of the base metal, because the yield strength of the base metal is larger than that of the clad metal. With an increase in repair width, the residual stresses are decreased. When the repair width is increased to 24 mm, the residual stresses in the weld have been decreased greatly and the peak residual stresses have been reduced to less than the yield strength. Therefore, the recommended repair width should not be less than 24 mm, which provides a reference for optimizing repair welding technology for this stainless steel clad pate. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Clad plate Repair weld Residual stress Repair width
1. Introduction Clad plate consisting of a thin layer of reactive (or refractory) metal integrally bonded onto a lower cost, thicker base metal, has been widely used in construction of corrosion resistant equipment. During the manufacture or service, cracks or crack-like defects are often formed in clad layer and penetrate into base metal [1]. The best method for repairing the cracked region is to remove and then refill that part by weld layers (repair weld technique). The repair weld of clad plate belongs to dissimilar steel weld, which causes difficulties in quality control [2,3]. For the repair of clad plate, a very important aspect is to control the welding residual stresses. This is because the tensile residual stress has an adverse effect on life [4e6] and corrosion [7,8]. When combined with stress generated by service load, a tensile residual stress reduces crack initiation life, accelerates growth rates of pre-existing or service-induced defects, and increases the susceptibility of structures to catastrophic failure by fracture [9]. Therefore, an accurate knowledge of residual stress in weld repairs is essential for structural integrity assessment [10,11].
* Corresponding author. Tel.: þ86 546 8391776; fax: þ86 546 8393620. E-mail address: [email protected] (W. Jiang). 0308-0161/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2010.06.003
Finite element method (FEM) has been widely used to predict the residual stress in the repair welding [12e16]. From the open literatures, it can be concluded that repair length [17], heat input [18], groove shape and pass number [19] have great effects on residual stress. Dong [13] found weld repairs typically increase the transverse stress compared with the initial weld, and the shorter the repair length the greater the increase in transverse stress. Welding procedure related parameters (pass lumping, heat input and inter-pass temperature) are more important in analyzing weld repairs than in initial fabrication welds [20e22]. Soanes [14] modeled residual stresses at a repair in a steam header to tubeplate, which supports the development and optimization of a successful repair and heat treatment strategy and thus underwrite the safety case for return to service. With respect to the repair weld of clad plate, Motarjemi [1] performed a fracture assessment of weld-repaired clad plate by European flaw assessment procedure. Rybicki [23] investigated the residual stress in a heat treated weld clad plate by FEM and experiment. Hein [24] researched the residual stress in repair welds in a clad plate by FEM and numerical simulation method, which supported the repair weld optimization by minimizing the residual stress. In this paper, a FEM model is developed to estimate the residual stress at repair weld in a stainless steel clad
458
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463 Table 1 Chemical composition of 304 and Q345R.
unit: mm 304
Composition
C
Si
Mn
S
P
Cr
Ni
304 Q345R
0.06 0.15
1.00 0.32
2.0 1.38
0.03 0.014
0.035 0.016
17.0e19.0 e
8.0e11.0 e
400
600
800
166 108 0.28 7840 18.2
150 82 0.28 7755 18.6
125 69 0.28 7667 19.5
20.2
22.8
25.4
Y Q345R
X(S11) Z(S33)
Table 2 Material properties for 304 and Q345R steel. Temperature( C)
Fig. 1. The geometrical model of repair weld in a clad plate.
plate. The effect of repair width on residual stress has been studied, which provides a reference for optimizing the repair welding technology. 2. FE analysis of repair welding residual stress A sequential coupling FE program is developed to calculate residual stress by general purpose FE software code ABAQUS. Firstly, a thermal analysis is carried out to determine temperature history field, and then the temperature results are applied incrementally to mechanical model to simulate residual stress. 2.1. Geometrical model and meshing Fig. 1 shows the geometrical model of the repair weld joint in a clad plate. The flaws generated on clad layer are removed and then overlay welded. Clad thickness and base metal thickness are 3 and 17 mm, respectively. The groove angle is 30 and the repair width is 8 mm. A 2-D plane strain FE model is built and the FE meshing is shown in Fig. 2. In total, 2144 nodes and 2022 elements are meshed.
20
200
a. Material properties for 304 stainless steel Young’s Modulus (GPa) 199 180 Yield strength (MPa) 206 153 Poisson’s ratio 0.28 0.28 3 8010 7931 Density (kg/m ) 16.0 17.2 Thermal expansion (1/ C 106) Thermal conductivity 15.26 17.6 (W/m C) Specific Heat (J/kg C) 500 544.3 b. Material properties for Q345R steel Young’s Modulus (GPa) 200 183 Yield strength (MPa) 345 310 Poisson’s ratio 0.3 0.3 7850 7840 Density (kg/m3) 14.0 14.2 Thermal expansion (1/ C 106) Thermal conductivity 53.17 47.73 (W/m C) Specific Heat (J/kg C) 461 523
582
634
686
160 280 0.3 7830 16.0
150 210 0.3 7820 16.6
125 160 0.3 7810 18
39.57 607
36.01 678
33 700
and mechanical properties are incorporated. The temperature dependency of physical and mechanical properties of 304 [25] and Q345 [26] are listed in Table 2. 2.3. Welding temperature analysis In thermal analysis, the welding process is primarily simulated by applying a distributed heat flux to weld elements. The distributed heat flux, DFLUX, is calculated by
2.2. Material properties The material of base metal is Q345R, which is a Chinese grade and meets the requirements of strength and stiffness. The clad metal is 304 stainless steel, which is used for corrosion resistance. The weld metal is assumed to have the same material properties as 304. Their chemical compositions are listed in Table 1. For thermal and mechanical analyses, temperature-dependent thermo-physical
DFLUX ¼
U$I$h V
(1)
where U is the voltage, I is current, h is the arc efficiency (0.6) and V is the weld pass volume. The net line energy is calculated by,
Q ¼
U$I$h v
(2)
where v is the welding speed. From Eqs. (1) and (2), the distributed heat flux can be calculate by,
DFLUX ¼
Q $v V
(3)
The voltage, current and speed used in the calculation are 24 V, 150I and 2 mm/s. The simulation of weld metal deposition is achieved by Element Birth and Death. Before welding, the weld metal elements are removed. Once the welding starts, the welded pass is added and heated, and then it is cooled down until the next pass cycle begins. The temperature history of all nodes is stored in a file for the subsequent residual stress calculation. The material properties relevant to thermal analysis are density, specific heat capacity, latent heat capacity, and solidus/liquidus temperatures. 2.4. Residual stress analysis
Fig. 2. FE meshing.
The residual stress is calculated by using the temperature distribution obtained from thermal analysis as input data. Birth and
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463
Death technology used in section 2.3 is also used in stress analysis. The material properties relevant to residual stress are elastic modulus, yield stress, Poisson’s ratio, and coefficient of thermal expansion. For 304 and Q345R steel, solid-state phase transformation does not occur. Therefore, the total strain rate can be decomposed into three components as follows:
3 ¼ 3e þ 3p þ 3ts
(4)
where 3 , 3 and 3 stand for elastic strain, plastic strain and thermal strain, respectively. Elastic strain is modeled using the isotropic Hooke’s law with temperature-dependent Young’s modulus and Poisson’s ratio. The thermal strain is calculated using temperaturedependent coefficient of thermal expansion. For the plastic strain, a rate-independent plastic model is employed with a von Mises yield surface, temperature-dependent mechanical properties and linear kinematic hardening model. Kinematic hardening is considered because material points undergo both loading and unloading in the welding process. The same FE mesh used in the temperature simulation is applied in residual stress analysis. e
p
ts
459
calculated the residual stresses in the butt-welded joints according to the same parameters in Ref [27]. Fig. 4 presents a comparison of the residual stresses computed by us and Chang. It is shown that our results are consistent with Chang’s experiment and FE results [27]. Therefore, the FE program developed here is suitable to be used for residual stress analysis in the repair weld of a clad plate. 4. Results and discussion 4.1. Residual stress distribution contour Fig. 5 shows the contours of transverse (S11) and longitudinal stress (S33). The peak S11 is 202 MPa, which is near the yield strength and shown in heat affected zone (HAZ) of clad metal. The peak S33 is 384 MPa, which is shown at base metal. The yield strength of base metal is larger than that of clad metal; therefore the peak stress is shown at the side of base metal.
2.5. Boundary and initial conditions The initial ambient temperature is 20 C. During thermal analysis, convection and radiation are both taken into consideration. During stress analysis, nodes of A and B at the end of bottom surface shown in Fig. 2 are constrained in X and Y-direction. Thus, the rigid body motion is avoided. 3. Verification of the FE simulation method A butt-welded joints the same as Chang’s [27], is developed to verify the present FE method as shown in Fig. 3. Chang measured the residual stresses along the surface by X-ray method. Here we
unit: mm
Fig. 5. The contours of transverse (a) and longitudinal stress (b).
Y X
Z
In order to discuss the results fully, five reference paths named P1, P2, P3, P4 and P5 have been selected to investigate the stress distribution, as shown in Fig. 2. P1 is the center line of the repair weld. P2 and P3 are located in the right and left of HAZ, respectively. P4 is along the HAZ beneath the weld. P5 is on the surface of the weld.
Fig. 3. A butt-weld joint model to verify the FE method.
b
60
400
FEM by us [27] Experiment by Chang
40
Longitudinal stress (MPa)
Transverse stress (MPa)
a
20
0
-20
0
10
20
30 X (mm)
40
50
FEM by us [27] Experiment by Chang
300 200 100 0 -100 -200 0
10
20
30 X (mm)
Fig. 4. A comparison of the transverse (a) and longitudinal residual stress (b) by us and Chang [27].
40
50
460
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250
400
S11 S33
200
Residual stress (MPa)
Residual stress (MPa)
350
150
100
300 250
S11 S33
200 150 100 50
50 0
1
2
3
4
0
2
4
Distance (mm)
6
8
Distance (mm)
Fig. 6. Residual stress distribution along P1.
Fig. 8. Residual Stress distribution along P4.
4.4. Effect of repair width on residual stress 4.2. Residual stress distribution in weld metal Fig. 6 shows the residual stress distribution along P1 in the weld center. Along the weld thickness, the transverse stress shows a gradual increase while longitudinal stress remains almost unchanged. The longitudinal stress in weld metal is 215 MPa, which has reached the yield strength of clad metal. 4.3. Residual stress distribution in HAZ Fig. 7 shows the residual stress distribution along P2 and P3 in HAZ. It is obviously observed that the stresses in the right and left HAZ are different. This is because the heat input is not symmetric along the weld center line P1. The subsequent heating of weld pass has effect on the previous pass, which leads to an asymmetric stress distribution. Fig. 8 shows the residual stress distribution along P4. Along the HAZ beneath the weld metal, the stresses in the right are larger than those in the left. S11 and S33 are about 55 MPa and 290 MPa in the left, respectively, but they jump to 90 MPa and 350 MPa in the right.
Keeping the rest of the welding parameters constant, five models with a repair width of 8.0, 12.0, 16.0, 20.0 and 24.0 mm were developed to discuss the influence of repair width. 4.4.1. Effect of repair width on peak stresses Fig. 9 shows the effect of repair width on peak residual stresses. It is shown that the peak stresses are decreased with repair width increase. When repair width increases from 8 to 24 mm, both the transverse and longitudinal stresses are decreased about 23%. 4.4.2. Effect of repair width on residual stress in the weld metal Fig. 10 shows the transverse and longitudinal stress with different repair length, with distance along the weld surface (P5), normalized by their length. It is shown that the stresses are decreased with repair width increase. When the repair width is 8 mm, the maximum S11 is 185 MPa, and S33 along the surface is about 210 MPa. When the repair width increases to 24 mm, the average of transverse stress in 70% of the width is about 120 MPa while stress in the rest 30% is very small. S33 in the half length has been less than 100 MPa and the maximum has been decreased to
400
360
Residual stress (MPa)
300
Peak residual stress (MPa)
S11 along P2 S11 along P3 S33 along P2 S33 along P3
240
180
120
60
350
300
S11 S33
250
200
150 0
1
2
3
Distance (mm) Fig. 7. Residual stress distribution along P2 and P3.
4
6
8
10
12
14
16
18
20
Repair width (mm) Fig. 9. Effect of repair width on peak stresses.
22
24
200 8mm 12mm 16mm 20mm 24mm
150
100
50
0.0
0.2
0.4
0.6
0.8
Longitudinal stress (MPa)
140
120
8mm 12mm 16mm 20mm 24mm
80
60
0
1
120 8mm 12mm 16mm 20mm 24mm
80 40 0 -40
360 320
0.0
0.2
0.4
0.6
0.8
3
4
8mm 12mm 16mm 20mm 24mm
280 240 200 160
-80
2 Distance (mm)
b
200 160
461
100
1.0
Normalized distance
b
a
Longitudinal stress (MPa)
Transverse stress (MPa)
a
Transverse stress (MPa)
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463
0
1
1.0
2
3
4
Distance (mm)
Normalized distance
Fig. 11. Effect of repair width on residual stress along P2.
Fig. 10. Effect of repair width on residual stress along P5.
165 MPa. Based on this, it can be shown that the repair width on residual stress is notable. 4.4.3. Effect of repair width on residual stress in HAZ Fig. 11 shows the effect of repair width on residual stress along path P2. It is shown that the stresses are decreased with an increase
b
200 8mm 12mm 16mm 20mm 24mm
160 120 80 40 0
0
1
2
3
4
350
8mm 12mm 16mm 20mm 24mm
280 Longitudinal stress (MPa)
Transverse stress (MPa)
a
in repair width. When the repair width is increased from 8 to 24 mm, the transverse stress has been less than 100 MPa and the longitudinal stress has been decreased about 20%. Fig.12 shows the effect of repair width on residual stress along P3. It can be observed that the stresses are greatly decreased when the repair width increase from 8 to 24 mm. When the repair width is
210 140 70 0 -70 -140
0
Distance (mm) Fig. 12. Effect of repair width on residual stress along P3.
1
2 Distance (mm)
3
4
462
Transverse stress (MPa)
a
W. Jiang et al. / International Journal of Pressure Vessels and Piping 87 (2010) 457e463
60
(1) During the repair weld of clad plate, the peak residual stresses are shown in HAZ of base metal with larger yield strength. The mismatching of yield strength between clad metal and base metal causes an unbalance stress distribution. (2) The repair width has great effect on residual stresses. With repair width increase, the residual stresses are decreased. In order to decrease the residual stress, the recommended repair width should not be less than 24 mm.
40
Acknowledgments
20
The authors gratefully acknowledge the support provided by Research Fund of Jiangsu Key Laboratory of Digital Manufacture for Industry Equipment and Control Technology (2010) and Young Teachers Fund of China University of Petroleum (2009).
100 8mm 12mm 16mm 20mm 24mm
80
0.0
Longitudinal stress (MPa)
b
0.2
0.4
0.6
0.8
1.0
Normalized Distance
References
350 300 250 200 8mm 12mm 16mm 20mm 24mm
150 100 50 0
0.0
0.2
0.4
0.6
0.8
1.0
Normalized distance Fig. 13. Effect of repair width on residual stress along P4.
24 mm, 80% of the transverse stress has been less than 80 MPa. The longitudinal stress in clad metal has been reduced to compressive stress, and the longitudinal stress in base metal is only about 50 MPa. Fig. 13 shows the effect of repair width on residual stress along path P4, with distance normalized by their length. It can be found that the stresses are also greatly decreased when the repair width increases to 24 mm. When the repair length is 24 mm, all the transverse stress has been less than 50 MPa, and half of the longitudinal stress has been less than 100 MPa. Based on the above analysis, it is found that the residual stresses are decreased with the repair width increasing. When the repair width is increased, the weld passes are increased. And the corresponding increase of heat input makes the cooling time increase, which leads to a decrease in residual stress. When the repair width is increased to 24 mm, the residual stress has been decreased greatly, and the peak residual stress has been reduced to below the yield strength. Therefore, it is proposed that the repair width is best not less than 24 mm. 5. Conclusion This study developed a sequential coupling finite element procedure to predict residual stresses in repair weld for a stainless steel clad plate. And the effect of repair width on residual stress has been discussed. Based on the obtained results, the following conclusions may be drawn:
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