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Verification of residual stresses in flash-butt-weld rails using neutron diffraction
ARTICLE IN PRESS
Physica B 385–386 (2006) 894–896 www.elsevier.com/locate/physb
Verification of residual stresses in flash-butt-weld rails using neutron diffraction David Tawfika,, Oliver Kirsteinb, Peter John Muttonc, Wing Kong Chiua a
Mechanical Engineering Department, Monash University, Melbourne, Australia Bragg Institute, Australian Nuclear Science Technology Organisation, Sydney, Australia c Institute of Railway Technology, Monash University, Melbourne, Australia
b
Abstract Residual stresses developed during flash-butt welding may play a crucial role in prolonging the fatigue life of the welded tracks under service loading conditions. The finished welds typically exhibit high levels of tensile residual stresses in the web region of the weld. Moreover, the surface condition of the web may contain shear drag or other defects resulting from the shearing process which may lead to the initiation and propagation of fatigue cracks in a horizontal split web failure mode under high axle loads. However, a comprehensive understanding into the residual stress behaviour throughout the complex weld geometry remains unclear and is considered necessary to establish the correct localised post-weld heat treatment modifications intended to lower tensile residual stresses. This investigation used the neutron diffraction technique to analyse residual stresses in an AS60 flash-butt-welded rail cooled under normal operating conditions. The findings will ultimately contribute to developing modifications to the flash-butt-welding procedure to lower tensile residual stresses which may then improve rail performance under high axle load. r 2006 Elsevier B.V. All rights reserved. PACS: 81.40.Jj Keywords: Residual stresses; Flash butt; Neutron diffraction
1. Introduction Flash-butt welds produced using both stationary (fixed) and mobile welders have been reported to incur high vertical tensile residual stress levels in the web region of the weld [1–4]. The resistance welding process involves preheating the rail ends, flashing and finally forging the ends through a large upset force. The magnitudes of these stresses, hardness distribution and HAZ across the weld (which in turn may influence the magnitude of impact loading) are influenced by a range of welding parameters, including preheating conditions, the magnitude of the final upset force, post-weld cooling conditions, and the characteristics of the rail material. The presence of any imperfections in the weld, such as flow lips or shear Corresponding author. Tel.: +6 139 905 1089; fax: +6 139 905 3122.
E-mail address: david.tawfi[email protected] (D. Tawfik). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.242
drag on the external surfaces, may initiate fatigue cracks such as the horizontal split web failure mode, which occurs during torsional loading of the rail head under high axle loads. Techniques such as hole drilling [1], trepanning [4] and numerical modelling [1,4] have been used to predict residual stresses in flash-butt weld. However, non-destructive techniques such as neutron diffraction [5] are still required to verify these stresses in the interior of the component. This paper investigates the distribution of residual stresses across an AS60 mobile flash-butt-welded rail using neutron diffraction. The aim of these measurements was to confirm the distribution of residual stresses as predicted by the numerical method, which could subsequently be used to establish the optimum localised rapid post weld heat treatment to alleviate tensile residual stresses and hence improve rail performance under high axle load conditions.
ARTICLE IN PRESS D. Tawfik et al. / Physica B 385–386 (2006) 894–896
895
2. Experimental procedure 2.1. Sample preparation The chemical composition of the AS60 rail steel [reference AS1085.1] used throughout the experimental program is presented in Table 1. The flash-butt weld was produced by a Plasser mobile welder operating under normal conditions. The thermomechanical history of a similar weld during post-weld cooling was previously shown both experimentally and numerically (3D FE modelling) [4]. A finished weld approximately 650 mm long was extracted from the track using an abrasive rail saw. A 630 170 11 mm plate section was then extracted (using cold saw cuts) along the centre line of the welded rail for neutron strain scanning (Fig. 1). The sectioning is expected to have altered the residual stress distribution from that present in the complete weld. Previous research [6] has shown that stresses in the vertical (Z) and longitudinal (Y) directions will remain largely unchanged, but stresses in the transverse (X) would be substantially relaxed. The plate was ground, and etched with 2% nital to reveal the location of the fusion line and the extent of the heat-affected zone (HAZ) (Fig. 2.) The full-width of the HAZ extended to approximately 50 mm in the mid-web region, while the head and foot regions showed a narrower HAZ width. This is typical of a mobile flash-butt-welded rail. A narrower and more uniform HAZ throughout the height of rail is typical for a stationary flash-butt-welded rail. This is due to differences in the electrode contact position (i.e. top and bottom surfaces, compared to both
Table 1 Chemical composition of rail (wt%) C
Mn
Si
S
P
Cr
Al
0.8
0.87
0.19
0.014
0.017
0.02
0.001
Fig. 1. Plate section for neutron scanning extracted from a normal cooled AS60 flash-butt-welded rail.
Fig. 2. Macroetched AS60 rail flash-butt weld.
sides of the rail web in mobile flash-butt-welding machines) and preheating conditions which result in a steeper thermal gradient from the rail end. 2.2. Neutron diffraction measurements Residual stress measurements were made using the Triple Axis Neutron Spectrometer located at ANSTO, Australia. A neutron wavelength of 1.4 A˚ using the (2 1 1) lattice reflection and the detector angle (2y) was set to approximately 741. The sampling gauge volume was set to 3 3 3 mm3. Measurements were taken both longitudinally (Y) across the weld at heights of 16, 79 and 126 mm above the rail foot and vertically (Z) along the fusion line. The number of neutron counts was set to 50,000 requiring approximately 45 min to ensure the intensity peaks had clearly been defined. Strain measurements were made in the three principal directions for each point in order to fully define the stress tensor given by Generalised Hooke’s Law: i n E h si ¼ i þ (1) ðx þ y þ z Þ , ð1 þ nÞ 1 2n where i ¼ [x, y, z] stands for the principal components and E (206 GPa) and n (0.33) are the elastic constants of the material. The stress free lattice spacing were measured along a comb section extracted from the head of the weld and prepared by EDM wire cutting (Fig. 3). The blocks were 4 mm wide by 4 mm deep separated by 0.5 mm slits. The measurements using a (2 1 1) lattice reflection in each principal direction (z, y, x) at the same relative point across the stress free comb section are expected to be similar. This was confirmed with two-directional measurements taken in
ARTICLE IN PRESS D. Tawfik et al. / Physica B 385–386 (2006) 894–896
896
180 160
Measured (ND) Vertical (Z) stress (weld cut out) Measured (ND) Longitudinal (Y) stress (weld cut out) Measured (ND) Transverse (X) stress(weld cut out)
Rail height (mm)
140
Fig. 3. Stress free lattice comb section.
Vertical (Z) stress (MPa)
300 250 200
80 60 40
height (79 mm)
20 0 -350 -300 -250 -200 -150 -100 -50
100
0
50 100 150 200 250 300
Stress (MPa)
50
Fig. 5. Vertical (Z), longitudinal (Y) and transverse (X) stress along the fusion line.
0 -50 -100 -150 0
10
20
30
40
50
60
70
80
90
foot fusion regions exhibited compressive longitudinal stresses. Transverse stresses along the weld were close to zero.
Distance from the fusion line (mm)
(a)
Longitudinal (Y) stress (MPa)
100
height (126mm) height (16 mm)
150
120
250 200 150 100 50 0 -50 -100 -150 -200 -250 -300
height (126mm) height (79mm) height (16 mm)
0 (b)
10
20
30
40
50
60
70
80
90
Distance the fusion line (mm)
Fig. 4. (a) Vertical and (b) longitudinal residual stresses at heights of 16, 79 and 126 mm above the rail foot.
4. Conclusions The neutron diffraction technique was utilised to measure the distribution of residual stresses in a plate specimen prepared from a normal cooled AS60 mobile flash-butt-welded rail. Peak tensile vertical and longitudinal stresses were measured close to the mid-section of the web region of the weld. Compressive longitudinal stresses were evident close to the head and foot regions. The magnitude of the vertical and longitudinal stresses appears to have been altered significantly as a result of sectioning the full weld to produce the plate specimen. These effects will need to be confirmed with FE modelling. The effect of welding parameters on the residual stress distribution will also need to be examined.
the vertical (Z) and longitudinal (Y) directions. The average stress free spacing was taken as reference.
Acknowledgements
3. Results
The authors would like to thank the financial assistance of AINSE and John Holland for supplying and fabricating the flash-butt-welded rails.
Fig. 4(a) and (b) shows the major vertical and longitudinal residual stresses, respectively, across the welded plate section at heights of 16, 79 and 126 mm above the rail foot. The results indicated that the tensile vertical stresses in the mid web region are slightly higher than the longitudinal stresses at the same position. Moreover, the magnitude of both these stresses in the mid-web region are lower than reported previously [1–4]. Low (tensile) vertical and (compressive) longitudinal stress levels were measured at 16 mm above the foot. Fig. 5 shows the vertical, longitudinal and transverse stresses along the fusion line. Tensile vertical and longitudinal residual stress were found to be concentrated predominately in the mid-web region of the weld. The head and
References [1] B.L. Skyttebol, Josefson, Proceedings of the Seventh International Seminar on Numerical Analysis of Weldability, 2003, pp. 21. [2] P.J. Mutton, Conference failure analysis in material engineering, Instit. Met. Mater. Australasia (1994). [3] H. Mansouri, A. Monshi, H. Hadavinia, J. Strain Anal. 39 (3) (2004) 271. [4] D. Tawfik, P.J. Mutton, W.K. Chiu, Proceedings of the Eighth International Conference on Heavy Haul, 2005, pp. 653–658. [5] A.N. Ezellio, G.A. Webster, P.J. Webster, Annual Report, The Institute Laue, Langevin, UK, 1996, p. 10. [6] P.J. Webster, G. Mills, X.D. Wang, W.P. Kang, T.M. Holden, J. Strain Anal. 32 (1997) 389.
Physica B 385–386 (2006) 894–896 www.elsevier.com/locate/physb
Verification of residual stresses in flash-butt-weld rails using neutron diffraction David Tawfika,, Oliver Kirsteinb, Peter John Muttonc, Wing Kong Chiua a
Mechanical Engineering Department, Monash University, Melbourne, Australia Bragg Institute, Australian Nuclear Science Technology Organisation, Sydney, Australia c Institute of Railway Technology, Monash University, Melbourne, Australia
b
Abstract Residual stresses developed during flash-butt welding may play a crucial role in prolonging the fatigue life of the welded tracks under service loading conditions. The finished welds typically exhibit high levels of tensile residual stresses in the web region of the weld. Moreover, the surface condition of the web may contain shear drag or other defects resulting from the shearing process which may lead to the initiation and propagation of fatigue cracks in a horizontal split web failure mode under high axle loads. However, a comprehensive understanding into the residual stress behaviour throughout the complex weld geometry remains unclear and is considered necessary to establish the correct localised post-weld heat treatment modifications intended to lower tensile residual stresses. This investigation used the neutron diffraction technique to analyse residual stresses in an AS60 flash-butt-welded rail cooled under normal operating conditions. The findings will ultimately contribute to developing modifications to the flash-butt-welding procedure to lower tensile residual stresses which may then improve rail performance under high axle load. r 2006 Elsevier B.V. All rights reserved. PACS: 81.40.Jj Keywords: Residual stresses; Flash butt; Neutron diffraction
1. Introduction Flash-butt welds produced using both stationary (fixed) and mobile welders have been reported to incur high vertical tensile residual stress levels in the web region of the weld [1–4]. The resistance welding process involves preheating the rail ends, flashing and finally forging the ends through a large upset force. The magnitudes of these stresses, hardness distribution and HAZ across the weld (which in turn may influence the magnitude of impact loading) are influenced by a range of welding parameters, including preheating conditions, the magnitude of the final upset force, post-weld cooling conditions, and the characteristics of the rail material. The presence of any imperfections in the weld, such as flow lips or shear Corresponding author. Tel.: +6 139 905 1089; fax: +6 139 905 3122.
E-mail address: david.tawfi[email protected] (D. Tawfik). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.242
drag on the external surfaces, may initiate fatigue cracks such as the horizontal split web failure mode, which occurs during torsional loading of the rail head under high axle loads. Techniques such as hole drilling [1], trepanning [4] and numerical modelling [1,4] have been used to predict residual stresses in flash-butt weld. However, non-destructive techniques such as neutron diffraction [5] are still required to verify these stresses in the interior of the component. This paper investigates the distribution of residual stresses across an AS60 mobile flash-butt-welded rail using neutron diffraction. The aim of these measurements was to confirm the distribution of residual stresses as predicted by the numerical method, which could subsequently be used to establish the optimum localised rapid post weld heat treatment to alleviate tensile residual stresses and hence improve rail performance under high axle load conditions.
ARTICLE IN PRESS D. Tawfik et al. / Physica B 385–386 (2006) 894–896
895
2. Experimental procedure 2.1. Sample preparation The chemical composition of the AS60 rail steel [reference AS1085.1] used throughout the experimental program is presented in Table 1. The flash-butt weld was produced by a Plasser mobile welder operating under normal conditions. The thermomechanical history of a similar weld during post-weld cooling was previously shown both experimentally and numerically (3D FE modelling) [4]. A finished weld approximately 650 mm long was extracted from the track using an abrasive rail saw. A 630 170 11 mm plate section was then extracted (using cold saw cuts) along the centre line of the welded rail for neutron strain scanning (Fig. 1). The sectioning is expected to have altered the residual stress distribution from that present in the complete weld. Previous research [6] has shown that stresses in the vertical (Z) and longitudinal (Y) directions will remain largely unchanged, but stresses in the transverse (X) would be substantially relaxed. The plate was ground, and etched with 2% nital to reveal the location of the fusion line and the extent of the heat-affected zone (HAZ) (Fig. 2.) The full-width of the HAZ extended to approximately 50 mm in the mid-web region, while the head and foot regions showed a narrower HAZ width. This is typical of a mobile flash-butt-welded rail. A narrower and more uniform HAZ throughout the height of rail is typical for a stationary flash-butt-welded rail. This is due to differences in the electrode contact position (i.e. top and bottom surfaces, compared to both
Table 1 Chemical composition of rail (wt%) C
Mn
Si
S
P
Cr
Al
0.8
0.87
0.19
0.014
0.017
0.02
0.001
Fig. 1. Plate section for neutron scanning extracted from a normal cooled AS60 flash-butt-welded rail.
Fig. 2. Macroetched AS60 rail flash-butt weld.
sides of the rail web in mobile flash-butt-welding machines) and preheating conditions which result in a steeper thermal gradient from the rail end. 2.2. Neutron diffraction measurements Residual stress measurements were made using the Triple Axis Neutron Spectrometer located at ANSTO, Australia. A neutron wavelength of 1.4 A˚ using the (2 1 1) lattice reflection and the detector angle (2y) was set to approximately 741. The sampling gauge volume was set to 3 3 3 mm3. Measurements were taken both longitudinally (Y) across the weld at heights of 16, 79 and 126 mm above the rail foot and vertically (Z) along the fusion line. The number of neutron counts was set to 50,000 requiring approximately 45 min to ensure the intensity peaks had clearly been defined. Strain measurements were made in the three principal directions for each point in order to fully define the stress tensor given by Generalised Hooke’s Law: i n E h si ¼ i þ (1) ðx þ y þ z Þ , ð1 þ nÞ 1 2n where i ¼ [x, y, z] stands for the principal components and E (206 GPa) and n (0.33) are the elastic constants of the material. The stress free lattice spacing were measured along a comb section extracted from the head of the weld and prepared by EDM wire cutting (Fig. 3). The blocks were 4 mm wide by 4 mm deep separated by 0.5 mm slits. The measurements using a (2 1 1) lattice reflection in each principal direction (z, y, x) at the same relative point across the stress free comb section are expected to be similar. This was confirmed with two-directional measurements taken in
ARTICLE IN PRESS D. Tawfik et al. / Physica B 385–386 (2006) 894–896
896
180 160
Measured (ND) Vertical (Z) stress (weld cut out) Measured (ND) Longitudinal (Y) stress (weld cut out) Measured (ND) Transverse (X) stress(weld cut out)
Rail height (mm)
140
Fig. 3. Stress free lattice comb section.
Vertical (Z) stress (MPa)
300 250 200
80 60 40
height (79 mm)
20 0 -350 -300 -250 -200 -150 -100 -50
100
0
50 100 150 200 250 300
Stress (MPa)
50
Fig. 5. Vertical (Z), longitudinal (Y) and transverse (X) stress along the fusion line.
0 -50 -100 -150 0
10
20
30
40
50
60
70
80
90
foot fusion regions exhibited compressive longitudinal stresses. Transverse stresses along the weld were close to zero.
Distance from the fusion line (mm)
(a)
Longitudinal (Y) stress (MPa)
100
height (126mm) height (16 mm)
150
120
250 200 150 100 50 0 -50 -100 -150 -200 -250 -300
height (126mm) height (79mm) height (16 mm)
0 (b)
10
20
30
40
50
60
70
80
90
Distance the fusion line (mm)
Fig. 4. (a) Vertical and (b) longitudinal residual stresses at heights of 16, 79 and 126 mm above the rail foot.
4. Conclusions The neutron diffraction technique was utilised to measure the distribution of residual stresses in a plate specimen prepared from a normal cooled AS60 mobile flash-butt-welded rail. Peak tensile vertical and longitudinal stresses were measured close to the mid-section of the web region of the weld. Compressive longitudinal stresses were evident close to the head and foot regions. The magnitude of the vertical and longitudinal stresses appears to have been altered significantly as a result of sectioning the full weld to produce the plate specimen. These effects will need to be confirmed with FE modelling. The effect of welding parameters on the residual stress distribution will also need to be examined.
the vertical (Z) and longitudinal (Y) directions. The average stress free spacing was taken as reference.
Acknowledgements
3. Results
The authors would like to thank the financial assistance of AINSE and John Holland for supplying and fabricating the flash-butt-welded rails.
Fig. 4(a) and (b) shows the major vertical and longitudinal residual stresses, respectively, across the welded plate section at heights of 16, 79 and 126 mm above the rail foot. The results indicated that the tensile vertical stresses in the mid web region are slightly higher than the longitudinal stresses at the same position. Moreover, the magnitude of both these stresses in the mid-web region are lower than reported previously [1–4]. Low (tensile) vertical and (compressive) longitudinal stress levels were measured at 16 mm above the foot. Fig. 5 shows the vertical, longitudinal and transverse stresses along the fusion line. Tensile vertical and longitudinal residual stress were found to be concentrated predominately in the mid-web region of the weld. The head and
References [1] B.L. Skyttebol, Josefson, Proceedings of the Seventh International Seminar on Numerical Analysis of Weldability, 2003, pp. 21. [2] P.J. Mutton, Conference failure analysis in material engineering, Instit. Met. Mater. Australasia (1994). [3] H. Mansouri, A. Monshi, H. Hadavinia, J. Strain Anal. 39 (3) (2004) 271. [4] D. Tawfik, P.J. Mutton, W.K. Chiu, Proceedings of the Eighth International Conference on Heavy Haul, 2005, pp. 653–658. [5] A.N. Ezellio, G.A. Webster, P.J. Webster, Annual Report, The Institute Laue, Langevin, UK, 1996, p. 10. [6] P.J. Webster, G. Mills, X.D. Wang, W.P. Kang, T.M. Holden, J. Strain Anal. 32 (1997) 389.