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Comparison of fracture properties of steel pipes
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 12 (2019) 521–528
www.materialstoday.com/proceedings
DAS35
Comparison of fracture properties of steel pipes Ľubomír Gajdoš*, Martin Šperl a
Institute of Theoretical and Applied Mechanics, Academy of Sciences of the Czech Republic,Prosecká 76, 190 00 Prague 9, Czech Republic, e-mail: [email protected]
Abstract
An investigation was made into mechanical and fracture – mechanical properties of seamless pipe Ø 525/8.5 mm from CSN 413126 steel and axially welded pipe Ø 508/6.3 mm from L360NE steel. The results pointed on the dominance of the welded pipe in basic tensile properties, notch toughness and fracture toughness. Prevailing properties of the welded pipe are due to extending the microalloying of the steel by V, Nb, and Ti with a lower content of S and P and also by a higher degree of working of the material during rolling sheet before winding the pipe. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of 35th Danubia Adria Symposium on Advances in Experimental Mechanics. Keywords: Seamless and welded pipes; L360NE steel; impact tests; fracture toughness; Paris law; pipe integrity
1. Introduction Hot rolled seamless pipes are usually aimed for small pipe diameters (less than 500 mm) and they are supplied in the condition after cooling down from the finish rolling temperature (above Ac3 - which is about 850°C for the steels investigated). This condition is admitted as an equivalent to normalization. The starting materials for manufacture of welded pipes with longitudinal or helical welds are strips or sheets of steel, usually in the condition after cooling down from the finish rolling temperature (above Ac3 ) or after heat treatment. In this case the material is better worked than when manufacturing seamless pipes so that mechanical and fracture mechanical properties are also better. In order to assess quantitatively the effect of basic technology of pipe manufacture on pipe integrity * Corresponding author. Tel.: 00420222363095; fax: 00420286884634 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of 35th Danubia Adria Symposium on Advances in Experimental Mechanics.
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L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
mechanical and fracture mechanical tests were carried out on specimens taken from two different pipes: (i) seamless pipe Ø 525/8.5 mm made from steel ČSN 413126 and (ii) longitudinally welded pipe Ø 508/6.3 mm made from steel L360NE. While seamless pipe (VTŽ Chomutov) was exploitated since the seventieths of the last century the welded pipe (Mannesmann) was not exploitated at all and it has been manufactured according to present technological standards. As for chemical composition and mechanical properties, the two materials are similar to one another except for their yield stress. Therefore it is not surprising that L360NE steel is the equivalent of CSN 413126 steel, and vice versa. In reference [1] it is L360NB steel that is referred to and not L360NE steel. The linepipe steels in [1] are referred to according to the EN 10208 standard [2]. However, L360NE steel is referred to in [3] according to the EN ISO 3183-2 standard. 2. Comparison of properties of the steels 2.1 Tensile properties Stress – strain properties were obtained on flat specimens in the hoop direction. Before machining of the specimens the curved segments were press straightened. The results of tensile tests are shown in Fig. 1.
600
sigma (MPa)
500 400 300 200 100 0 0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,1
epsilon (mm/mm) seamless pipe
welded pipe
Fig. 1. Stress – strain diagrams for hoop direction
Strength parameters for steel 13126 are less than for steel L360NE but, however, the gradient of the hardening above the yield stress is higher, so that for reaching a certain strain in the elasto-plastic region a higher stress is required than for steel L360NE. An approximate measure of plastic strain energy in elasto-plastic region is the yield stress to ultimate strength ratio, i.e. Rp0.2/Rm. The lower this ratio the higher is the plastic strain stored energy in elasto-plastic region. For the specific specimen from the seamless pipe this ratio is 0.59 whilst for the specimen from the welded pipe this ratio is 0.72. This clearly demonstrates a higher plastic strain stored energy in the specimen from the seamless pipe. 2.2 Notch impact properties Impact notch toughness was measured on pendulum impact testing machine 300 J using Charpy specimens with the thickness ~ 5.3 mm for the seamless pipe and with thickness ~ 4.1 mm for the welded pipe. Notch toughness values were calculated from the impact energy. Higher magnitudes of KCV were found for L360NE steel (124.5 J/cm2) in contrast to 101.3 J/cm2 for 13126 steel. The significance of the notch toughness KCV consists in principle
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in two aspects: a) although the notch toughness impact test is a dynamic test its results can be correlated with fracture toughness in terms of the stress intensity factor Kc which is a static characteristics. This correlation is possible, providing the results of the notch toughness are concerned with the upper plateau of the transition curve [4], b) the results of notch toughness tests on Charpy specimens with a 2/3 thickness make it possible to qualify whether a running crack in the wall of a pressure pipeline would stop in a short distance (length of a single pipe) or whether it will continue to run in the pipeline wall to a long distance. The appropriate empirical relations are digestedly shown in [5]. 2.3 Fracture properties Fracture properties of material are best characterized by fracture toughness. This quantity was investigated for both pipes on the basis of the so called J-based R curve. It is a common procedure and is a subject of several international standards, e.g. [6] or [7]. Compact tension (CT) specimens were used in fracture tests. After cycling CT specimens to obtain a sharp fatigue crack all CT specimens were monotonically loaded at room temperature to various magnitudes of the force-point displacement with a simultaneous record of the „force (F) – force-point displacement (f) “. Areas under F – f curves (Apl) were used to determine the plastic component Jpl of the J integral according to relation (1).
J pl
Apl
(1)
Bw a
Elastic component of the J integral is
J el
K 2 1 2 E
(2)
The total magnitude of J integral is then J J el J pl
(3)
Parameter η in Eq. (1) is given by expression
2 0,522w a / w In situations when loading of a CT specimen continued to the descending part of the F – f curve an area Apl,m was also determined. This area is limited from the right by a parallel to the initial elastic straight line crossing the F – f curve in the point of the maximum force. This enabled the J integral Jm to be determined. After unloading the specimens they were placed at the furnace heated to 200°C for 2 hours to obtain an oxidation colouring of the faces of the crack. After that the specimens were cooled down to the temperature of a liquid nitrogen and subsequently they were broken open. Crack extensions Δa which occurred during monotonic loading of CT specimens were then determined from microscopic photographs of fracture surfaces. For illustration, a fracture surface of a specific CT specimen from the seamless pipe and a fracture surface of another CT specimen from the welded pipe are represented on Fig. 2 and Fig. 3. Three different areas can be seen at the fracture surfaces (from the bottom to the top): the fatigue area, the crack extension area, and the final rupture area.
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Fig. 2. Fracture surface of CT specimen
Fig. 3. Fracture surface of CT specimen
(seamless pipe)
(welded pipe)
We are interested here in the crack extension area. As can be seen crack extension varies across the thickness of a specimen. Therefore a mean value should be considered in constructing a J-based R curve. According to the Standard [7] this value is taken as an average of nine measurements of Δa across the specimen thickness with an average of side surface crack extensions taken as one measurement only. After evaluating crack extension Δa for each specimen (J- Δa) couples were plotted at the diagram J - Δa and a power law curve was fitted to these points (see Fig. 4). The power law curves obtained in this way are the R curves. Besides the R curves in the figure there are also so called blunting lines represented by dashed straight lines. Cross points of these lines with R curves define the J value Jin at which an initiation of a stable subcritical crack growth starts. There are also drawn 0.2 mm offset lines which intersect the R curves in a point the y axis of which defines the J value J0.2. The final results are in the Table 1.
Fig. 4. R curves for the pipes
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Table 1. Magnitudes of fracture parameters Pipe Seamless Welded
Jin 87 262
Fracture parameter (N/mm) J0.2 Jm 214 324 436 404
As can be seen from this table, fracture parameters Jin, J0.2, and Jm for the welded pipe are higher than those for the seamless pipe indicating thus higher resistance against fracture and subsequently higher relative critical depth of an eventual crack in the pipe wall of the welded pipe. 2.3 Kinetic properties of the growth of fatigue cracks The kinetics of growth of fatigue cracks was investigated during pre-cracking of CT specimens. From both the stress range applied to a specimen and the instantaneous crack length in a specimen an effective value of the stress intensity factor range ΔKef was determined. This made it possible to calculate the constant C and exponent n in the Paris law:
da C. K ef dN
n
(4)
The results obtained are presented in a graphical form in Fig. 5 for the seamless pipe and in Fig. 6 for the welded pipe.
Fig. 5. Paris law dependence for the seamless pipe
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L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
Fig. 6. Paris law dependence for the welded pipe
Paris law parameters are presented in Table 2. Table 2. Magnitudes of Paris parameters for both pipes Pipe
C
n -12
Seamless
2.125 ͯ 10
5.30
Welded
2.244 ͯ 10-9
3.24
From the viewpoint of fatigue crack growth rate both Paris law parameters are important although the Paris law exponent n is considered to be of a greater importance. The magnitude of this exponent for steels ranges usually between two and four. From Table 2 it can be seen that exponent n for the seamless pipe deviates from this range. On the other hand, it should be mentioned that the high value of the exponent n for the seamless pipe is compensated by a much smaller magnitude of the Paris constant C. In order to qualify how this result can influence the resultant fatigue crack growth rate we have compared growth rates da/dN for both pipes in the range of the effective stress intensity factor Kef = 15 – 25 MPa√m. This range corresponds to the relative crack depth a/t = 0.1 – 0.2 for a pressure of 6.3 MPa and a/t = 0.18 – 0.27 for a pressure of 4.0 MPa. The results are shown in Fig. 7. As it follows from the figure the curve for the welded pipe lies above that for the seamless pipe in the whole stress intensity factor range considered. This indicates that a fatigue crack in the welded pipe propagates faster than in the seamless pipe for all ranges ΔKef considered. However, as far as we look more closely to the right side of the figure we can find that the curve for the seamless pipe becomes steeper so that for greater ranges ΔKef it will exceed that for the welded pipe. A more detailed calculation shows that this happens when the stress intensity factor range reaches a value of ΔKef = 29,3 MPam0.5. To illustrate differences in the fatigue life of seamless pipe and welded pipe we can consider an initial longitudinal part-through crack of the surface length e.g. 2c = 120 mm and depth 2 mm in each pipe.
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Fig. 7. A comparison of fatigue crack growth rate for both pipes
Both pipes are cyclically loaded by a fluctuating internal pressure in the limits pmin = 1.0 MPa and pmax = 6.3 MPa. First we shall determine critical crack depths for both pipes employing fracture toughness J0.2 = 214 N/mm for the seamless pipe and J0.2 = 436 N/mm for the welded pipe. On the basis of engineering methods, e.g. [8, 9] we can arrive at these magnitudes of the critical depths: acr ~ 4.2 mm for the seamless pipe and acr ~ 3.6 mm for the welded pipe. The integrity of a pipe is generally connected with the total time of operation of a linepipe. During this period a crack depth increases up to the critical depth. Therefore, by integrating Eq. (4) by suitable methods we can arrive at the following fatigue lives: Nf = 4 307 cycles for the seamless pipe and Nf = 1 651 cycles for the welded pipe. This means that the fatigue life of the seamless pipe would be longer than that for the welded pipe. 3. Conclusions On the basis of experimental results obtained it can be stated that tensile, notch toughness and fracture toughness properties of the welded pipe are better than those of the seamless pipe. The yield stress of the welded pipe is higher by almost 50% and the ultimate tensile strength by more than 20%. Similarly, notch toughness was found to be higher by roughly 23% and fracture toughness in terms of J0.2 was found to be a double that of the seamless pipe. The only exception were kinetic properties of the growth of fatigue cracks in the working scale of gas pipelines (ΔKef = 15 – 25 MPa√m). The fatigue crack growth rate for the seamless pipe is less than for the welded pipe up to roughly ΔKef = 29 MPa√m. It means that for a specific part-through crack of the same dimensions in both pipes subjected to the same pressure fluctuation the number of cycles to fracture is higher for the seamless pipe. It can be concluded that old gas pipelines composed of pipes which were manufactured in seventieths of the last century by seamless technology can, from the viewpoint of integrity, compete with gas pipelines composed of modern pipes manufactured by welding from rolled out sheets. This is due to outweighing the reduced strength and fracture properties by a higher wall thickness. Acknowledgements This work was supported by the grant project No. TE020000162 of the Technological Grant Agency of the Czech Republic. The authors are grateful for the financial means.
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References [1] I. Fürbacher, K. Macek, J. Seidl, Encyclopedia of engineering materials with foreign equivalents (in Czech), Verlag Dashöfer, Prague, 2001. [2] EN 10208-2: 1996, Steel pipes for pipelines for combustible fluids – Technical delivery conditions – Part 2: Pipes of requirement class B. [3] ISO 3183-2:1996, Petroleum and natural gas industries – Steel pipe for pipelines – Technical delivery conditions – Part 2: Pipes of requirements class B. [4] S.T. Rolfe, J.M. Barsom, Fracture and fatigue control in structures – Applications of fracture mechanics, Prentice-Hall International, Inc., London, 1977. [5] L. Gajdoš et al., Reliability of gas pipelines (in Czech), ČVUT, Prague, 2000. [6] ASTM E1820-01: 2001, Standard test method for measurement of fracture toughness. [7] ISO 12135:2002, Metallic materials – Unified method of test for the determination of quasistatic fracture toughness. [8] L. Gajdoš, M. Srnec, Acta Tech. CSAV, 39 (1994) 151–171. [9] RCC-MR: Design and construction rules for mechanical components of FBR nuclear island, AFCEN-3-5, Paris, 1985.
ScienceDirect Materials Today: Proceedings 12 (2019) 521–528
www.materialstoday.com/proceedings
DAS35
Comparison of fracture properties of steel pipes Ľubomír Gajdoš*, Martin Šperl a
Institute of Theoretical and Applied Mechanics, Academy of Sciences of the Czech Republic,Prosecká 76, 190 00 Prague 9, Czech Republic, e-mail: [email protected]
Abstract
An investigation was made into mechanical and fracture – mechanical properties of seamless pipe Ø 525/8.5 mm from CSN 413126 steel and axially welded pipe Ø 508/6.3 mm from L360NE steel. The results pointed on the dominance of the welded pipe in basic tensile properties, notch toughness and fracture toughness. Prevailing properties of the welded pipe are due to extending the microalloying of the steel by V, Nb, and Ti with a lower content of S and P and also by a higher degree of working of the material during rolling sheet before winding the pipe. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of 35th Danubia Adria Symposium on Advances in Experimental Mechanics. Keywords: Seamless and welded pipes; L360NE steel; impact tests; fracture toughness; Paris law; pipe integrity
1. Introduction Hot rolled seamless pipes are usually aimed for small pipe diameters (less than 500 mm) and they are supplied in the condition after cooling down from the finish rolling temperature (above Ac3 - which is about 850°C for the steels investigated). This condition is admitted as an equivalent to normalization. The starting materials for manufacture of welded pipes with longitudinal or helical welds are strips or sheets of steel, usually in the condition after cooling down from the finish rolling temperature (above Ac3 ) or after heat treatment. In this case the material is better worked than when manufacturing seamless pipes so that mechanical and fracture mechanical properties are also better. In order to assess quantitatively the effect of basic technology of pipe manufacture on pipe integrity * Corresponding author. Tel.: 00420222363095; fax: 00420286884634 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of 35th Danubia Adria Symposium on Advances in Experimental Mechanics.
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L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
mechanical and fracture mechanical tests were carried out on specimens taken from two different pipes: (i) seamless pipe Ø 525/8.5 mm made from steel ČSN 413126 and (ii) longitudinally welded pipe Ø 508/6.3 mm made from steel L360NE. While seamless pipe (VTŽ Chomutov) was exploitated since the seventieths of the last century the welded pipe (Mannesmann) was not exploitated at all and it has been manufactured according to present technological standards. As for chemical composition and mechanical properties, the two materials are similar to one another except for their yield stress. Therefore it is not surprising that L360NE steel is the equivalent of CSN 413126 steel, and vice versa. In reference [1] it is L360NB steel that is referred to and not L360NE steel. The linepipe steels in [1] are referred to according to the EN 10208 standard [2]. However, L360NE steel is referred to in [3] according to the EN ISO 3183-2 standard. 2. Comparison of properties of the steels 2.1 Tensile properties Stress – strain properties were obtained on flat specimens in the hoop direction. Before machining of the specimens the curved segments were press straightened. The results of tensile tests are shown in Fig. 1.
600
sigma (MPa)
500 400 300 200 100 0 0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,1
epsilon (mm/mm) seamless pipe
welded pipe
Fig. 1. Stress – strain diagrams for hoop direction
Strength parameters for steel 13126 are less than for steel L360NE but, however, the gradient of the hardening above the yield stress is higher, so that for reaching a certain strain in the elasto-plastic region a higher stress is required than for steel L360NE. An approximate measure of plastic strain energy in elasto-plastic region is the yield stress to ultimate strength ratio, i.e. Rp0.2/Rm. The lower this ratio the higher is the plastic strain stored energy in elasto-plastic region. For the specific specimen from the seamless pipe this ratio is 0.59 whilst for the specimen from the welded pipe this ratio is 0.72. This clearly demonstrates a higher plastic strain stored energy in the specimen from the seamless pipe. 2.2 Notch impact properties Impact notch toughness was measured on pendulum impact testing machine 300 J using Charpy specimens with the thickness ~ 5.3 mm for the seamless pipe and with thickness ~ 4.1 mm for the welded pipe. Notch toughness values were calculated from the impact energy. Higher magnitudes of KCV were found for L360NE steel (124.5 J/cm2) in contrast to 101.3 J/cm2 for 13126 steel. The significance of the notch toughness KCV consists in principle
L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
523
in two aspects: a) although the notch toughness impact test is a dynamic test its results can be correlated with fracture toughness in terms of the stress intensity factor Kc which is a static characteristics. This correlation is possible, providing the results of the notch toughness are concerned with the upper plateau of the transition curve [4], b) the results of notch toughness tests on Charpy specimens with a 2/3 thickness make it possible to qualify whether a running crack in the wall of a pressure pipeline would stop in a short distance (length of a single pipe) or whether it will continue to run in the pipeline wall to a long distance. The appropriate empirical relations are digestedly shown in [5]. 2.3 Fracture properties Fracture properties of material are best characterized by fracture toughness. This quantity was investigated for both pipes on the basis of the so called J-based R curve. It is a common procedure and is a subject of several international standards, e.g. [6] or [7]. Compact tension (CT) specimens were used in fracture tests. After cycling CT specimens to obtain a sharp fatigue crack all CT specimens were monotonically loaded at room temperature to various magnitudes of the force-point displacement with a simultaneous record of the „force (F) – force-point displacement (f) “. Areas under F – f curves (Apl) were used to determine the plastic component Jpl of the J integral according to relation (1).
J pl
Apl
(1)
Bw a
Elastic component of the J integral is
J el
K 2 1 2 E
(2)
The total magnitude of J integral is then J J el J pl
(3)
Parameter η in Eq. (1) is given by expression
2 0,522w a / w In situations when loading of a CT specimen continued to the descending part of the F – f curve an area Apl,m was also determined. This area is limited from the right by a parallel to the initial elastic straight line crossing the F – f curve in the point of the maximum force. This enabled the J integral Jm to be determined. After unloading the specimens they were placed at the furnace heated to 200°C for 2 hours to obtain an oxidation colouring of the faces of the crack. After that the specimens were cooled down to the temperature of a liquid nitrogen and subsequently they were broken open. Crack extensions Δa which occurred during monotonic loading of CT specimens were then determined from microscopic photographs of fracture surfaces. For illustration, a fracture surface of a specific CT specimen from the seamless pipe and a fracture surface of another CT specimen from the welded pipe are represented on Fig. 2 and Fig. 3. Three different areas can be seen at the fracture surfaces (from the bottom to the top): the fatigue area, the crack extension area, and the final rupture area.
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L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
Fig. 2. Fracture surface of CT specimen
Fig. 3. Fracture surface of CT specimen
(seamless pipe)
(welded pipe)
We are interested here in the crack extension area. As can be seen crack extension varies across the thickness of a specimen. Therefore a mean value should be considered in constructing a J-based R curve. According to the Standard [7] this value is taken as an average of nine measurements of Δa across the specimen thickness with an average of side surface crack extensions taken as one measurement only. After evaluating crack extension Δa for each specimen (J- Δa) couples were plotted at the diagram J - Δa and a power law curve was fitted to these points (see Fig. 4). The power law curves obtained in this way are the R curves. Besides the R curves in the figure there are also so called blunting lines represented by dashed straight lines. Cross points of these lines with R curves define the J value Jin at which an initiation of a stable subcritical crack growth starts. There are also drawn 0.2 mm offset lines which intersect the R curves in a point the y axis of which defines the J value J0.2. The final results are in the Table 1.
Fig. 4. R curves for the pipes
L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
525
Table 1. Magnitudes of fracture parameters Pipe Seamless Welded
Jin 87 262
Fracture parameter (N/mm) J0.2 Jm 214 324 436 404
As can be seen from this table, fracture parameters Jin, J0.2, and Jm for the welded pipe are higher than those for the seamless pipe indicating thus higher resistance against fracture and subsequently higher relative critical depth of an eventual crack in the pipe wall of the welded pipe. 2.3 Kinetic properties of the growth of fatigue cracks The kinetics of growth of fatigue cracks was investigated during pre-cracking of CT specimens. From both the stress range applied to a specimen and the instantaneous crack length in a specimen an effective value of the stress intensity factor range ΔKef was determined. This made it possible to calculate the constant C and exponent n in the Paris law:
da C. K ef dN
n
(4)
The results obtained are presented in a graphical form in Fig. 5 for the seamless pipe and in Fig. 6 for the welded pipe.
Fig. 5. Paris law dependence for the seamless pipe
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L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
Fig. 6. Paris law dependence for the welded pipe
Paris law parameters are presented in Table 2. Table 2. Magnitudes of Paris parameters for both pipes Pipe
C
n -12
Seamless
2.125 ͯ 10
5.30
Welded
2.244 ͯ 10-9
3.24
From the viewpoint of fatigue crack growth rate both Paris law parameters are important although the Paris law exponent n is considered to be of a greater importance. The magnitude of this exponent for steels ranges usually between two and four. From Table 2 it can be seen that exponent n for the seamless pipe deviates from this range. On the other hand, it should be mentioned that the high value of the exponent n for the seamless pipe is compensated by a much smaller magnitude of the Paris constant C. In order to qualify how this result can influence the resultant fatigue crack growth rate we have compared growth rates da/dN for both pipes in the range of the effective stress intensity factor Kef = 15 – 25 MPa√m. This range corresponds to the relative crack depth a/t = 0.1 – 0.2 for a pressure of 6.3 MPa and a/t = 0.18 – 0.27 for a pressure of 4.0 MPa. The results are shown in Fig. 7. As it follows from the figure the curve for the welded pipe lies above that for the seamless pipe in the whole stress intensity factor range considered. This indicates that a fatigue crack in the welded pipe propagates faster than in the seamless pipe for all ranges ΔKef considered. However, as far as we look more closely to the right side of the figure we can find that the curve for the seamless pipe becomes steeper so that for greater ranges ΔKef it will exceed that for the welded pipe. A more detailed calculation shows that this happens when the stress intensity factor range reaches a value of ΔKef = 29,3 MPam0.5. To illustrate differences in the fatigue life of seamless pipe and welded pipe we can consider an initial longitudinal part-through crack of the surface length e.g. 2c = 120 mm and depth 2 mm in each pipe.
L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
527
Fig. 7. A comparison of fatigue crack growth rate for both pipes
Both pipes are cyclically loaded by a fluctuating internal pressure in the limits pmin = 1.0 MPa and pmax = 6.3 MPa. First we shall determine critical crack depths for both pipes employing fracture toughness J0.2 = 214 N/mm for the seamless pipe and J0.2 = 436 N/mm for the welded pipe. On the basis of engineering methods, e.g. [8, 9] we can arrive at these magnitudes of the critical depths: acr ~ 4.2 mm for the seamless pipe and acr ~ 3.6 mm for the welded pipe. The integrity of a pipe is generally connected with the total time of operation of a linepipe. During this period a crack depth increases up to the critical depth. Therefore, by integrating Eq. (4) by suitable methods we can arrive at the following fatigue lives: Nf = 4 307 cycles for the seamless pipe and Nf = 1 651 cycles for the welded pipe. This means that the fatigue life of the seamless pipe would be longer than that for the welded pipe. 3. Conclusions On the basis of experimental results obtained it can be stated that tensile, notch toughness and fracture toughness properties of the welded pipe are better than those of the seamless pipe. The yield stress of the welded pipe is higher by almost 50% and the ultimate tensile strength by more than 20%. Similarly, notch toughness was found to be higher by roughly 23% and fracture toughness in terms of J0.2 was found to be a double that of the seamless pipe. The only exception were kinetic properties of the growth of fatigue cracks in the working scale of gas pipelines (ΔKef = 15 – 25 MPa√m). The fatigue crack growth rate for the seamless pipe is less than for the welded pipe up to roughly ΔKef = 29 MPa√m. It means that for a specific part-through crack of the same dimensions in both pipes subjected to the same pressure fluctuation the number of cycles to fracture is higher for the seamless pipe. It can be concluded that old gas pipelines composed of pipes which were manufactured in seventieths of the last century by seamless technology can, from the viewpoint of integrity, compete with gas pipelines composed of modern pipes manufactured by welding from rolled out sheets. This is due to outweighing the reduced strength and fracture properties by a higher wall thickness. Acknowledgements This work was supported by the grant project No. TE020000162 of the Technological Grant Agency of the Czech Republic. The authors are grateful for the financial means.
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L. Gajdoš and M. Šperl / Materials Today: Proceedings 12 (2019) 521–528
References [1] I. Fürbacher, K. Macek, J. Seidl, Encyclopedia of engineering materials with foreign equivalents (in Czech), Verlag Dashöfer, Prague, 2001. [2] EN 10208-2: 1996, Steel pipes for pipelines for combustible fluids – Technical delivery conditions – Part 2: Pipes of requirement class B. [3] ISO 3183-2:1996, Petroleum and natural gas industries – Steel pipe for pipelines – Technical delivery conditions – Part 2: Pipes of requirements class B. [4] S.T. Rolfe, J.M. Barsom, Fracture and fatigue control in structures – Applications of fracture mechanics, Prentice-Hall International, Inc., London, 1977. [5] L. Gajdoš et al., Reliability of gas pipelines (in Czech), ČVUT, Prague, 2000. [6] ASTM E1820-01: 2001, Standard test method for measurement of fracture toughness. [7] ISO 12135:2002, Metallic materials – Unified method of test for the determination of quasistatic fracture toughness. [8] L. Gajdoš, M. Srnec, Acta Tech. CSAV, 39 (1994) 151–171. [9] RCC-MR: Design and construction rules for mechanical components of FBR nuclear island, AFCEN-3-5, Paris, 1985.