- Email: [email protected]
Microstructural aspects of intergranular and transgranular crack propagation in an API X65 steel pipeline related to fatigue failure
Engineering Failure Analysis 94 (2018) 214–225
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Microstructural aspects of intergranular and transgranular crack propagation in an API X65 steel pipeline related to fatigue failure
T
⁎
M.A. Mohtadi-Bonaba, , M. Eskandarib, M. Sanayeic, S. Dasd a
Department of Mechanical Engineering, University of Bonab, Velayat Highway, Bonab, Iran Department of Materials Science &Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, S7N5A9 Saskatoon, Saskatchewan, Canada d Centre for Engineering Research in Pipelines (CERP), University of Windsor, Windsor, Ontario, Canada b c
A R T IC LE I N F O
ABS TRA CT
Keywords: Steel pipeline Fatigue crack Energy backscatter diffraction Scanning electron microscopy
In this research, we investigated microstructural aspects of intergranular and transgranular fatigue crack propagation in an API X65 steel pipelines. For this purpose, the fatigue testing, based on ASTM E647 Standard, was carried out on CT specimens. The micro-texture measurement results showed that fatigue micro-cracks propagates dominantly in transgranular manner through differently oriented grains. It was also observed that most of the grains involved in crack propagation are broken during the crack growth. Coincidence site lattice (CSL) boundaries are considered as crack resistant paths; however, there was an accumulation of Σ3 boundaries around the fatigue micro-cracks showing that such boundaries acts as high energy boundaries during crack propagation. Elongated manganese sulphide inclusion was determined as the most detrimental inclusion for crack propagation due to the highly disordered boundaries between metal matrix and this inclusion. In other words, this type of inclusion may initiate the fatigue crack due to a high stress concentration factor that can provide in some sharp edges. Moreover, it can facilitate fatigue crack growth by reducing the local fracture toughness. Many manganese sulphide inclusions were found on fatigue fracture surfaces.
1. Introduction Since steel pipelines carry oil and natural gas in a safe and economical way, they are very important to the nation's economy. In the United States, pipelines carried more than four trillion barrel-miles in 2001. Mostly, steel pipelines are used to carry sour hydrocarbons in severe environments. Therefore, they are exposed to different types of failure modes. For instance, hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) have been recognized as the most important failure modes in sour environments [1–3]. Moreover, due to the some external factors such as wind, ground movements, pressure fluctuations inside the pipe body, such steels are exposed to the fatigue failure as well. Since long cracks initiate from structural defects, crack initiation is very important to scientists and industry as well. It is well accepted among researchers that structural defects, such as non-coherent inclusions, precipitates and hard phases, play a crucial role during crack nucleation in all types of failures [4–8]. Aside from the crack nucleation sites, crack propagation is also important in fatigue failure. There are several works in the literature which have been focused on crack propagation sites in steel pipelines [9–12]. However, most of such works concentrated on HIC and SCC related failure and there are not sufficient studies on fatigue crack propagation in steel pipelines. It is worth-mentioning that the mentioned cyclic loads
⁎
Corresponding author. E-mail address: [email protected] (M.A. Mohtadi-Bonab).
https://doi.org/10.1016/j.engfailanal.2018.08.014 Received 19 December 2017; Received in revised form 14 March 2018; Accepted 13 August 2018 Available online 16 August 2018 1350-6307/ © 2018 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
weaken the pipe body by crack nucleation and propagation. There are some parameters involving crack propagation in steel pipelines. First, the microstructure of pipe has a key role in crack propagation. Crack growth can easily occur through hard phases, such as martensite and bainite. Fatigue failures are divided to three steps. In the first step, cracks nucleate from structural defects, such as inclusion and precipitates. Some inclusions such as manganese sulphide are hard and brittle and have an elongated shape as well. This type of inclusions may provide regions with high stress concentration factor and can be considered as crack initiation site [13]. The second step in fatigue failures is the crack propagation step in which crack growth occurs through the thickness where crack propagates easily. Basically, crack chooses easily paths for propagation which are grain boundaries, hard phases and special grain orientations. Several researchers studied fatigue crack propagation in steel pipeline. However, most of them discussed the role of microstructural parameters on fatigue failure. The effect of crystallographic texture and grain boundary character on fatigue crack propagation has been less considered in the literature. The third step in fatigue failure is the final fracture. Even though most researchers believe that the crack initiation and fracture life times are negligible compared with the crack propagation time in steels [14], most pipeline designers do not consider crack initation to be negligible. Some researches focused on fatigue failure in steel pipelines. For instance, the finding of Sowards et al. illustrated that the thermal residual stresses retards the fatigue crack propagation rate in friction stir weld region in steel pipeline. The role of inclusions and precipitates on HIC and SCC in steel pipelines have been investigated by several researchers. Yu et al. investigated the role of different microstructural parameters on fatigue failure in steel pipelines and concluded that some types of inclusions, such as manganese sulphide, have a key role in fatigue crack propagation [13]. More likely, fatigue cracks initiate from inclusions due to the highly disordered boundaries between metal matrix and inclusions. Moreover, some types of inclusions with an elongated shape may provide regions with high stress concentration factor and consequently are prone for crack nucleation. In another study, the findings of Hayne et al. [14] documented that the orientation of manganese sulphide is very important on fatigue failure of induction hardened 4140 steel. The same authors concluded that the effect of manganese sulphide orientation on fatigue behavior outweigh the effect of different orientations of the banded microstructure. The effect of sulphide inclusion, its level and orientation on fatigue properties of SAE 4140 steel was investigated by Cyril et al. [15] and they concluded that this type of inclusion can be considered as the fatigue crack initiation site. It is notable that aside from the role of inclusions, hard phases, such as martensite-austenite constituents, play a critical role in crack initiation in steel pipeline [16]. In this study, we investigated the microstructure of an API X65 steel pipeline with an FEI Quanta 200 FEG environmental scanning electron microscope (SEM) under high vacuum. We also performed the fatigue tests on compact-tension (CT) specimens. As mentioned earlier, the role of texture and micro-texture on fatigue crack propagation has been less considered in the literature. Therefore, we also carried out EBSD measurements on the fatigue cracked regions to deeply understand the role of texture and grain boundary character on fatigue crack propagation in steel pipeline. Moreover, the effect of manganese sulphide inclusion on fatigue crack propagation was investigated. Finally, we discussed the microstructural aspects of intergranular and transgranular of fatigue cracks in X65 steel pipelines. 2. Experimental procedure 2.1. Tested material We investigate an as-received API X65 steel pipeline with the thickness t = 8.5 mm, modulus elasticity E = 200 GPa, yield strength σy = 568 MPa and ultimate tensile strength σUTS = 650 MPa in this work. Table 1 shows the chemical composition of the X65 specimen. We simply abbreviated the rolling, transverse and normal directions of X65 steel as RD, TD and ND, respectively. We grinded the surface and the cross section of steel with MD-Piano 120, 220, 500, 1200, 2000 and 4000 discs and then polished with the MD-Dac 3 μm and MD-Nap1μm polishing discs to observe the microstructure of steel. Then, we etched the surface and cross section of polished areas with 2% nital solution. We performed the SEM observations using with a FEI Quanta 200 FEG environmental SEM equipped with electron backscatter diffraction (EBSD) detector. We used EBSD measurements to investigate the role of texture and grain boundary character in fatigue crack propagation. 2.2. Fatigue experiments We carried out fatigue experiments on six typical CT specimens using an Instron Fatigue Testing Machine. Fig. 1 shows the dimension of X65 specimen that was used in fatigue testing. Based on the ASTM E647 (2008) standard, CT specimens were provided from X65 steel pipeline with an outer diameter of 762 mm and thickness of 8.5 mm using a wire electrical discharge machining (EDM). As shown in Fig. 1, a notch was created in the circumferential (rolling) direction in order to achieve a fatigue crack. In this test, the applied load was from sinusoidal type and varied from 5 kN to 10 kN (stress ratio = 0.50) and the frequency of the test was 50 Hz. We used an electrical discharge machine (EDM) to create a notch on the outer surface of each specimen. The notch was Table 1 Chemical composition of the X65 specimen. Pipeline Steels
C
Mn
Si
Nb
Ti
Cu
V
S
P
N
X65
0.081
1.54
0.33
0.04
0.002
0.18
0.001
0.003
0.01
0.009
215
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 1. Dimension of CT specimens used for fatigue testing.
oriented in the longitudinal direction. Based on Fig. 1, notch depth was approximately 3.9 mm throughout its length which was 44 mm. The purpose was to achieve a fatigue crack in a special location. Therefore, we created a stress concentration at the same location. Since residual stresses may have had some effects on crack propagation in the vicinity of welds, notches were created away from welds. For four specimens, the test was continued until complete fracture occurred. For other two specimens, when the fatigue crack reached to 8 mm in length, the test was stopped to investigate the crack propagation. It is notable that every specimen in the same group had the similar results. In these experiments, a digital microscope and the controller of the fatigue testing machine were used to measure the crack length and number of cycles associated with the measured crack length, respectively.
Fig. 2. SEM images of microstructure from surface (RD-TD plane) of API X65 steel pipeline at (a) 2000× and (b) 10,000×. 216
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 3. (a) SEM image of microstructure and (b) SEM image of microstructure from another region (Both images were provided from surface (RD-TD plane) of API X65 steel pipeline showing pearlite microstructure).
3. Results and discussion 3.1. Microstructure of tested material Figs. 2a and b show the microstructure of surface (RD-TD plane) of X65 steel pipeline. The microstructure of API X65 steel pipeline has been mostly composed of ferrite phase which has high resistance against cracking since this phase has low stored energy of deformation. This phase has been recognized as the softest phase in steel pipeline. As shown in Fig. 2b, one can observe equiaxed grains which are a poof of full recrystallization achievement in such grains which has occurred during hot rolling process. Ferrite phase is not the only present phase in X65 steel pipeline. As clearly shown in Fig. 3a and b, some pearlite structure is also seen in the microstructure of steel. However, pearlite and ferrite bands have a key role in determining the crack direction and mechanical properties in steel pipeline [17,18]. It is worth-mentioning that concentration of manganese is higher in pearlite bands than ferrite phase [18].
Fig. 4. (a) EBSD map, (b) its ODF at Φ2 = 45°, (c) EBSD map from another region and (d) its ODF at Φ2 = 45° (Both EBSD maps were measured at surface (RD-TD plane) of API X65 steel pipeline). 217
Fig. 5. (a) Grain boundary map, (b) its relative frequency vs. misorientation diagram, (c) grain boundary map from another region and (d) its relative frequency vs. misorientation diagram (Both maps were provided from surface (RD-TD plane) of API X65 steel pipeline).
M.A. Mohtadi-Bonab et al.
Engineering Failure Analysis 94 (2018) 214–225
218
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
3.2. EBSD measurements Fig. 4a and c show the EBSD map measured on the surface (RD-TD plane) of X65 steel. It is notable that grains with 〈111〉//ND, 〈011〉//ND and 〈001〉//ND orientation were shown with blue, green and red colors, respectively. As clearly seen in this image, the orientations have been randomly distributed in this image. In other word, the texture of this steel is weak or random. It is generally accepted among researchers that grains with 〈001〉//ND orientation are more prone to cracking due to the lack of slip systems [20–24]. As seen in Fig. 4a and c, there are many grains with < 111 > //ND and 〈011〉//ND orientations. Based on the crystallographic texture analyses, crack chooses easy regions for propagation which are composed of high angle grain boundaries, < 001 > //ND oriented grains, areas with no coincidence site lattice (CSL) boundaries. Orientation distribution function (ODF) at Φ2 = 45° plays an important role in steel structures [19]. As seen in ODF map shown in Fig. 4b and d, the intensity of (110) [1–10] orientation is higher than others. This orientation may act as a resistance path against fatigue crack propagation due to enough slip systems. Fig. 5a and c show the distribution of high angle grain boundaries (HAGBs), medium angle grain boundaries (MAGBs) and low angle grain boundaries (LAGBs) at the surface of X65 steel. A grain boundary with a misorientation angle of 15° < ϴ < 62.8°, 5° < ϴ < 15° and 1° < ϴ < 5° is considered as a HAGB, MAGB and LAGB, respectively. In Fig. 5a and c, HAGBs, MAGBs and LAGBs are illustrated with black, blue and green lines colors. As we see in Fig. 5a and c, the low angle grain boundaries are distributed uniformly through the microstructure of steel. The accumulation of such boundaries through the microstructure of steel is a sign of a high dislocation density at the surface of X65 steel. This also shows that the full recrystallization was not achieved during the dynamic recrystallization that occurred during hot rolling. Moreover, this proves that there is residual stresses in this steel. In other words, the residual stresses have not been completely removed during the hot deformation and manufacturing process. These stresses are very important for fatigue crack nucleation and may change the steel behavior to cyclic loading. As shown in Fig. 5b and d, the relative frequency variations vs. misorientations, the relative frequency of 1° misorientation is highest at the surface of asreceived specimen. Such misorientations are converted to high angle grain boundaries if subsequent annealing is applied to the specimen.
Fig. 6. (a) KAM map, (b) its relative frequency vs. local misorientation diagram, (c) KAM map from another region and (d) its relative frequency vs. local misorientation diagram (Both maps were provided from surface (RD-TD plane) of API X65 steel pipeline). 219
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 7. (a) EBSD map on fatigue micro-crack and (b) EBSD map on fatigue micro-crack from another region (Both maps were provided from surface (RD-TD plane) of API X65 steel pipeline).
Fig. 6a and c also represent the Kernel Average Misorientation (KAM) map in the as-received specimen. These maps show the misorientation difference between a given point inside a grain and its neighbors inside the same grain. The result of these maps is in a good agreement with the grain boundary maps data as presented in Fig. 5a and c. Here, as seen in Fig. 6b and d, the relative frequency of local misorientations with < 1° is highest compared with that of other misorientations. This image also proves that the density of dislocations is high in the as-received specimen. Role of crystallographic texture on fatigue crack propagation in steel pipelines has been less considered by researchers. However, some researchers investigated the role of texture on fatigue crack growth resistance in Al-Cu-Mg alloys [20,21]. They concluded that the intensity of Goss texture ({011} 〈100〉) controls the fatigue crack propagation rate. In other words, since Goss grains have large twist angles with neighbor grains, they are able to make crack deflections. Consequently, they consume a high amount of energy during crack nucleation and propagation. Therefore, such grains may improve fatigue crack propagation resistance. Most researchers believe that crystallographic texture has undeniable role during crack propagation. For instance, Venegas et al. and Mohtadi-Bonab et al. [22–25] believe that {111} dominant texture improves hydrogen-induced cracking resistance. Venegas. et al. showed that {111}//ND, {011}//ND and {112}//ND fibers, ND is the specimen normal direction, improve HIC resistance in steel pipeline since they decrease the number of available trans and inter-granular low resistance paths and increase the number of high resistance intergranular crack paths created by CSL boundaries and LAGBs. However, we should consider that the fatigue crack propagation is under consideration in the current research and the results may be different from what have been observed on HIC crack propagation in steel pipelines. To observe the role of grain orientation on fatigue crack propagation, a fatigue micro-crack has been shown in Fig. 7a and b. The EBSD map, as shown in Fig. 7a and b, show a fatigue micro-crack which propagated through the surface of X65 steel pipeline. In this map, the grains oriented in < 111 > //ND, < 011 > //ND and < 001 > //ND directions have been simply shown with blue, green and red colors, respectively. Clearly, one can observe that fatigue crack propagated through the differently oriented grains. However, the number of grains with < 111 > //ND orientation involved with fatigue crack is higher than the grains with other orientation. This may be with the contradiction with the statement that the < 111 > //ND oriented grains are basically crack resistant. This is true when the cracks grow in intergranular manner. However, since the orientation of grains beside both sides of crack is the same in most cases, it is concluded that the fatigue crack propagates dominantly in transgranular manner. Another point is that the fatigue crack propagation is accompanied with grain fragmentation. This is also another reason for transgranular manner of crack propagation in X65 steel. Fig. 8a and c show distribution of CSL boundaries around the fatigue crack. CSL boundaries are recognized as crack resistant path since they are categorized as low angle grain boundaries which have a low stored energy of deformation. MingFei et al. investigated fatigue crack propagation in low carbon steels and showed that CSL boundaries in ferritebainite steel increases the fatigue crack propagation resistance [26]. Based on Fig. 8b and d, the distribution of CSL boundaries around the crack is low. Comparatively, the relative frequency of ∑3 boundaries is higher than that of other boundaries. Such boundaries are categorized as high angle boundaries since twining does not occur in Σ3 type boundaries in steel pipeline [27]. The point is that the role of CSL boundaries becomes more important when the type of cracking is from intergranular. Fig. 9a, b, c and d show KAM map and recrystallization map in fatigue tested specimen. The accumulation of dislocations around the fatigue crack, as
220
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 8. (a) CSL boundaries around fatigue micro-crack, (b) frequency vs. Sigma values diagram, (c) CSL boundaries around fatigue micro-crack from another region and (d) frequency vs. Sigma values diagram (Both figures were provided from surface (RD-TD plane) of API X65 steel pipeline).
shown in Kernel map, is shown with yellow and red colors. The recrystallized, recovered and deformed regions in Fig. 9b and d are also shown with blue, yellow and red colors, respectively. Basically, fatigue crack prefer to choose an easy path for propagation. Dislocations have a high amount of stored energy and may be considered a suitable sites for fatigue crack propagation. When steel specimens are subjected to fatigue loadings, dislocations are generated even before macro-scale plasticity. Accumulation of dislocations form channel-vein and ladder-like structures which carry most of the plastic deformation inside slip bands [28]. It is also more probable that some of such dislocations have been created during crack propagation by plastic deformation. This trend is also completely true about the recrystallization map. All maps, presented in Fig. 9a, b, c and d, have a good correlation since the regions with high dislocation density, shown with yellow color in Fig. 9a and c, are deformed as seen in Figs. 9b and 9d. As mentioned, the crack prefers to propagate through the deformed region since this region has a high amount of stored energy. It is more probable that such regions, as seen in Fig. 9b and d, have been deformed during the crack propagation. However, some small regions have been deformed during manufacturing process and full recrystallization was not achieved during subsequent recrystallization. These deformed regions are considered as the suitable regions for crack propagation. Beside the effect of texture and micro-texture on fatigue crack growth, the role of inclusions and precipitates on fatigue crack propagation in steel pipelines is undeniable [29]. In this study, the effect of various inclusions and precipitates on fatigue crack initiation and propagation in X65 steel pipeline was accurately studied. The results showed that manganese sulphide type inclusion has an important role on fatigue crack initiation and nucleation. Fig. 10a shows a Backscattered electron (BSE) image of a fatigue crack in X65 steel pipeline. As clearly seen in this figure, two types of inclusion are seen in the crack propagation path which may have a substantial effect on fatigue crack nucleation by providing regions with high stress concentration factors. Since the type of 221
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 9. (a) KAM map and (b) recrystallization map around fatigue micro-crack, (c) KAM map from another region and (d) recrystallization map around fatigue micro-crack (Both maps were provided from surface (RD-TD plane) of API X65 steel pipeline).
inclusions plays a crucial role in crack propagation, an energy dispersive spectroscopy (EDS) point scan on these two inclusions was carried out to determine their types. Fig. 10b is EDS point scan diagrams which proves that the type of both inclusions is from manganese sulphide. More probably, this type of inclusion has a key role in determining the direction of fatigue crack path. This inclusion is elongated during manufacturing of pipe when the cast slabs are subjected to the hot rolling process. Elongated manganese sulphide inclusions are able to provide regions with high stress concentration factor and may initiate fatigue cracks. Moreover, this type of inclusion is hard and brittle and therefore facilitates crack growth. Fig. 10c also shows an inclusion around a fatigue crack in X65 steel pipeline. Fig. 10d proves that the type of this inclusion is manganese sulphide as-well. The role of such inclusion is to weaken the strength of steel by decreasing the local fracture toughness. In this case, fatigue cracks can easily propagate through the region with high accumulation of such inclusions. Fig. 11a and b show the fatigue fracture surface of X65 steel pipeline. In Fig. 11a, a large elongated manganese sulphide is observed. Moreover, an SEM image with a low magnification, as shown in Fig. 11b, shows that there are many elongated manganese sulphide inclusions in the fracture surface of fatigue tested specimen. As mentioned before, one of the role of inclusions is to reduce of the fracture toughness of the local region. Based on Fig. 11b, one can see that fracture has occurred from a region with high concentration of inclusions. This finding is in good agreement with the point that accumulation of inclusions in a region reduces the strength of steel.
222
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 10. (a) BSE image of fatigue micro-crack, (b) EDS point scan from inclusion (1) along the crack path and, EDS point scan from inclusion (2) along the crack path, (c) backscattered electron (BSE) image of fatigue micro-crack from another region and (d) EDS point scan from an inclusion around the crack (Both images were provided from surface (RD-TD plane) of API X65 steel pipeline).
4. Conclusion Based on the fatigue testing on an API X65 steel pipeline, SEM and EBSD measurements, the following conclusions are made: (1) The dominant texture of X65 steel is weak and random. Also, fatigue crack can propagate through differently oriented grains. However, the {001} local texture may facilitate fatigue crack propagation due to the lack of slip systems. (2) Even though the fatigue crack can propagate in both intergranular and transgranular manners. In this study, the dominant mode of fatigue crack is transgranular. (3) CSL boundaries are considered as the crack resistant path since they reduce the probability of fatigue crack growth by decreasing 223
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 11. SEM images of fatigue fracture surfaces showing (a) a large manganese sulphide inclusion and (b) many manganese sulphide inclusions.
the number of available transgraular and intergranular low resistance cleavage paths. However, the accumulation of Σ3 boundaries was high around the fatigue crack. Such boundaries are categorized as HAGBS since twining does not occur in Σ3 type boundaries in steel pipeline. They may decrease fatigue crack resistance in steel pipeline. (4) Inclusions paly two important role during fatigue crack propagation. First, they may initiate fatigue crack by providing regions with high stress concentration factor. Second, their accumulation decreases the local fracture toughness and then facilitate crack growth. The manganese sulphide inclusion has been recognized the most detrimental for fatigue crack propagation. Acknowledgment We would like to thank Natural Sciences and Engineering Research Council of Canada for the financial support of this project. We also sincerely thank support received from Centre for Engineering Research in Pipelines (CERP) located in Windsor, ON, Canada. We would like to thank the Research Center of University of Bonab for the financial support of this project. References [1] E. Gamboa, V. Linton, M. Law, Fatigue of stress Corrosion Cracks in X65 Steel pipelines, Int. J. Fatigue 30 (2008) 850–860. [2] M.A. Mohtadi-Bonab, M. Eskandari, R. Karimdadashi, J.A. Szpunar, Effect of different microstructural parameters on hydrogen induced cracking in an API X70 pipeline steel, Met. Mat. Int. 23 (2017) 726–735. [3] M.A. Mohtadi-Bonab, M. Eskandari, A focus on different factors affecting hydrogen induced cracking in oil and natural gas pipeline steel, Eng. Fail. Anal. 79 (2017) 351–360. [4] M.A. Arafin, J.A. Szpunar, A new understanding of intergranular stress corrosion cracking resistance of Steel pipeline steel through grain boundary character and crystallographic texture studies, Corros. Sci. 51 (2009) 119–128. [5] M.A. Mohtadi-Bonab, R. Karimdadashi, M. Eskandari, J.A. Szpunar, Hydrogen-induced cracking assessment in pipeline steels through permeation and crystallographic texture measurements, J. Mater. Eng. Perform. 25 (2016) 1781–1793. [6] V.M. Pleskach, P.A. Averchenko, Effect of gaseous erosion on a decrease of the fatigue strength of specimens of titanium alloy VT8, Strength Mater. 7 (1975) 1036–1037. [7] V. Olden, A. Alvaro, O.M. Akselsen, Hydrogen diffusion and hydrogen influenced critical stress intensity in an api x70 pipeline steel welded joint – experiments and FE simulations, Int. J. Hydrog. Energy 37 (2012) 11474–11486. [8] H.J. Christ, A. Jung, H.J. Maier, R. Teteruk, Thermomechanical fatigue–damage mechanisms and mechanism-based life prediction methods, Sadhana 28 (2003) 147–165. [9] S. Hassanifard, M.A. Mohtadi-Bonab, Gh. Jabbari, Investigation of fatigue crack propagation in spot-welded joints based on fracture mechanics approach, J. Mater. Eng. Perform. 22 (2013) 245–250. [10] J.F. Cooper, R.A. Smith, The measurement of fatigue cracks at spot-welds, Int. J. Fatigue 7 (1985) 137–140. [11] J.W. Sowards, T. Gnäupel-Herold, J.D. McColskey, V.F. Pereira, A.J. Ramirez, Characterization of mechanical properties, fatigue-crack propagation, and residual stresses in a microalloyed pipeline-steel friction-stir weld, Mater. Des. 88 (2015) 632–642. [12] J.A. Ronevich, B.P. Somerday, C.W.S. Marchi, Effects of microstructure banding on hydrogen assisted fatigue crack growth in X65 pipeline steels, Int. J. Fatigue 82 (2016) 497–504. [13] M. Yu, W. Chen, R. Kania, G. Boven, J. Been, Crack propagation of pipeline steel exposed to a near-neutral ph environment under variable pressure fluctuations, Int. J. Fatigue 82 (2016) 658–666. [14] B.T. Lu, Further study on crack growth model of buried pipelines exposed to concentrated carbonate–bicarbonate solution, Eng. Fract. Mech. 131 (2014) 296–314. [15] F. Huang, J. Liu, Z.J. Deng, J.H. Cheng, Z.H. Lu, X.G. Li, Effect of microstructure and inclusions on hydrogen induced cracking susceptibility and hydrogen trapping efficiency of X120 pipeline steel, Mater. Sci. Eng. A 527 (2010) 6997–7001. [16] J. Moon, C. Park, S.J. Kim, Influence of Ti addition on the hydrogen induced cracking of API 5L X70 Hot-rolled pipeline steel in acid sour media, Met. Mater. Int. 18 (2012) 613–617. [17] D. Hardie, E.A. Charles, A.H. Lopez, Hydrogen embrittlement of high strength pipelines steel, Corros. Sci. 48 (2006) 4378–4385. [18] F.A. Khalid, M. Farooque, A. Ul Haq, A.Q. Khan, Role of ferrite/pearlite banded structure and segregation on mechanical properties of microalloyed hot rolled steel, Mater. Sci. Tech. 15 (1999) 1209–1215. [19] R. Jamaati, M.R. Toroghinejad, M.A. Mohtadi-Bonab, H. Edris, J.A. Szpunar, M.R. Salmani, Texture development of ARB-processed steel-based nanocomposite, J. Mater. Eng. Perfor. 23 (2014) 4436–4445. [20] W. Wu, Z. Liu, Y. Hu, F. Li, S. Bai, P. Xia, A. Wanga, C. Ye, Goss texture intensity effect on fatigue crack propagation resistance in an Al-Cu-Mg alloy J. Alloys
224
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Compds, Accepted Manuscript, DOI: https://doi.org/10.1016/j.jallcom.2017.09.320. [21] F. Li, Z. Liu, W. Wu, P. Xia, P. Ying, Q. Zhao, J. Li, S. Bai, C. Ye, On the role of texture in governing fatigue crack propagation behavior of 2524 Aluminum Alloy, Mater. Sci. Eng. A 669 (2016) 367–378. [22] V. Venegas, F. Caleyo, T. Baudin, J.H. Espina-Hernández, J.M. Hallen, On the role of crystallographic texture in mitigating hydrogen-induced cracking in pipeline steel, Corros. Sci. 53 (2011) 4204–4212. [23] M.A. Mohtadi-Bonab, J.A. Szpunar, R. Basu, M. Eskandari, The mechanism of failure by hydrogen induced cracking in an acidic environment for API 5L X70 pipeline steel, Int. J. Hydrog. Energy 40 (2015) 1096–1107. [24] M.A. Mohtadi-Bonab, M. Eskandari, J.A. Szpunar, Effect of arisen dislocation density and texture components during cold rolling and annealing treatments on hydrogen induced cracking susceptibility in pipeline steel, J. Mater. Res. 31 (2016) 3390–3400. [25] M.A. Mohtadi-Bonab, M. Eskandari, J.A. Szpunar, Texture, local misorientation, grain boundary and recrystallization fraction in pipeline steels related to hydrogen induced cracking, Mater. Sci. Eng. A 620 (2015) 97–106. [26] G. Mingfei, Y. Hao, In-situ investigation on the fatigue crack propagation behavior in ferrite-pearlte and dual-phase ferrite-bainite low carbon steels, Science China 26 (2013) 71–79. [27] V. Venegas, F. Caleyo, J.M. Hallen, T. Baudin, R. Penelle, Role of crystallographic texture in hydrogen-induced cracking of low carbon steels for sour service piping, Miner. Metal. Mater. Soci. ASM Int. 38 (2007) 1022–1031. [28] S. Brinckman, On the Role of Dislocations in Fatigue Crack Initiation, PhD Thesis University of Groningen, 2005. [29] M.A. Mohtadi-Bonab, M. Eskandari, H. Ghaednia, S. Das, Effect of microstructural parameters on fatigue crack propagation in an API X65 pipeline steel, J. Mater. Eng. Perfor. 25 (2016) 4933–4940.
225
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Microstructural aspects of intergranular and transgranular crack propagation in an API X65 steel pipeline related to fatigue failure
T
⁎
M.A. Mohtadi-Bonaba, , M. Eskandarib, M. Sanayeic, S. Dasd a
Department of Mechanical Engineering, University of Bonab, Velayat Highway, Bonab, Iran Department of Materials Science &Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, S7N5A9 Saskatoon, Saskatchewan, Canada d Centre for Engineering Research in Pipelines (CERP), University of Windsor, Windsor, Ontario, Canada b c
A R T IC LE I N F O
ABS TRA CT
Keywords: Steel pipeline Fatigue crack Energy backscatter diffraction Scanning electron microscopy
In this research, we investigated microstructural aspects of intergranular and transgranular fatigue crack propagation in an API X65 steel pipelines. For this purpose, the fatigue testing, based on ASTM E647 Standard, was carried out on CT specimens. The micro-texture measurement results showed that fatigue micro-cracks propagates dominantly in transgranular manner through differently oriented grains. It was also observed that most of the grains involved in crack propagation are broken during the crack growth. Coincidence site lattice (CSL) boundaries are considered as crack resistant paths; however, there was an accumulation of Σ3 boundaries around the fatigue micro-cracks showing that such boundaries acts as high energy boundaries during crack propagation. Elongated manganese sulphide inclusion was determined as the most detrimental inclusion for crack propagation due to the highly disordered boundaries between metal matrix and this inclusion. In other words, this type of inclusion may initiate the fatigue crack due to a high stress concentration factor that can provide in some sharp edges. Moreover, it can facilitate fatigue crack growth by reducing the local fracture toughness. Many manganese sulphide inclusions were found on fatigue fracture surfaces.
1. Introduction Since steel pipelines carry oil and natural gas in a safe and economical way, they are very important to the nation's economy. In the United States, pipelines carried more than four trillion barrel-miles in 2001. Mostly, steel pipelines are used to carry sour hydrocarbons in severe environments. Therefore, they are exposed to different types of failure modes. For instance, hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) have been recognized as the most important failure modes in sour environments [1–3]. Moreover, due to the some external factors such as wind, ground movements, pressure fluctuations inside the pipe body, such steels are exposed to the fatigue failure as well. Since long cracks initiate from structural defects, crack initiation is very important to scientists and industry as well. It is well accepted among researchers that structural defects, such as non-coherent inclusions, precipitates and hard phases, play a crucial role during crack nucleation in all types of failures [4–8]. Aside from the crack nucleation sites, crack propagation is also important in fatigue failure. There are several works in the literature which have been focused on crack propagation sites in steel pipelines [9–12]. However, most of such works concentrated on HIC and SCC related failure and there are not sufficient studies on fatigue crack propagation in steel pipelines. It is worth-mentioning that the mentioned cyclic loads
⁎
Corresponding author. E-mail address: [email protected] (M.A. Mohtadi-Bonab).
https://doi.org/10.1016/j.engfailanal.2018.08.014 Received 19 December 2017; Received in revised form 14 March 2018; Accepted 13 August 2018 Available online 16 August 2018 1350-6307/ © 2018 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
weaken the pipe body by crack nucleation and propagation. There are some parameters involving crack propagation in steel pipelines. First, the microstructure of pipe has a key role in crack propagation. Crack growth can easily occur through hard phases, such as martensite and bainite. Fatigue failures are divided to three steps. In the first step, cracks nucleate from structural defects, such as inclusion and precipitates. Some inclusions such as manganese sulphide are hard and brittle and have an elongated shape as well. This type of inclusions may provide regions with high stress concentration factor and can be considered as crack initiation site [13]. The second step in fatigue failures is the crack propagation step in which crack growth occurs through the thickness where crack propagates easily. Basically, crack chooses easily paths for propagation which are grain boundaries, hard phases and special grain orientations. Several researchers studied fatigue crack propagation in steel pipeline. However, most of them discussed the role of microstructural parameters on fatigue failure. The effect of crystallographic texture and grain boundary character on fatigue crack propagation has been less considered in the literature. The third step in fatigue failure is the final fracture. Even though most researchers believe that the crack initiation and fracture life times are negligible compared with the crack propagation time in steels [14], most pipeline designers do not consider crack initation to be negligible. Some researches focused on fatigue failure in steel pipelines. For instance, the finding of Sowards et al. illustrated that the thermal residual stresses retards the fatigue crack propagation rate in friction stir weld region in steel pipeline. The role of inclusions and precipitates on HIC and SCC in steel pipelines have been investigated by several researchers. Yu et al. investigated the role of different microstructural parameters on fatigue failure in steel pipelines and concluded that some types of inclusions, such as manganese sulphide, have a key role in fatigue crack propagation [13]. More likely, fatigue cracks initiate from inclusions due to the highly disordered boundaries between metal matrix and inclusions. Moreover, some types of inclusions with an elongated shape may provide regions with high stress concentration factor and consequently are prone for crack nucleation. In another study, the findings of Hayne et al. [14] documented that the orientation of manganese sulphide is very important on fatigue failure of induction hardened 4140 steel. The same authors concluded that the effect of manganese sulphide orientation on fatigue behavior outweigh the effect of different orientations of the banded microstructure. The effect of sulphide inclusion, its level and orientation on fatigue properties of SAE 4140 steel was investigated by Cyril et al. [15] and they concluded that this type of inclusion can be considered as the fatigue crack initiation site. It is notable that aside from the role of inclusions, hard phases, such as martensite-austenite constituents, play a critical role in crack initiation in steel pipeline [16]. In this study, we investigated the microstructure of an API X65 steel pipeline with an FEI Quanta 200 FEG environmental scanning electron microscope (SEM) under high vacuum. We also performed the fatigue tests on compact-tension (CT) specimens. As mentioned earlier, the role of texture and micro-texture on fatigue crack propagation has been less considered in the literature. Therefore, we also carried out EBSD measurements on the fatigue cracked regions to deeply understand the role of texture and grain boundary character on fatigue crack propagation in steel pipeline. Moreover, the effect of manganese sulphide inclusion on fatigue crack propagation was investigated. Finally, we discussed the microstructural aspects of intergranular and transgranular of fatigue cracks in X65 steel pipelines. 2. Experimental procedure 2.1. Tested material We investigate an as-received API X65 steel pipeline with the thickness t = 8.5 mm, modulus elasticity E = 200 GPa, yield strength σy = 568 MPa and ultimate tensile strength σUTS = 650 MPa in this work. Table 1 shows the chemical composition of the X65 specimen. We simply abbreviated the rolling, transverse and normal directions of X65 steel as RD, TD and ND, respectively. We grinded the surface and the cross section of steel with MD-Piano 120, 220, 500, 1200, 2000 and 4000 discs and then polished with the MD-Dac 3 μm and MD-Nap1μm polishing discs to observe the microstructure of steel. Then, we etched the surface and cross section of polished areas with 2% nital solution. We performed the SEM observations using with a FEI Quanta 200 FEG environmental SEM equipped with electron backscatter diffraction (EBSD) detector. We used EBSD measurements to investigate the role of texture and grain boundary character in fatigue crack propagation. 2.2. Fatigue experiments We carried out fatigue experiments on six typical CT specimens using an Instron Fatigue Testing Machine. Fig. 1 shows the dimension of X65 specimen that was used in fatigue testing. Based on the ASTM E647 (2008) standard, CT specimens were provided from X65 steel pipeline with an outer diameter of 762 mm and thickness of 8.5 mm using a wire electrical discharge machining (EDM). As shown in Fig. 1, a notch was created in the circumferential (rolling) direction in order to achieve a fatigue crack. In this test, the applied load was from sinusoidal type and varied from 5 kN to 10 kN (stress ratio = 0.50) and the frequency of the test was 50 Hz. We used an electrical discharge machine (EDM) to create a notch on the outer surface of each specimen. The notch was Table 1 Chemical composition of the X65 specimen. Pipeline Steels
C
Mn
Si
Nb
Ti
Cu
V
S
P
N
X65
0.081
1.54
0.33
0.04
0.002
0.18
0.001
0.003
0.01
0.009
215
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 1. Dimension of CT specimens used for fatigue testing.
oriented in the longitudinal direction. Based on Fig. 1, notch depth was approximately 3.9 mm throughout its length which was 44 mm. The purpose was to achieve a fatigue crack in a special location. Therefore, we created a stress concentration at the same location. Since residual stresses may have had some effects on crack propagation in the vicinity of welds, notches were created away from welds. For four specimens, the test was continued until complete fracture occurred. For other two specimens, when the fatigue crack reached to 8 mm in length, the test was stopped to investigate the crack propagation. It is notable that every specimen in the same group had the similar results. In these experiments, a digital microscope and the controller of the fatigue testing machine were used to measure the crack length and number of cycles associated with the measured crack length, respectively.
Fig. 2. SEM images of microstructure from surface (RD-TD plane) of API X65 steel pipeline at (a) 2000× and (b) 10,000×. 216
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 3. (a) SEM image of microstructure and (b) SEM image of microstructure from another region (Both images were provided from surface (RD-TD plane) of API X65 steel pipeline showing pearlite microstructure).
3. Results and discussion 3.1. Microstructure of tested material Figs. 2a and b show the microstructure of surface (RD-TD plane) of X65 steel pipeline. The microstructure of API X65 steel pipeline has been mostly composed of ferrite phase which has high resistance against cracking since this phase has low stored energy of deformation. This phase has been recognized as the softest phase in steel pipeline. As shown in Fig. 2b, one can observe equiaxed grains which are a poof of full recrystallization achievement in such grains which has occurred during hot rolling process. Ferrite phase is not the only present phase in X65 steel pipeline. As clearly shown in Fig. 3a and b, some pearlite structure is also seen in the microstructure of steel. However, pearlite and ferrite bands have a key role in determining the crack direction and mechanical properties in steel pipeline [17,18]. It is worth-mentioning that concentration of manganese is higher in pearlite bands than ferrite phase [18].
Fig. 4. (a) EBSD map, (b) its ODF at Φ2 = 45°, (c) EBSD map from another region and (d) its ODF at Φ2 = 45° (Both EBSD maps were measured at surface (RD-TD plane) of API X65 steel pipeline). 217
Fig. 5. (a) Grain boundary map, (b) its relative frequency vs. misorientation diagram, (c) grain boundary map from another region and (d) its relative frequency vs. misorientation diagram (Both maps were provided from surface (RD-TD plane) of API X65 steel pipeline).
M.A. Mohtadi-Bonab et al.
Engineering Failure Analysis 94 (2018) 214–225
218
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
3.2. EBSD measurements Fig. 4a and c show the EBSD map measured on the surface (RD-TD plane) of X65 steel. It is notable that grains with 〈111〉//ND, 〈011〉//ND and 〈001〉//ND orientation were shown with blue, green and red colors, respectively. As clearly seen in this image, the orientations have been randomly distributed in this image. In other word, the texture of this steel is weak or random. It is generally accepted among researchers that grains with 〈001〉//ND orientation are more prone to cracking due to the lack of slip systems [20–24]. As seen in Fig. 4a and c, there are many grains with < 111 > //ND and 〈011〉//ND orientations. Based on the crystallographic texture analyses, crack chooses easy regions for propagation which are composed of high angle grain boundaries, < 001 > //ND oriented grains, areas with no coincidence site lattice (CSL) boundaries. Orientation distribution function (ODF) at Φ2 = 45° plays an important role in steel structures [19]. As seen in ODF map shown in Fig. 4b and d, the intensity of (110) [1–10] orientation is higher than others. This orientation may act as a resistance path against fatigue crack propagation due to enough slip systems. Fig. 5a and c show the distribution of high angle grain boundaries (HAGBs), medium angle grain boundaries (MAGBs) and low angle grain boundaries (LAGBs) at the surface of X65 steel. A grain boundary with a misorientation angle of 15° < ϴ < 62.8°, 5° < ϴ < 15° and 1° < ϴ < 5° is considered as a HAGB, MAGB and LAGB, respectively. In Fig. 5a and c, HAGBs, MAGBs and LAGBs are illustrated with black, blue and green lines colors. As we see in Fig. 5a and c, the low angle grain boundaries are distributed uniformly through the microstructure of steel. The accumulation of such boundaries through the microstructure of steel is a sign of a high dislocation density at the surface of X65 steel. This also shows that the full recrystallization was not achieved during the dynamic recrystallization that occurred during hot rolling. Moreover, this proves that there is residual stresses in this steel. In other words, the residual stresses have not been completely removed during the hot deformation and manufacturing process. These stresses are very important for fatigue crack nucleation and may change the steel behavior to cyclic loading. As shown in Fig. 5b and d, the relative frequency variations vs. misorientations, the relative frequency of 1° misorientation is highest at the surface of asreceived specimen. Such misorientations are converted to high angle grain boundaries if subsequent annealing is applied to the specimen.
Fig. 6. (a) KAM map, (b) its relative frequency vs. local misorientation diagram, (c) KAM map from another region and (d) its relative frequency vs. local misorientation diagram (Both maps were provided from surface (RD-TD plane) of API X65 steel pipeline). 219
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 7. (a) EBSD map on fatigue micro-crack and (b) EBSD map on fatigue micro-crack from another region (Both maps were provided from surface (RD-TD plane) of API X65 steel pipeline).
Fig. 6a and c also represent the Kernel Average Misorientation (KAM) map in the as-received specimen. These maps show the misorientation difference between a given point inside a grain and its neighbors inside the same grain. The result of these maps is in a good agreement with the grain boundary maps data as presented in Fig. 5a and c. Here, as seen in Fig. 6b and d, the relative frequency of local misorientations with < 1° is highest compared with that of other misorientations. This image also proves that the density of dislocations is high in the as-received specimen. Role of crystallographic texture on fatigue crack propagation in steel pipelines has been less considered by researchers. However, some researchers investigated the role of texture on fatigue crack growth resistance in Al-Cu-Mg alloys [20,21]. They concluded that the intensity of Goss texture ({011} 〈100〉) controls the fatigue crack propagation rate. In other words, since Goss grains have large twist angles with neighbor grains, they are able to make crack deflections. Consequently, they consume a high amount of energy during crack nucleation and propagation. Therefore, such grains may improve fatigue crack propagation resistance. Most researchers believe that crystallographic texture has undeniable role during crack propagation. For instance, Venegas et al. and Mohtadi-Bonab et al. [22–25] believe that {111} dominant texture improves hydrogen-induced cracking resistance. Venegas. et al. showed that {111}//ND, {011}//ND and {112}//ND fibers, ND is the specimen normal direction, improve HIC resistance in steel pipeline since they decrease the number of available trans and inter-granular low resistance paths and increase the number of high resistance intergranular crack paths created by CSL boundaries and LAGBs. However, we should consider that the fatigue crack propagation is under consideration in the current research and the results may be different from what have been observed on HIC crack propagation in steel pipelines. To observe the role of grain orientation on fatigue crack propagation, a fatigue micro-crack has been shown in Fig. 7a and b. The EBSD map, as shown in Fig. 7a and b, show a fatigue micro-crack which propagated through the surface of X65 steel pipeline. In this map, the grains oriented in < 111 > //ND, < 011 > //ND and < 001 > //ND directions have been simply shown with blue, green and red colors, respectively. Clearly, one can observe that fatigue crack propagated through the differently oriented grains. However, the number of grains with < 111 > //ND orientation involved with fatigue crack is higher than the grains with other orientation. This may be with the contradiction with the statement that the < 111 > //ND oriented grains are basically crack resistant. This is true when the cracks grow in intergranular manner. However, since the orientation of grains beside both sides of crack is the same in most cases, it is concluded that the fatigue crack propagates dominantly in transgranular manner. Another point is that the fatigue crack propagation is accompanied with grain fragmentation. This is also another reason for transgranular manner of crack propagation in X65 steel. Fig. 8a and c show distribution of CSL boundaries around the fatigue crack. CSL boundaries are recognized as crack resistant path since they are categorized as low angle grain boundaries which have a low stored energy of deformation. MingFei et al. investigated fatigue crack propagation in low carbon steels and showed that CSL boundaries in ferritebainite steel increases the fatigue crack propagation resistance [26]. Based on Fig. 8b and d, the distribution of CSL boundaries around the crack is low. Comparatively, the relative frequency of ∑3 boundaries is higher than that of other boundaries. Such boundaries are categorized as high angle boundaries since twining does not occur in Σ3 type boundaries in steel pipeline [27]. The point is that the role of CSL boundaries becomes more important when the type of cracking is from intergranular. Fig. 9a, b, c and d show KAM map and recrystallization map in fatigue tested specimen. The accumulation of dislocations around the fatigue crack, as
220
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 8. (a) CSL boundaries around fatigue micro-crack, (b) frequency vs. Sigma values diagram, (c) CSL boundaries around fatigue micro-crack from another region and (d) frequency vs. Sigma values diagram (Both figures were provided from surface (RD-TD plane) of API X65 steel pipeline).
shown in Kernel map, is shown with yellow and red colors. The recrystallized, recovered and deformed regions in Fig. 9b and d are also shown with blue, yellow and red colors, respectively. Basically, fatigue crack prefer to choose an easy path for propagation. Dislocations have a high amount of stored energy and may be considered a suitable sites for fatigue crack propagation. When steel specimens are subjected to fatigue loadings, dislocations are generated even before macro-scale plasticity. Accumulation of dislocations form channel-vein and ladder-like structures which carry most of the plastic deformation inside slip bands [28]. It is also more probable that some of such dislocations have been created during crack propagation by plastic deformation. This trend is also completely true about the recrystallization map. All maps, presented in Fig. 9a, b, c and d, have a good correlation since the regions with high dislocation density, shown with yellow color in Fig. 9a and c, are deformed as seen in Figs. 9b and 9d. As mentioned, the crack prefers to propagate through the deformed region since this region has a high amount of stored energy. It is more probable that such regions, as seen in Fig. 9b and d, have been deformed during the crack propagation. However, some small regions have been deformed during manufacturing process and full recrystallization was not achieved during subsequent recrystallization. These deformed regions are considered as the suitable regions for crack propagation. Beside the effect of texture and micro-texture on fatigue crack growth, the role of inclusions and precipitates on fatigue crack propagation in steel pipelines is undeniable [29]. In this study, the effect of various inclusions and precipitates on fatigue crack initiation and propagation in X65 steel pipeline was accurately studied. The results showed that manganese sulphide type inclusion has an important role on fatigue crack initiation and nucleation. Fig. 10a shows a Backscattered electron (BSE) image of a fatigue crack in X65 steel pipeline. As clearly seen in this figure, two types of inclusion are seen in the crack propagation path which may have a substantial effect on fatigue crack nucleation by providing regions with high stress concentration factors. Since the type of 221
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 9. (a) KAM map and (b) recrystallization map around fatigue micro-crack, (c) KAM map from another region and (d) recrystallization map around fatigue micro-crack (Both maps were provided from surface (RD-TD plane) of API X65 steel pipeline).
inclusions plays a crucial role in crack propagation, an energy dispersive spectroscopy (EDS) point scan on these two inclusions was carried out to determine their types. Fig. 10b is EDS point scan diagrams which proves that the type of both inclusions is from manganese sulphide. More probably, this type of inclusion has a key role in determining the direction of fatigue crack path. This inclusion is elongated during manufacturing of pipe when the cast slabs are subjected to the hot rolling process. Elongated manganese sulphide inclusions are able to provide regions with high stress concentration factor and may initiate fatigue cracks. Moreover, this type of inclusion is hard and brittle and therefore facilitates crack growth. Fig. 10c also shows an inclusion around a fatigue crack in X65 steel pipeline. Fig. 10d proves that the type of this inclusion is manganese sulphide as-well. The role of such inclusion is to weaken the strength of steel by decreasing the local fracture toughness. In this case, fatigue cracks can easily propagate through the region with high accumulation of such inclusions. Fig. 11a and b show the fatigue fracture surface of X65 steel pipeline. In Fig. 11a, a large elongated manganese sulphide is observed. Moreover, an SEM image with a low magnification, as shown in Fig. 11b, shows that there are many elongated manganese sulphide inclusions in the fracture surface of fatigue tested specimen. As mentioned before, one of the role of inclusions is to reduce of the fracture toughness of the local region. Based on Fig. 11b, one can see that fracture has occurred from a region with high concentration of inclusions. This finding is in good agreement with the point that accumulation of inclusions in a region reduces the strength of steel.
222
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 10. (a) BSE image of fatigue micro-crack, (b) EDS point scan from inclusion (1) along the crack path and, EDS point scan from inclusion (2) along the crack path, (c) backscattered electron (BSE) image of fatigue micro-crack from another region and (d) EDS point scan from an inclusion around the crack (Both images were provided from surface (RD-TD plane) of API X65 steel pipeline).
4. Conclusion Based on the fatigue testing on an API X65 steel pipeline, SEM and EBSD measurements, the following conclusions are made: (1) The dominant texture of X65 steel is weak and random. Also, fatigue crack can propagate through differently oriented grains. However, the {001} local texture may facilitate fatigue crack propagation due to the lack of slip systems. (2) Even though the fatigue crack can propagate in both intergranular and transgranular manners. In this study, the dominant mode of fatigue crack is transgranular. (3) CSL boundaries are considered as the crack resistant path since they reduce the probability of fatigue crack growth by decreasing 223
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Fig. 11. SEM images of fatigue fracture surfaces showing (a) a large manganese sulphide inclusion and (b) many manganese sulphide inclusions.
the number of available transgraular and intergranular low resistance cleavage paths. However, the accumulation of Σ3 boundaries was high around the fatigue crack. Such boundaries are categorized as HAGBS since twining does not occur in Σ3 type boundaries in steel pipeline. They may decrease fatigue crack resistance in steel pipeline. (4) Inclusions paly two important role during fatigue crack propagation. First, they may initiate fatigue crack by providing regions with high stress concentration factor. Second, their accumulation decreases the local fracture toughness and then facilitate crack growth. The manganese sulphide inclusion has been recognized the most detrimental for fatigue crack propagation. Acknowledgment We would like to thank Natural Sciences and Engineering Research Council of Canada for the financial support of this project. We also sincerely thank support received from Centre for Engineering Research in Pipelines (CERP) located in Windsor, ON, Canada. We would like to thank the Research Center of University of Bonab for the financial support of this project. References [1] E. Gamboa, V. Linton, M. Law, Fatigue of stress Corrosion Cracks in X65 Steel pipelines, Int. J. Fatigue 30 (2008) 850–860. [2] M.A. Mohtadi-Bonab, M. Eskandari, R. Karimdadashi, J.A. Szpunar, Effect of different microstructural parameters on hydrogen induced cracking in an API X70 pipeline steel, Met. Mat. Int. 23 (2017) 726–735. [3] M.A. Mohtadi-Bonab, M. Eskandari, A focus on different factors affecting hydrogen induced cracking in oil and natural gas pipeline steel, Eng. Fail. Anal. 79 (2017) 351–360. [4] M.A. Arafin, J.A. Szpunar, A new understanding of intergranular stress corrosion cracking resistance of Steel pipeline steel through grain boundary character and crystallographic texture studies, Corros. Sci. 51 (2009) 119–128. [5] M.A. Mohtadi-Bonab, R. Karimdadashi, M. Eskandari, J.A. Szpunar, Hydrogen-induced cracking assessment in pipeline steels through permeation and crystallographic texture measurements, J. Mater. Eng. Perform. 25 (2016) 1781–1793. [6] V.M. Pleskach, P.A. Averchenko, Effect of gaseous erosion on a decrease of the fatigue strength of specimens of titanium alloy VT8, Strength Mater. 7 (1975) 1036–1037. [7] V. Olden, A. Alvaro, O.M. Akselsen, Hydrogen diffusion and hydrogen influenced critical stress intensity in an api x70 pipeline steel welded joint – experiments and FE simulations, Int. J. Hydrog. Energy 37 (2012) 11474–11486. [8] H.J. Christ, A. Jung, H.J. Maier, R. Teteruk, Thermomechanical fatigue–damage mechanisms and mechanism-based life prediction methods, Sadhana 28 (2003) 147–165. [9] S. Hassanifard, M.A. Mohtadi-Bonab, Gh. Jabbari, Investigation of fatigue crack propagation in spot-welded joints based on fracture mechanics approach, J. Mater. Eng. Perform. 22 (2013) 245–250. [10] J.F. Cooper, R.A. Smith, The measurement of fatigue cracks at spot-welds, Int. J. Fatigue 7 (1985) 137–140. [11] J.W. Sowards, T. Gnäupel-Herold, J.D. McColskey, V.F. Pereira, A.J. Ramirez, Characterization of mechanical properties, fatigue-crack propagation, and residual stresses in a microalloyed pipeline-steel friction-stir weld, Mater. Des. 88 (2015) 632–642. [12] J.A. Ronevich, B.P. Somerday, C.W.S. Marchi, Effects of microstructure banding on hydrogen assisted fatigue crack growth in X65 pipeline steels, Int. J. Fatigue 82 (2016) 497–504. [13] M. Yu, W. Chen, R. Kania, G. Boven, J. Been, Crack propagation of pipeline steel exposed to a near-neutral ph environment under variable pressure fluctuations, Int. J. Fatigue 82 (2016) 658–666. [14] B.T. Lu, Further study on crack growth model of buried pipelines exposed to concentrated carbonate–bicarbonate solution, Eng. Fract. Mech. 131 (2014) 296–314. [15] F. Huang, J. Liu, Z.J. Deng, J.H. Cheng, Z.H. Lu, X.G. Li, Effect of microstructure and inclusions on hydrogen induced cracking susceptibility and hydrogen trapping efficiency of X120 pipeline steel, Mater. Sci. Eng. A 527 (2010) 6997–7001. [16] J. Moon, C. Park, S.J. Kim, Influence of Ti addition on the hydrogen induced cracking of API 5L X70 Hot-rolled pipeline steel in acid sour media, Met. Mater. Int. 18 (2012) 613–617. [17] D. Hardie, E.A. Charles, A.H. Lopez, Hydrogen embrittlement of high strength pipelines steel, Corros. Sci. 48 (2006) 4378–4385. [18] F.A. Khalid, M. Farooque, A. Ul Haq, A.Q. Khan, Role of ferrite/pearlite banded structure and segregation on mechanical properties of microalloyed hot rolled steel, Mater. Sci. Tech. 15 (1999) 1209–1215. [19] R. Jamaati, M.R. Toroghinejad, M.A. Mohtadi-Bonab, H. Edris, J.A. Szpunar, M.R. Salmani, Texture development of ARB-processed steel-based nanocomposite, J. Mater. Eng. Perfor. 23 (2014) 4436–4445. [20] W. Wu, Z. Liu, Y. Hu, F. Li, S. Bai, P. Xia, A. Wanga, C. Ye, Goss texture intensity effect on fatigue crack propagation resistance in an Al-Cu-Mg alloy J. Alloys
224
Engineering Failure Analysis 94 (2018) 214–225
M.A. Mohtadi-Bonab et al.
Compds, Accepted Manuscript, DOI: https://doi.org/10.1016/j.jallcom.2017.09.320. [21] F. Li, Z. Liu, W. Wu, P. Xia, P. Ying, Q. Zhao, J. Li, S. Bai, C. Ye, On the role of texture in governing fatigue crack propagation behavior of 2524 Aluminum Alloy, Mater. Sci. Eng. A 669 (2016) 367–378. [22] V. Venegas, F. Caleyo, T. Baudin, J.H. Espina-Hernández, J.M. Hallen, On the role of crystallographic texture in mitigating hydrogen-induced cracking in pipeline steel, Corros. Sci. 53 (2011) 4204–4212. [23] M.A. Mohtadi-Bonab, J.A. Szpunar, R. Basu, M. Eskandari, The mechanism of failure by hydrogen induced cracking in an acidic environment for API 5L X70 pipeline steel, Int. J. Hydrog. Energy 40 (2015) 1096–1107. [24] M.A. Mohtadi-Bonab, M. Eskandari, J.A. Szpunar, Effect of arisen dislocation density and texture components during cold rolling and annealing treatments on hydrogen induced cracking susceptibility in pipeline steel, J. Mater. Res. 31 (2016) 3390–3400. [25] M.A. Mohtadi-Bonab, M. Eskandari, J.A. Szpunar, Texture, local misorientation, grain boundary and recrystallization fraction in pipeline steels related to hydrogen induced cracking, Mater. Sci. Eng. A 620 (2015) 97–106. [26] G. Mingfei, Y. Hao, In-situ investigation on the fatigue crack propagation behavior in ferrite-pearlte and dual-phase ferrite-bainite low carbon steels, Science China 26 (2013) 71–79. [27] V. Venegas, F. Caleyo, J.M. Hallen, T. Baudin, R. Penelle, Role of crystallographic texture in hydrogen-induced cracking of low carbon steels for sour service piping, Miner. Metal. Mater. Soci. ASM Int. 38 (2007) 1022–1031. [28] S. Brinckman, On the Role of Dislocations in Fatigue Crack Initiation, PhD Thesis University of Groningen, 2005. [29] M.A. Mohtadi-Bonab, M. Eskandari, H. Ghaednia, S. Das, Effect of microstructural parameters on fatigue crack propagation in an API X65 pipeline steel, J. Mater. Eng. Perfor. 25 (2016) 4933–4940.
225