A new understanding of intergranular stress corrosion cracking resistance of pipeline steel through grain boundary character and crystallographic texture studies

Corrosion Science 51 (2009) 119–128

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A new understanding of intergranular stress corrosion cracking resistance of pipeline steel through grain boundary character and crystallographic texture studies M.A. Arafin *, J.A. Szpunar 1 Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, Canada H3A2B2

a r t i c l e

i n f o

Article history: Received 30 June 2008 Accepted 3 October 2008 Available online 14 October 2008 Keywords: A. Steel B. SEM B. X-ray diffraction C. Stress corrosion B. EBSD

a b s t r a c t The roles of grain boundary character and crystallographic texture on the intergranular stress corrosion cracking (IGSCC) of API X-65 pipeline steel has been studied using scanning electron microscope (SEM) based electron backscattered diffraction (EBSD) and X-ray texture measurements. It has been found that low angle and special coincident site lattice (CSL) boundaries, mainly R11 and R13b and, possibly R5, are crack-resistant while the CSL boundaries beyond R13b and the random high angle boundaries are prone to cracking. However, several cracks were found to have been arrested even when the random high angle grain boundaries were available for them, both at the crack-tips and areas immediately ahead of them, to continue propagating. Texture studies in the vicinities of these crack-arrest regions, as well as in the cracked areas, provided a new understanding of crystallographic orientation-dependent IGSCC resistance: the boundaries of {1 1 0}krolling plane (RP) and {1 1 1}kRP textured grains, mainly associated with h1 1 0i and h1 1 1i rotation axes, respectively, were crack-resistant due to their low energy configurations, while the cracked boundaries were mainly linked to the {1 0 0}kRP textured grains. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Intergranular stress corrosion cracking (IGSCC) has long been an issue of serious concern in the pipeline industry. Despite the fact that several catastrophic failures have been reported so far due to this reason all over the world in the last forty years or so (see, e.g., [1–3]), the structural characteristics of pipeline materials that greatly influence the cracking process are still not clearly understood. This paper reports some key findings on the grain boundary character and crystallographic texture dependent IGSCC resistance of API X65 pipeline steel. The mechanism of formation of IGSCC in pipeline steel material can be summarized as following: the underground pipelines utilize cathodic protection to prevent the loss of metals but, unfortunately, the cathodic current breaks the ground water into hydroxyl ions and thus increases the pH [3]. This high pH solution reacts with CO2 and form a complex carbonate–bicarbonate solution [4,5]. Extensive efforts have been expended to understand the mechanism of formation of classical SCC in this high pH carbonate–bicarbonate environment. Perhaps, the most frequently given explanation is that when the concentration of carbonate is high en-

* Corresponding author. Tel.: +1 514 398 4755x09516; fax: +1 514 398 4492. E-mail addresses: muhammad.arafi[email protected] (M.A. Arafin), jerzy. [email protected] (J.A. Szpunar). 1 Tel.: +1 514 398 2050; fax: +1 514 398 4492. 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.10.006

ough in the solution to passivate the pipe surface, intergranular cracking occurs by anodic dissolution mechanism [6–8]. Parkins [6] also suggested that plastic strain ahead of the crack-tip prevents the formation of protective oxide films and cracks continue to propagate. However, regardless of the mechanisms, it is, no doubt, obvious that the susceptibilities of the pipeline to IGSCC are material and environment dependent. Therefore, it is imperative that the structure of the grain boundaries that facilitate or resist such cracking be identified. Unfortunately, this fundamental information which can be exploited to produce pipeline steels with superior IGSCC resistance still remains unexplored. It is well-accepted that random high angle grain boundaries (HAB) have higher energy than low angle (LAB) and special coincidence site lattice (CSL) boundaries and provide relatively easy path for crack propagation [9–12]. The role of grain boundary character on the IGSCC of high purity nickel, austenitic nickel and iron based stainless alloys and, Ni3Al intermetallic was studied by several researchers (see, e.g., [13–18]). But, except the immunity of R1 (LAB) and R3 (twin) boundaries, no general consensus was found among them on the specialty of other CSL boundaries in providing resistance to IGSCC and, each material has to be assessed separately. However, a common practice to start with is to exclude the CSL boundaries with R value higher than 29 from the list of boundaries that might provide resistance to cracking, as was originally suggested by Watanabe [19,20]. One must also remember

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that the immunity of the grain boundaries to cracking is greatly affected by the segregation of impurity elements, e.g., sulphur, phosphorus, etc., as well as by the harsh service environment and, on many occasions, the group of special boundaries that are IGSCCresistant are only a few (see, e.g., [13,15–17]). The API X65 pipeline steel, commonly used in pipeline industry, is ferritic/pearlitic which does not exhibit noticeable twinning; however, Venegas et al. [21] found that most of the low R CSL boundaries, up to R33, were resistant to hydrogen induced cracking (HIC) in API X46 pipeline steel; however, in a latter paper [22], they indicated that these boundaries were mainly R13b, R11 and R29a types although, the role of these boundaries and whether any other types of CSL boundaries could be considered as special in providing resistance to IGSCC is not known for this steel let alone the API X65 pipeline steel. It is also well-known from the literature that texture plays a key role in different types of cracking such as fatigue cracking, hydrogen induced cracking (HIC), deformation cracking, etc. [21–30]. Therefore, it would be of interest to find out whether crystallographic texture has any effect on the intergranular stress corrosion cracking of pipeline steel. Alexandreanu and Was [31] studied the effect of grain orientation on IGSCC but, the inclusion of texture in that study was only limited to assessing whether the cracked boundaries belonged to the grains of similar or dissimilar orientations and, also, the material of their study was austenitic nickelbased (Ni–16Cr–9Fe) alloy which is not used in the pipeline industry. The authors concluded that boundaries associated with grains of dissimilar orientations are susceptible to cracking and the ones with similar orientations are less vulnerable. This approach has very limited practical value because it does not take into consideration the energy aspect of the boundaries which is the main driving force for crack propagation nor does it identify the crack-resistant boundaries that can be generated by certain textured grains. Recently, King et al. [32] showed that, besides the LAB and low R CSL boundaries, grain boundaries adjacent to low {h k l} index planes could resist IGSCC in austenitic stainless steel, as was originally suggested by Rohrer et al. [33] that such boundaries might have special properties. Indeed, based on Rohrer et al.’s work [33], Randle [34] proposed a new terminology called ‘‘grain boundary plane engineering” and indicated that more of these boundaries could be incorporated in polycrystals through grain shape and texture control. On the other hand, it might be possible that boundaries associated with the grains of certain orientations are resistant to IGSCC while some are conducive to such cracking which was the case with HIC in API X46 pipeline steel [22]. It is worth mentioning here that, since the IGSCC follows the grain boundary path, the temptation to focus on the grain boundary character, based on the misorientation angle between the neighboring grains (LAB and HAB) and the definition of CSL boundaries, is obvious. However, crystallographic texture can control grain misorientation and, therefore, indirectly the grain boundary structure and energy. In addition, the grain boundary energy is not solely dependent on the misorientation between grains but is also dictated by the axis of misorientation and, thus, texture. In other words, for the same misorientation angle but different rotation axis, the boundary energy can be significantly different. One should, thus, expect that the local texture, namely the grain orientation in the vicinity of the crack, may affect crack propagation or arrest. The objective of the current study is to examine and identify

the roles of texture and grain boundary character in intergranular stress corrosion crack propagation and arrest of API X65 pipeline steel. Better understanding of these processes is the key to improving and optimizing the structures of both current and future pipeline steels. 2. Experimental investigations 2.1. Materials The API X65 samples were taken out from an in-service pipeline, used to transport natural gas from Alberta to Eastern Canada and USA, which contained several intergranular stress corrosion cracks. The wall-thickness of the pipe was approximately 1 cm and the composition of the steel is given in Table 1. 2.2. Microstructure, micro- and meso-texture studies The samples were cut along the TD–ND section, as shown in Fig. 1, to study the microstructure, grain boundary character distribution and micro-texture in order to identify the characteristics of the grain boundaries and texture that are susceptible to IGSCC and the ones that resist such cracking. This would also allow examining the possible crack macro-branching and deflection. For optical microscopy, samples were prepared by grinding with SiC papers up to 1200 grit and then polishing with 3 and 1 lm diamond paste. The samples were etched with 2% nital solution for approximately 40 s and examined with the Clemex Imaging System. Micro- and meso-texture studies have been conducted using Philips XL30 S FEG SEM equipped with EBSD detector and TSL OIM Analysis Software. The sample preparation for EBSD was quite extensive. The samples were first ground up to 1200 grit, then polished with 3 and 1 lm diamond paste suspension and, finally, polished with 0.05 lm colloidal silica slurry for 6 h. 2.3. Macro-texture studies Macro-texture measurements have been carried out at different layers of the pipe thickness along the RD–TD sections to study the non-homogeneity of texture and grain boundary misorientation distributions through the thickness. These studies were carried out in a Siemens D-500 X-ray Diffractometer equipped with Texture Goniometer. The {1 1 0}, {2 0 0} and {2 1 1} incomplete pole figures were obtained, using Mo radiation, in the reflection mode on a 5° grid up to 80° sample tilt. The recorded experimental texture data were analyzed using the TexTools Software [35]. 3. Results and discussions 3.1. Nature of SCC in the pipeline steel In the investigated samples, a total of 10 cracks were found and, among them, six were of significant length (>10% of the pipe thickness). These big cracks were often macro-branched or, deflected or, branched and deflected and then merged with subsequent deflection as shown in Fig. 2. All the cracks were of intergranular nature which is an indication of high pH SCC [4,6]. It has been reported by several researchers that the characteristics of high pH SCC include frequent micro- and macro-branching [18,36–38]. The micro-

Table 1 Composition of API X65 pipeline steel (wt%). C

Mn

Si

S

P

Ni

Cr

Mo

Cu

V

Nb

Ti

Al

Fe

0.07

1.36

0.19

0.002

0.013

0.01

0.2

<0.01

<0.01

<0.01

0.04

<0.01

0.011

Bal

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Fig. 1. Sample preparation for macro- and micro-texture studies (RP; rolling Plane, RD; rolling direction, ND; normal direction and TD; transverse direction).

Fig. 2. Examples of SCC cracks (a) branched, merged and then deflected; (b) deflected (ND and TD are pipe’s normal (radial) direction and transverse direction, respectively).

graphs shown in Fig. 2 are examples of such branching and deflection. Since the cracks are of intergranular nature, it is expected that the crack propagation path is heavily dependent on the associated grain boundary characteristics, as discussed earlier. It is also of interest to see which crystallographic textures have influences on these grain boundaries that retard the crack propagation and bring the crack to an arrest. 3.2. Grain boundary character distribution Four different areas were chosen starting from the outer edge towards the center of the pipe thickness. Interestingly, the number of high angle grain boundaries was found to be much higher on the outer edge which gradually decreased towards the mid-thickness region, as shown in Fig. 3. It should be pointed out here that the longest crack was of 1.5 mm length which corresponds to location 3 in Fig. 3. As can be seen, in addition to the decreasing number of HAB, the number of CSL boundaries, satisfying the Brandon’s criterion [39] of maximum angular deviation (Dh = 15°R1/2), increased gradually from the outer surface towards the mid-thickness of the pipe. The observed CSL boundaries in the crack-tip regions were mainly R3, R11 and R13b types as shown in Fig. 4. However, except R11 and R13b, no R3n boundaries were found at the specific crack-ar-

rest points. The R3n boundaries in low carbon ferritic/pearlitic steels are essentially texture induced non-coherent high energy grain boundaries since true twinning/multiple twining, in general, does not occur in these steels [22] and, hence, cannot be considered special boundaries that resist cracking. One important point here is that there is no direct relationship of grain boundary energy with the R value [40] and, hence, it cannot be said that other CSL boundaries having R values less than 13b have lower energy and, automatically qualify to be included in the special category. The fractions of R5, R7 and R13a boundaries in the steel samples were very small to positively conclude on their ability to resist cracking. Nevertheless, besides the immunity of R11 and R13b boundaries, R5 boundaries seemed to be crack-resistant as well, an example of this will be given in the following section. Unfortunately, no R7 and R13a boundaries were observed at the specific triple junctions where the cracks were found to have been arrested, nor were present along the cracked paths, and thus, their specialty in providing resistance to IGSCC could not be ascertained. 3.3. Crack propagation, branching and deflection – local analyses The grains of interests in the EBSD IQ map (Fig. 5a) were numbered and misorientations between the neighboring grains were calculated. Some of the triple junctions, extracted from Fig. 5a,

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Fig. 3. (a) Bar chart showing the grain boundary character distributions from the pipe outer edge towards the mid-thickness (b) measured positions in the pipe thickness (locations 1, 2, 3 and 4 are 250 lm, 1.1 mm, 1.5 mm and 2 mm from the outer surface of the pipe, respectively).

are also presented schematically in Fig. 5b for better visualization. The crack branched into two when it approached grains 7 and 8. It was found that the misorientations between grains 7 and 13 and, 7 and 10 were 56.3° and 41.2°, respectively, both of which fall into the classification of HAB (h > 15°); also, neither of these boundaries were special CSL boundaries. Therefore, both of them could be identified as high energy grain boundaries which provide easy crack propagation paths and, this might be the reason why the initial crack branched into two segments. The lower branched crack (see Fig. 5a) got deflected at an angle of almost 45°, shown in the circle, when it reached grain 30. Calculation of misorientations between grains 30 and 31, and, 31 and 32 revealed that the latter is a

Fig. 4. Examples of CSL boundary distributions in the crack-tip regions.

low angle boundary (h = 12.3°) whereas the misorientation of the former is 36.1° which is a HAB. This is probably the reason the crack took a sharp deflected path although the boundary between grains 31 and 32 was the most favorably oriented, or, in other words, perpendicular to the direction of circumferential hoop stress. A sharp change of direction was again observed when the crack arrived at the triple junction consists of grains 36, 37 and 38. It was expected that the crack would go through the boundary between the grains 37 and 38 but, instead, it chose to break only the boundary between 36 and 38 which was unfavorably oriented with respect to the stress axis. CSL analyses revealed that the boundary between the former grain pair was a R11 boundary which obviously had much lower energy than the random high angle boundaries and, therefore, crack-resistant. When the crack reached the triple junction associated with grains 39, 40 and 41, it seemed to have jumped over the boundary between grains 39 and 40, without cracking it, and reappeared in the next grain which is essentially a 3D effect. The boundary between grains 39 and 40 was a LAB (h = 9°) but the one between 40 and 41 was a HAB (h = 43°) and also, favorably oriented. This seemed to be quite unusual according to the conventional understanding of grain boundary characteristics; however, texture analysis revealed that these grains had orientations close to {1 1 0}krolling plane (RP) with h1 1 0i boundary rotation axis which might have played a role in resisting the crack propagation along this path. The cracking resistance of this type of boundaries will be discussed in details latter in Section 3.6. The deflected crack continued in the same direction as the favorably oriented boundaries were R11 and R13b special CSL boundaries, as indicated by arrows in Fig. 5a. No crack was observed between grains 1 0 0 and 1 0 1 but it re-emerged again in the next grain. The boundary between grains 1 0 0 and 1 0 1 was a R5 boundary and this might be the reason why it was not fractured but, considering the fact that grains are 3D, the crack probably took a path underneath or above the investigated surface where the boundary energy was higher and got back to the surface in the next grain. It should be pointed out here that CSL boundaries beyond R13b were not found to be particularly crack-resistant, e.g., some R17a and R29a boundaries were observed among the cracked boundaries. Although the fractions of these boundaries were very small to get statistically valid information, but in no case, any of these boundaries showed resistance to crack propagation which made us believe that they should be excluded from the list of special boundaries that are IGSCC-resistant. Several other crack sites, including the upper branched crack in Fig. 5a, were studied which also supported the observations that the LAB and special CSL boundaries, mainly R11 and R13b and

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22 23 25

11

8 10

6

2 3 5 28

13 12

34

14 29

30

7 15

4

16 17

1 27

Σ13b

9

24

26

19

18

35

TD

31 32 33

36

37 38

Σ11

21

ND 41

20

39 40

Σ11

Σ13b Σ11

Σ5 100 101

25 μm

R

10

R

7

13

R

31 30

R

R

L

L

{110}//RP

36

32

R

37 Σ11 38

TD

{110}//RP

41 R <110> R 39 40

R

R 100

Σ5 101 R

ND

Fig. 5. Grain boundary character analyses for crack propagation, branching and deflection: (a) EBSD IQ map, (b) local analyses (extracted from image (a): R ? HAB and L ? LAB).

perhaps R5, are crack-resistant and, deviations from the ideal linear direction of crack propagation occurs when the crack-tip meet random high misorientation grain boundaries which have, in general, high energy. 3.4. Role of texture on IGSCC-micro-texture studies Many of the observed cracks were arrested in the regions where the fractions of HAB were reasonably high and, it would be of interest to find out whether crystallographic texture played a role in arresting these cracks despite the fact that high angle grain boundaries were available for them to propagate. One such example is shown in Fig. 6. The grain boundary character distribution immediately ahead of the crack-tip region showed strong presence of random high angle boundaries, however, the crack got arrested. Specific triple junction where the crack was arrested also showed two non-cracked random high angle boundaries, as presented schematically (extracted from the EBSD IQ map) in Fig. 6c. In steels, ODF at u2 = 45° section displays the major texture components as shown schematically in Fig. 7a. Therefore, in the current study, ODFs from the EBSD scan data have been calculated at this cross-section along the crack paths and just beyond the

crack-arrest points to evaluate the role of texture on the SCC. As can be seen that {1 0 0}kRP has the highest intensity along the crack propagation path (Fig. 7b) while {1 1 0}kRP was dominant in the latter case (Fig. 7c). In order to verify this observation, a total of 18 cracked and immediately ahead of the crack-tip areas were also investigated. Representative examples of the inverse pole figures, for the regions just beyond the crack-tips, are shown in Fig. 8 which exhibit dominant {1 1 0}kRP texture and, to some extent, {1 1 1}kRP texture. In contrast to the crack-arrest areas, the cracked regions showed mainly {1 0 0}kRP texture, as evident in Fig. 9 where two representative examples are shown. Therefore, it seems that cracks tend to follow the grain boundaries associated with {1 0 0}kRP textured grains and get arrested when they encounter the boundaries associated with {1 1 0}kRP or {1 1 1}kRP textured grains. It should be pointed out here that the crack-arrest regions with higher CSL boundary fractions also exhibited dominant {1 1 0}kRP and {1 1 1}kRP textures since R11 and R13b boundaries, largely present in these areas, are ideally defined with h1 1 0i/50.48° and h1 1 1i/ 27.8° rotation-axis/misorientation-angle [41], respectively, the fractions of which could be increased significantly by the presence of {1 1 0}kRP and {1 1 1}kRP textured grains.

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Fig. 6. Example of HAB dominant crack-arrest region: (a) EBSD IQ map, (b) GBCD and (c) schematic of triple junction showing the cracked and non-cracked segments.

{100}//RP

a {100}<110>

ϕ 2 = constant = 45˚

{112}//RP

ϕ 1 (0˚ - 90˚)

{112}<110>

{111}<112>

{111}//RP {111}//RP

ϕ

{111}<110>

(0˚ - 90˚)

{111}<110> {111}<110>

{110}<110> {110}//RP

b

c

Fig. 7. u2 = 45° section of the ODFs: (a) schematic of major texture components, (b) along the cracked path and (c) in the region immediately ahead of the crack-tip.

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Fig. 8. Examples of inverse pole figures for immediately ahead of the crack-tip areas.

Fig. 9. Examples of inverse pole figures for the cracked regions.

3.5. Macro-texture studies through the pipe thickness Macro-texture studies have been conducted on a non-cracked sample of the same pipe in order to further verify the roles of grain boundary character and crystallographic texture observed through the micro-texture studies presented earlier. Grain boundary misorientation distributions were calculated for different layers (RD– TD sections) along the pipe thickness using the ODFs obtained from the X-ray texture measurements. The procedure for estimating such distribution was described by Morawiec et al. [42] and the calculation module is available in the TexTools Software [35]. The change of high angle grain boundary fraction, in the interval

of 25–55°, from the surface of the pipe towards the inner surface is shown in Fig. 10. CSL boundaries are also high misorientation boundaries that can fall in the selected interval but the fraction of these boundaries (excluding R3n types) was found to be very low in this sample (maximum of 6% for up to R13b types); therefore, the reported fraction of high angle boundaries could be considered as random high angle boundaries. It is obvious from Fig. 10 that high angle grain boundary fraction is very high at the pipe surface and did not decrease in any significant way from the outer to the inner surface. However, in this sample, only small corrosion pits were observed on the surface layer and no cracks were registered.

Fig. 10. HAB fractions through the pipe thickness.

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Fig. 11. ODF at u2 = 45° section at the 150 lm beneath the surface of a non-cracked sample.

As mentioned earlier, many of the observed cracks, in our cracked specimen, were arrested even when the fractions of HABs were quite high, but a closer look at the texture revealed that {1 1 0}kRP and, to some extent, {1 1 1}kRP were dominant in the areas immediately ahead of these crack-tips regardless of their distance from the pipe surface and the fractions of high angle boundaries; whereas, {1 0 0}kRP texture was dominant along the crack propagation paths. These observations were further confirmed by studying the through-thickness texture of the non-cracked sample

of the same pipe in which the random HABs were dominant throughout the thickness. X-ray macro-texture analyses showed that {1 0 0}kRP texture was very weak (approximately 0.5 times the random intensity) at the surface, however, the {1 1 0}kRP texture had the highest intensity and, the {1 1 1}kRP fiber, as shown in Fig. 11, was well-defined. One must not compare the intensities with those of the ODFs presented earlier for the cracked and immediately ahead of the crack-tip regions. Unlike the EBSD results that represent very local textures, the X-ray texture measurements were carried out with 2.2 cm  1.38 cm specimens to reveal the macroscopic texture. The point to consider here is that {1 1 0}kRP and {1 1 1}kRP textures have five times and three times higher intensities, respectively, than the {1 0 0}kRP texture in the noncracked sample. If this result is interpreted in conjunction with the facts that, this sample did not have any crack although it encountered the same service conditions and length of deployment as the cracked sample, the conclusion that {1 1 0}kRP and {1 1 1}kRP textures could resist the crack propagation appears to be justified. The presence of a strong {1 0 0}kRP texture, as well as {1 1 1}kRP fiber texture, in the mid-thickness region of the non-cracked sample is documented in Fig. 12. Although {1 1 1}kRP could resist IGSCC, {1 0 0}kRP texture is associated with higher probability of cracking, as discussed earlier; however, stress corrosion cracks initiate at the surface where a crack-resistant texture will prevent the nucleation and growth of cracks, provided that the sample is thick enough. Therefore, it can be concluded that the cracks can be arrested either by incorporating high fraction of LAB and special CSL boundaries or, by creating {1 1 0}kRP and {1 1 1}kRP textures close to the pipe surface. 3.6. Crack propagation resistance for {1 1 0}kRP and {1 1 1}kRP textured steels

Fig. 12. ODF at u2 = 45° section at the mid-thickness region of the non-cracked sample.

In order to better understand how {1 1 0}kRP and {1 1 1}kRP textures are preventing the crack propagation, the rotation axes of the boundaries in the crack-arrest regions have been determined. The relative grain boundary energies of API X65 steel is not known for different misorientation axes, however, Hayakawa and Szpunar [43] estimated these energies for Fe–3%Si steel. In this steel, h1 1 0i and h1 1 1i misorientation axes (both for tilt and twist type boundaries), mainly associated with the {1 1 0}kRP and {1 1 1}kRP grains, have lower grain boundary energies than the ones with h1 0 0i misorientation axis associated with the {1 0 0}kRP textured grains. This is true for both low and high angle boundaries. The

Fig. 13. Axis/angle misorientation distribution of the grain boundaries in the crack-arrest region.

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Fig. 14. Elastic modulus anisotropy for (a) {1 0 0}kRP, (b) {1 1 1}kRP, and (c) {1 1 0}kRP texture dominated steels (contour intensities  100 = unit in GPa).

grain boundaries of API X65 steel is also expected to show similar results since its composition is also predominantly Fe-based (98 wt%). Similar calculations, although for low angle boundaries in pure aluminum only, were also presented by Yang et al. [44]. The axis/ angle misorientation distribution in the crack-arrest regions, a representative example of which is shown in Fig. 13, indeed revealed that the rotation axes of these grain boundaries were mainly h1 1 0i and h1 1 1i. This, again, indicates that boundaries associated with {1 1 0}kRP and {1 1 1}kRP textured grains are more likely to resist crack propagation. One exception to note here is that R5 boundaries are defined with h1 0 0i/36.87° rotation-axis/misorientationangle pair [41]. This suggests that an energy cusp might exist in the relative grain boundary energy vs. misorientation angle diagram at 36.87° for h1 0 0i boundary rotation axis. However, as mentioned earlier, the fraction of R5 boundaries were not large enough to confirm their immunity to IGSCC and, hence, the possible low energy configuration of R5 boundaries should be considered only as a prediction. Interestingly, other CSL boundaries that were observed in the crack paths such as, R17a and R29a, must have h1 0 0i rotation axis [41] which supports the validity of the estimated grain boundary energies reported in [43] and its applicability in API X65 steel. One might argue that R19 and R21a also have h1 1 1i and h1 1 0i rotation axes, respectively, and therefore, will have low energy and, thus, might be crack-resistant. But, no such boundaries were observed in the crack-arrest points to assign specialty to these boundaries in providing resistance to IGSCC, nor were observed along the crack propagation paths. 3.7. Anisotropy of Young’s modulus for {1 0 0}kRP, {1 1 1}kRP and {1 1 0}kRP steels The anisotropy of elastic modulus calculations have been carried out for {1 1 0}kRP, {1 1 1}kRP and {1 0 0}kRP textured steels in order to assess their relative toughness.

Cubic crystals have four threefold and three fourfold axes of symmetry and, there are only three elastic stiffness constants that are independent. The three independent elastic stiffness coefficients for a-Fe are well-known, C11 = 233.1 GPa, C44 = 117.83 GPa and C12 = 135.44 GPa [45]. The C-coefficients from the ODF and the elastic stiffness constants can be utilized to obtain the anisotropy of Young’s modulus for the given material, e.g., C-coefficients of {1 1 0}kRP texture dominated steel could be obtained from its ODF; these coefficients along with the three elastic stiffness coefficients could then be used to obtain the anisotropy of Young’s moduli for {1 1 0}kRP textured steel. The elastic modulus calculation module of the TexTools Software [35] was used to perform the calculations. It is evident from Fig. 14 that the elastic modulus is higher in {1 0 0}kRP textured steel (maximum of 230 GPa) than those of {1 1 1}kRP and {1 1 0}kRP textured steels (maximum of 210 and 170 GPa, respectively). This should indicate that {1 0 0}kRP textured steel is more likely to crack. 4. Conclusions Besides the role of grain boundary character on the IGSCC of pipeline steel, a new understanding of texture dependent cracking resistance has been gained which will act as a guideline to produce such steels with superior IGSCC resistance. The conclusions of this study can be summarized as following: (1) The grain boundary character plays a key role on the intergranular stress corrosion cracking of pipeline steel. Low angle and special CSL boundaries (R11, R13b and, probably, R5) are crack-resistant while the random high angle grain boundaries are susceptible to cracking. (2) CSL boundaries beyond R13b type were not found to be particularly strong in providing resistance to intergranular crack propagation. Although the fractions of such bound-

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aries were very small in the investigated samples, some of these boundaries were observed in the crack paths and none in the crack-arrest points. (3) Crack branching and deflection are controlled mainly by the structure of the grain boundaries at the junctions where these deviations from the initial crack propagation path take place. (4) Both the macro- and micro-texture studies confirmed that the crystallographic texture significantly affect the intergranular stress corrosion cracking of pipeline steel. The boundaries of {1 1 0}kRP and, to some extent, {1 1 1}kRP textured grains, associated with h1 1 0i and h1 1 1i rotation axes, respectively, provide high resistance to IGSCC while the boundaries of {1 0 0}kRP textured grains are the most susceptible. (5) This study indicates that the initiation and subsequent propagation of IGSCC might be avoided either by providing a large fraction of low angle and special CSL boundaries at the pipe surface or by modifying the surface texture.

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