Microstructure evaluation after hydrogen charging

After 12 h of hydrogen charging

The microstructure of both test specimens after hydrogen charging showed similar phases compared to the starting material, except that the onset of HIC along the mid-thickness was different for both specimens as presented in Fig. 6.6. After hydrogen charging for 12 hrs HIC was observed to have nucleated only in WD specimen. This is shown in Fig. 6.6b as discontinuous intergranular microcracks, which propagated along the mid-thickness. The observed crack appeared to be non-orientation dependent in Fig. 6.6c, as crack nucleated at inter-grain boundaries and propagated through boundaries and grains associated with various crystallographic planes. Also, notice in Fig. 6.6d that the crack initiation and propagation path was mainly along the deformed grain regions. This area has been clearly highlighted by the red colored grains aligned according to the crack zone.

Fig. 6.6 Microstructural images of mid-thickness section after 12 h of hydrogen charging indicating no cracks along banding in WE (a, b), onset of discontinuous cracks in WD (c, d), EBSD orientation map (e) and recrystallization map where deformed regions = red, recovered

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After 16 h of hydrogen charging

Hydrogen charging for 16 h showed that more prominent HIC was propagated along the mid- thickness of both specimens. Cracks were aligned parallel to the transverse direction as indicated in Fig. 6.7. It was not clear where exactly the crack started in these specimens. Nevertheless, HIC was clearly formed and propagated through significant portions of both test specimens. In Fig. 6.7a, crack loops can be seen around inclusions in specimen WE. Also, many aggregates of small inclusion are found along crack path in specimen WD (Fig. 6.7b). This suggests that inclusions combined with other microstructural features played a significant role in HIC initiation and propagation across test specimens. A plausible explanation for HIC observed in these steel plates will be that cracks initiated at the mid-thickness as a result of hydrogen attack on inclusions and microstructural phases. Thereafter, crack propagation was stabilized in this region by the presence of susceptible phases and banded deformed grains that were segregated around the mid-thickness following thermomechanical processing.

Fig. 6.7 SEM micrographs of mid-thickness section after 16 h of hydrogen charging indicating HIC in (a) WE and (b) WD

Fig. 6.8 shows EBSD maps for both specimens following 16 h of hydrogen charging. It is typical for HIC to propagate in a mixed fracture mode depending on microstructure and grain characteristics [14]. Unlike the nucleation of intergranular hydrogen assisted cracks in WD

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specimen after 12 h of hydrogen charging (Fig. 6.6b-d), crack propagation became predominantly trans-granular in both specimens after 16 h The belief is that the nucleated micro-cracks became ‘aggressive’ at extended charging time. Figs. 6.8a and 6.8f shows similar orientation maps for WE and WD. Again, the same dominance of 〈011〉||RD and 〈001〉||RD fiber textures in as hot rolled samples were observed after hydrogen charging. In a different study on X65 pipeline steel, grain boundaries of 〈011〉||rolling plane (RP) and 〈111〉||RP textured grains offered resistance to cracking, while boundaries of 〈001〉||RP textured grains were more susceptible to crack propagation [147]. However, there is no evidence in this study that cracks propagated preferentially along grains of specific crystallographic orientation. This confirms the importance of other microstructural features in increasing HIC susceptibility of WE and WD specimens. Most especially, crack susceptible phases, inclusions, and banded deformed grains along the mid- thickness of both specimens will limit the possibility of crack arrest at 〈011〉||RD oriented grains. Grain boundary distribution maps in Figs. 6.8b and 6.8g shows a mixture of HAGB and LAGB in the mid-thickness regions of both pipeline steel specimens. In as hot rolled specimens, higher fraction of HAGB was established in WE and WD compared to their LAGB (Table 6.1). Some researchers have associated HAGB with high stored energy, which facilitates initiation and propagation of HIC in steels [15]. However, it is difficult to estimate the exact amount of these boundaries situated along the crack propagation path. Therefore, the predominance of HAGB may not be regarded as the sole driving force for HIC across the mid-thickness of WE and WD, even though more HAGB are observed in WD. Comparing other microstructural features will further elucidate the reasons for early crack initiation in specimen WD and subsequent crack propagation along the mid-thickness region of both test specimens.

Calculating KAM (Kernel Average Misorientation) distribution for both specimen from Figs. 6.8c and 6.8h could be useful in analysis of HIC. Local misorientation values of 0.58° and 0.66° were determined for WE and WD respectively. This also demonstrates that grains in WD have higher number of defects or stored energy, than those in WE. Also, Figs. 6.8c and 6.8h shows that majority of such defects are located along the crack path where local misorientation is higher in both specimens. This finding corroborates earlier observation of high dislocation density and banded deformed grains along the mid-thickness in as hot rolled specimens (Figs. 6.2c,h and Figs. 6.3c,h). In view of this, it is expected that in-grain misorientation and distortions between

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neighboring grains in the mid-thickness region will increase cracking susceptibility, especially in WD. Thermomechanical processing impart stored energy in pipeline steel, but recrystallization enables grain recovery. Therefore, recrystallized and recovered grains show less susceptibility to HIC, while deformed grains are highly prone to cracking [417]. Figs. 6.8d and 6.8i shows recrystallization maps for WE and WD respectively after hydrogen charging. When comparing the extent of recrystallization, deformation and recovery in both specimens, higher fraction of deformed grains is found in WD, with less recrystallized and recovered grains as presented in Table 6.4. Considering the cooling rate of 51.5 ℃/ s applied to WD and 42.75 ℃/ s applied to WE, less time will be available for complete recrystallization and recovery to occur in WD. This means that more deformed grains are bound to remain in the structure of WD, whereas less deformed grains, with more recrystallized and recovered grains will develop in WE.

Table 6.4 Area fraction of recrystallized, recovered and deformed grains in pipeline steel specimens after hydrogen charging for 16 h

Specimens Deformed area

fraction (%) Recovered area fraction (%) Recrystallized area fraction (%) WE 16 76 6 WD 30 64 5

The deformed grains in WE are mainly located along the crack path in the mid-thickness, while in WD they are seen both on the crack path and outside. A similar alignment of deformed grains in the mid-thickness region was also seen in as hot rolled specimens (Figs. 6.2d,i and Figs. 6.3d,i), and can be a reason for crack propagation in this area. One can correlate the presence of deformed grains with high local misorientation, and segregation of martensite and cementite along the mid- thickness of specimens. There are more particles of cementite segregated outside the crack path in specimen WE (Fig. 6.8e) in comparison to WD (Fig. 6.8j) which featured more martensite phase. This recalls that diffusion of hydrogen must have been lower in specimen WD when compared to WE. It is possible that higher presence of deformed grains played a significant role in limiting the amount of hydrogen that permeated through specimen WD in relation to specimen WE. The lower fraction of recovered and recrystallized grains within the cracked area of specimen WD supports the idea of lower hydrogen diffusivity and increased tendency for HIC, in contrast to specimen WE with higher fraction of recovered and recrystallized grains.

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Fig. 6.8 EBSD maps for hydrogen charged pipeline steel specimens WE (a, b, c, d, e), and WD (f, g, h, I, j) after 16 hrs. Orientation maps are a, f. Grain boundary distribution maps are b, g. Local misorientaion maps are c, h. Recrystallization fraction maps d, I, (deformed regions = red,

recovered regions = yellow, and recrystallized regions = blue). Phase maps are e, j, (blue = ferrites, black spots = martensite, yellow spots = cementite, red spots = austenite)

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