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Hydrogen Embrittlement Susceptibility and Safety Control of Reheated CGHAZ in X80 Welded Pipeline Qiushi Deng, Weimin Zhao, Wei Jiang, Timing Zhang, Tingting Li, and Yujiao Zhao (Submitted June 30, 2017; in revised form November 21, 2017) Coarse-grained heat-affected zone (CGHAZ) exhibits the highest hydrogen embrittlement (HE) susceptibility, which changes under the influence of thermal cycle. In this study, slow strain rate tension (SSRT) tests were conducted to investigate the HE susceptibility of reheated CGHAZs and the critical hydrogen pressure for fracture failure. Results show that intercritically reheated CGHAZ (ICCGHAZ) possesses the lowest HE resistance. Analyses of HE index and fracture indicate that the critical hydrogen pressure is 3.5 MPa. Microstructure analysis reveals that HE susceptibility is associated with multiple factors, such as phase composition, grain coarsening, HAB density, and MA constituent. Blocky necklace-like MA constituent along prior austenite boundaries plays a predominant role in intensifying the HE susceptibility of ICCGHAZ. Keywords

critical pressure, heat-affected zones, embrittlement, MA constituent, X80

hydrogen

1. Introduction With new energy development and increasing power demands, hydrogen energy has become a hot topic (Ref 1-3) and could be a substitute for fossil fuel, which causes environmental damages. However, difficulties in transport of hydrogen gas must be solved first before promoting hydrogen energy. Pipeline steel is the most effective tool for transmitting high-pressure gas and has been widely exploited in pipeline construction. In this regard, the potential of X80 pipelines for H2 transport has been investigated. A previous study (Ref 4) mentioned that a certain amount of H2 can be transported by mixing it with natural gas. Another study (Ref 5) pointed out that some currently used pipelines can be directly applied for H2 transmission to reduce construction costs. Nevertheless, one of the most serious issues that should be considered is hydrogen damage and mechanical property degradation of low-carbon steel (Ref 6), such as X80. Many works were conducted on hydrogen embrittlement behavior of pipeline steel. Moro et al. (Ref 7) reported that hydrogen impact starts to rise in X80 steel when the H2 pressure is beyond 0.1 MPa. Meng et al. (Ref 8) tested the mechanical property of X80 in natural gas/hydrogen mixtures with 5.0, 10.0, 20.0, and 50.0 vol.% hydrogen at the pressure of 12 MPa. Results indicated that the fatigue life of X80 steel dramatically decreased when the partial hydrogen pressure reached 5.0 vol.% of 12 MPa. Thick-walled pipelines, such as X80, which exhibit long transmission distance, require welding to assemble each section; in this case, hydrogen embrittlement (HE) becomes complex on the welded joint because of microstructure heterogeneity. Many

Qiushi Deng, Weimin Zhao, Wei Jiang, Timing Zhang, Tingting Li, and Yujiao Zhao, Department of Mechanical and Electronic Engineering, China University of Petroleum, Qingdao 266580, China. Contact e-mail: [email protected].

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researchers (Ref 9-12) found that in different types of steel, heataffected zone (HAZ) exhibits high HE susceptibility; in particular, the coarse-grained heat-affected zone (CGHAZ) of X80 possesses poor HE resistance (Ref 13, 14). However, dual pass or multi-pass welding is needed for thick-walled pipes and involves secondary thermal cycle, leading to a complicated CGHAZ microstructure. After reheating to different peak temperatures, CGHAZ can be divided into sub-regions, including subcritically reheated CGHAZ (SCGHAZ), intercritically reheated CGHAZ (ICCGHAZ), supercritically reheated CGHAZ (SCCGHAZ), and unaltered reheated CGHAZ (UACGHAZ) (Ref 15). Thus far, research has focused on the reheated CGHAZ region and the results elucidated that drastic drop of toughness in ICCGHAZ threatens the whole partÕs usability (Ref 16-19). MA constituents that are distributed along the grain boundaries in a connected blocky morphology are responsible for the mechanical property degradation of ICCGHAZ (Ref 20). The increase in the MA content also affects HE susceptibility. Han et al. (Ref 21) revealed that the retained austenite and local enrichment of MA led to low hydrogen effective diffusivity and improved HE susceptibility. Zhao et al. (Ref 22) confirmed this viewpoint and stressed that coarse blocky MA can trap stress and hydrogen, facilitating the initiation of hydrogen-induced crack. Nonetheless, the impact degree of MA constituent on HE and the critical HE parameter for safety design has been rarely reported. This study investigated the HE susceptibility of the reheated CGHAZ sub-regions, namely ICCGHAZ, SCCGHAZ, and UACGHAZ, of X80 welded joints. The relationship between HE and microstructural changes was studied. The most susceptive sub-region was selected to confirm the critical pressure for HE failure. Results provide valuable reference for pipeline safety design and practical application on H2 transmission.

2. Materials and Procedures 2.1 Materials The studied material was a high-strength steel grade API X80 used for pipeline manufacturing, and its chemical

Table 1 Chemical composition of X80 steel, in wt.% Element

Fig. 1

C

Si

Mn

Cu

Mo

Ni

Cr

Nb

V

Fe

0.06

0.27

1.81

0.28

0.31

0.3

0.02

0.07

0.01

Balance

Sub-region location of the X80 reheated CGHAZ zone

composition is shown in Table 1. The test areas were located in the HAZ of the X80 welded joint (Fig. 1). After the secondary welding in the thermal cycle process, CGHAZ was further divided into sub-regions marked with A, B, and C, where A is for UACGHAZ, B is for SCCGHAZ, and C is for ICCGHAZ. The reheated CGHAZ of real weld represents a very small portion of the welded joint; as such, microstructure examination and preparation of samples for tensile tests are difficult without encountering interferences caused by adjacent sub-regions. Hence, thermal simulation was applied to reproduce a large volume of sub-regions of the reheated CGHAZ. Ac1 and Ac3 can be obtained by Eq 1 and 2, respectively (Ref 23): Ac1 ð CÞ ¼ 723  10:7wðMnÞ  3:9wðNiÞ þ 29wðSiÞ þ 16:7wðCrÞ þ 290wðAsÞ þ 6:38wðWÞ ðEq 1Þ Ac3 ð CÞ ¼ 910230wðCÞ0:5 15:2wðNiÞ þ 44:7wðSiÞ þ 104wðVÞ þ 31:5wðMnÞ þ 13:1wðWÞ ðEq 2Þ The values of Ac1 and Ac3 are 711 and 872 C, respectively. Typical peak temperatures of 800, 950, and 1350 C were selected for welding thermal simulation to obtain ICCGHAZ, SCCGHAZ, and UACGHAZ, respectively. In this study, Gleeble 3500 was utilized to fabricate different simulated sub-regions through the process shown in Fig. 2. The same heating speed (150 C/s), holding time at peak temperature (5 s), and cooling rate (20 C/s) time were adopted for both first and second thermal cycle processes.

2.2 Experimental Procedures 2.2.1 Slow Strain Rate Tension Test. Slow strain rate tension test (SSRT) was carried out according to the ASTM standard test method (G142-98). Sub-region samples were processed into specific shapes (Fig. 3). The samples for tensile tests were lightly grounded with a 600 grade SiC paper along tensile direction to eliminate the influence of the surface oxide film and unify surface roughness. Environmental test was conducted under certain H2 pressure (10.0 vol.% of 12 MPa), which was applied first to examine the HE susceptibility of each sub-region. The surface of the sample was coated with nickel, which enhanced the hydrogen adsorption and intensified the contrast of the hydrogen embrittlement index (HEI) of each sub-region under a relatively low test pressure level (Ref 24). The test was conducted at 40 ± 1 C and strain rate of 105/s. The reduction in the cross-sectional area was obtained from the test and used for calculating HEI to compare HE susceptibility. The relative reduction in the elongation rate can also theoretically reflect the HE change and thus used to determine HEI. However, the entire loading system may exhibit elastic deformation during the tensile process, thereby affecting the HEI results. In this case, the relative reduction in area can be used for direct and accurate calculation. HEI can be obtained by Eq 3: u  uH  100% ðEq 3Þ HEI ¼ 0 u0 where u0 and uH are the reduction in the cross-sectional area in nitrogen and hydrogen gas environments, respectively. The most susceptive sub-region was confirmed by comparing the HEI results. Then, environmental tensile tests were

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Fig. 2

Welding thermal cycle curves in the simulated welding process. (a) ICCGHAZ, (b) SCCGAHZ, and (c) UACGHAZ

conducted under different H2 pressure levels to determine the most vulnerable sub-region. The critical pressure for HE failure was determined by investigating HEI value, HEI increment, and fracture morphology. The samples for the test were uncoated to simulate the real working condition. Test temperature and strain rate were the same as in the former part. The specimens for both parts were pre-charged in hydrogencontaining atmosphere for 24 h prior to SSRT tests. 2.2.2 Microstructure Analysis. Microstructure was studied through optical microscopy (OM), color metallography, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) analyses. For OM and color metallography study, samples were ground up to 2000 grit and polished with 3.5 lm diamond paste suspension. The samples were etched with 4 vol.% Nital solution and Lepera for OM and color metallography analyses, respectively. Leica DM 2500 micro-

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scope was used to observe the morphology of the microstructure. For EBSD study, samples were ground up to 3000 grit, electrically polished with 5 vol.% perchloric acid, and observed by ZEISS ULTRA 55 electron microscope.

3. Results and Discussion 3.1 Hydrogen Embrittlement Susceptibility Figure 4 shows the tensile test results of CGHAZ and reheated CGHAZs. The curves are similar before the yield limit for both hydrogen and nitrogen gas environments. Plastic deformation decreased in different degrees in samples in the H2 environment compared with that in the nitrogen environment, implying that plasticity was lost due to hydrogen influence.

Fig. 3

Standard tension specimen for SSRT

Fig. 4

Stress–strain curves in the nitrogen and hydrogen environments of CGHAZ and different simulated sub-regions

Table 2 Mechanical properties of different sub-regions in nitrogen and hydrogen environments

Tensile strength/MPa Reduction of area/% HE Index (HEI)/%

Test environment

CG HAZ

ICCG HAZ

SCCG HAZ

UACG HAZ

N2 H2 N2 H2 …

762.13 760.12 73.71 45.25 38.60

755.14 747.41 71.03 42.64 39.95

701.72 692.22 75.51 52.17 30.90

723.33 710.46 74.00 46.55 37.09

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Fig. 5

Microstructure morphology of different sub-regions. (a) CGHAZ, (b) ICCGHAZ, (c) SCCGHAZ, and (d) UACGHAZ

Table 2 shows the data of tensile strength, reduction in crosssectional area, and HEI value. The tensile strength slightly decreased, and the reduction in the cross-sectional area decreased when H2 was introduced. The calculated HEI values were arranged from high to low as ICCGHAZ, CGHAZ, UACGHAZ, and SCCGHAZ. ICCGHAZ possessed the highest HE susceptibility and thus became the weakest part of the entire reheated CGHAZ region.

3.2 Microstructural Impact on HE Failure 3.2.1 Microstructure Evolution. The microstructure morphologies of the simulated samples of CGHAZ, ICCGHAZ, SCCGHAZ, and UACGHAZ are shown in Fig. 5. Phase composition is pointed with white arrows. Microstructural changes occurred after secondary reheating at different peak temperatures (Fig. 5b-d). Figure 5a shows that CGHAZ mainly consists of lath bainite ferrite (BF), with grain size of 30-50 lm. Bainite lath and discontinuously distributed carbonenriched constituents were distinctly observed. Figure 5(b) demonstrates the microstructure morphology of ICCGHAZ, which underwent secondary reheating at 800 C. The coarse bainitic microstructure remained, but necklace-shaped carbonrich constituent was formed along grain boundaries and confirmed as MA constituent (Ref 15, 25). ICCGHAZ under-

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went incomplete phase transformation because the secondary transformation temperature is slightly higher than Ac1. The microstructure partially reverted to austenite along grain boundaries due to the low transformation temperature in a short time. The diffusion of carbon was difficult, and it accumulated at grain boundaries to stabilize austenite, which was formed between Ac1 and Ac3, during which MA constituent was generated and distributed along grain boundaries in a necklace-like shape (Ref 16, 25). By contrast, notable changes were observed after CGHAZ was reheated to 950 C (Fig. 5c). Lath bainite ferrite (BF) with coarse grain size was replaced by uniform massive ferrite (MF) and a small amount of granular bainite (GB). Grain size was refined to less than 20 lm, and the grain boundary area increased because of intermediate-temperature transformation. According to the Hall–Petch equation, small grain size in SCCGHAZ improved the mechanical property of the material. Figure 5(d) reveals the microstructure morphology of UACGHAZ that was subjected to secondary heating at 1350 C. The microstructure still mainly consists of lath bainite ferrite (BF), with the largest grain size (over 50 lm in average) and the smallest grain boundary area, compared with the other microstructures. Grain boundary is affected not only by grain size changes but also by grain reorientation (Ref 24, 26). All Euler maps of the sub-regions were obtained through EBSD technique

Fig. 6

EBSD all Euler maps of different sub-regions. (a) CGHAZ, (b) ICCGHAZ, (c) SCCGHAZ, and (d) UACGHAZ

Fig. 7 Distribution, fraction, and morphology of the MA constituent of different sub-regions. (a) CGHAZ, (b) ICCGHAZ, (c) SCCGHAZ, and (d) UACGHAZ

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Fig. 8

SSRT results under different H2 pressure levels

Fig. 9 levels

Hydrogen embrittlement index under different H2 pressure

(Fig. 6); in the image, the high-angle grain boundaries (greater than 15) are sketched with black lines, and similar or same colored grain represents similar or same grain orientation. The content of high-angle grain boundaries (HAB) was counted through HKL software. The densities of HAB are 18.8, 23.2, 35.8, and 17.7% for CGHAZ, ICCGHAZ, SCCGHAZ, and UACGHAZ, respectively. 3.2.2 Factors Influencing HE. Diffusible hydrogen mainly affects plastic deformation, resulting in plasticity loss and fracture failure. When plastic deformation occurs, dislocation starts to move massively. The trapped hydrogen can be migrated through dislocation motion (Ref 27) and locally accumulate when the dislocation piles up or stops. Hydrogen diffuses and accumulates to different degrees due to microstructural differences in different samples, resulting in changes in the HE susceptibility at different levels. From the perspective of phase composition, the bainitic microstructure in CGHAZ, ICCGHAZ, and UACGHAZ is more susceptible to HE than the ferritic or granular bainitic microstructure in SCCGHAZ (Ref 28, 29). Furthermore, the grain boundary exhibits a loose structure and facilitates hydrogen accumulation (Ref 30).

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Analysis of the microstructure morphology showed that the grain size showed a significantly higher decrease in SCCGHAZ than that in ICCGHAZ and UACGHAZ. The grain boundary length in SCCGHAZ is higher than that in the other regions; hence, the amount of hydrogen trapped per unit length of the grain boundary decreased under the same hydrogen charging condition (Ref 31). In this case, SCCGHAZ exhibited low hydrogen enrichment along grain boundaries. Meanwhile, UACGHAZ, the sample with the smallest amount of grain boundary area supposedly possessed high HE susceptibility. Noticeably, the diffusible hydrogen can diffuse into the microcavity generated by dislocation movement and crack would initiate here if [H] accumulates to a critical value (Ref 32). The boundaries with high angle (> 15), known as irreversible hydrogen trap, have high cohesive energy to bind hydrogen from diffusion. When load is applied, the hydrogen trapped is extremely difficult to move toward the micro-cavities, therefore mitigating the HE impact to some extent. From the result of the EBSD study, SCCGHAZ exhibited the highest density of HAB. By contrast, the HAB values in CGHAZ, ICCGHAZ, and UACGHAZ are relatively low, which may engage more hydrogen to diffuse and aggregate, thereby increasing the HE susceptibility. CGHAZ, ICCGHAZ, and UACGHAZ consist of bainitic microstructure, and UACGHAZ possesses the highest grain size, the highest grain boundary area reduction, and the lowest HAB content. The HEI of UACGHAZ is supposed to be the highest, but the HE susceptibility of this sample is slightly lower than that of CGHAZ and ICCGHAZ. The most susceptive sub-region was confirmed to be ICCGHAZ. This finding may be associated with the existence of MA constituent because the retained austenite and the interfaces between the constituents can trap hydrogen from transporting (Ref 21) and enrich diffusible hydrogen locally. The hard phase-like MA can cause local stress concentration, which facilitates easy accumulation of hydrogen. The effect of the existence of MA constituent on HE susceptibility was investigated. Lepera color etching technique was used to identify and quantify the distribution, fraction, and morphology of MA constituent in the sub-regions. Figure 7 shows the micrographs of CGHAZ, ICCGHAZ, SCCGHAZ, and UACGHAZ after Lepera etching, where the prominently bright phases were identified as the MA constituent. The contents of MA were calculated to be 3.09, 4.61, 2.96, and 2.15% (the result was averaged by counting three areas of each sample) in CGHAZ, ICCGHAZ, SCCGHAZ, and UACGHAZ, respectively. Compared with that in CGHAZ, the fraction of the MA constituent decreased in the samples reheated at 1350 C. The MA constituents in UACGHAZ are dispersed along the bainite lath, and some fine-sized components are discontinuously distributed along the grain boundaries. This finding implied that less hydrogen was trapped by MA constituents in UACGHAZ and less diffusible hydrogen was concentrated at these areas than the other three sub-regions. The slightly lower HE susceptibility of UACGHAZ than that of CGHAZ can be explained from the perspective of MA constituent. The content of MA constituent of ICCGHAZ is the highest among the sub-regions tested. The morphology and distribution changed, and blocky MA constituents were observed, aggregating along the prior austenite boundaries in a necklace-like

Fig. 10

Fracture surface morphology of ICCGHAZ specimen: (a) under nitrogen environment and (b) detail of rectangular box in (a)

Fig. 11 Fracture surface morphology of ICCGHAZ specimen under different hydrogen pressure levels: (a) 3 MPa, (b) 3.5 MPa, (c) 4 MPa, and (d) 6 MPa

Table 3 Brittle area proportion under different hydrogen pressure levels Hydrogen pressure/MPa Brittle area proportion/%

3 51.3

shape. This type of distribution damaged the integrality of the microstructure and thus deteriorated its mechanical property. Grain boundaries were weakened by the agglomeration of large blocky MA constituent. When stress was applied on this area, stress concentration was massively generated at the location

3.5 52.1

4 65.1

6 73.5

where the blocky MA constituents accumulated. In consequence, micro-cracks may be induced along the interface between the MA constituent and the matrix; such cracks can assist the debonding of the MA constituents from the matrix and the crack propagation (Ref 16, 21, 33). Hydrogen near this

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area can be easily attracted due to the high triaxial tensile stress generated. Hence, blocky MA constituents with necklace-like shape along the grain boundaries improved hydrogen enrichment locally and increased HE susceptibility. The morphology and distribution of MA constituents play a predominant role in HE over other factors mentioned above. ICCGHAZ exhibited the highest HEI value and the greatest HE susceptibility.

3.5 MPa. Hence, the H2 transmission pressure should be limited below 3.5 MPa for safety concern.

4. Conclusions (1)

3.3 Critical Parameter for HE Failure Based on the cask effect, the region that shows the greatest HE susceptibility should be selected to investigate the safety working pressure for hydrogen gas transmission from the design and application perspectives. ICCGHAZ was selected according to the previous result. Usually, if the relative reduction in the crosssectional area (RAH2/RAN2) is greater than 0.7 (or HEI is less than 0.3), then the material shows acceptable HE resistance. However, the criterion cannot be unified due to differences in material property, microstructure, and working/experiment conditions. For example, Kong et al. (Ref 34) defined the HE sensitive zone where HEI was beyond 35% of X80 steel welded joints under H2S-containing environment. Therefore, the critical value for HE in this study should be investigated accordingly. Figure 8 shows the tensile test results of the simulated ICCGHAZ samples under different H2 pressure levels. The tensile strength slightly fluctuated, but the plasticity decreased in different degrees when the H2 pressure increased. The HEI under each pressure level was calculated (Fig. 9). The HEI exceeded 30% when the hydrogen pressure reached 3 MPa, indicating possible HE effect. Based on the overall trend, the HEI value increased with increasing hydrogen pressure. The greatest increment appeared when the hydrogen pressure increased from 3.5 to 4 MPa. The increment slowed down and mildly changed when the pressure exceeded 4 MPa. However, the sample with HEI exceeding 40% became intolerable, and the material became very brittle because of the 4 MPa hydrogen pressure. Figure 10(a) shows the macroscopic fracture morphology of ICCGHAZ specimen tested in nitrogen gas environment. The area within the white box was zoomed in and detailed in Fig. 10(b). The cup-cone-shaped fracture consists of dimples with different sizes and depths and secondary cracks formed by micro-void coalescence, showing a typical ductile characteristic. However, the brittle fracture characteristic appeared and increased with increasing H2 pressure. As shown in Fig. 11, each fracture surface was divided into three sections, which were designated by I, II, and III, representing ductile area, transition area, and brittle area, respectively. The typical characteristic of each area was pointed out with white box; area I consists of continuous dimples, area II includes ductile– brittle junction, and area III is mainly made of quasi-cleavage. The junction of the brittle–ductile transition area was distinguished with red dotted line. The ductile area reduced with increasing H2 pressure. Given that the absorbed hydrogen concentration is positively correlated with hydrogen pressure, diffusible hydrogen atoms can be more introduced under higher pressure and have high chance for hydrogen enrichment inside the material, thereby increasing the HE susceptibility. The brittle area of each specimen was counted, and the proportion was calculated (Table 3). The relatively massive increment of the brittle area occurred from 3.5 to 4 MPa hydrogen pressure. Combined with the HEI values and fracture analysis results, HE susceptibility increased, and the highest risk of HE failure was obtained when the H2 pressure exceeded

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(2)

(3)

Plasticity was lost in different degrees in different subregions in reheated CGHAZ under hydrogen environment. The calculated HEI values were arranged from high to low: ICCGHAZ, CGHAZ, UACGHAZ, and SCCGHAZ, where ICCGHAZ possesses the greatest HE susceptibility and thus becomes the weakest part of the entire reheated CGHAZ region. The effect of hydrogen on embrittlement susceptibility of the reheated CGHAZ of X80 welded joint is associated with microstructural characteristics, including phase composition, grain size, HAB density, and MA constituent. The blocky necklace-like MA constituent along austenite boundaries plays a predominant role in intensifying the HE susceptibility of ICCGHAZ. HE susceptibility is positively correlated with hydrogen pressure. For ICCGHAZ, the possibility of HE failure greatly increases when the hydrogen pressure is beyond 3.5 MPa. According to the cask effect, the H2 transmission pressure should be limited below 3.5 MPa for safety concern.

Acknowledgments This work was financially funded by the National Natural Science Foundation of China (No. 51705535), the China Postdoctoral Science Foundation (No. 2016M602218), and the Natural Science Foundation of Shandong Province (No. ZR2017MEE005).

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