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Hydrogen Embrittlement Behavior at Different Strain Rates in Low-carbon Martensitic Steel
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2S (2015) S735 – S738
International Conference on Martensitic Transformations, ICOMAT-2014
Hydrogen embrittlement behavior at different strain rates in lowcarbon martensitic steel Y. Momotania,*, A. Shibataa,b, D. Teradab,c, N. Tsujia,b a
Department of Materials Science and Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan Elements Strategy Initative for Structural Materials, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan c Department of Mechanical Science and Engineering, Chiba institute of technology, 2-17-1 Tsudanuma, Narashino-shi, Chiba 275-0016, Japan b
Abstract This study investigated hydrogen embrittlement behavior in a low-carbon martensitic steel by uniaxial tensile test at different strain rates. With increasing the strain rate, fracture surface morphology changed from intergranular-like manner at prior austenite grain boundaries to ductile one inside prior austenite grains. The hydrogen microprint treatment revealed that hydrogen tended to accumulate mainly on prior austenite grain boundaries when the strain rate was low. On the other hand, in the case of higher strain rate, hydrogen distribution was rather uniform presumably because there was no enough time for hydrogen to accumulate on prior austenite grain boundaries during the tensile test. Accordingly, we concluded that the change of hydrogenrelated fracture manner with increasing the strain rate was due to the difference in the hydrogen accumulation behavior. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). an open access under the license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. This Selection andisPeer-review underarticle responsibility ofCC the BY-NC-ND chairs of the International Conference on Martensitic Transformations 2014. Keywords: hydrogen embrittlement; hydrogen accumulation; hydrogen microprint technique; lath martensitic microstructure; strain rate
1. Introduction The demands for high strength steels have been increasing due to economic and environmental reasons. However, it is well known that high strength steels are highly sensitive to hydrogen embrittlement. Lath martensite is a structure that appears in many high strength steels. Lath martensite structure consists of several structural units, i.e., lath, block, packet and prior austenite grain. The lath is a single crystal of martensite plate. The block consists of
* Y. Momotani. Tel.: +81-75-753-4868; fax: +81-75-753-4978. E-mail address: [email protected]
2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.387
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martenisite laths with identical orientation, and the packet consists of several blocks whose habit planes are nearly parallel. In order to improve hydrogen embrittlement properties of high strength steels, several groups investigated hydrogen-related fracture behavior from a viewpoint of microstructure of martensite. Takai et al. [1] revealed using secondary ion mass spectroscopy that prior austenite grain boundaries were one of the predominant hydrogen trapping sites. Kim et al. [2] and Nagao et al. [3] observed microstructures just beneath the quasi-cleavage fracture surface in low-carbon martensitic steels by transmission electron microscopy (TEM). Their results suggested that quasi-cleavage fracture surface was parallel to lath boundaries of martensite. Shibata et al. [4] investigated relationship between hydrogen-related crack propagation and lath martensite microstructure in a low-carbon steel, and reported that micro-cracks, which formed on or in the vicinity of prior austenite grain boundaries, propagated along certain {011}M plane, leading to hydrogen-related fracture. In addition, Ohnishi et al. [5] and Brown et al. [6] reported that hydrogen embrittlement behavior depended on not only microstructure of the material but also deformation conditions. According to their results, the susceptibility to hydrogen embrittlement increased with decreasing strain rate at any temperature ranges. So far, however, the reason why the deformation condition affects hydrogen embrittlement has not yet been elucidated. The present study investigated hydrogen-related fracture behaviors at different strain rates from a viewpoint of microstructure of martensite and hydrogen accumulation. 2. Experimental procedure An Fe-0.2C alloy (C: 0.21, Si: <0.02, Mn: <0.02, P: <0.005, S: 0.0005, Fe: bal. (wt.%)) was used in the present study. The specimen was austenitized at 1323 K for 1.8 ks in vacuum, followed by iced-brine quenching in order to obtain a fully lath martensite structure. Sheet-type tensile test specimens with a gauge length of 10 mm, width of 5 mm, and thickness of 1 mm were cut from the heat-treated specimen, and then cathodically charged with hydrogen in a 3 % NaCl aqueous solution containing 3 g L-1 NH4SCN at a current density of 1.0 A m-2 for 86.4 ks. The 4 wt.ppm. diffusible hydrogen content of the specimen measured by thermal desorption spectrometry was 4.94 Hydrogen embrittlement behavior of the specimens was evaluated by uniaxial tensile tests at strain rates ( ) of 8.3 × 10-6 s-1 and 8.3 × 10-1 s-1. The tensile tests were conducted at room temperature in air. Fracture surfaces of the specimens after the tensile test were observed by scanning electron microscopy (SEM, FEI: XL30S-FEG). Crystallographic features of the hydrogen-related fracture surface was analyzed using an electron backscattering diffraction (EBSD) analyzer equipped in SEM operated at 15 kV. The hydrogen accumulation behavior during tensile test was evaluated by hydrogen microprint technique [7]. The hydrogen microprint technique is a method to make hydrogen-trapping sites visible by the use of a reaction between silver ion coated on the specimen surface and hydrogen emitted from the specimen. After hydrogen charging, a liquid emulsion containing AgBr particles (Ilford L-4) and gelatin diluted by 10 mass% NaNO2 aqueous solution was applied on the surface of the specimen. After a tensile test, the specimens were immersed in formalin (HCHO aqueous solution) for 3 s, and then put in a fixing solution (15 mass% Na2S2O3 aqueous solution diluted by 10 mass% NaNO2 aqueous solution) for 10 min. The procedures of the hydrogen microprint technique was carried out in a dark room in order to prevent AgBr from reacting with light to form silver (Ag). The relationship between the location of precipitated silver particles and microstructure of martensite was confirmed by EBSD. 3. Results 8.3 × 10-6 s-1 Figure 1 shows fracture surfaces of the hydrogen-charged specimens after the tensile test at (a) -1 -1 8.3 × 10 s . As shown in Fig. 1 (a), macroscopic view of the fracture surface after the tensile test at and (b) 8.3 × 10-6 s-1 is intergranular-like. However, some components of the fracture surface are not smooth but striations or 8.3 × 10-1 sfine dimple patterns can be observed. On the other hand, the fracture surface after the tensile test at 1 mainly consists of dimple patterns (Fig. 1 (b)). Figure 2 (a) and (c) are SEM images around the fracture surface 8.3 × 10-6 s-1 and 8.3 × 10-1 s-1, while (b) and (d) are EBSD orientation maps of the after the tensile test at areas (a) and (c), respectively. The observations were carried out on the sections perpendicular to sheet normal. Ni layers were electrodeposited on the fracture surfaces before polishing to preserve the surfaces. In Fig. 2 (b) and (d), the block boundaries, the packet boundaries, and the prior austenite grain boundaries characterized by EBSD orientation analysis are drawn in black thin lines, black thick lines, and white broken lines, respectively. Although
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Y. Momotani et al. / Materials Today: Proceedings 2S (2015) S735 – S738
parts of the fracture surface at lower region in Fig. 2 (a) and (c) are close to the prior austenite grain boundaries, it is clear that the fracture surface does not coincide with the prior austenite grain boundaries. This indicates that the hydrogen-related fracture was not an exact intergranular fracture at prior austenite grain boundaries, but a fracture in 8.3 × 10-1 s-1, on the other hand, the ductile the vicinity of prior austenite grain boundaries. In the case of fracture surface seems to have no relationship with specific boundaries, such as the block boundaries, the packet boundaries, and the prior austenite grain boundaries. Figure 1 (b) and Fig. 2 (c,d) indicated that the specimen tested 8.3 × 10-1 s-1 showed ductile fracture. at
Fig. 1. SEM images showing fracture surfaces of the specimens after tensile tests at (a)
8.3 × 10-6 s-1 and (b)
8.3 × 10-1 s-1.
Fig. 2 SEM images ((a) ((a), (c)) and corresponding EBSD orientation maps ((b), (d)) around fracture surfaces after tensile test at (a), (b) 8.3 × 10-6 s-1 and (c), (d) 8.3 × 10-1 s-1. The gray regions (left hand sides) in (a), (c) are Ni layers plated. The block boundaries, the packet boundaries, and the prior austenite grain boundaries determined by orientation analysis are drawn in black thin lines, black thick lines, and white broken lines, respectively.
Fig. 3 SEM images ((a), (b), (d)) and EBSD orientation maps ((c), (e)) of the specimen after hydrogen microprint treatment: (a) held for 4.68 ks without loading: (b) – (e) tensile tested at 8.3 × 10-6 s-1: (c), (e) EBSD orientation maps corresponding to (b), (d). The block boundaries, the packet boundaries, and the prior austenite grain boundaries identified by orientation analysis are drawn by black thin lines, black thick lines, and white broken lines, respectively.
Fig. 4. An SEM image of the specimen tensile tested at
8.3 × 10-1 s-1. After the hydrogen microprint treatment.
Figure 3 (a) is an SEM image of the specimen after hydrogen microprint treatment without loading. The exposure time for the hydrogen microprint treatment was 4.68 ks, which corresponded to the time required for fracture in the
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tensile test at 8.3 × 10-6 s-1. It is observed in Fig. 3 (a) that the silver particles distribute randomly. Figures 3 (b– e) are SEM images after hydrogen microprint treatment and corresponding EBSD orientation maps of the specimens 8.3 × 10-6 s-1. In the EBSD orientation maps, the block boundaries, packet boundaries, and prior tensile tested at austenite grain boundaries are also drawn in black thin lines, black thick lines, and white broken lines, respectively. It was found that the distributions of silver particles in Fig. 3 (b) and (d) were completely different from that in Fig. 3 (a). The silver particles in Fig. 3 (b) and (c) were likely to precipitate along various boundaries of lath martensite structure, such as lath boundaries, block boundaries, packet boundaries, and prior austenite grain boundaries. On the other hand, a network of the silver particles was observed in the SEM image of Fig. 3 (d). The corresponding EBSD orientation map shown in Fig. 3 (e) revealed that the silver particles preferentially formed along the prior austenite grain boundaries. These results suggest that the applied tensile stress enhanced hydrogen accumulation on specific boundaries, in particular prior austenite grain boundaries. On the other hand, in the case of higher strain rate (8.3 × 10-1 s-1), silver particles precipitated rather randomly as shown in Fig. 4. This indicates that hydrogen did not 8.3 × 10-1 s-1 was 0.2 s, which was much accumulate on specific sites. The period of time during tensile test at -6 -1 8.3 × 10 s (4.68 ks). It can be considered that there was no enough time for hydrogen to shorter than that at accumulate on prior austenite grain boundaries or other boundaries during the tensile test at 8.3 × 10-1 s-1. Accordingly, the hydrogen-related fracture changed from an intergranular-like manner at prior austenite grain boundaries to a ductile manner inside prior austenite grains with increasing the strain rate. 4. Conclusions The present study investigated the effect of strain rate on hydrogen embrittlement behaviors in a low-carbon martensitic steel. The main results obtained are as follows: When the strain rate was 8.3 × 10-6 s-1, the hydrogen-charged specimen showed mainly intergranular-like fracture in the vicinity of prior austenite grain boundaries. In the case of 8.3 × 10-1 s-1, on the other hand, the hydrogen-charged specimen showed ductile fracture surfaces within prior austenite grain. Hydrogen microprint technique and EBSD analysis suggested that hydrogen tended to preferentially accumulate on specific boundaries, in particular prior austenite grain boundaries, during tensile deformation at slow strain rates. On the other hand, in the case of higher strain rate, hydrogen distributed uniformly because there was no enough time for hydrogen to accumulate on specific sites during the tensile test. Hydrogen-related fracture changed from an intergranular-like manner at prior austenite grain boundaries to a ductile manner inside prior austenite grains with increasing the strain rate because of the change in the hydrogen accumulation behavior. Acknowledgements This study was financially supported by the Grant-in-Aid for Scientific Research on Innovative Area, “Bulk Nanostructured Metals” (area No. 2201), the Grant-in-Aid for Scientific Research (A) (No. 24246114), the Grant-inAid for Young Scientists (A) (No. 24686082), the Grant-in-Aid for Challenging Exploratory Research (No. 24656440), the Grant-in-Aid for JSPS Fellows (No. 26·2927) and the Elements Strategy Initiative for Structural Materials (ESISM), all through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. NT and AS were also supported by the ISIJ Research Promotion Grant. The authors gratefully appreciate all the supports. References [1] K. Takai, J. Seki, Y. Homma, Mater. Trans. 36 (1995) 1134-1139. [2] Y. H. Kim, J. W. Morris, Jr., Metall. Trans. A 14 (1983) 1883-1888. [3] A. Nagao, A.D. Smith, M. Dadfarnia, P. Sofronis, I.M. Robertson, Acta Mater. 60 (2012) 5182-5189. [4] A. Shibata, H. Takahashi, N. Tsuji, ISIJ Inter. 52 (2012) 208-212. [5] T. Ohnishi, K. Higashi, N. Inoue, Y. Nakatani, J. Japan Inst. Metals 45 (1981) 972-976. [6] J.T. Brown, W.M. Baldwin, Trans. AIME 200 (1954) 298-303. [7] J. Ovejero-Garcia, J. Mater. Sci. 80 (1985) 2623-2629.
ScienceDirect Materials Today: Proceedings 2S (2015) S735 – S738
International Conference on Martensitic Transformations, ICOMAT-2014
Hydrogen embrittlement behavior at different strain rates in lowcarbon martensitic steel Y. Momotania,*, A. Shibataa,b, D. Teradab,c, N. Tsujia,b a
Department of Materials Science and Engineering, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan Elements Strategy Initative for Structural Materials, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan c Department of Mechanical Science and Engineering, Chiba institute of technology, 2-17-1 Tsudanuma, Narashino-shi, Chiba 275-0016, Japan b
Abstract This study investigated hydrogen embrittlement behavior in a low-carbon martensitic steel by uniaxial tensile test at different strain rates. With increasing the strain rate, fracture surface morphology changed from intergranular-like manner at prior austenite grain boundaries to ductile one inside prior austenite grains. The hydrogen microprint treatment revealed that hydrogen tended to accumulate mainly on prior austenite grain boundaries when the strain rate was low. On the other hand, in the case of higher strain rate, hydrogen distribution was rather uniform presumably because there was no enough time for hydrogen to accumulate on prior austenite grain boundaries during the tensile test. Accordingly, we concluded that the change of hydrogenrelated fracture manner with increasing the strain rate was due to the difference in the hydrogen accumulation behavior. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). an open access under the license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. This Selection andisPeer-review underarticle responsibility ofCC the BY-NC-ND chairs of the International Conference on Martensitic Transformations 2014. Keywords: hydrogen embrittlement; hydrogen accumulation; hydrogen microprint technique; lath martensitic microstructure; strain rate
1. Introduction The demands for high strength steels have been increasing due to economic and environmental reasons. However, it is well known that high strength steels are highly sensitive to hydrogen embrittlement. Lath martensite is a structure that appears in many high strength steels. Lath martensite structure consists of several structural units, i.e., lath, block, packet and prior austenite grain. The lath is a single crystal of martensite plate. The block consists of
* Y. Momotani. Tel.: +81-75-753-4868; fax: +81-75-753-4978. E-mail address: [email protected]
2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.387
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Y. Momotani et al. / Materials Today: Proceedings 2S (2015) S735 – S738
martenisite laths with identical orientation, and the packet consists of several blocks whose habit planes are nearly parallel. In order to improve hydrogen embrittlement properties of high strength steels, several groups investigated hydrogen-related fracture behavior from a viewpoint of microstructure of martensite. Takai et al. [1] revealed using secondary ion mass spectroscopy that prior austenite grain boundaries were one of the predominant hydrogen trapping sites. Kim et al. [2] and Nagao et al. [3] observed microstructures just beneath the quasi-cleavage fracture surface in low-carbon martensitic steels by transmission electron microscopy (TEM). Their results suggested that quasi-cleavage fracture surface was parallel to lath boundaries of martensite. Shibata et al. [4] investigated relationship between hydrogen-related crack propagation and lath martensite microstructure in a low-carbon steel, and reported that micro-cracks, which formed on or in the vicinity of prior austenite grain boundaries, propagated along certain {011}M plane, leading to hydrogen-related fracture. In addition, Ohnishi et al. [5] and Brown et al. [6] reported that hydrogen embrittlement behavior depended on not only microstructure of the material but also deformation conditions. According to their results, the susceptibility to hydrogen embrittlement increased with decreasing strain rate at any temperature ranges. So far, however, the reason why the deformation condition affects hydrogen embrittlement has not yet been elucidated. The present study investigated hydrogen-related fracture behaviors at different strain rates from a viewpoint of microstructure of martensite and hydrogen accumulation. 2. Experimental procedure An Fe-0.2C alloy (C: 0.21, Si: <0.02, Mn: <0.02, P: <0.005, S: 0.0005, Fe: bal. (wt.%)) was used in the present study. The specimen was austenitized at 1323 K for 1.8 ks in vacuum, followed by iced-brine quenching in order to obtain a fully lath martensite structure. Sheet-type tensile test specimens with a gauge length of 10 mm, width of 5 mm, and thickness of 1 mm were cut from the heat-treated specimen, and then cathodically charged with hydrogen in a 3 % NaCl aqueous solution containing 3 g L-1 NH4SCN at a current density of 1.0 A m-2 for 86.4 ks. The 4 wt.ppm. diffusible hydrogen content of the specimen measured by thermal desorption spectrometry was 4.94 Hydrogen embrittlement behavior of the specimens was evaluated by uniaxial tensile tests at strain rates ( ) of 8.3 × 10-6 s-1 and 8.3 × 10-1 s-1. The tensile tests were conducted at room temperature in air. Fracture surfaces of the specimens after the tensile test were observed by scanning electron microscopy (SEM, FEI: XL30S-FEG). Crystallographic features of the hydrogen-related fracture surface was analyzed using an electron backscattering diffraction (EBSD) analyzer equipped in SEM operated at 15 kV. The hydrogen accumulation behavior during tensile test was evaluated by hydrogen microprint technique [7]. The hydrogen microprint technique is a method to make hydrogen-trapping sites visible by the use of a reaction between silver ion coated on the specimen surface and hydrogen emitted from the specimen. After hydrogen charging, a liquid emulsion containing AgBr particles (Ilford L-4) and gelatin diluted by 10 mass% NaNO2 aqueous solution was applied on the surface of the specimen. After a tensile test, the specimens were immersed in formalin (HCHO aqueous solution) for 3 s, and then put in a fixing solution (15 mass% Na2S2O3 aqueous solution diluted by 10 mass% NaNO2 aqueous solution) for 10 min. The procedures of the hydrogen microprint technique was carried out in a dark room in order to prevent AgBr from reacting with light to form silver (Ag). The relationship between the location of precipitated silver particles and microstructure of martensite was confirmed by EBSD. 3. Results 8.3 × 10-6 s-1 Figure 1 shows fracture surfaces of the hydrogen-charged specimens after the tensile test at (a) -1 -1 8.3 × 10 s . As shown in Fig. 1 (a), macroscopic view of the fracture surface after the tensile test at and (b) 8.3 × 10-6 s-1 is intergranular-like. However, some components of the fracture surface are not smooth but striations or 8.3 × 10-1 sfine dimple patterns can be observed. On the other hand, the fracture surface after the tensile test at 1 mainly consists of dimple patterns (Fig. 1 (b)). Figure 2 (a) and (c) are SEM images around the fracture surface 8.3 × 10-6 s-1 and 8.3 × 10-1 s-1, while (b) and (d) are EBSD orientation maps of the after the tensile test at areas (a) and (c), respectively. The observations were carried out on the sections perpendicular to sheet normal. Ni layers were electrodeposited on the fracture surfaces before polishing to preserve the surfaces. In Fig. 2 (b) and (d), the block boundaries, the packet boundaries, and the prior austenite grain boundaries characterized by EBSD orientation analysis are drawn in black thin lines, black thick lines, and white broken lines, respectively. Although
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Y. Momotani et al. / Materials Today: Proceedings 2S (2015) S735 – S738
parts of the fracture surface at lower region in Fig. 2 (a) and (c) are close to the prior austenite grain boundaries, it is clear that the fracture surface does not coincide with the prior austenite grain boundaries. This indicates that the hydrogen-related fracture was not an exact intergranular fracture at prior austenite grain boundaries, but a fracture in 8.3 × 10-1 s-1, on the other hand, the ductile the vicinity of prior austenite grain boundaries. In the case of fracture surface seems to have no relationship with specific boundaries, such as the block boundaries, the packet boundaries, and the prior austenite grain boundaries. Figure 1 (b) and Fig. 2 (c,d) indicated that the specimen tested 8.3 × 10-1 s-1 showed ductile fracture. at
Fig. 1. SEM images showing fracture surfaces of the specimens after tensile tests at (a)
8.3 × 10-6 s-1 and (b)
8.3 × 10-1 s-1.
Fig. 2 SEM images ((a) ((a), (c)) and corresponding EBSD orientation maps ((b), (d)) around fracture surfaces after tensile test at (a), (b) 8.3 × 10-6 s-1 and (c), (d) 8.3 × 10-1 s-1. The gray regions (left hand sides) in (a), (c) are Ni layers plated. The block boundaries, the packet boundaries, and the prior austenite grain boundaries determined by orientation analysis are drawn in black thin lines, black thick lines, and white broken lines, respectively.
Fig. 3 SEM images ((a), (b), (d)) and EBSD orientation maps ((c), (e)) of the specimen after hydrogen microprint treatment: (a) held for 4.68 ks without loading: (b) – (e) tensile tested at 8.3 × 10-6 s-1: (c), (e) EBSD orientation maps corresponding to (b), (d). The block boundaries, the packet boundaries, and the prior austenite grain boundaries identified by orientation analysis are drawn by black thin lines, black thick lines, and white broken lines, respectively.
Fig. 4. An SEM image of the specimen tensile tested at
8.3 × 10-1 s-1. After the hydrogen microprint treatment.
Figure 3 (a) is an SEM image of the specimen after hydrogen microprint treatment without loading. The exposure time for the hydrogen microprint treatment was 4.68 ks, which corresponded to the time required for fracture in the
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tensile test at 8.3 × 10-6 s-1. It is observed in Fig. 3 (a) that the silver particles distribute randomly. Figures 3 (b– e) are SEM images after hydrogen microprint treatment and corresponding EBSD orientation maps of the specimens 8.3 × 10-6 s-1. In the EBSD orientation maps, the block boundaries, packet boundaries, and prior tensile tested at austenite grain boundaries are also drawn in black thin lines, black thick lines, and white broken lines, respectively. It was found that the distributions of silver particles in Fig. 3 (b) and (d) were completely different from that in Fig. 3 (a). The silver particles in Fig. 3 (b) and (c) were likely to precipitate along various boundaries of lath martensite structure, such as lath boundaries, block boundaries, packet boundaries, and prior austenite grain boundaries. On the other hand, a network of the silver particles was observed in the SEM image of Fig. 3 (d). The corresponding EBSD orientation map shown in Fig. 3 (e) revealed that the silver particles preferentially formed along the prior austenite grain boundaries. These results suggest that the applied tensile stress enhanced hydrogen accumulation on specific boundaries, in particular prior austenite grain boundaries. On the other hand, in the case of higher strain rate (8.3 × 10-1 s-1), silver particles precipitated rather randomly as shown in Fig. 4. This indicates that hydrogen did not 8.3 × 10-1 s-1 was 0.2 s, which was much accumulate on specific sites. The period of time during tensile test at -6 -1 8.3 × 10 s (4.68 ks). It can be considered that there was no enough time for hydrogen to shorter than that at accumulate on prior austenite grain boundaries or other boundaries during the tensile test at 8.3 × 10-1 s-1. Accordingly, the hydrogen-related fracture changed from an intergranular-like manner at prior austenite grain boundaries to a ductile manner inside prior austenite grains with increasing the strain rate. 4. Conclusions The present study investigated the effect of strain rate on hydrogen embrittlement behaviors in a low-carbon martensitic steel. The main results obtained are as follows: When the strain rate was 8.3 × 10-6 s-1, the hydrogen-charged specimen showed mainly intergranular-like fracture in the vicinity of prior austenite grain boundaries. In the case of 8.3 × 10-1 s-1, on the other hand, the hydrogen-charged specimen showed ductile fracture surfaces within prior austenite grain. Hydrogen microprint technique and EBSD analysis suggested that hydrogen tended to preferentially accumulate on specific boundaries, in particular prior austenite grain boundaries, during tensile deformation at slow strain rates. On the other hand, in the case of higher strain rate, hydrogen distributed uniformly because there was no enough time for hydrogen to accumulate on specific sites during the tensile test. Hydrogen-related fracture changed from an intergranular-like manner at prior austenite grain boundaries to a ductile manner inside prior austenite grains with increasing the strain rate because of the change in the hydrogen accumulation behavior. Acknowledgements This study was financially supported by the Grant-in-Aid for Scientific Research on Innovative Area, “Bulk Nanostructured Metals” (area No. 2201), the Grant-in-Aid for Scientific Research (A) (No. 24246114), the Grant-inAid for Young Scientists (A) (No. 24686082), the Grant-in-Aid for Challenging Exploratory Research (No. 24656440), the Grant-in-Aid for JSPS Fellows (No. 26·2927) and the Elements Strategy Initiative for Structural Materials (ESISM), all through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. NT and AS were also supported by the ISIJ Research Promotion Grant. The authors gratefully appreciate all the supports. References [1] K. Takai, J. Seki, Y. Homma, Mater. Trans. 36 (1995) 1134-1139. [2] Y. H. Kim, J. W. Morris, Jr., Metall. Trans. A 14 (1983) 1883-1888. [3] A. Nagao, A.D. Smith, M. Dadfarnia, P. Sofronis, I.M. Robertson, Acta Mater. 60 (2012) 5182-5189. [4] A. Shibata, H. Takahashi, N. Tsuji, ISIJ Inter. 52 (2012) 208-212. [5] T. Ohnishi, K. Higashi, N. Inoue, Y. Nakatani, J. Japan Inst. Metals 45 (1981) 972-976. [6] J.T. Brown, W.M. Baldwin, Trans. AIME 200 (1954) 298-303. [7] J. Ovejero-Garcia, J. Mater. Sci. 80 (1985) 2623-2629.