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Hydrogen embrittlement in a Fe–Mn–C ternary twinning-induced plasticity steel
Corrosion Science 54 (2012) 1–4
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Letter
Hydrogen embrittlement in a Fe–Mn–C ternary twinning-induced plasticity steel Motomichi Koyama a,b,⇑, Eiji Akiyama b, Kaneaki Tsuzaki a,b a b
Doctoral Program in Materials Science and Engineering, University of Tsukuba, 1-2-1 Sengen, Tsukuba, Ibaraki 305-8571, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 1 September 2011 Accepted 14 September 2011 Available online 21 September 2011 Keywords: A. Steel B. Galvanostatic C. Hydrogen embrittlement
a b s t r a c t The influence of hydrogen entry on ductility was evaluated in a ternary twinning-induced plasticity (TWIP) steel with a composition of Fe–18Mn–0.6C in wt.% using tensile tests. The samples with a thickness of 1.2 mm were charged with hydrogen galvanostatically during the tensile tests. Significant hydrogen content was introduced by the hydrogen-charging. The total elongation was significantly deteriorated from approx. 60% to 30% by the hydrogen-charging. A clear intergranular fracture surface was observed in a vicinity of the sample surface in the hydrogen-charged samples. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Twinning-induced plasticity (TWIP) steels show superior tensile elongation and ultimate strength due to deformation twinning of a face centered cubic structure, and have drawn attention as automobile materials [1–7]. The typical compositions of the TWIP steels are Fe–Mn–C based austenitic steels: Fe–17Mn–0.6C [8], Fe–18Mn– 0.6C [2], Fe–22Mn–0.6C [4,6], and Fe–18Mn–0.6C–1.5Al steels [5] (wt.%). To apply the steels into a practical use, various researches are required. For instance, evaluation of hydrogen embrittlement is crucially important. So et al. reported that the Fe–18Mn–0.6C–1.5Al TWIP steel did not show deterioration of tensile properties by electrochemical hydrogen-charging [10]. In contrast, delayed fracture in cup-forming test specimens of Fe–15Mn–0.6C [7], Fe–16Mn–0.6C [7], Fe–17Mn–0.6C [7], and Fe–22Mn–0.6C [7,9] TWIP steels was reported. In case of the typical TWIP steel with compositions of Fe–22Mn–0.6C, the delayed fracture occurred after cup-forming and subsequent exposure in air for 7 days [9]. One of the important causes seems to be hydrogen entry [7,9]. However, the effect of the hydrogen entry on ductility in ternary Fe–Mn–C TWIP steels has not been clarified yet. To understand the influence of hydrogen entry on ductility, tensile tests of a Fe–Mn–C ternary TWIP steel has been performed under hydrogen-charging, and the changes in total elongation, tensile strength, and fracture mode with introduced hydrogen have
⇑ Corresponding author at: Doctoral Program in Materials Science and Engineering, University of Tsukuba, 1-2-1 Sengen, Tsukuba, Ibaraki 305-8571, Japan. Tel.: +81 29 859 2000; fax: +81 29 859 2101. E-mail address: [email protected] (M. Koyama). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.09.022
been investigated in the present study. To our knowledge, there is no publication on this subject. 2. Experimental A TWIP steel with a chemical composition of Fe–18Mn–0.6C (wt.%) was prepared by vacuum induction melting. The steel was hot forged and rolled. Then it was solution treated and water quenched. All of the specimens were cut with spark erosion. The grain size of about 3 lm including annealing twin boundaries and no significant texture were observed by an electron back scattering diffraction (EBSD) analysis as shown in Fig. 1. The EBSD analysis was performed at 20 kV with a beam step size of 0.3 lm on a mechanically-polished surface of the as-solution-treated sample. Tensile tests were conducted at ambient temperature and at an initial strain rate of 5.1 10 5 s 1 for the specimens with gauge dimensions of 4 1.2 10 mm with a grip section on both ends using an Instron type machine. The configuration of the specimen is shown in Fig. 2(a). Total elongations were determined by measuring the gauge length before and after the tests. All the stress– strain curves were fitted to the measured elongations. Hydrogen was introduced into the specimens during the tensile tests by electrochemical charging in a 3% NaCl aqueous solution containing 3 g/ L of NH4SCN at a current density of 10 A/m2. A platinum wire was used for the counter electrode. A schematic of the experimental setup is shown in Fig. 2(b). The solution was continually added to cover the gauge part of the sample during the tensile tests. The hydrogen desorption was measured by a thermal desorption analysis (TDA) from room temperature to 900 K. The TDA was conducted as soon as possible after the tensile test. The time between the tensile test and the TDA was within 20 min. The heating rate was 100 K/h. Diffusible hydrogen content was determined by
2
M. Koyama et al. / Corrosion Science 54 (2012) 1–4
Fig. 1. Inverse pole figure map of the rolling direction (RD) and the corresponding orientation distribution.
(a)
32
measuring cumulative desorbed hydrogen from room temperature to 450 K. The diffusible hydrogen is defined as hydrogen that diffuses at room temperature. The diffusible hydrogen is reported to play a key role on hydrogen embrittlement [11].
32
10
t= 1.2 4.0
10
R=3.0
3. Results and discussion
80
(b)
Fig. 3(a) shows stress–strain curves in the hydrogen-uncharged and charged samples. The serrations on the stress–strain curves are known to be caused by dynamic strain aging [2,12]. The stress– strain curve of the hydrogen-uncharged sample is typical in Fe–18Mn–0.6C TWIP steel [2,7]. The total elongation and ultimate tensile strength of the hydrogen-uncharged sample were 62% and 1200 MPa, respectively. However, the total elongation was obviously deteriorated by the hydrogen-charging. The average total elongation and ultimate tensile strength of three tests of the hydrogen-charged samples were 32% and 1010 MPa, respectively. Fig. 3(b) shows the corresponding work hardening rates of the stress–strain curves shown in Fig. 3(a). The broken line exhibits the plastic instability condition, indicating that premature fracture occurred before meeting the plastic instability condition in the hydrogen-charged sample. Fig. 4 shows hydrogen desorption rate curves of the hydrogenuncharged and charged samples. The diffusible hydrogen contents of the hydrogen-uncharged and charged samples were 0.02 and 0.36 wt. ppm, respectively, indicating that hydrogen was significantly introduced by the tensile tests under hydrogen-charging.
Upper grip Plastic container Electrode (Pt)
Solution level
Gauge part
Sample Rubber closure Lower grip
Fig. 2. (a) Configuration of the sample. (b) Installation of tools into a cell for the hydrogen-charge.
1400
(a)
(b)
Hydrogen-uncharged 5000 Work hardening rate (MPa)
Engineering stress (MPa)
1200
6000
1000 800
Hydrogen-charged
600 400 200 0
Hydrogen-uncharged
4000 3000
Hydrogen-charged
2000 1000 0
0
10
20 30 40 50 60 Engineering strain (%)
70
80
0
500
1000 1500 2000 True stress (MPa)
2500
3000
Fig. 3. (a) Engineering stress–strain curves of hydrogen-charged and uncharged steels, and (b) the corresponding work hardening rates.
M. Koyama et al. / Corrosion Science 54 (2012) 1–4
Hydrogen desorption rate, wt. ppm/s
1.5×10-4
Hydrogen-charged
1.2×10-4
0.9×10-4
0.6×10-4
0.3×10-4
Hydrogen-uncharged
0
300
400
500
600
700
800
900
Temperature, K Fig. 4. Hydrogen desorption rate curves of the hydrogen-uncharged and charged steels during the tensile tests.
The influence of the hydrogen entry appeared on abovementioned total elongation as well as characteristics of fracture surfaces. Fig. 5(a) shows a ductile fracture surface showing considerable dimples in the hydrogen-uncharged sample. In contrast, a clear intergranular fracture surface was observed in the vicinity of the surface of the hydrogen-charged sample as shown in Fig. 5(b) and (c). The fracture mode is the same as that shown in the delayed fracture of the cup-formed Fe–22Mn–0.6C TWIP steel [9]. Though the intergranular fracture surface was observed near the surface of the sample as indicated by the square in Fig. 5(b), the fracture surface of the central part of the hydrogen-charged sample showed considerable dimples (Fig. 5(d)) which were similar to those of the
3
hydrogen-uncharged sample (Fig. 5(a)). The localization of intergranular fracture near the surface is because of a difference in the hydrogen concentration from the surface to the internal part. It is considered that the high concentration of hydrogen in the vicinity of the surface induced the intergranular fracture, while the relatively low hydrogen concentration in the internal part could not contribute to induce the intergranular fracture. The depth of the intergranularly fractured part is about 100 lm, indicating the depth of the hydrogen affected zone. The hydrogen was probably introduced by simple diffusion as well as dislocation motions [13,14]. The intergranularly fractured part in the hydrogen affected zone is the initiation sites of the premature fracture shown in Fig. 3(b). The possible causes of the intergranular fracture are localized slip [15], reduction in cohesive energy of grain boundary [16,17], deformation twinning [18,19], or martensitic transformation [7,20,21], promoted by hydrogen entry. Plate-like products such as martensite [22] and deformation twins [23,24] are generally considered to cause a stress concentration at grain boundaries. Thus, the hydrogen-promoted deformation twinning and martensitic transformation may induce the intergranular fracture. In order to reveal the mechanism of the intergranular fracture, further investigations are required. In summary, the hydrogen embrittlement property of the Fe– Mn–C TWIP steel could be successfully evaluated with good reproducibility from the reduction of elongation measured by using tensile tests under hydrogen-charging. The hydrogen-charging during the tensile test introduced a significant amount of hydrogen, decreasing the total elongation by half due to the transition of the fracture mode from ductile to intergranular fracture.
4. Conclusion The influence of hydrogen entry was investigated in the Fe– 18Mn–0.6C steel as one of the typical TWIP steels using tensile tests under hydrogen-charging. A clear difference in the tensile
Fig. 5. (a) Fracture surface of the hydrogen-uncharged steel. Fracture surface of the hydrogen-charged steel (b) at a low magnification, (c) at a high magnification of the part surrounded by the square in (b) and (d) at a high magnification on the center part.
4
M. Koyama et al. / Corrosion Science 54 (2012) 1–4
properties was observed between the hydrogen-uncharged and charged samples. The total elongation was deteriorated from approx. 60% to 30% because of the change in the fracture mode from ductile to intergranular fracture.
[9]
[10]
Acknowledgments [11]
M. Koyama acknowledges the Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists. We would also like to acknowledge Dr. Takahiro Sawaguchi for taking part in the discussions during the experiments. POSCO supported this work through providing the samples and funding under a Contract No. (22-4119).
[12]
[13] [14] [15]
References [1] O. Grassel, G. Frommeyer, Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–Al steels, Mater. Sci. Technol. 14 (1998) 1213–1217. [2] L. Chen, H.-S. Kim, S.-K. Kim, B.C. De Cooman, Localized deformation due to Portevin–LeChatelier effect in 18Mn–0.6C TWIP austenitic steel, ISIJ Int. 47 (2007) 1804–1812. [3] R. Ueji, N. Tsuchida, D. Terada, N. Tsuji, Y. Tanaka, A. Takemura, K. Kunishige, Tensile properties and twinning behavior of high manganese austenitic steel with fine-grained structure, Scr. Mater. 59 (2008) 963–966. [4] I.G. Urrutia, S. Zaefferer, D. Raabe, The effect of grain size and grain orientation on deformation twinning in a Fe–22 wt.% Mn–0.6 wt.% C TWIP steel, Mater. Sci. Eng. A 527 (2010) 3552–3560. [5] S. Kang, Y.-S. Jung, J.-H. Jun, Y.-K. Lee, Effects of recrystallization annealing temperature on carbide precipitation, microstructure, and mechanical properties in Fe–18Mn–0.6C–1.5Al TWIP steel, Mater. Sci. Eng. A 527 (2010) 745–751. [6] O. Bouaziz, S. Allain, C.P. Scott, P. Cugy, D. Barbier, High manganese austenitic twinning induced plasticity steels: a review of the microstructure properties relationships, Curr. Opin. Solid State Mater. Sci., in press. [7] B.C. De Cooman, K.-G. Chin, J. Kim, High Mn TWIP Steels for Automotive Applications, New Trends and Developments in Automotive System Engineering, M. Chiaberge (ed.), 2011, ISBN: 978-953-307-517-4, InTech. [8] M. Koyama, T. Sawaguchi, T.-K. Lee, C.-S. Lee, K. Tsuzaki, Work hardening associated with e-martensitic transformation, deformation twinning and
[16] [17]
[18]
[19] [20]
[21] [22]
[23]
[24]
dynamic strain aging in Fe–17Mn–0.6C and Fe–17Mn–0.8C TWIP steels, Mater. Sci. Eng. A 528 (2011) 7310–7316. K.-G. Chin, C.-Y. Kang, S.Y. Sin, S. Hong, S. Lee, H.-S. Kim, K.-H. Kim, N.J. Kim, Effects of Al addition on deformation and fracture mechanisms in two high manganese TWIP steels, Mater. Sci. Eng. A 528 (2011) 2922–2928. K.H. So, J.S. Kim, Y.S. Shun, K.-T. Park, Y.-K. Lee, C.S. Lee, Hydrogen delayed fracture properties and internal hydrogen behavior of a Fe–18Mn–1.5Al–0.6C TWIP steel, ISIJ Int. 49 (2009) 1952–1959. K. Takai, R. Watanuki, Hydrogen in trapping states innocuous to environmental degradation of high strength steel, ISIJ Int. 43 (2003) 520–526. S. Allain, P. Cugy, C. Scott, J.P. Chateau, A. Rusinek, A. Deschamps, The influence of plastic instabilities on the mechanical properties of a high-manganese austenitic FeMnC steel, Int. J. Mat. Res. 99 (2008) 734. G.S. Frankel, R.M. Latanision, Hydrogen transport during deformation in nickel: part 1. Single crystal nickel, Metal. Trans. A 17A (1986) 869–875. J.K. Tien, A.W. Thompson, I.M. Bernstein, R.J. Richards, Hydrogen transport by dislocations, Metal. Trans. A 7A (1976) 821–829. D.G. Ulmer, C.J. Altstetter, Hydrogen-induced strain localization and failure of austenitic stainless-steels at high hydrogen concentrations, Acta Mater. 39 (1991) 1237–1248. J. Kameda, C.J. McMahon Jr., Solute segregation and hydrogen-induced intergranular fracture in an alloy steel, Metal. Trans. A 14A (1983) 903–911. M. Yamaguchi, K. Ebihara, M. Itakura, T. Kadoyoshi, T. Suzudo, H. Kaburaki, First-principles study on the grain boundary embrittlement of metals by solute segregation: part II. Metal (Fe, Al, Cu)-hydrogen (H) systems, Metal. Mater. Trans. A 42A (2011) 330–339. J.M. Rigsbee, R.B. Benson Jun, TEM investigation of hydrogen-induced deformation twinning and associated martensitic phases in 304-type stainless-steel, J. Mater. Sci. 12 (1977) 406–409. E.G. Astafurova, G.G. Zakharova, H.J. Maier, Hydrogen-induced twinning in <0 0 1> Hadfield steel single crystals, Scr. Mater. 63 (2010) 1189–1192. Y. Kim, N. Kang, Y. Park, G. Kim, S. Kim, K. Cho, Effects of the strain induced martensite transformation on the delayed fracture for Al-added TWIP Steel, J. Korean Inst. Met. Mater. 46 (2008) 780–787. P. Rozenak, L. Zevin, D. Eliezer, Hydrogen effects on phase-transformations in austenitic stainless-steels, J. Mater. Sci. 19 (1984) 567–573. H. Nakatsu, S. Takaki, Effect of austenite grain size in Fe–Mn alloys on epsilon martensitic transformation and their mechanical properties, J. Japan Inst. Metal. 60 (1996) 141–148. Y. Tomota, M. Strum, J.W. Morris Jr., The relationship between toughness and microstructure in Fe-high Mn binary-alloys, Metal. Trans. A 18A (1987) 1073– 1081. H. Suto, B.S. Chun, Tech. Report, Tohoku University 44 (1979) 317.
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Letter
Hydrogen embrittlement in a Fe–Mn–C ternary twinning-induced plasticity steel Motomichi Koyama a,b,⇑, Eiji Akiyama b, Kaneaki Tsuzaki a,b a b
Doctoral Program in Materials Science and Engineering, University of Tsukuba, 1-2-1 Sengen, Tsukuba, Ibaraki 305-8571, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 1 September 2011 Accepted 14 September 2011 Available online 21 September 2011 Keywords: A. Steel B. Galvanostatic C. Hydrogen embrittlement
a b s t r a c t The influence of hydrogen entry on ductility was evaluated in a ternary twinning-induced plasticity (TWIP) steel with a composition of Fe–18Mn–0.6C in wt.% using tensile tests. The samples with a thickness of 1.2 mm were charged with hydrogen galvanostatically during the tensile tests. Significant hydrogen content was introduced by the hydrogen-charging. The total elongation was significantly deteriorated from approx. 60% to 30% by the hydrogen-charging. A clear intergranular fracture surface was observed in a vicinity of the sample surface in the hydrogen-charged samples. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Twinning-induced plasticity (TWIP) steels show superior tensile elongation and ultimate strength due to deformation twinning of a face centered cubic structure, and have drawn attention as automobile materials [1–7]. The typical compositions of the TWIP steels are Fe–Mn–C based austenitic steels: Fe–17Mn–0.6C [8], Fe–18Mn– 0.6C [2], Fe–22Mn–0.6C [4,6], and Fe–18Mn–0.6C–1.5Al steels [5] (wt.%). To apply the steels into a practical use, various researches are required. For instance, evaluation of hydrogen embrittlement is crucially important. So et al. reported that the Fe–18Mn–0.6C–1.5Al TWIP steel did not show deterioration of tensile properties by electrochemical hydrogen-charging [10]. In contrast, delayed fracture in cup-forming test specimens of Fe–15Mn–0.6C [7], Fe–16Mn–0.6C [7], Fe–17Mn–0.6C [7], and Fe–22Mn–0.6C [7,9] TWIP steels was reported. In case of the typical TWIP steel with compositions of Fe–22Mn–0.6C, the delayed fracture occurred after cup-forming and subsequent exposure in air for 7 days [9]. One of the important causes seems to be hydrogen entry [7,9]. However, the effect of the hydrogen entry on ductility in ternary Fe–Mn–C TWIP steels has not been clarified yet. To understand the influence of hydrogen entry on ductility, tensile tests of a Fe–Mn–C ternary TWIP steel has been performed under hydrogen-charging, and the changes in total elongation, tensile strength, and fracture mode with introduced hydrogen have
⇑ Corresponding author at: Doctoral Program in Materials Science and Engineering, University of Tsukuba, 1-2-1 Sengen, Tsukuba, Ibaraki 305-8571, Japan. Tel.: +81 29 859 2000; fax: +81 29 859 2101. E-mail address: [email protected] (M. Koyama). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.09.022
been investigated in the present study. To our knowledge, there is no publication on this subject. 2. Experimental A TWIP steel with a chemical composition of Fe–18Mn–0.6C (wt.%) was prepared by vacuum induction melting. The steel was hot forged and rolled. Then it was solution treated and water quenched. All of the specimens were cut with spark erosion. The grain size of about 3 lm including annealing twin boundaries and no significant texture were observed by an electron back scattering diffraction (EBSD) analysis as shown in Fig. 1. The EBSD analysis was performed at 20 kV with a beam step size of 0.3 lm on a mechanically-polished surface of the as-solution-treated sample. Tensile tests were conducted at ambient temperature and at an initial strain rate of 5.1 10 5 s 1 for the specimens with gauge dimensions of 4 1.2 10 mm with a grip section on both ends using an Instron type machine. The configuration of the specimen is shown in Fig. 2(a). Total elongations were determined by measuring the gauge length before and after the tests. All the stress– strain curves were fitted to the measured elongations. Hydrogen was introduced into the specimens during the tensile tests by electrochemical charging in a 3% NaCl aqueous solution containing 3 g/ L of NH4SCN at a current density of 10 A/m2. A platinum wire was used for the counter electrode. A schematic of the experimental setup is shown in Fig. 2(b). The solution was continually added to cover the gauge part of the sample during the tensile tests. The hydrogen desorption was measured by a thermal desorption analysis (TDA) from room temperature to 900 K. The TDA was conducted as soon as possible after the tensile test. The time between the tensile test and the TDA was within 20 min. The heating rate was 100 K/h. Diffusible hydrogen content was determined by
2
M. Koyama et al. / Corrosion Science 54 (2012) 1–4
Fig. 1. Inverse pole figure map of the rolling direction (RD) and the corresponding orientation distribution.
(a)
32
measuring cumulative desorbed hydrogen from room temperature to 450 K. The diffusible hydrogen is defined as hydrogen that diffuses at room temperature. The diffusible hydrogen is reported to play a key role on hydrogen embrittlement [11].
32
10
t= 1.2 4.0
10
R=3.0
3. Results and discussion
80
(b)
Fig. 3(a) shows stress–strain curves in the hydrogen-uncharged and charged samples. The serrations on the stress–strain curves are known to be caused by dynamic strain aging [2,12]. The stress– strain curve of the hydrogen-uncharged sample is typical in Fe–18Mn–0.6C TWIP steel [2,7]. The total elongation and ultimate tensile strength of the hydrogen-uncharged sample were 62% and 1200 MPa, respectively. However, the total elongation was obviously deteriorated by the hydrogen-charging. The average total elongation and ultimate tensile strength of three tests of the hydrogen-charged samples were 32% and 1010 MPa, respectively. Fig. 3(b) shows the corresponding work hardening rates of the stress–strain curves shown in Fig. 3(a). The broken line exhibits the plastic instability condition, indicating that premature fracture occurred before meeting the plastic instability condition in the hydrogen-charged sample. Fig. 4 shows hydrogen desorption rate curves of the hydrogenuncharged and charged samples. The diffusible hydrogen contents of the hydrogen-uncharged and charged samples were 0.02 and 0.36 wt. ppm, respectively, indicating that hydrogen was significantly introduced by the tensile tests under hydrogen-charging.
Upper grip Plastic container Electrode (Pt)
Solution level
Gauge part
Sample Rubber closure Lower grip
Fig. 2. (a) Configuration of the sample. (b) Installation of tools into a cell for the hydrogen-charge.
1400
(a)
(b)
Hydrogen-uncharged 5000 Work hardening rate (MPa)
Engineering stress (MPa)
1200
6000
1000 800
Hydrogen-charged
600 400 200 0
Hydrogen-uncharged
4000 3000
Hydrogen-charged
2000 1000 0
0
10
20 30 40 50 60 Engineering strain (%)
70
80
0
500
1000 1500 2000 True stress (MPa)
2500
3000
Fig. 3. (a) Engineering stress–strain curves of hydrogen-charged and uncharged steels, and (b) the corresponding work hardening rates.
M. Koyama et al. / Corrosion Science 54 (2012) 1–4
Hydrogen desorption rate, wt. ppm/s
1.5×10-4
Hydrogen-charged
1.2×10-4
0.9×10-4
0.6×10-4
0.3×10-4
Hydrogen-uncharged
0
300
400
500
600
700
800
900
Temperature, K Fig. 4. Hydrogen desorption rate curves of the hydrogen-uncharged and charged steels during the tensile tests.
The influence of the hydrogen entry appeared on abovementioned total elongation as well as characteristics of fracture surfaces. Fig. 5(a) shows a ductile fracture surface showing considerable dimples in the hydrogen-uncharged sample. In contrast, a clear intergranular fracture surface was observed in the vicinity of the surface of the hydrogen-charged sample as shown in Fig. 5(b) and (c). The fracture mode is the same as that shown in the delayed fracture of the cup-formed Fe–22Mn–0.6C TWIP steel [9]. Though the intergranular fracture surface was observed near the surface of the sample as indicated by the square in Fig. 5(b), the fracture surface of the central part of the hydrogen-charged sample showed considerable dimples (Fig. 5(d)) which were similar to those of the
3
hydrogen-uncharged sample (Fig. 5(a)). The localization of intergranular fracture near the surface is because of a difference in the hydrogen concentration from the surface to the internal part. It is considered that the high concentration of hydrogen in the vicinity of the surface induced the intergranular fracture, while the relatively low hydrogen concentration in the internal part could not contribute to induce the intergranular fracture. The depth of the intergranularly fractured part is about 100 lm, indicating the depth of the hydrogen affected zone. The hydrogen was probably introduced by simple diffusion as well as dislocation motions [13,14]. The intergranularly fractured part in the hydrogen affected zone is the initiation sites of the premature fracture shown in Fig. 3(b). The possible causes of the intergranular fracture are localized slip [15], reduction in cohesive energy of grain boundary [16,17], deformation twinning [18,19], or martensitic transformation [7,20,21], promoted by hydrogen entry. Plate-like products such as martensite [22] and deformation twins [23,24] are generally considered to cause a stress concentration at grain boundaries. Thus, the hydrogen-promoted deformation twinning and martensitic transformation may induce the intergranular fracture. In order to reveal the mechanism of the intergranular fracture, further investigations are required. In summary, the hydrogen embrittlement property of the Fe– Mn–C TWIP steel could be successfully evaluated with good reproducibility from the reduction of elongation measured by using tensile tests under hydrogen-charging. The hydrogen-charging during the tensile test introduced a significant amount of hydrogen, decreasing the total elongation by half due to the transition of the fracture mode from ductile to intergranular fracture.
4. Conclusion The influence of hydrogen entry was investigated in the Fe– 18Mn–0.6C steel as one of the typical TWIP steels using tensile tests under hydrogen-charging. A clear difference in the tensile
Fig. 5. (a) Fracture surface of the hydrogen-uncharged steel. Fracture surface of the hydrogen-charged steel (b) at a low magnification, (c) at a high magnification of the part surrounded by the square in (b) and (d) at a high magnification on the center part.
4
M. Koyama et al. / Corrosion Science 54 (2012) 1–4
properties was observed between the hydrogen-uncharged and charged samples. The total elongation was deteriorated from approx. 60% to 30% because of the change in the fracture mode from ductile to intergranular fracture.
[9]
[10]
Acknowledgments [11]
M. Koyama acknowledges the Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists. We would also like to acknowledge Dr. Takahiro Sawaguchi for taking part in the discussions during the experiments. POSCO supported this work through providing the samples and funding under a Contract No. (22-4119).
[12]
[13] [14] [15]
References [1] O. Grassel, G. Frommeyer, Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–Al steels, Mater. Sci. Technol. 14 (1998) 1213–1217. [2] L. Chen, H.-S. Kim, S.-K. Kim, B.C. De Cooman, Localized deformation due to Portevin–LeChatelier effect in 18Mn–0.6C TWIP austenitic steel, ISIJ Int. 47 (2007) 1804–1812. [3] R. Ueji, N. Tsuchida, D. Terada, N. Tsuji, Y. Tanaka, A. Takemura, K. Kunishige, Tensile properties and twinning behavior of high manganese austenitic steel with fine-grained structure, Scr. Mater. 59 (2008) 963–966. [4] I.G. Urrutia, S. Zaefferer, D. Raabe, The effect of grain size and grain orientation on deformation twinning in a Fe–22 wt.% Mn–0.6 wt.% C TWIP steel, Mater. Sci. Eng. A 527 (2010) 3552–3560. [5] S. Kang, Y.-S. Jung, J.-H. Jun, Y.-K. Lee, Effects of recrystallization annealing temperature on carbide precipitation, microstructure, and mechanical properties in Fe–18Mn–0.6C–1.5Al TWIP steel, Mater. Sci. Eng. A 527 (2010) 745–751. [6] O. Bouaziz, S. Allain, C.P. Scott, P. Cugy, D. Barbier, High manganese austenitic twinning induced plasticity steels: a review of the microstructure properties relationships, Curr. Opin. Solid State Mater. Sci., in press. [7] B.C. De Cooman, K.-G. Chin, J. Kim, High Mn TWIP Steels for Automotive Applications, New Trends and Developments in Automotive System Engineering, M. Chiaberge (ed.), 2011, ISBN: 978-953-307-517-4, InTech. [8] M. Koyama, T. Sawaguchi, T.-K. Lee, C.-S. Lee, K. Tsuzaki, Work hardening associated with e-martensitic transformation, deformation twinning and
[16] [17]
[18]
[19] [20]
[21] [22]
[23]
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