Effect of hydrogen content on the embrittlement in a Fe–Mn–C twinning-induced plasticity steel

Corrosion Science 59 (2012) 277–281

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Effect of hydrogen content on the embrittlement in a Fe–Mn–C 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

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Article history: Received 22 November 2011 Accepted 4 March 2012 Available online 10 March 2012 Keywords: A. Steel B. Galvanostatic C. Hydrogen embrittlement

a b s t r a c t The hydrogen embrittlement of a Fe–18Mn–0.6C austenitic steel (wt.%) was examined using tensile tests under hydrogen charging at various current densities. The tensile properties deteriorated due to the occurrence of intergranular fracture above a specific current density. The work hardening behavior was not affected by the hydrogen charging, indicating that the embrittlement was independent of the change in behavior of slip deformation, martensitic transformation, and twinning deformation. The relationship between the fracture stresses for the intergranular fracture and the diffusible hydrogen content of the austenitic steel was approximated to the power law similarly to ferritic high strength steels. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Fe–Mn–C twinning induced plasticity (TWIP) steels have received attention as an automobile material [1,2], since they show superior elongation and strength due to deformation twinning [3–5]. However, various Fe–Mn–C austenitic steels have reported hydrogen embrittlement in recent works [1,6–11]. The improvement and evaluation of the hydrogen embrittlement property have become urgent issues that need to be resolved quickly so that the steels can be applied to automobile materials. The hydrogen embrittlement was caused by the cup-forming tests and the subsequent exposure in air in Fe–15Mn–0.6C [1,9], Fe–16Mn–0.6C [1,9], Fe–15Mn–0.7C–3Al [10], Fe–17Mn–0.6C [1,9], Fe–18Mn–0.6C [11], and Fe–22Mn–0.6C [1,8,9] TWIP steels. Tensile tests after electrochemical hydrogen charging also showed hydrogen embrittlement in Fe–17Mn–0.4C–2.7Al austenitic steels [6]. According to the literatures, the fracture mode was intergranular fracture [6,8]. In our previous work, tensile tests during hydrogen charging produced a significant amount of diffusible hydrogen and a clear intergranular fracture in the Fe–18Mn–0.6C TWIP steel [11]. Hydrogen was introduced to the specimens effectively during the tensile tests by simple diffusion as well as dislocation motions [12,13], resulting in a clear embrittlement phenomenon. Factors affected by the hydrogen include the mobility of slip dislocations [14], the behavior of martensitic transformation [9,10,15] and deformation twinning [16,17] and cohesive energy of the grain boundary [18,19] in austenitic steels. When slip deformation, ⇑ Corresponding author at: National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Tel.: +81 29 859 2000; fax: +81 29 859 2101. E-mail address: [email protected] (M. Koyama). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.03.009

martensitic transformation, and deformation twinning behavior are influenced by the hydrogen, a change in the work hardening behavior is expected. Therefore, the work hardening behavior and the fracture mode are essential for studying the hydrogen embrittlement in TWIP steels. In the present work, the hydrogen embrittlement property of the TWIP steel was evaluated by the tensile test during hydrogen charging at various current densities to understand the correspondence between the diffusible hydrogen content, work hardening behavior, and fracture mode. These factors should reveal intrinsic controlling factors for the hydrogen embrittlement. So far, there has not been any systematic research on the quantitative relationship between the hydrogen content and the embrittlement property in Fe–Mn–C austenitic steels. 2. Experimental A steel with a chemical composition of Fe–17.6Mn–0.61C (wt.%) was prepared by vacuum induction melting. The thickness of the steel was reduced from 60 to 2.6 mm by hot rolling at 1273 K and reduced further to 1.4 mm by cold rolling. Then, it was solution treated at 1073 K for 3.6 ks under an argon atmosphere. All the specimens were cut with spark erosion. The average grain size was about 3 lm including the annealing twin boundaries [11]. In our previous study, the depth of the hydrogen-affected zone showing intergranular fracture was about 0.1 mm from each surface of the 1-mm-thick specimen when the hydrogen was introduced during the tensile tests at a current density of 10 A/m2in a 3% NaCl aqueous solution containing 3 g/L of NH4SCN at ambient temperature and an initial strain rate of 5.1  105 s1in the same Fe–18Mn–0.6C steel [11]. The tensile tests in the present study

M. Koyama et al. / Corrosion Science 59 (2012) 277–281

were conducted under the same conditions as that in the previous report; however, the thickness of the specimen was reduced to 0.3 mm. A thinner specimen was used to increase the ratio of the hydrogen-affected zone to the specimen thickness and to allow the tensile tests to become more sensitive to the influence of hydrogen. The gauge dimensions of the specimen in the present study was 4 mmw  0.3 mmt  10 mml with a grip section on both ends that can be fixed in an Instron type machine. The thickness was reduced from 1.4 to 0.3 mm by mechanical polishing. The total elongations were determined by measuring the gauge length before and after the tests, and all the stress–strain curves were fitted to the measured elongations. Work hardening rates were obtained from the stress–strain curves that were smoothened by the software, KaleidaGraph, since serrations occur on the stress–strain curves of Fe–Mn–C austenitic steels. Hydrogen was introduced into the specimens by electrochemical charging during the tensile tests with current densities of 1, 3, 5, 10 A/m2. A platinum wire was used as the counter electrode. A solution was added continuously to cover the gauge part of the sample during the tensile tests. The hydrogen content was measured by the thermal desorption analysis (TDA) from room temperature to 550 K. The TDA was conducted within 20 min after the tensile test. The heating rate was 100 K/h. The diffusible hydrogen content was determined by measuring the cumulative desorbed hydrogen from ambient temperature to 473 K. In this work, we determined the diffusible hydrogen to be the amount of hydrogen that diffuses at room temperature in 10 days. The hydrogen in the form of an interstitial solid solution and the hydrogen attached to the reversible traps are included. The diffusible hydrogen is reported to play a key role in hydrogen embrittlement [20]. 3. Results Fig. 1 shows the engineering stress–strain curves in the present steel with hydrogen charging at various current densities. The hydrogen charging did not cause any differences in the serrated flow behavior associated with dynamic strain aging [21,22]. The elongation decreased when the current density was 3 A/m2 and higher. The true stress–strain curves shown in Fig. 2 were obtained from the engineering stress–strain curves shown in Fig. 1. The true

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fracture stresses are exhibited on the true stress–strain curves. The elongation and true fracture stress decreased with increasing current density from 3 to 10 A/m2. However, the hydrogen charging did not affect the work hardening behavior much even when the hydrogen embrittlement occurred. Fig. 3 shows the work hardening rates obtained from the true stress–strain curves. The broken line refers to the plastic instability condition (r = dr/de where r = true stress and e = true strain). The work hardening behavior of the stress–strain responses with and without hydrogen charging at 1 A/m2 met the plastic instability condition as indicated by the arrows. However, when the current density was equal to or higher than 3 A/m2, a fracture occurred before satisfying the plastic instability condition, indicating the occurrence of a premature fracture in the tensile tests at relatively high current densities. The introduced hydrogen content at the various current densities was measured by the TDA as shown in Fig. 4. The hydrogen content was shown to increase with increasing current density.

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Fig. 3. Work hardening rates calculated from Fig. 2.

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intergranular fracture surface was even observed in the central part of the 0.3 mm-thick sample.

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4. Discussion 4.1. Work hardening and diffusible hydrogen content

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Fig. 5. Desorption of hydrogen after exposure in air for 10 days. The first peak indicates diffusible hydrogen.

Fig. 5 shows a thermal desorption curve of the steel tensile-tested under hydrogen charging at 5 A/m2 and that subsequently exposed in air for 10 days. The peak of the hydrogen desorption rate appearing around 373 K disappeared after 10 days, indicating that the peak corresponds to diffusible hydrogen. Consequently, the diffusible hydrogen content shown hereafter was determined by measuring the cumulative hydrogen desorption from room temperature to 473 K. The diffusible hydrogen content at various current densities is summarized in Fig. 6. The diffusible hydrogen content increased significantly with increasing current density. The fracture modes in the present conditions are shown in Fig. 7. When the work hardening behavior met the plastic instability condition shown in Fig. 3, at a current density of 1 A/m2 or less, a large number of dimples appeared on the entire fracture surface. In contrast, intergranularly fractured parts were observed near the surface when a premature fracture occurred. The depth of the hydrogen affected zone increased with increasing current density. Fig. 8 shows a magnified image of the area marked by the square in Fig. 7d, consisting of a more apparent intergranular fracture. The

Many mechanisms for hydrogen embrittlement in austenitic steels can be proposed from different viewpoints. The localization of slip dislocations suggests the hydrogen enhanced localized plasticity (HELP) model [14], and fractography and ab initio calculation suggest the decohesion model [18,19]. Effects of hydrogen on phase stability as well as stacking fault energy may indicate martensitic transformation- or deformation twinning-induced cracking. In this section, we discuss the mechanism of hydrogen embrittlement from the viewpoint of work hardening. The hydrogen charging produced a significant diffusible hydrogen content and was varied by changing the applied current density. The hydrogen-affected zone at a current density of 10 A/m2 is about 0.05–0.15 mm from each surface, which was estimated from the depth of the intergranularly fractured part. The depth of the hydrogen-affected zone agrees with the previous result [11]. The depth is significant compared to the sample thickness of 0.3 mm; therefore, if hydrogen-assisted or -degraded work hardening occurs, then the hydrogen-affected zone should affect the macroscopic stress–strain responses. However, the work hardening behavior of the present steel did not show any significant differences arising from the hydrogen charging as shown in Fig. 2. In general, the work hardening rate changes drastically when the behavior of the slip, martensitic transformation [23–25], and deformation twinning [3–5,24,25] are altered. For example, an increase in the mobility of slip dislocations by the existence of hydrogen decreases the work hardening rates significantly in stable austenitic steels [14,26]. Additionally, the work hardening rates were reported to decrease due to the suppression of the a0 -martensitic transformation by hydrogen charging during the tensile tests in a metastable austenitic steel [27]. There was no change in the work hardening behavior in the present study which indicates that the hydrogen charging did not affect the behavior of the dislocation slip, martensitic transformation, and deformation twinning significantly.

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Fig. 7. Fractographs after deformation (a) without hydrogen-charge, with hydrogen-charge at current densities of (b) 1 A/m2, (c) 5 A/m2, and (d) 10 A/m2.

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Diffusible hydrogen content (wt. ppm) In the absence of an occurrence or a promotion of the localization of a slip, martensitic transformation, and deformation twinning, the possible cause of hydrogen embrittlement is the reduction in the cohesive energy of the grain boundaries which explains the lack of change in work hardening (Fig. 2), the occurrence of premature fracture (Fig. 3), and the clear intergranular fracture (Fig. 8).

4.2. Fracture and hydrogen content In hydrogen-induced intergranular fracture which did not show any change in the work hardening behavior, the important controlling factors are fracture stress and diffusible hydrogen content. A plot of the true fracture stress against diffusible hydrogen content is shown in Fig. 9. The two regions where ductile fracture and intergranular fracture were observed are indicated on the plot.

Fig. 9. True fracture stress plotted against diffusible hydrogen content.

The relationship between diffusible hydrogen content and fracture stress is approximated to the power law in ferritic high strength steels in Wang’s report [28,29]. Fig. 10 shows double natural logarithmic plots of Fig. 9. The region for intergranular fracture shows a linear relationship between diffusible hydrogen content and true fracture stress, indicating that the hydrogen embrittlement of the present TWIP steel could also be approximated to the power law. TWIP steels show a completely different work hardening behavior, initial and deformation microstructure as well as crystal structure from that of the conventional high strength steels. However, the hydrogen embrittlement of the present TWIP steel can be discussed in the same manner as that of the conventional high strength steels through the application of the power law in the present TWIP steel.

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5. Conclusion The effect of diffusible hydrogen content on the tensile properties was investigated in the Fe–18Mn–0.6C TWIP steel. Hydrogen was introduced by electrochemical charging at various current densities during the tensile tests. The work hardening behavior was independent of diffusible hydrogen content. The influence of diffusible hydrogen content on the tensile properties and fracture modes can be classified into two cases. The sample with a relatively low diffusible hydrogen content (e.g. 0.33 wt.ppm), showed a ductile fracture and tensile properties that were similar to those of the hydrogen-uncharged sample. In contrast, the samples with relatively high diffusible hydrogen content (e.g. 1.24 wt.ppm) showed an intergranular fracture and a degradation of the true fracture stress. The true fracture stress showing intergranular fracture decreased with increasing diffusible hydrogen content in accordance with the power law. Acknowledgements M. Koyama acknowledges the Research Fellowship of NIMS Junior Researcher (20092010) and the Japan Society for the Promotion of Science for Young Scientists (2011). We would also like to acknowledge Dr. Takahiro Sawaguchi for taking part in the discussions during the experiments. POSCO supported this work through the provision of the samples and funding. References [1] O. Kwon, Development of high performance high manganese TWIP steels in POSCO, in: Proceedings of HMnS, CD-ROM, 2011. [2] C. Scott, S. Allain, M. Faral, N. Guelton, The development of a Fe–Mn–C austenitic steel for automobile applications, Rev. Metall./Cah. Inf. Tech. 103 (2006) 293–302.

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