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Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels Oluwole Kazum a, Mihail Ionescu b, Hossein Beladi c, M. Bobby Kannan a,* a
Disciple of Chemical Engineering, College of Science and Engineering, James Cook University, Townsville, Queensland, 4811, Australia b Australian Nuclear Science and Technology Organisation, Centre for Accelerator Science, Sydney, 2234, Australia c Institute for Frontier Materials, Deakin University, Victoria, 3126, Australia
article info
abstract
Article history:
Hydrogen depth profiles and microhardness of the electrochemically hydrogen-charged
Received 3 October 2018
nanostructured bainitic steels (produced at two different transformation temperatures,
Received in revised form
i.e. 200 C (NBS200) and 350 C (NBS350)) were obtained using elastic recoil detection
19 February 2019
analysis (ERDA) technique and Vickers microhardness testing, respectively, and compared
Accepted 21 March 2019
to that of mild steel. The ERDA results showed that the subsurface hydrogen concentration
Available online xxx
was higher in NBS200, followed by NBS350 and mild steel. However, the microhardness data of the hydrogen-charged steels revealed material softening in NBS200 and NBS350,
Keywords:
whereas the mild steel exhibited material hardening effect. The microhardness along the
Nanostructured bainitic steels
cross-sectional depth of the steels showed that the softening effect in NBS200 was closer to
Hydrogen-induced softening
the hydrogen-charged surface compared to that of NBS350. The plausible mechanisms for
Microhardness
the softening effect in the NBS200 and NBS350, and hardening effect in mild steel have
Electrochemical hydrogen-charging
been discussed in this paper.
Hydrogen depth profile
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nanostructured bainitic steels have potential applications in hydrogen transportation infrastructure due to their excellent mechanical properties [1,2]. However, it widely reported in the literature that hydrogen in high-strength steels under mechanical loading could lead to a catastrophic form of corrosion failure known as hydrogen-induced cracking (HIC) [3].
Hydrogen concentration, distribution and diffusivity in steels influence their HIC behaviour. Generally, depending on the source of hydrogen (welding, mineral acid cleaning or cathodic protection), inhibitor or heat-treatment (baking) is used to prevent hydrogen diffusion or remove the diffused hydrogen [4,5]. Recently, we reported that the effective hydrogen diffusivities of nanostructured bainitic steels (NBS200: 6.26 108 cm2 s1 and NBS350: 1.09 106 cm2 s1) were
* Corresponding author. E-mail address: [email protected] (M.B. Kannan). https://doi.org/10.1016/j.ijhydene.2019.03.172 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
2
international journal of hydrogen energy xxx (xxxx) xxx
lower than that of mild steel (1.99 106 cm2 s1) [6]. It was also concluded that the high dislocation density in the bainitic ferrite phase and the fine microstructure of NBS200 compared to NBS350 contributed to the lower effective hydrogen diffusivity in NBS200. Generally, an increase in the hydrogen concentration in steels affects their mechanical properties such as tensile strength, ductility [7,8] and hardness [9e12]. Hydrogen trapping sites (reversible and irreversible) could concentrate hydrogen and lead to crack initiation and also facilitate propagation [13,14]. Crack initiation in steels can be due to various factors such as surface imperfections and pitting corrosion [15e17]. However, hydrogen concentration in the subsurface can also lead to crack initiation by hydrogen blistering. Further, hydrogen concentration along the crosssectional depth of the material could influence the crack propagation rate [18e24]. Studies have reported an increase in hardness at the hydrogen-induced crack tips in steels [25,26]. It is critical to study the hydrogen-depth profile and microhardness in these high-strength nanostructured bainitic steels to understand the mechanism of hydrogen diffusion and embrittlement, and thereby evaluate their suitability for hydrogen transport applications. Hence in this study, the hydrogen depth profiles and microhardness of the nanostructured bainitic steels (NBS200 and NBS350) were obtained using elastic recoil detection analysis (EDRA) technique and microhardness testing, respectively. The hydrogen diffusion mechanisms in these steels were discussed.
was vigorously purged with nitrogen gas for 15 min and then gently purged throughout the experiment to reduce the oxygen concentration in the electrolyte. Elastic recoil detection analysis (ERDA) was done on the hydrogen-charged steels at the Australian Nuclear Science and Technology Organisation (ANSTO) ion beam facility within the Centre for Accelerator Science (CAS), which was installed on a beamline serviced by the 2 MV STAR tandem accelerator. The subsurface hydrogen profile was done using 1.8 MeV Heþ ions impinging on the sample surface at an angle of 72 relative to the sample, which have enough energy to recoil the lighter hydrogen atoms from the sample up to a depth of ~1.5 mm. The energy of recoiled hydrogen was measured using a silicon surface barrier detector placed at a scattering angle of 32 to the incoming beam, with an 8.5 mm Mylar foil filter used to stop the helium scattered by the sample surface and by a 1 2 mm2 4-way slits that reduced the energy spread and defined a solid angle of 0.8 msr. The number of helium ions impinging upon the sample was monitored by measuring the drain current of the sample holder, which was around 5 nA, and accounting for secondary electrons. The result of hydrogen yield versus energy (or channel number) was compared with a calculated result generated by SIMNRA software [28]. The results of hydrogen measurement on the samples at room temperature were obtained by integrating the area under the hydrogen peak and comparing it with Kapton, which contains 24 at% hydrogen, and it is stable under the 1.8 MeV helium beam. The depth distribution of hydrogen was obtained from the energy loss of helium in the sample, by making use of the stopping power data incorporated in the SIMNRA analysis software. Microhardness measurements were carried out using an automated Zwick hardness tester (Model: ZHVm). The hardness measurements were performed immediately after hydrogen-charging by applying a load of 200 gf with a load time of 10 s. The hardness test was done on three test samples for the surface and the cross-sectional depth hardness on the uncharged and hydrogen-charged steels. For the crosssectional hardness, the measurements were done at a distance interval of 0.2 mm from charged surface.
Experimental procedure The chemical composition of the nanostructured bainitic steels (NBS200 and NBS350) and the mild steel used in this study is given in Table 1. To obtain different bainitic microstructure, the NBS steel was initially reheated to 1100 C and held isothermally for 30 min, followed by isothermal heat treatment at different temperatures. The NBS200 was isothermally held at 200 C for 10 days, whereas NBS350 was held at 350 C for 1 day [27]. The samples were ground using SiC paper up to 2400 grits, washed with distilled water and ultrasonically cleaned in acetone and then in ethanol. Three test samples were used for each condition to confirm reproducibility. The hydrogen charging was carried out in an electrochemical cell, with the steel sample (dimensions: 20 mm 20 mm 2 mm, mounted in a resin to expose one side of the sample) as the working electrode and a platinum mesh as the counter electrode. The electrolyte contained 0.1 M NaOH and a hydrogen promoter (10 g/l of Na2S.9H2O). Hydrogen was generated by electrochemical cathodic charging using a DC source (Model: Powertech MP-3084). A constant current density of 10 mA cm2 was applied for a period of 6 h. Prior to the hydrogen charging, the electrolyte
Results and discussion Fig. 1 shows the hydrogen depth profiles of the mild steel and nanostructured bainitic steels and the corresponding hydrogen concentrations. All the steel samples showed a relatively high hydrogen yield close to the hydrogen-charged surface. However, the hydrogen yield close to the hydrogencharged surface was significantly higher for NBS200 compared to NBS350 and.mild steel. The NBS200 exhibited a peak hydrogen yield of ~20 (±2.75) cts at 0.14 mm subsurface
Table 1 e The chemical composition of the steels used in this study (wt. %). Steels Mild steel (nominal) Nanostructured bainitic steel
C
Mn
Co
Si
Al
Cr
Mo
Fe
0.20 0.79
0.90 1.98
e 1.58
0.10 1.5
e 1.06
0.40 0.98
0.15 0.24
Bal Bal
Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e (a) Hydrogen depth profiles, and (b) the corresponding hydrogen concentration of hydrogen-charged mild steel, NBS200 and NBS350.
depth, whereas NBS350 steel showed a peak hydrogen yield of ~13 (±1.00) cts at ~ 0.17 mm subsurface depth. For the mild steel, the peak hydrogen yield was ~12 (±0.25) cts at ~0.18 mm subsurface depth. It was noted that the peak hydrogen yield for NBS200 was closer to the charged surface (0.14 mm) compared to that of NBS350 (0.17 mm) and mild steel (0.18 mm). Within the subsurface depth of 1.6 mm, the hydrogen concentration for NBS200 was higher by 37% and 48% compared to NBS350 and mild steel, respectively. The observed high subsurface hydrogen concentration and peak hydrogen yield closer to the hydrogen-charged surface in NBS200 suggest that more hydrogen was trapped in the subsurface compared to NBS350 and mild steel. The images of the Vickers microhardness indent on the surface of the uncharged and hydrogen-charged steels are shown in Fig. 2a and the corresponding surface hardness values are presented in Fig. 2b. For the mild steel, the indent diagonal length for the hydrogen-charged was smaller in comparison with the uncharged, which suggests that the surface hardness of mild steel increased with hydrogencharging. The Vickers hardness value of the hydrogencharged mild steel was ~22% higher than the uncharged sample. In contrast, the nanostructured bainitic steels showed an increase in the indent diagonal length under hydrogen-charged condition. Thus, NBS200 and NBS350 exhibited ~5% and ~12% decrease in the hardness with hydrogen-charging, respectively. It was noted that the decrease in the hardness was higher in NBS350 compared to NBS200. Fig. 3 shows the microhardness depth profile of the uncharged and hydrogen-charged steels along the cross-section. The hydrogen-charged mild steel showed a higher hardness value (26%) close to the hydrogen-charged surface (~0.2 mm), compared to that of the uncharged sample. The hardness value reduced gradually along the cross-sectional depth, however, the hardness values were higher across the sample thickness in comparison with the uncharged mild.steel. In the case of the nanostructured bainitic steels, the hardness value
was significantly lower close to the hydrogen-charged surface compared to the uncharged samples. At a cross-sectional depth of ~0.2 mm, the hydrogen-charged NBS200 exhibited a hardness value of ~707 HV, whereas the uncharged sample showed a hardness value of ~774 HV. Similarly, the hydrogencharged NBS350 exhibited a hardness value of ~300 HV, which was lower than the uncharged sample (~353 HV) at a crosssectional depth of ~0.2 mm. The hardness drop in NBS200 (~8%) was lower than that of NBS350 (~14%). The hardness in NBS200 continued to drop up to ~0.6 mm cross-sectional depth of the sample from the hydrogen-charged side, however, no significant difference in hardness value was recorded beyond 0.8 mm cross-sectional depth. Interestingly, in NBS350, the hardness of the hydrogen-charged sample was lower across the sample thickness compared to the uncharged sample. The hardness drop in NBS350 was significantly high up to ~0.8 mm cross-sectional depth, however, thereafter the difference in the hardness values decreased. The ERDA and microhardness results suggest that the microstructural features of the steels influenced their hydrogen concentration and distribution in the subsurface and beyond. Previously, we reported that the nanostructured bainitic steels consist of nanolayers of bainitic ferrite and retained austenite. The thickness of these layers were finer in NBS200 compared to NBS350, and the volume fraction of the bainitic ferrite was higher in NBS200 (72%) than in NBS350 (42%). The bainitic ferrite in NBS200 exhibited higher dislocation density (4.7 10 15 m2) in comparison with NBS350 (2 10 15 m2) due to their different isothermal transformation temperatures [27]. Hence, we suggested that the finer microstructure with higher dislocation density in the bainitic ferrite phase of NBS200 trapped more hydrogen and was responsible for the lower effective hydrogen diffusivity in NBS200 (6.26 108 cm2 s1) compared with NBS350 (1.09 106 cm2 s1) [6]. In this study, the higher hydrogen concentration observed in the subsurface of NBS200 can be attributed to its lower effective hydrogen diffusivity, which concentrated the hydrogen in the subsurface and thus the
Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
4
international journal of hydrogen energy xxx (xxxx) xxx
Fig. 3 e Microhardness along the cross-section of the uncharged and hydrogen-charged mild steel, NBS200 and NBS350.
Fig. 2 e a) Micrographs of the microhardness indents on the surface of uncharged and hydrogen-charged mild steel, NBS200 and NBS350. b) Surface microhardness of uncharged and hydrogen-charged mild steel, NBS200 and NBS350.
peak hydrogen yield was higher and closer to the hydrogencharged surface. In the case of mild steel, hydrogen was not concentrated in the subsurface since the effective hydrogen diffusivity was high in the mild steel (1.99 106 cm2 s1) [6]. Further, the dislocation density (hydrogen trap sites) in low carbon mild steel was reported to be 0.9 1014 m2 [29], which is significantly lower than that in NBS200 and NBS350. Thus,
the hydrogen concentration was relatively uniform in the mild steel. The hydrogen-induced hardening and softening effects in the mild steel and nanostructured bainitic steels, respectively, can be attributed to the presence and interaction of hydrogen atoms with the steels. Literature suggests that a large concentration of dissolved hydrogen in the lattices of iron reduces plasticity, and supersaturation of hydrogen, which lead to the formation of blisters, void and cracks, which generate stress and new dislocation and thereby enhances hardening [30,31]. Hydrogen atoms in the crystal lattice sites enhance solid solution strengthening in mild steel. Basically, solute atoms create stress fields around them which causes lattice distortion and impede dislocation motion and thereby increases the yield stress and hardness of the material. On the other hand, the interaction of a small amount of hydrogen atoms with dislocations can enhance dislocation mobility and lead to the softening effect [32,33]. Hence, in the mild steel, the high concentration of hydrogen in the lattice of the body centred cubic (BCC) structure has led to the material embrittlement (hardening effect). For the nanostructured bainitic steels, however, the hydrogen atoms were trapped in the subsurface by the high number of dislocations, which led to hydrogen enhanced local plasticity (HELP) [34,35]. This caused softening effect in nanostructured bainitic steels. The softening was lower in the case of NBS200 compared with NBS350 due to the relatively high hydrogen concentration in the subsurface of NBS200 resulting in hydrogen environment around the dislocations, which pinned the dislocations and increased the stress required for plastic deformation and thus reduced the softening effect [31,32]. A schematic representation of the hydrogen concentration and distribution in the subsurface and beyond is shown in Fig. 4. This study suggests that nanostructured bainitic steels (NBS200 and NBS350) would perform better than the mild steel
Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 4 e Schematic representation of hydrogen concentration and distribution in the subsurface and beyond of NBS200, NBS350 and mild steel.
under hydrogen environment such as in hydrogen transport infrastructures, and NBS200 appears superior to NBS350.
Conclusions Hydrogen depth profiles and microhardness values showed that hydrogen concentration in the subsurface of NBS200 was higher compared with NBS350 steel and mild steel due to the lower hydrogen diffusivity in NBS200, which largely limited the hydrogen diffusion to the subsurface. Microhardness along the cross-sectional depth revealed hydrogen-induced softening effect in NBS200 and NBS350 and hydrogeninduced hardening effect in mild steel. The interactions between the hydrogen atoms and the dislocations present in the bainitic ferrite contributed to the softening effect in NBS200 and NBS350, whereas hydrogen dissolved in the crystal lattice of mild steel exhibited hardening effect (embrittlement). The difference in the degree of hydrogen-induced softening observed between NBS200 and NBS350 can be attributed to the difference in the effective hydrogen diffusivity and dislocation density in the bainitic ferrite phase.
Acknowledgement The authors acknowledge the financial support provided by the Australian Government through NCRIS funding scheme for the Centre for Accelerator Science facility, where the hydrogen depth profiling was carried out. The authors also grateful for the support provided by the AINSE PGRA and JCUIPRS scholarship scheme towards the first author's research work at James Cook University.
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[2] Yoozbashi MN, Yazdari S. Mechanical properties of nanostructured, low temperature bainitic steel designed using a thermodynamic model. Mater Sci Eng 2010;527:3200e5. [3] Escobar DP, Minambres C, Duprez L, Verbenken K, Verhaege M. Internal and Surface damage of multiphase steels and pure iron after electrochemical hydrogen charging. Corros Sci 2011;53:3166e76. [4] Khare A, Vishwakarama M, Parashar V. A review on failures of industrial components due to hydrogen embrittlement and temperature for damage prevention. Int J Appl Eng Res 2017;12:1784e92. [5] Agrawal R, Namboodhiri TKG. The inhibition of corrosion and hydrogen embrittlement of AISI 410 stainless steel. J Appl Electrochem 1992;22:383e9. [6] Kazum O, Beladi H, Timokhina IB, He Y, Kannan MB. Hydrogen permeation in nanostructured bainitic steel. Metall Mater Trans 2016;47:4896e903. [7] Arafin MA, Szpunar JA. Effect of bainitic microstructure on the susceptibility of pipeline steels to hydrogen induced cracking. Mater Sci Eng 2011;528:4927e40. [8] Depover T, Eeckhout EV, Verbeken K. The impact of hydrogen on the ductility loss of bainitic Fe-C alloys. Mater Sci Technol 2016;32:1625e31. [9] Yokoyama K, Kaneko K, Ogawa T, Moriyama K, Asaoka K, Sakai J. Hydrogen embrittlement of work-hardened Ni-Ti alloy in fluoride solutions. Biomaterials 2005;26:101e8. [10] Latifi A, Miresmaeili R, Abdollah-Zadeh A. The mutual effects of hydrogen and microstructure on hardness and impact energy of SMA welds in X65 steel. Mater Sci Eng 2017;679:87e94. [11] Murakami Y, Kanezaki T, Mine Y. Hydrogen effect against hydrogen embrittlement. Metall Mater Trans 2010;41:2548e62. [12] Ismail NM, Khatif NAA, Kecik MAA, Shaharudin MAH. The effect of heat treatment on the hardness and impact dium carbo n Steel. Mater Sci Eng properties of me 2016;114:1e10. [13] Zakroczymski T, Glowacka A, Swiatnicki W. Effect of hydrogen concentration on the embrittlement of a du´plex stainless Steel. Corros Sci 2005;47:1403e14. [14] Pressouyre GM, Bernstein IM. A kinetic trapping model for hydrogen-induced cracking. Acta Metall Matter 1979;27:89e100. [15] Cheng YF. Wiley series in corrosion: stress corrosion cracking of pipelines. Somerset NJ, USA: Wiley; 2013.
Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
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[16] Ali Rajabipour, Melchers RE. Capacity of pitting corroded pipes under hydrogen assisted cracking. Int J Hydrogen Energy 2015;30:9388e99. [17] Bo LL, Jin S, Zhanbing C, Cai YZ. Effects of vanadium precipitates on hydrogen trapping efficiency and hydrogen induced cracking resistance in X80 pipeline steel. Int J Hydrogen Energy 2018;36:17353e63. [18] Kannan M, Dietzel W, Blawert C, Riekehr S, Kocak M. Stress corrosion cracking behaviour of Nd: YAG laser butt welded AZ31 Mg sheet. Mater Sci Eng 2007;444:220e6. [19] Kheradmand N, Johnsen R, Olsen JS, Barnoush A. Effect of hydrogen on the hardness of different phases in super duplex stainless steel. Int J Hydrogen Energy 2016;41:704e12. [20] Kim Y, Kim Y, Kim D, Kim S, Nam W, Choe H. Effects of hydrogen diffusion the mechanical properties of austenite 316L steel at ambient temperature. Mater Trans 2011;52:507e13. [21] Han DK, Kim YM, Han HN, Bhadeshia HKDH, Suh DW. Hydrogen and aluminium in high-manganese twinninginduced plasticity Steel. Scripta Mater 2014;80:9. [22] Weilunski LS, Beneson RE, Lanford WA. Helium-induced hydrogen recoil analysis for metallurgical applications. Nucl Instrum Methods 1983;218:120e4. [23] Turos A, Meyer O. Depth profiling of hydrogen by detection recoiled protons. Nucl Instrum Methods B 1984;4:92e7. [24] Baglin JEE, Kellock AJ, Deline VR, Crockett MA, Shih AH. Absolute cross section for hydrogen forward scattering. Nucl Instrum Methods B 1992;64:469e74. [25] Djukic MB, Zeravcic VS, Bakic G, Sedmak A, Rajicic B. Hydrogen embrittlement of low carbon structural steel. Procedia Mater Sci 2014;3:1167e72.
[26] Li Y, Zhao L, Pan H. Hydrogen permeation behaviour and associated phase transformation in annealed AISI304 stainless steels. Mater Struct 2013;46:621e7. [27] Timokhina IB, Beladi H, Xiong XY, Adachi, Hodgson PD. Nanoscale microstructural characterization of nanobainitic steel. Acta Mater 2011;59:5511e22. [28] Mayer M. SIMNRA user's guide, report IPP9/113. Garching, Germany: Max-Plank Institut fur Plasmaphysik; 1997. [29] Yin F, Hanamura T, Umezawa O, Nagai K. Phosphorusinduced dislocation structure variation in the warm-rolled ultrafine-grained low-carbon steel. Mater Sci Eng 2003;354:31e9. [30] Siddiqui RA, Abdullah HA. Hydrogen embrittlement in 0.3% carbon steel used for petrochemical applications. J Mater Process Technol 2005;170:430e5. [31] Asano S, Otsuka R. The lattice hardening due to dissolved hydrogen in iron and steel. Scripta Metall 1976;10:1015e9. [32] Kirchheim R. Solid solution softening and hardening by mobile solute atoms with special focus on hydrogen. Scripta Mater 2012;67:767e70. [33] Barnoush A, Asgari M, Johnsen R. Resolving the hydrogen effect on dislocation nucleation and mobility by electrochemical nanoindentation. Scripta Mater 2012;66:414e7. [34] Cwiek J, Zielinski A. Mechanism of hydrogen enhancedcracking of high-strength steel welded joints. J Achiev Mater Manuf Eng 2006;18:207e10. [35] Gavriljuk VG, Shivanyuk VN, Foct J. Diagnostic experimental results on the hydrogen embrittlement of austenitic steels. Acta Mater 2003;51:1293e305.
Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels Oluwole Kazum a, Mihail Ionescu b, Hossein Beladi c, M. Bobby Kannan a,* a
Disciple of Chemical Engineering, College of Science and Engineering, James Cook University, Townsville, Queensland, 4811, Australia b Australian Nuclear Science and Technology Organisation, Centre for Accelerator Science, Sydney, 2234, Australia c Institute for Frontier Materials, Deakin University, Victoria, 3126, Australia
article info
abstract
Article history:
Hydrogen depth profiles and microhardness of the electrochemically hydrogen-charged
Received 3 October 2018
nanostructured bainitic steels (produced at two different transformation temperatures,
Received in revised form
i.e. 200 C (NBS200) and 350 C (NBS350)) were obtained using elastic recoil detection
19 February 2019
analysis (ERDA) technique and Vickers microhardness testing, respectively, and compared
Accepted 21 March 2019
to that of mild steel. The ERDA results showed that the subsurface hydrogen concentration
Available online xxx
was higher in NBS200, followed by NBS350 and mild steel. However, the microhardness data of the hydrogen-charged steels revealed material softening in NBS200 and NBS350,
Keywords:
whereas the mild steel exhibited material hardening effect. The microhardness along the
Nanostructured bainitic steels
cross-sectional depth of the steels showed that the softening effect in NBS200 was closer to
Hydrogen-induced softening
the hydrogen-charged surface compared to that of NBS350. The plausible mechanisms for
Microhardness
the softening effect in the NBS200 and NBS350, and hardening effect in mild steel have
Electrochemical hydrogen-charging
been discussed in this paper.
Hydrogen depth profile
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nanostructured bainitic steels have potential applications in hydrogen transportation infrastructure due to their excellent mechanical properties [1,2]. However, it widely reported in the literature that hydrogen in high-strength steels under mechanical loading could lead to a catastrophic form of corrosion failure known as hydrogen-induced cracking (HIC) [3].
Hydrogen concentration, distribution and diffusivity in steels influence their HIC behaviour. Generally, depending on the source of hydrogen (welding, mineral acid cleaning or cathodic protection), inhibitor or heat-treatment (baking) is used to prevent hydrogen diffusion or remove the diffused hydrogen [4,5]. Recently, we reported that the effective hydrogen diffusivities of nanostructured bainitic steels (NBS200: 6.26 108 cm2 s1 and NBS350: 1.09 106 cm2 s1) were
* Corresponding author. E-mail address: [email protected] (M.B. Kannan). https://doi.org/10.1016/j.ijhydene.2019.03.172 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
2
international journal of hydrogen energy xxx (xxxx) xxx
lower than that of mild steel (1.99 106 cm2 s1) [6]. It was also concluded that the high dislocation density in the bainitic ferrite phase and the fine microstructure of NBS200 compared to NBS350 contributed to the lower effective hydrogen diffusivity in NBS200. Generally, an increase in the hydrogen concentration in steels affects their mechanical properties such as tensile strength, ductility [7,8] and hardness [9e12]. Hydrogen trapping sites (reversible and irreversible) could concentrate hydrogen and lead to crack initiation and also facilitate propagation [13,14]. Crack initiation in steels can be due to various factors such as surface imperfections and pitting corrosion [15e17]. However, hydrogen concentration in the subsurface can also lead to crack initiation by hydrogen blistering. Further, hydrogen concentration along the crosssectional depth of the material could influence the crack propagation rate [18e24]. Studies have reported an increase in hardness at the hydrogen-induced crack tips in steels [25,26]. It is critical to study the hydrogen-depth profile and microhardness in these high-strength nanostructured bainitic steels to understand the mechanism of hydrogen diffusion and embrittlement, and thereby evaluate their suitability for hydrogen transport applications. Hence in this study, the hydrogen depth profiles and microhardness of the nanostructured bainitic steels (NBS200 and NBS350) were obtained using elastic recoil detection analysis (EDRA) technique and microhardness testing, respectively. The hydrogen diffusion mechanisms in these steels were discussed.
was vigorously purged with nitrogen gas for 15 min and then gently purged throughout the experiment to reduce the oxygen concentration in the electrolyte. Elastic recoil detection analysis (ERDA) was done on the hydrogen-charged steels at the Australian Nuclear Science and Technology Organisation (ANSTO) ion beam facility within the Centre for Accelerator Science (CAS), which was installed on a beamline serviced by the 2 MV STAR tandem accelerator. The subsurface hydrogen profile was done using 1.8 MeV Heþ ions impinging on the sample surface at an angle of 72 relative to the sample, which have enough energy to recoil the lighter hydrogen atoms from the sample up to a depth of ~1.5 mm. The energy of recoiled hydrogen was measured using a silicon surface barrier detector placed at a scattering angle of 32 to the incoming beam, with an 8.5 mm Mylar foil filter used to stop the helium scattered by the sample surface and by a 1 2 mm2 4-way slits that reduced the energy spread and defined a solid angle of 0.8 msr. The number of helium ions impinging upon the sample was monitored by measuring the drain current of the sample holder, which was around 5 nA, and accounting for secondary electrons. The result of hydrogen yield versus energy (or channel number) was compared with a calculated result generated by SIMNRA software [28]. The results of hydrogen measurement on the samples at room temperature were obtained by integrating the area under the hydrogen peak and comparing it with Kapton, which contains 24 at% hydrogen, and it is stable under the 1.8 MeV helium beam. The depth distribution of hydrogen was obtained from the energy loss of helium in the sample, by making use of the stopping power data incorporated in the SIMNRA analysis software. Microhardness measurements were carried out using an automated Zwick hardness tester (Model: ZHVm). The hardness measurements were performed immediately after hydrogen-charging by applying a load of 200 gf with a load time of 10 s. The hardness test was done on three test samples for the surface and the cross-sectional depth hardness on the uncharged and hydrogen-charged steels. For the crosssectional hardness, the measurements were done at a distance interval of 0.2 mm from charged surface.
Experimental procedure The chemical composition of the nanostructured bainitic steels (NBS200 and NBS350) and the mild steel used in this study is given in Table 1. To obtain different bainitic microstructure, the NBS steel was initially reheated to 1100 C and held isothermally for 30 min, followed by isothermal heat treatment at different temperatures. The NBS200 was isothermally held at 200 C for 10 days, whereas NBS350 was held at 350 C for 1 day [27]. The samples were ground using SiC paper up to 2400 grits, washed with distilled water and ultrasonically cleaned in acetone and then in ethanol. Three test samples were used for each condition to confirm reproducibility. The hydrogen charging was carried out in an electrochemical cell, with the steel sample (dimensions: 20 mm 20 mm 2 mm, mounted in a resin to expose one side of the sample) as the working electrode and a platinum mesh as the counter electrode. The electrolyte contained 0.1 M NaOH and a hydrogen promoter (10 g/l of Na2S.9H2O). Hydrogen was generated by electrochemical cathodic charging using a DC source (Model: Powertech MP-3084). A constant current density of 10 mA cm2 was applied for a period of 6 h. Prior to the hydrogen charging, the electrolyte
Results and discussion Fig. 1 shows the hydrogen depth profiles of the mild steel and nanostructured bainitic steels and the corresponding hydrogen concentrations. All the steel samples showed a relatively high hydrogen yield close to the hydrogen-charged surface. However, the hydrogen yield close to the hydrogencharged surface was significantly higher for NBS200 compared to NBS350 and.mild steel. The NBS200 exhibited a peak hydrogen yield of ~20 (±2.75) cts at 0.14 mm subsurface
Table 1 e The chemical composition of the steels used in this study (wt. %). Steels Mild steel (nominal) Nanostructured bainitic steel
C
Mn
Co
Si
Al
Cr
Mo
Fe
0.20 0.79
0.90 1.98
e 1.58
0.10 1.5
e 1.06
0.40 0.98
0.15 0.24
Bal Bal
Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
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Fig. 1 e (a) Hydrogen depth profiles, and (b) the corresponding hydrogen concentration of hydrogen-charged mild steel, NBS200 and NBS350.
depth, whereas NBS350 steel showed a peak hydrogen yield of ~13 (±1.00) cts at ~ 0.17 mm subsurface depth. For the mild steel, the peak hydrogen yield was ~12 (±0.25) cts at ~0.18 mm subsurface depth. It was noted that the peak hydrogen yield for NBS200 was closer to the charged surface (0.14 mm) compared to that of NBS350 (0.17 mm) and mild steel (0.18 mm). Within the subsurface depth of 1.6 mm, the hydrogen concentration for NBS200 was higher by 37% and 48% compared to NBS350 and mild steel, respectively. The observed high subsurface hydrogen concentration and peak hydrogen yield closer to the hydrogen-charged surface in NBS200 suggest that more hydrogen was trapped in the subsurface compared to NBS350 and mild steel. The images of the Vickers microhardness indent on the surface of the uncharged and hydrogen-charged steels are shown in Fig. 2a and the corresponding surface hardness values are presented in Fig. 2b. For the mild steel, the indent diagonal length for the hydrogen-charged was smaller in comparison with the uncharged, which suggests that the surface hardness of mild steel increased with hydrogencharging. The Vickers hardness value of the hydrogencharged mild steel was ~22% higher than the uncharged sample. In contrast, the nanostructured bainitic steels showed an increase in the indent diagonal length under hydrogen-charged condition. Thus, NBS200 and NBS350 exhibited ~5% and ~12% decrease in the hardness with hydrogen-charging, respectively. It was noted that the decrease in the hardness was higher in NBS350 compared to NBS200. Fig. 3 shows the microhardness depth profile of the uncharged and hydrogen-charged steels along the cross-section. The hydrogen-charged mild steel showed a higher hardness value (26%) close to the hydrogen-charged surface (~0.2 mm), compared to that of the uncharged sample. The hardness value reduced gradually along the cross-sectional depth, however, the hardness values were higher across the sample thickness in comparison with the uncharged mild.steel. In the case of the nanostructured bainitic steels, the hardness value
was significantly lower close to the hydrogen-charged surface compared to the uncharged samples. At a cross-sectional depth of ~0.2 mm, the hydrogen-charged NBS200 exhibited a hardness value of ~707 HV, whereas the uncharged sample showed a hardness value of ~774 HV. Similarly, the hydrogencharged NBS350 exhibited a hardness value of ~300 HV, which was lower than the uncharged sample (~353 HV) at a crosssectional depth of ~0.2 mm. The hardness drop in NBS200 (~8%) was lower than that of NBS350 (~14%). The hardness in NBS200 continued to drop up to ~0.6 mm cross-sectional depth of the sample from the hydrogen-charged side, however, no significant difference in hardness value was recorded beyond 0.8 mm cross-sectional depth. Interestingly, in NBS350, the hardness of the hydrogen-charged sample was lower across the sample thickness compared to the uncharged sample. The hardness drop in NBS350 was significantly high up to ~0.8 mm cross-sectional depth, however, thereafter the difference in the hardness values decreased. The ERDA and microhardness results suggest that the microstructural features of the steels influenced their hydrogen concentration and distribution in the subsurface and beyond. Previously, we reported that the nanostructured bainitic steels consist of nanolayers of bainitic ferrite and retained austenite. The thickness of these layers were finer in NBS200 compared to NBS350, and the volume fraction of the bainitic ferrite was higher in NBS200 (72%) than in NBS350 (42%). The bainitic ferrite in NBS200 exhibited higher dislocation density (4.7 10 15 m2) in comparison with NBS350 (2 10 15 m2) due to their different isothermal transformation temperatures [27]. Hence, we suggested that the finer microstructure with higher dislocation density in the bainitic ferrite phase of NBS200 trapped more hydrogen and was responsible for the lower effective hydrogen diffusivity in NBS200 (6.26 108 cm2 s1) compared with NBS350 (1.09 106 cm2 s1) [6]. In this study, the higher hydrogen concentration observed in the subsurface of NBS200 can be attributed to its lower effective hydrogen diffusivity, which concentrated the hydrogen in the subsurface and thus the
Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
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Fig. 3 e Microhardness along the cross-section of the uncharged and hydrogen-charged mild steel, NBS200 and NBS350.
Fig. 2 e a) Micrographs of the microhardness indents on the surface of uncharged and hydrogen-charged mild steel, NBS200 and NBS350. b) Surface microhardness of uncharged and hydrogen-charged mild steel, NBS200 and NBS350.
peak hydrogen yield was higher and closer to the hydrogencharged surface. In the case of mild steel, hydrogen was not concentrated in the subsurface since the effective hydrogen diffusivity was high in the mild steel (1.99 106 cm2 s1) [6]. Further, the dislocation density (hydrogen trap sites) in low carbon mild steel was reported to be 0.9 1014 m2 [29], which is significantly lower than that in NBS200 and NBS350. Thus,
the hydrogen concentration was relatively uniform in the mild steel. The hydrogen-induced hardening and softening effects in the mild steel and nanostructured bainitic steels, respectively, can be attributed to the presence and interaction of hydrogen atoms with the steels. Literature suggests that a large concentration of dissolved hydrogen in the lattices of iron reduces plasticity, and supersaturation of hydrogen, which lead to the formation of blisters, void and cracks, which generate stress and new dislocation and thereby enhances hardening [30,31]. Hydrogen atoms in the crystal lattice sites enhance solid solution strengthening in mild steel. Basically, solute atoms create stress fields around them which causes lattice distortion and impede dislocation motion and thereby increases the yield stress and hardness of the material. On the other hand, the interaction of a small amount of hydrogen atoms with dislocations can enhance dislocation mobility and lead to the softening effect [32,33]. Hence, in the mild steel, the high concentration of hydrogen in the lattice of the body centred cubic (BCC) structure has led to the material embrittlement (hardening effect). For the nanostructured bainitic steels, however, the hydrogen atoms were trapped in the subsurface by the high number of dislocations, which led to hydrogen enhanced local plasticity (HELP) [34,35]. This caused softening effect in nanostructured bainitic steels. The softening was lower in the case of NBS200 compared with NBS350 due to the relatively high hydrogen concentration in the subsurface of NBS200 resulting in hydrogen environment around the dislocations, which pinned the dislocations and increased the stress required for plastic deformation and thus reduced the softening effect [31,32]. A schematic representation of the hydrogen concentration and distribution in the subsurface and beyond is shown in Fig. 4. This study suggests that nanostructured bainitic steels (NBS200 and NBS350) would perform better than the mild steel
Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172
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Fig. 4 e Schematic representation of hydrogen concentration and distribution in the subsurface and beyond of NBS200, NBS350 and mild steel.
under hydrogen environment such as in hydrogen transport infrastructures, and NBS200 appears superior to NBS350.
Conclusions Hydrogen depth profiles and microhardness values showed that hydrogen concentration in the subsurface of NBS200 was higher compared with NBS350 steel and mild steel due to the lower hydrogen diffusivity in NBS200, which largely limited the hydrogen diffusion to the subsurface. Microhardness along the cross-sectional depth revealed hydrogen-induced softening effect in NBS200 and NBS350 and hydrogeninduced hardening effect in mild steel. The interactions between the hydrogen atoms and the dislocations present in the bainitic ferrite contributed to the softening effect in NBS200 and NBS350, whereas hydrogen dissolved in the crystal lattice of mild steel exhibited hardening effect (embrittlement). The difference in the degree of hydrogen-induced softening observed between NBS200 and NBS350 can be attributed to the difference in the effective hydrogen diffusivity and dislocation density in the bainitic ferrite phase.
Acknowledgement The authors acknowledge the financial support provided by the Australian Government through NCRIS funding scheme for the Centre for Accelerator Science facility, where the hydrogen depth profiling was carried out. The authors also grateful for the support provided by the AINSE PGRA and JCUIPRS scholarship scheme towards the first author's research work at James Cook University.
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Please cite this article as: Kazum O et al., Hydrogen depth profiles and microhardness of electrochemically hydrogen-charged nanostructured bainitic steels, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.172