Materials Science and Engineering A 527 (2010) 3259–3263
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Hydrogen embrittlement of microalloyed rail steels A.P. Moon a,∗ , R. Balasubramaniam a , B. Panda b a b
Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur-208016, India R & D Center, EWAC Alloys Ltd., Larsen & Toubro, Mumbai-400072, India
a r t i c l e
i n f o
Article history: Received 22 May 2009 Received in revised form 31 January 2010 Accepted 3 February 2010
Keywords: Rail steel Hydrogen embrittlement Hydrogen diffusivity Fractograph
a b s t r a c t The effect of hydrogen on mechanical properties of three different rail steels of composition in weight percent, C–Mn (0.71C–1.04Mn), Cu–Mo (0.69C–1.16Mn–0.24Cu–0.18Mo) and Cr–Cu–Ni (named as NCC) (0.71C–1.15Mn–0.59Cr–0.40Cu–0.20Ni) was studied. Hydrogen was charged electrochemically in 0.5 mol l−1 H2 SO4 solution at a current density of 0.1 A cm−2 for 12 h. Duplicate tensile tests were performed before and after hydrogen charging. The tests revealed that the rail steels were susceptible to hydrogen embrittlement as confirmed by the reduction in ductility. The degree of embrittlement was higher in C–Mn rail steel compared to the Cu–Mo and NCC rail steels. The hydrogen-charged samples revealed brittle features after hydrogen charging. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Railways are important part of the transit system of any country. One of the important components of the railway transportation system is the rail. Rails are routinely checked and are maintained in proper service condition because any premature failure of rails can result in accidents. The replacement of rails due to failures is an economic concern. Additionally, problems related to corrosion also affect the safety of rails. In the Indian context, a recent survey has shown that approximately one third of all failures are caused by corrosion [1,2]. Hydrogen is generally produced during corrosion of metals. Depending on conditions, hydrogen reduction can occur in neutral and alkaline solutions as well as acidic solutions [3]. As regards the cathodic reaction during corrosion of rail steels, the environment is not very acidic, so the primary cathodic reaction is probably oxygen reduction. However, if the potential is low enough, hydrogen evolution will also occur to some extent. The hydrogen generated by the cathodic reaction can enter the steel and cause embrittlement. Hydrogen, due to its small size, can diffuse relatively rapidly in metallic materials [4]. In certain conditions, this can lead to the build up of hydrogen concentration within the material, which can subsequently result in hydrogen embrittlement (HE) [5]. This phenomenon is well known in materials science. The effects of hydrogen on the mechanical properties of iron and steel have been widely investigated [6]. The susceptibility to hydrogen embrittle-
∗ Corresponding author. Tel.: +91 512 2597974 (lab). E-mail address:
[email protected] (A.P. Moon). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.02.013
ment is higher in presence of tensile loading conditions and more severe when the steel possesses high strength and low ductility. Since the rail steels are of relatively high strength and exposed to conditions that favor hydrogen entry (through corrosion), it was important to study the embrittlement behaviour due to hydrogen. The hydrogen content in the rail steel is specified to be below 3 ppm [7], which is achieved by special degassing arrangements during processing of rail steel. The present communication will address the effect of hydrogen on mechanical properties of three-rail steels at ambient temperature (30–32 ◦ C), as it is well known that embrittlement is most severe at temperatures near ambient temperature [7]. The C–Mn is the standard rail steel, while the Cu–Mo and Cr–Cu–Ni rail steels are novel rail steels that have been developed for improved atmospheric and crevice corrosion resistance [8]. It is known that embrittlement due to hydrogen is more around room temperature and therefore, the present study will address embrittlement at ambient conditions.
2. Experimental procedure The rail steels used in the study were designated as C–Mn, Cu–Mo and Cr–Cu–Ni (named as NCC). The detailed compositions are presented in Table 1. The rail foot possesses the thinnest cross section and is also the location most susceptible to corrosion due to the accumulation of corrosive media, emanating from various sources. Apart from contaminants from atmosphere, the discharge from open lavatories of the Indian passenger coaches is of great concern. The corrosive media, flowing along the rail head and rail web finally gets accumulated at the rail foot region, due to the
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Table 1 Compositions of rail steels (in wt%). Sample
C
Mn
Cu
Mo
Cr
Ni
Si
S
P
C–Mn Cu–Mo NCC
0.71 0.69 0.71
1.04 1.16 1.15
– 0.24 0.40
– 0.18 –
– – 0.59
– – 0.20
0.21 0.19 0.35
0.013 0.022 0.026
0.022 0.024 0.027
Fig. 1. Schematic showing tensile sample orientation with respect to rail.
presence of rail fixtures [8,9]. These fixtures act as barriers to the flow of the corrosive media. Therefore, this portion of the rail is highly prone to the aggressive conditions. The entry of hydrogen into the material results in deterioration of mechanical properties. In view of this, the effect of hydrogen on the tensile behaviour of samples obtained from the rail foot region of rails was the focus. Tensile samples were prepared from the foot portion of rails in the rolling direction as shown in (Fig. 1). The rail steel studied in this research was processed in the following manner at SAIL, Bhilai, India. The steel from the basic oxygen furnace was degassed in a degasser. After degassing, the heat was cast into blooms by continuous casting process. The dimensions of blooms were 300 mm × 335 mm (width × thickness). The blooms were reheated in reheating furnaces consisting of 3 zones, charging zone, heating zone (1280–1300 ◦ C) and soaking zone (1260–1280 ◦ C). The soaking time at 1260–1280 ◦ C was 1 h. The next process involved rolling of the blooms to rails in roughing, intermediate and finishing rolling mills. The finish rolling temperature was maintained at around 900 ◦ C. The rails were finally cooled in soaking pits after heating the rails to a temperature of 500 ◦ C. The slow cooling rate ensured the formation of pearlitic microstructure, which gives optimum combination of strength and ductility as the chemical composition of rail steel is close to the eutectoid composition [10]. Cylindrical tensile specimens of gage length to diameter ratio of 4:1 were prepared according to ASTM E-8 standard, as shown in Fig. 2. Hydrogen was charged by cathodic polarization of the tensile sample in an electrochemical cell in 0.5 mol l−1 H2 SO4 solutions using a current density of 0.1 A cm−2 at room temperature (30–32 ◦ C). A few drops of sodium arsenite concentration of 100 mg/l (NaAsO2 ) were introduced from a chemical dropper to serve as a hydrogen recombination ‘poison’. Its role is to prevent the formation of molecular hydrogen thereby accelerating the entry of
hydrogen into the material. The hydrogen charging time was 12 h. The specimen served as the cathode and a stainless steel strip, the anode. Special care was taken to ensure that hydrogen was charged only in the gage section, by covering the other free surfaces with Teflon tape. The influence of hydrogen on mechanical properties was studied by comparing the mechanical properties before and after hydrogen charging. Duplicate tensile tests were performed as per specification ASTM E-8 on tensile machine of 20 kN capacity (Hounsfield model H20 K-W, England). The tests were started immediately after hydrogen charging to minimize escape of hydrogen. All the tests were conducted in uni-axial tensile loading condition at an engineering strain rate of 0.1 s−1. The strain rate at which tests were conducted after hydrogen charging was kept the same as that for samples without hydrogen charging, to allow comparison of test results. After failure, the final gauge length and the final diameter were measured. All tensile tests were duplicated and the average mechanical properties are reported in this communication. The fracture surfaces were observed in a scanning electron microscope (Carl Zeiss EVO50, Germany). Micro-hardness profiles across the cross section of separate samples was obtained after hydrogen charging for 24 h in 0.5 mol l−1 H2 SO4 solution at a current density of 0.1 A cm−2 . In this procedure, the cross sections were ground to remove 1 mm from one of the side surfaces immediately after charging and the micro-hardness data was obtained as a function of distance into the sample from the sides of this section. 3. Results and discussion The engineering stress–strain curves of sample, before and after hydrogen charging, are presented in Figs. 3 and 4, respectively. As the rail steels did not show definite yield points, the 0.2% offset yield stress was determined, according to ASTM E-8. Mechanical properties (average of two tests) like yield strength (YS), ultimate tensile strength (UTS), percentage elongation and percentage reduction in area, are tabulated in Table 2. There is a reduction in UTS after hydrogen charging for all threerail steels, in decreasing order as C–Mn, Cu–Mo and NCC steel. Since the hydrogen-charged samples failed without any necking, it is clear that the UTS in this case represented the condition when the embrittlement process tended to fracture the sample. After hydrogen charging, a higher yield stress (YS) was noted in the case of C–Mn and Cu–Mo while there was a marginal decrease in YS in the case of NCC steel. The increase in yield stress can be related to the strain in the lattice created by the presence of hydrogen [11]. Micro-hardness profiling experiments verified the effect of hydrogen and also indicated the distance into the sample where
Fig. 2. Standard cylindrical tensile specimen according to ASTM E-8 Length of reduced section (A) is 20 mm, distance between shoulders (B) is 28 mm, diameter of reduced section (D1) is 4 mm, grip diameter (D2) = 8 mm, radius of curvature (R) = 4 mm.
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Table 2 Average mechanical properties of rail steels before and after hydrogen charging. Rail Steel
Condition
Y.S (MPa)
UTS (MPa)
Elongation (%)
RA (%)
HEI based on % RA
C–Mn
Uncharged H2 -Charged
420 437
893 740
11.72 6.47
23.00 4.20
82
Cu–Mo
Uncharged H2 -Charged
556 634
964 872
15.75 7.50
38.92 14.35
63
NCC steel
Uncharged H2 -Charged
699 635
1105 984
12.75 9.00
20.97 10.85
48
this effect was significant. The hardness profiles are shown in Fig. 5 for the rail steel samples. The increase in hardness after hydrogen charging cannot be held responsible due to superabundant vacancy as such vacancies can be generated at a higher temperature and predominant in FCC crystal system hence the increase in the hardness in the near-surface regions must be related to hydrogen in solution, essentially in ferrite phase of pearlite [12]. A complete discussion of these results is presented elsewhere [13]. The bulk hardness of the samples are not in direct proportion to the yield stress measured in the tensile tests, which could be due to variation in the samples
used for the testing purpose. Another significant result is that the effect due to hydrogen can be appreciated by observing the fracture surface in the near-surface region for a distance approximately up to 300 microns from the surface. The fracture surfaces in the near-surface regions are shown in Fig. 6. In the case of samples that were not charged with hydrogen, the lower ductility in case of C–Mn rail steels was revealed by facets features resembled like cleavage (Fig. 6a) while the Cu–Mo and NCC rail steels exhibited cup-and-cone ductile fracture features with dimple morphology (Fig. 6b and c). The fracture surface showed more brittle character after hydrogen charging as revealed by the larger fraction of cleavage-like facets appeared on the surface (Fig. 6d–f). In order to obtain a quantitative estimate of the effect of hydrogen, the hydrogen embrittlement index (HEI) was estimated using ductility, measured as %RA (percentage reduction in area), which reduced after hydrogen charging. This can be defined as the proportional change of ductility after hydrogen charging, given by the following equation: HEI =
(R0 − RH ) × 100 R0
(1)
where R0 is the ductility before hydrogen charging and RH is ductility after hydrogen charging. A larger value of HEI signifies a larger effect of hydrogen on ductility. The degree of embrittlement was highest in C–Mn steel followed by Cu–Mo. The embrittlement was lowest in case of NCC steel (see Table 2). The reasons for the higher degree of embrittlement in C–Mn steel needs to be investigated in the future. Fig. 3. Engineering stress–strain curves for rail steels before hydrogen charging.
Fig. 4. Engineering stress–strain curves for rail steels after hydrogen charging.
Fig. 5. Average micro-hardness profiles as a function of distance into the surface for rail steels after hydrogen charging.
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Fig. 6. Fractographs from fracture surfaces of tensile samples of (a) reference C–Mn, (b) reference Cu–Mo, (c) reference NCC, (d) hydrogen-charged C–Mn, (e) hydrogen-charged Cu–Mo and (f) hydrogen-charged NCC steel.
4. Conclusions
Acknowledgements
The effect of hydrogen on mechanical properties of three different rail steels of composition C–Mn in weight percent, (0.71C–1.04Mn), Cu–Mo (0.69C–1.16Mn–0.24Cu–0.18Mo) and Cr–Cu–Ni (0.71C–1.15Mn–0.59Cr–0.40Cu–0.20Ni) was studied. Hydrogen was charged electrochemically in 0.5 mol l−1 H2 SO4 solution at a current density of 0.1 A cm−2 for 12 h. Duplicate tensile tests performed before and after hydrogen charging revealed that the steels were susceptible to hydrogen embrittlement based on reduction in ductility. The degree of embrittlement was higher in C–Mn steel compared to other alloy rail steels. The hydrogen-charged samples revealed brittle cleavage-like features on the fracture surface.
The authors would like to thank the Technology Mission for Railway Safety (TMRS) under the aegis of Ministry of Human Resources Development and Ministry of Railways, India for financial support in research work.
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A.P. Moon et al. / Materials Science and Engineering A 527 (2010) 3259–3263 [5] N. Ansari, R. Balasubramaniam, Mater. Sci. Eng. 293A (2000) 292–295. [6] T. Tabata, H.K. Birnbaum, Scr. Met. 17 (1983) 947–950. [7] A. Moon, Effect of hydrogen on mechanical properties of novel rail steels, Master of Technology thesis, Indian Institute of Technology, Kanpur, India, 2009. [8] B. Panda, On the corrosion of novel rail steels, PhD thesis, Indian Institute of Technology, Kanpur, India, 2008.
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[9] B. Panda, R. Balasubramaniam, G. Dwivedi, Corros. Sci. 50 (2008) 1684– 1692. [10] B. Panda, R. Balasubramaniam, A.P. Moon, Mater. Sci. Technol. 25 (2009) 1375–1382. [11] N. Lasseigne, D.L. Olson, Mater. Eval. 66 (2008) 1077–1083. [12] Y. Fukai, K. Mori, H. Shinomiya, J. Alloys Compd. 348 (2003) 105–109. [13] A. Moon, R. Balasubramaniam, Defect Diffusion Forum 293 (2009) 41–45.
