Microscopic investigation of surface layers on rails

Applied Surface Science 239 (2005) 132–141

Microscopic investigation of surface layers on rails Y. Jira´skova´a,*, J. Svobodaa, O. Schneeweissa, W. Davesb,c, F.D. Fischerb,d Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zˇizˇkova 22, CZ 616 62 Brno, Czech Republic b Institute of Mechanics, Montanuniversita¨t Leoben, Franz-Josef-Straße 18, A–8700 Leoben, Austria c Materials Center Leoben, Franz-Josef-Straße 13, A-8700 Leoben, Austria d Erich Schmid Institute for Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700 Leoben, Austria

a

Accepted 12 May 2004 Available online 25 August 2004

Abstract Reflection g-ray and conversion electron Mo¨ssbauer spectroscopy (MS) completed by optical (OM), transmission electron (TEM) and atomic force microscopy (AFM) and microhardness measurements are applied in order to study the load and temperature driven changes in the phase composition and micro-structure of rail specimens at the surface and in the bulk. It is shown that severe plastic deformation due to the normal pressure as well as shearing together with a rapid change of temperature at service conditions lead to decomposition of the initial pearlite structure accompanied by surface oxidation, defect formation, carbon clustering, precipitation of nanosize carbide particles and austenitisation of the material. A severe plastic deformation is simulated using ball milling of specimens machined from a virgin rail and the results of phase analysis are compared. # 2004 Elsevier B.V. All rights reserved. PACS: 61.18.Fs; 68.35.Dv; 81.40.
z Keywords: Mo¨ssbauer phase analysis; TEM; Defects; Hardening; Ball milling; Rail; Nanocrystalline structure; White etching layer

1. Introduction The development of high-speed trains increases demands on the quality of materials and micro-structural stability of the rail and wheel surfaces. The surrounding atmosphere, contact temperatures which can reach several hundred 8C due to slipping and friction between wheel and rail and their rapid changes in wheel/rail operating conditions may cause severe thermal stresses [1]. Moreover, the contact pressure, *

Corresponding author. Tel.: þ420-532290446; fax: þ420-541218657. E-mail address: [email protected] (Y. Jira´skova´).

which can exceed 1 GPa, and surface shear stresses upto several hundreds of MPa cause severe plastic deformations [2]. Consequently, the modifications of the rail surface structure and phase composition lead to surface corrugations and formation of micro- and macrocracks [3–5]. To minimise damage on train and track and to avoid the risk of transport accidents, one has to minimise changes in the surface properties and to stabilise the surface structure and phase composition of rails and wheels. Therefore, experimental investigations of real rail and wheel specimens as well as specimens loaded in laboratories, for e.g. by ball milling, supplemented by theoretical modelling of

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.289

Y. Jira´ skova´ et al. / Applied Surface Science 239 (2005) 132–141

thermal and mechanical stresses are still of importance [6–9]. The aim of the present paper is to follow changes in the structure of the rail surfaces induced by different service loadings and by ball milling in laboratory conditions. The results obtained from optical (OM) and transmission electron microscopy (TEM) are completed by microhardness measurements and mainly by Mo¨ ssbauer spectroscopy (MS), which is sensitive to surface phase composition. The microcracks are detected by atomic force microscopy (AFM).

2. Experimental details 2.1. Material The chemical composition of the typical rail steel UIC 900 A is the following: C, 0.60–0.82; Si, 0.13– 0.60; Mn, 0.65–1.25; P(max), 0.03; S, 0.008–0.03; Al(max), 0.004; N(max), 0.01, all in wt.%. The specimens were machined from a new, unused (I) rail and from a rail sector loaded in operating conditions (II, III), taken from a curved track with a radius of 280 m; static axle load 22.5 t; speed 80 km/h; inclination 26%; accumulated load 137 Miot. These specimens were drawn from the same curve in the track at various positions, showing zones affected mechanically and thermally in different ways. 2.2. Optical, electron and atomic force microscopy The macroscopic optical surface inspections were done on the rail specimens I, II and III. The cross-sections of selected specimens were prepared by a standard metallography procedure. They were also used for microscopic observations by scanning electron microscopy (SEM) and by AFM, which was performed in air in a contact mode by scanning an area of approximately (100 mm  100 mm). A surface thin film prepared from the rail specimen III was studied also by TEM. For this purpose, a 3 mm thick specimen was cut parallel to the rail axis by spark erosion. The specimen was thinned from the side of bulk material by mechanical thinning down to a thickness of approximately 100 mm followed by ion beam thinning until perforation.

133

2.3. Mo¨ ssbauer spectroscopy Discs with a thickness of 4 mm and a diameter of approximately 25 mm were prepared from the supplied pieces of rails. The specimen of type I was cut from the bulk of the new, unused rail. The surface was carefully ground and polished with 1 mm diamond paste and finally cleaned in ethylalcohol. The other two specimens were cut from the top of the loaded rail (II and III) without any surface treatment to obtain the real surface phase composition. Conversion electron Mo¨ ssbauer spectroscopy (CEMS) and g-ray Mo¨ ssbauer spectroscopy in backscattering geometry (gBMS) using 57 Co in Rh matrix as a source were applied for investigations of the surface layers at room temperature. CEMS spectra were measured using the gas flow (94%He, 6%CH4) electron counter. The thickness of the analysed surface layer was approximately 300 nm. For gBMS the 2p-proportional detector was used and surface layers approximately 30 mm thick were studied. Calibration of the velocity scale in both modes was performed using a standard pure a-iron specimen. Mo¨ ssbauer parameters were obtained by computer fitting the data to a number of Lorentzian lines using the CONFIT program package [10]. The discrete single- and/or double-line components represent the paramagnetic (pm) phases as e.g. austenite, e-martensite, some carbide, FeO. The six-line components indicate the ferromagnetic (fm) phases, e.g. ferrite, martensite, cementite, hematite, magnetite. The individual phases (fm, pm) are identified on the basis of hyperfine parameters: hyperfine field B [T] (fm), isomer shift IS [mm/s] (fm, pm) and quadrupole splitting EQ [mm/s] (fm, pm) by comparing them with literature data. The phase contents are determined according to the corresponding sub-spectra intensities I [%] (fm, pm), i.e. from the iron atomic fraction supposing identical Lamb–Mo¨ ssbauer factors for all phases present. In the surface studies the Mo¨ ssbauer atoms are often located at many different sites, each with a different set of Mo¨ ssbauer parameters. As a result, the experimental spectra consist of broadened lines, reflecting variations in the surrounding of Mo¨ ssbauer atoms. Such spectra are analysed using a distribution of Mo¨ ssbauer parameters. In the present case, a hyperfine field distribution characterised by a mean value of hyperfine field, Bmean and a width of

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distribution, DB is used. Details concerning the application of the Mo¨ ssbauer effect can be found, e.g. in Ref. [11]. 2.4. Ball milling Cubic specimens with an edge length of 25 mm were cut out from the new, unused rail (I). Milling was performed using 50 steel balls of 20 mm in diameter and 5 cubic specimens of the rail. The milling was carried out in the high-energy mode for 1, 20 and 100 h in a closed, air-filled container with a volume 0.5 dm3. During the milling process the temperature of balls and specimens rose upto approximately 100 8C and decreased to room temperature within a short time after the milling was stopped. 2.5. Microhardness The hardness measurements were done at 20 g loading to reflect the hardness of the thin surface layer. Furthermore, higher loads upto 1000 g were used on the surface of the ball milled specimens to estimate the hardness profile near the surface.

3. Results and discussion

Fig. 2. Cross-section of the new, unused rail surface (specimen I).

well visible at the surface. The structure of the crosssection through this surface as seen in the OM is depicted in Fig. 2. It is formed by lamellar pearlite. The thin white lines (denoted by arrows) can be ascribed to globular cementite precipitated at ferrite grain boundaries. Mo¨ ssbauer spectra, shown in Fig. 3, are decomposed into six ferromagnetic sub-spectra with hyperfine parameters summarised in Table 1. The ferrite is represented by ferromagnetic sub-spectra M1–M4. Their hyperfine parameters change in dependence

3.1. Specimen I (virgin rail) Fig. 1 shows a rough surface of the new, unused rail. Oxides (full arrow) and impurities (dotted arrow) are

Fig. 1. Macroscopic view of the new, unused rail surface (specimen I).

Fig. 3. CEMS and gBMS spectra of the new, unused rail surface (specimen I).

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Table 1 Phase analysis and hyperfine parameters of the new, unused rail steel specimen I derived from gBMS and CEMS spectra at 300 K P Sub-spectrum B [T] IS [mm/s] EQ [mm/s] I [%] Phase I [%] gBMS

CEMS

gBMS

CEMS

gBMS

CEMS

gBMS

CEMS

M1 M2 M3 M4

34.4 32.9 31.1 29.5

35.0 33.1 30.4 27.4

0.06 0.00 0.02 0.23

0.05 0.00 0.07 0.32


0.06 0.00
0.01
0.22


0.07 0.00
0.03
0.19

6.8 62.8 12.8 4.8

4.7 68.4 11.1 2.7

MC1 MC2

19.6 17.0

21.8 17.9

0.07 0.19

0.05 0.21


0.02
0.16


0.17 0.07

7.2 5.6

5.5 7.70

on a random distribution of the alloying elements (Mn, Si, Al), present in the steel in small quantities and forming substitution solid solutions, and on defects arising from a machining process. The sub-spectra MC1 and MC2 can be ascribed to cementite Fe3C with imperfect structure, in accordance with data reported in Ref. [12]. The stoichiometric Fe3C carbide has an orthorhombic crystal structure with two crystallographically inequivalent iron sites characterised by two slightly different hyperfine fields of 20.5 and 20.7 T and by the same isomer shifts [13]. Therefore, these sites are usually not distinguishable in the room temperature Mo¨ ssbauer spectrum. Precipitated cementite, originating during transformation from austenite between 690 and 610 8C in the cooling process of the rail, shows an imperfect structure that could imply certain differences in the hyperfine fields (MC1, MC2–Table 1). The phase analyses by CEMS and gBMS of the 300 nm and 30 mm thick surface layers yields identical results within the experimental error (Table 1). In case of carbon free ferrite and of stoichiometric Fe3C the relative amount of 13% of the integral intensity of the cementite (Table 1) corresponds to a slightly higher value (0.9 wt.%) of the carbon content in the steel compared to its nominal content. The mean value of microhardness calculated from 5 measurements is 267 HV 0.02.

gBMS

CEMS

Ferrite 87.2

Ferrite 86.9

Cementite 12.8

Cementite 13.2

their connection into continuous white bands, as seen at the surface of specimen III (Section 3.3) and in e.g. Ref. [14], result from subsequent repeated loading in operating conditions. The scanning electron micrographs demonstrate deformed and in some places broken cementite lamellae denoted by the arrow in Fig. 5. The results of Mo¨ ssbauer measurements are summarised in Fig. 6 and Tables 2 and 3. The CEMS spectrum (Fig. 6 above) documents the disappearance of the original phase composition (compare CEMS spectra in Figs. 3 and 6) in the layers close to the surface. The formation of paramagnetic austenite, represented by double-line components in the central part of the spectrum (PA components in Tables), is clearly proved by Mo¨ ssbauer measurement even if some authors, e.g. [9], are in doubt about an increase in contact rail/wheel temperature on the value (600 8C) required for thermally induced phase transformations however a recent study [1] gives evidence of 600 8C

3.2. Specimen II (loaded rail) Fig. 4 shows a macroscopic view of a section of the rail surface. Light spots, denoted by arrows, correspond to white-etching layers (WEL) formed on the surface of the running rail. The growth of the spots and

Fig. 4. Macroscopic view of the loaded rail surface (specimen II).

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Y. Jira´ skova´ et al. / Applied Surface Science 239 (2005) 132–141 Table 2 Phase analysis and hyperfine parameters of the CEMS spectrum (300 nm) derived at 300 K for the loaded rail specimen II

Fig. 5. SCAN micrograph of cross section of the loaded rail surface (specimen II).

and more. It is to note that not only an increase in temperature but also a very high local hydrostatic pressure of more than 1 GPa provides a significant mechanical driving force for the transformation to austenite. Next to austenite the a-Fe2O3 (fm) and FeO (pm) oxides, represented by MO and PO components in the spectrum analysis, are detected. Oxidation and corrosion contribute to a decrease in the ferrite content. Not fully clear is the presence of PX1 and PX2 components with the similar values

Sub-spectrum

B [T]

IS EQ I [%] [mm/s] [mm/s]

Phase, vol.%

MO

51.9

0.01


0.45

13

M1 M2 M3 M4

33.1 30.1 28.6 22.6

0.00 0.02 0.08 0.02


0.03
0.23 0.25
0.02

10 8 13 6

MC

19.1

0.21


0.28

8

Cementite

PA1 PA2


0.11
0.16

4 7

Austenite, 11

1.08

PX1 PX2

0.39 0.35

0.56 1.24

15 9

PO

1.52

0.36

7

a-Fe2O3 Ferrite, 37

FeO

of isomer shifts and different large values of quadrupole splitting. A decrease in a relative abundance of cementite in CEMS spectrum observed during deformation [15] reflects a transition of iron and carbon atoms into ferrite. It can be supposed that the solubility of carbon in ferrite with high defect density is markedly higher due to its trapping at dislocations (defects can act as traps for carbon and/or nitrogen atoms [16]). The very high carbon content cannot be dissolved in a low defect density ferrite and therefore a standard

Table 3 Phase analysis and hyperfine parameters of the gBMS spectrum (30 mm) derived at 300 K for the loaded rail specimen II

Fig. 6. CEMS and gBMS spectra of the loaded rail surface (specimen II).

Sub-spectrum

B [T]

IS [mm/s]

EQ [mm/s]

I [%]

Phase, vol.%

MO

50.3 47.3

0.11 0.19


0.19
0.02

6 4

M0 M1 M2 M3 M4

34.6 33.2 32.1 32.0 30.9


0.50 0.06 0.01 0.08 0.05

0.21
0.03
0.29 0.06 0.12

12 30 10 8 6

Ferrite, 66

MC

20.6


0.34

0.15

4

Cementite

a-Fe2O3, 10

PA1

0.11

0.52

6

Austenite

PX1 PO

0.35 0.87

0.56 0.08

9 5

FeO

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recrystallization of ferrite is hindered. On the other hand the solubility of carbon in g-Fe is higher then in a-Fe and therefore a formation of nearly defect free austenite from ferrite with excess carbon content and high dislocation density is more probable even at much lower temperatures in comparison to the equilibrium phase diagram. An increase in temperature in the service conditions, e.g. during sliding followed by fast cooling, influences the defect as well as element redistribution in the close surface layers of the rail and can result repeatedly to the decomposition of the austenite into ferrite and cementite and/or other type of carbides. The analysis of gBMS data (Table 3) yields slightly lower contents of oxides (components MO and PO, 15%), austenite (PA1, 6%), and also cementite (MC, 4%) in comparison to the CEMS data. The mean value of microhardness, i.e. 1140 HV 0.02, measured on white spots is much higher in comparison to the microhardness of specimen I. 3.3. Specimen III (heavily loaded rail) The macroscopic surface observation, Fig. 7, shows marked traces of deformation in the direction (full arrow) of shear loading and a high content of oxides and impurities (dotted arrow). Fig. 8 shows an example of the structure of a cross section through the rail specimen. The cementite lamellae are not visible in the layers close to the surface but a brightly coloured band is clearly seen at the surface of the rail. This white etching layer is not present over the entire upper face of the rail and it is not of uniform thickness as

Fig. 7. Macroscopic view of the loaded rail surface (specimen III).

Fig. 8. Cross-sections of the loaded rail surface (specimen III).

documented in sections in Fig. 8. It ranges from a few micrometres (Fig. 8a) to approximately 100 mm (Fig. 8b). This depends probably on the irregularity of the surface of the new, unused rail and the intensity and conditions of service loading. The interface between the WEL and the initial lamellar structure is rough and a mixing of both structures is evident. TEM observations of WEL Fig. 9, show the mostly needle type structure with very fine carbides (arrow in Fig. 9a) and a small amount of initial lamellar pearlite structure (arrow in Fig. 9b). The WEL can be taken as highly deformed martensite with dispersed fine carbide particles, pressed-in impurities and only traces of initial pearlite. Formation of such a nanostructure evokes a hardening effect [17–19]. The microhardness of the WEL, 1142 HV 0.02, is the same as obtained for specimen II. The CEMS and gBMS spectra of specimen III are drawn in Fig. 10; the results of their analysis are summarised in Tables 4 and 5. The layers close to

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Y. Jira´ skova´ et al. / Applied Surface Science 239 (2005) 132–141 Table 4 Phase analysis and hyperfine parameters of the CEMS spectrum (300 nm) derived at 300 K for the loaded rail specimen III Sub-spectrum B/DB [T]

Fig. 9. TEM patterns taken from the loaded rail surface (specimen III).

IS EQ I [mm/s] [mm/s] [%]

M1 M2 M3

32.9 31.1 30.0

0.06 0.01 0.08


0.03
0.29 0.06

MFD

15.1/30.0

Phase, vol.%

30 10 8

Ferrite, 48 Defect region

0.72


0.60

23

PX1

0.42

0.69

18

PA1 PA2


0.05 0.22

0.27 0.75

6 2

Austenite, 8

the surface (CEMS phase analysis) do not show any evidence of cementite. Martensite (ferrite) is represented by sub-spectra M1–M4. It has to be noted here that the hyperfine parameters of ferrite and martensite are the same and, therefore it is impossible to distinguish these phases only by Mo¨ ssbauer spectroscopy. But considering the TEM results (previous section), the high value of microhardness and keeping in mind the arguments of other authors [4], the presence of martensite is more probable. PA1 and PA2 doublets represent austenite. The distribution of hyperfine fields (MFD) represents the highly deformed phase with a high content of defects, pressed-in impurities, etc. The parameters of PX1 component are similar to those detected in spectra of specimen II. Table 5 Phase analysis and hyperfine parameters of the gBMS spectrum (30 mm) derived at 300 K for the loaded rail specimen III

Fig. 10. CEMS and gBMS spectra of the loaded rail surface (specimen III).

Sub-spectrum

B [T]

IS EQ I [mm/s] [mm/s] [%]

Phase, vol.%

M1 M2 M3 M4 M5

34.3 33.0 31.7 30.4 27.0

0.00 0.00
0.05 0.02 0.00

0.10 0.00 0.04
0.02
0.02

10 45 8 8 7

MC1

21.4


0.12

0.17

8

MC2 MC3

16.2 11.6

0.04 0.10

0.04 0.05

2 3

PX1

0.35

0.56

9

PA1

0.11

0.52

6

Austenite

PO

0.87

0.08

5

FeO

Ferrite, 78

w-Fe5C2 carbide, 13

Y. Jira´ skova´ et al. / Applied Surface Science 239 (2005) 132–141

The analysis of gBMS spectrum in Table 5 does not show the highly deformed component. Towards the bulk, ferrite or martensite (M1–M5) forms 78% of studied volume. Hyperfine parameters of MC1–MC3 components agree well with data in Ref. [19,20] for the w-Fe5C2 (Ha¨ gg) carbide. This carbide has a structure with some similarities to cementite as can be seen from lattice parameters: a ¼ 1.1562 nm, b ¼ 0.4573 nm (a in Fe3C), c ¼ 0.5060 nm (b in Fe3C) [21,22]. In Ha¨ gg carbide the carbon atoms are surrounded by trigonal prisms of iron atoms but the arrangement of the prisms differs from that of Fe3C. This corresponds with different components in the Mo¨ ssbauer spectrum of the w-Fe5C2 phase. The structure of w-Fe5C2 carbide can be obtained by introducing stacking faults into the cementite structure during the deformation and tempering processes to which the rail is exposed. w-Fe5C2 carbide is detected also in ball milled specimens (Section 3.4), and the structural change of Fe3C into Fe5C2 is observed by Mo¨ ssbauer spectroscopy in mechanically alloyed specimens in Ref. [23]. An approximately 10 mm thick layer was removed mechanically from the surface of the specimen III and an analysis of CEMS spectrum yielded the same phase composition as shown in Table 5 (gBMS spectrum). It documents that the highly deformed phase is concentrated in the layers very close to the surface. Two places within the cross-section of specimen III were examined at room temperature by atomic force microscopy in contact mode. Approximately 100 mm  100 mm squares were scanned. Some micro-cracks, not visible by optical microscopy, can be seen in the upper as well as the lower part of the 2D image in Fig. 11. It can be supposed that these micro-cracks are formed at grain boundaries where also the small particles of carbides are observed. The arithmetic average roughness Ra for the upper image is 40.8 nm, for the lower one 32.1 nm. 3.4. Ball milled specimens The ball milling was performed to simulate changes in the surface phase composition of the rail steel specimens by controlled loading. Both CEMS (300 nm) and gBMS (30 mm) spectra were measured after 1, 20 and 100 h of milling time. The hyperfine parameters of phases detected in the Mo¨ ssbauer spec-

139

Fig. 11. 2D image of cross-section of the loaded rail surface (specimen III).

tra are the same as those in specimen III, within the range of experimental error. The relative abundances of the phases are summarised in Table 6. The CEMS and gBMS spectra as obtained after 100 h of milling are depicted in Fig. 12. After the first hour of milling approximately 10% of the iron atoms at the surface are bounded in oxide as detected by CEMS. A prolongation of milling time leads to refinement of the surface defect structure and to the disappearance of oxides (during milling the surface of the specimen is gradually powderised and the oxides remain in the powder). CEMS results also point to a decomposition of cementite followed by formation of regions with excess carbon content,

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140

Table 6 Phase analysis of 300 nm (CEMS at 300 K) and 30 mm (gBMS at 300 K) thick surface layers of specimens after the ball milling process Time of milling [h]

I [%] Ferrite

Cementite

0

CEMS gBMS

87 87

13 13

1

CEMS gBMS

80 84

4 10

20

CEMS gBMS

88 82

100

CEMS gBMS

81 83

w-Fe5C2 carbide

Austenite

Oxide

0 0

6 3

10 3

5 6

0 9

3 3

4 0

5 5

10 8

4 4

0 0

Fig. 12. CEMS and gBMS spectra taken after 100 h of ball milling.

Fig. 13. Dependence of hardness and depth of indentation on loads and times of milling.

favouring the formation of austenite along the same arguments as outlined in Section 3.2. Towards the bulk the presence of w-Fe5C2 is observed next to cementite as given by gBMS analysis. The measurements of the hardness and depth of indentation at loads up to 1000 g are summarised in Fig. 13. The microhardness obtained after 20 and 100 h of milling reaches the same value (1100 HV0.02) as measured for WEL on the loaded rail specimens II and III. They reflect the hardening effect due to the decomposition of the initial pearlite structure and formation of carbide precipitates as detected

by Mo¨ ssbauer measurements. The hardness values at loads above 200 g tend to reflect the properties of bulk rail material equivalent to new, unused rail specimen I.

4. Conclusions This paper summarises the results of microscopic surface investigations of new, unused (I) and loaded (II, III) rail specimens. It is shown that an application of atomic-resolution methods of Mo¨ ssbauer spectroscopy and AFM can yield detailed information.

Y. Jira´ skova´ et al. / Applied Surface Science 239 (2005) 132–141

Micro-cracks seen by AFM, in fact, become sources of stress concentration and may cause the nucleation and growth of macroscopic cracks. Mo¨ ssbauer spectroscopy allows detection of small amounts of austenite (2–3%) in the ferrite- or martensite substrate material and identification of very small particles with a degree of accuracy that is often not possible by other methods. On the other hand it does not allow distinguishing ferrite and martensite because of the very similar hyperfine parameters of both phases. The investigations have confirmed different microstructures at layers close to the surface in dependence on the loading conditions. When the pearlite structure is exposed to severe plastic deformation, the cementite decomposes and carbon atoms are trapped at dislocations of very high density in the ferrite phase. This fact, together with high compressive stresses and local heating, favours the formation of g-Fe phase (austenite). In consequent fast cooling stage austenite can decompose into ferrite and newly-created small particles of cementite and/or other type of carbides, such as Ha¨ gg carbide. As the crystalline structure of the Ha¨ gg carbide is similar to that of the cementite and the carbide particles are extremely small, one cannot expect a different influence of Ha¨ gg carbide on mechanical properties compared to cementite. Ball milling can be applied to obtain controlled severe plastic deformation of surface layers of the rail material but it provides only partial simulation of the service conditions of real rails. As the Mo¨ ssbauer phase analysis has shown, the precipitation of Ha¨ gg carbide after 20 and 100 h of ball milling is similar to the one observed in heavily loaded rail specimen III. The hardness measurements have yielded the same increase in the surface microhardness (1100 HV0.02). The formation of the WEL should be included in finite element method (FEM) modelling of the deformation and fracture of the rail. The WEL has to be taken as a material with considerably different mechanical properties (yield stress, ultimate strength, low ductility) in comparison to the original pearlite steel and this can influence the results of the deformation and life-time FEM predictions. Moreover, it should be taken into account that the WEL continuously grows into the pearlite while being continuously ground at the surface.

141

Acknowledgements The authors wish to thank voestalpine Schienen GmbH, represented by Dr. Peter Pointner, for supplying the rail specimens, J. Brezina for fruitful discussions, and the Project No. S2041105 of AS CR for financial support.

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