TRIBOLOGICALLY INDUCED CHANGES IN THE MICROSTRUCTURE OF RAIL SURFACES W. STADLBAUER Christian-Doppler-Laboratorium für Moderne Mehrphasenstähle am Lehrstuhl für Mechanik, Technische Universität München, Boltzmannstraße 15, 85747 Garching, GERMANY,e-mail: stadlbauer@lam.mw.tu-muenchen.de W. LOOS, E.A. WERNER Lehrstuhl für Mechanik, Technische Universität München, Boltzmannstraße 15, 85747 Garching, GERMANY; e-mail: loos@lam.mw.tu-muenchen.de, werner@lam.mw.tu-muenchen.de SUMMARY In the present work, the morphology as well as special features within “white etching layers” (WELs) on rails are observed by scanning electron microscopy and X-ray diffraction (XRD) analysis. An appropriate classification of the WEL-morphology is provided and possible mechanisms of formation are discussed.

Keywords: white etching layer, rails, X-ray diffraction analysis

1 INTRODUCTION High hardness surface layers appearing as “white etching layers” (WELs) are frequently observed on rails exposed to railway service. Detailed analyses of special features within WELs as well as the determination of the microstructure by X-ray diffraction (XRD) provide the basis for the discussion of possible mechanisms of WEL formation. 2 MORPHOLOGY AND SPECIAL

FEATURES OF WHITE LAYERS White etching layers do not appear always as completely homogeneous structures, but occasionally consist of different layers like apparent from Fig. 1, which shows the WEL on top of a cross sectioned rail head.

Figure 1: Internal structure of a WEL: “PBM” – pearlitic bulk material, “BL” – basis layer, “SL” –

structured layer, “NSL” – non-structured layer, “F” – remnant of grain boundary ferrite; right side: detail

from the NSL-SL interface; transverse section.

The different layers exhibit different hardness levels and it is interesting to note, that the “structured layer” (SL) is somewhat softer than the neighbouring “non-structured layer” (NSL) and “basis layer” (BL) (as evident from the width of the vertical scar from a Vickers diamond). While the NSL and the SL appear in combination with the BL only, the latter is sometimes observed to extend from the WEL-bulk interface to the surface of the rail without the NSL or SL being present.

Although, mostly, fragmentation of the cementite lamellae of the pearlitic material is observed on the WEL-bulk interface, there is evidence that the formation of the BL can also proceed without prior fragmentation of the cementite lamellae. As shown in Fig. 2, the lamellar morphology of the initially pearlitic material may still be present in the BL of the WEL.

100 µm

a

c b

500 nm

Figure 2: WEL formation (BL-type) without fragmentation of cementite lamellae. Optical

micrograph a), scanning electron micrograph b),c).

Grain boundary (pro-eutectoid) ferrite is often observed near the surface of rails and, surprisingly, remnants of grain boundary ferrite are partly visible in the WELs (though in the BL exclusively), too (Fig. 1).

Figure 3: Concentration of plastic strain on the very top layer of the rails’ surface; longitudinal section a),

transverse section of a different specimen b).

These remnants of grain boundary ferrite serve as indicators for plastic deformation as shown in the left part of Fig. 3, where the concentration of shear in a zone of some micrometers below the surface becomes apparent.

a b

3 X-RAY DIFFRACTION ANALYSIS XRD analysis was performed using CoKα-radiation. In order to minimize the influence of the bulk material below the WELs, a shallow angle for the incident beam (10°) was chosen, hence 90 percent of the diffracted intensity stems from a depth less than five micrometers. Analysis of WELs by XRD revealed the presence of approximately 5 to 9 vol.% of austenite, while the cementite phase, which is a constituent of the initially pearlitic material, was not detectable in any of the observed WELs (Fig. 4).

Figure 4: Comparison of X-ray diffraction patterns derived from the WEL and the bulk material (pearlite).

Analysis of the angular position of the austenite reflections reveals, that the carbon content in the austenite does not differ much from the average carbon content of the rail steel. Considerable broadening of the ferrite diffraction peaks suggests that high strains and a small crystallite size are further characteristics of the WELs. Using the Warren-Averbach method, it is possible to evaluate average microstrains to about 0.5 % and an “average column length” of approximately 7 nm which may be taken as measure for the grain size. Thus, it can be concluded, that the formation of WELs is accompanied by significant grain refinement and considerable lattice distortion which is owing to high dislocation densities. It is noteworthy, that the ferrite diffraction peaks from the WELs appear only slightly asymmetric and differ therefore significantly from those derived from the same material in the as-quenched, non-deformed (martensitic) condition. 4 POSSIBLE MECHANISMS OF WHITE

LAYER FORMATION To date there is no general agreement about the maximum temperature which is needed for the formation of WELs on the surface of rails (e.g. [1,2,4,6]). Therefore, in the following discussion both low and high temperature mechanisms are taken into account.

4.1 Low-temperature mechanisms Cementite dissolution is, besides austenite formation, one of the two striking incidents accompanying WEL formation. As most probable sources capable to provide

the energy which is required for carbide dissolution are regarded:

Formation of excess ferrite-cementite boundary area due to the formation of slip steps [5]. Trapping of carbon atoms by dislocations

[6,7,12]. Languillaume et al. [5] gave an explanation for cementite dissolution in wire drawing assuming that plastic shear deformation of the cementite lamellae produces additional area of cementite/ferrite phase boundaries. At a certain plastic strain, the energy which is stored in the phase boundaries exceeds the difference in Gibbs free energy of ferrite and cementite. Dissolution of cementite will eliminate these phase boundaries and, therefore, leads to smaller values of the Gibbs free energy of the (now fully ferritic) material.

Another possible source of energy promoting cementite dissolution at temperatures below Ac1 refers to the interaction of carbon atoms and dislocations. As a consequence of plastic deformation, dislocations pile up at the ferrite/cementite phase boundaries, thereby exerting attractive forces to the carbon atoms of the cementite phase (and altering the value of the phase boundary energy). By comparison of the binding energy of carbon atoms in the cementite phase (~0.5 eV) with the carbon-dislocation interaction energy, Kalish and Cohen [12] formulated the conditions for cementite dissolution in a dislocated ferrite (+cementite) structure, see Fig. 5.

Figure 5: Stability of cementite in a ferritic structure as a function of dislocation density and carbon content.

The number of carbon atoms which can be trapped by edge dislocations is about one half of that which can be

trapped by screw dislocations (data from [12]).

The large dislocation density required for complete dissolution of cementite in a 0.7 wt.% C steel due to dislocation-carbon interaction suggests, that the influence of excess interface area should not be omitted.

Concerning the incidence of low-temperature formation of austenite from an initially pearlitic material (which is not excluded a priori), it has to be noted, that no models comparable to those for cementite dissolution are available to date. However, it is possible to make some qualitative considerations for the austenite formation from pearlite partly on the basis of experimental observations: Using atom probe field ion microscopy, Hong et al. [8] found, that the carbon content of the

austenite (A)

cementite(CM)

CM

CM

CM

CM

CM

CM

A

bulk material (pearlite)

white layer

CM

40° 60°50°Diffraction Angle (2 )θ

Inte

nsity

(110

)

(200

)

0,1

1

1E+12 1E+13 1E+14

Dislocation Density [1/cm2]

Carb

on C

onten

t [wt

.%]

α + Fe3C

α + carbon in solution

screw edge

0.1

0.2

0.4

0.6

0.8

1.0

cementite in a heavily cold worked pearlitic material may be as low as 16 at.%, while the carbon content in the surrounding ferrite may raise up to 3 at.%. Provided that railway service leads to a similar redistribution of carbon in the top layer of the rail steel, the austenite which is observed in the WELs of rails (containing approximately 3 at.% C) might have formed from the carbon enriched ferrite by a shear mechanism. A different possibility would be that, due to the severely disturbed binding conditions in the carbon depleted cementite, the iron atoms are able to move into the positions of the austenite lattice. This might occur directly or via the movement of the iron atoms into the ferrite lattice as an intermediate step [11]. Subsequent carbon diffusion would be required in order to obtain austenite with 3 at.% C similar to the one found in the WELs on rails.

In fact, the mechanisms of austenite formation from pearlite due to cold working are highly speculative and formation of austenite from pearlite has, to our knowledge, never been observed under low temperature conditions like e.g. in wire drawing [5,8], powder milling [10] or severe plastic deformation experiments [9]. All of these experimental investigations provide evidence for the possibility of cementite dissolution below the Ac1 temperature. 4.2 High-temperature mechanisms Under sliding contact conditions, reasonable values for the sliding velocity and contact pressure will result in contact temperatures above Ac1 thereby leading to the formation of WELs [13]. For pin on disc configurations, calculation of bulk (mean) surface temperatures and flash temperatures (at asperities) is well established and has been incorporated in wear mechanism maps [14] which are based on numberless experimental investigations. As an experimental detail it is interesting to note, that WELs resulting from pure sliding may contain considerable amounts of retained austenite, as was found on the surface of steel pins after dry sliding [13]. Furthermore, these investigations [13] have shown that precipitation of fine carbides within the WELs readily occurs along plastic flow lines, dislocation nets and subgrain boundaries.

High temperatures even up to the melting point of steels are also assumed to appear in so called “adiabatic shear bands” (ASBs) which are, concerning their etching response and hardness, very similar to WELs on rails. ASBs are an outcome of dynamic loading conditions and originate as a consequence of thermal softening due to the heat which is generated by plastic deformation [16]. Although plain carbon steels in the pearlitic condition seem to be rather insensitive to localized shear deformation when compared to the quenched and tempered condition, WELs on rails have been interpreted also as overlapping ASBs [15].

Finally, both the WEL-formation due to sliding and the ASB-formation lead to grain refinement and dissolution

of cementite, if it was present in the initial microstructure [13,17].

5 DISCUSSION Since the WELs on rails are rather inhomogeneous, the following discussion is based on the classification which is suggested by Fig. 1.

Remnants of grain boundary ferrite never appear in the NSL which is occasionally observed at the top of the WEL. The reason for this may be, that the NSL originates from plastic deformation at temperatures sufficiently high for complete cementite dissolution and austenite formation. Extensive plastic deformation will eliminate all remnants of grain boundary ferrite in the top layer of the WEL, as is obvious from Fig. 3. After the wheel has passed, the austenitic layer, which now contains a considerable amount of dislocations, is cooled rapidly to ambient temperature. The subsequent martensite formation is hindered by the high amount of dislocations and therefore, a considerable amount of retained austenite can be found by XRD analysis. Furthermore, stress induced migration of carbon atoms to dislocations [12] will occur and give rise for the lack of tetragonality in the body centered phase. The mechanism of heat generation due to plastic deformation of a thin layer, similar to the conditions of sliding contact or similar to localized shear within ASBs seems to dominate the formation of the NSL.

In most cases, the NSL is surrounded by the somewhat softer SL. It has been suggested by Clayton et al. [18], that the reduced hardness is an outcome of tempering due to successive formation of overlapping WELs. Tempering of an initially martensitic structure is most likely the cause for the microstructure shown on the right hand side of Fig. 1. The amount of carbides formed in the course of tempering is very probably too small to be detected by XRD techniques, and therefore, tempering of a part of the WEL is not in contradiction to the results from XRD measurements. Decay of retained austenite then takes place in the course of the tempering reaction within the SL. The microstructure of the SL is therefore assumed to consist of tempered martensite containing finely dispersed carbides.

The BL finally, which was found at least in the boundary region between WEL and the pearlitic bulk material of every WEL, occasionally contains remnants of grain boundary ferrite what has led to the conclusion [18] that the austenitizing temperature is low or that the duration of the thermal cycle is very short. Re-austenitization of pearlite will start at ferrite/cementite interfaces. As long as the temperature is above Ac1, the austenite/ferrite boundaries stemming from adjacent cementite plates will proceed towards each other. When the material is rapidly cooled down again before the pearlite to austenite transformation is completed, the microstructure will consist of consecutive lamellae of ferrite and martensite. This martensite should contain some cementite since the ferrite-austenite reaction should, according to the continuous time temperature austenitization diagram (TTA), be completed before the

cementite is fully dissolved in the austenite (Provided that extrapolation of the TTA diagram to ~0.5 10-4 s (duration of contact) is qualitatively possible.). There are some indications from scanning electron microscopy observations that such a structure might be present at the boundary region between pearlite and BL. However, some micrometers away from the boundary region, no evidence was found for the presence of cementite within the BL (although cementite particles have occasionally been observed in the WEL of rails [1]). A second possibility to explain the structure shown in Fig. 2 b) and c) could be that, again according to the TTA diagram, the material has become already fully austenitic but carbon is still not distributed homogeneously. Provided that carbon depleted regions are etched more readily than carbon enriched ones, the structure would also consist of consecutive lamellae, in this case consisting of martensite of different carbon content. Independently of the above somewhat speculative considerations, it has to be noted that plastic deformation of pearlite leads to fragmentation and/or thinning of the cementite lamellae [19]. If the BL would have been formed due to a low temperature mechanism, the phase which can be hardly etched (cementite) should appear even thinner than in the originally prearlitic structure, which is in contradiction to the observations (Fig. 2). Moreover, plastic deformation of pearlite would lead to a reduction of the lamellae spacing [19]. The lamellae spacing obvious from Fig. 2 is approximately 250 nm which is about the same as the cementite lamellae spacing of the non-deformed material. Therefore, it can be concluded that severe plastic deformation is not necessary for the formation of the BL and temperature effects are thus likely do dominate. 6 ACKNOWLEDGEMENTS The authors are grateful to the VOEST-ALPINE SCHIENEN GmbH & CO KG, the VOEST-ALPINE STAHL DONAWITZ GmbH and the Christian-Doppler Forschungsgesellschaft for their financial support. 7 REFERENCES [1] Österle, W.: Rooch, H.: Pyzalla, A.: Wang, L.: Investigation of White Etching Layers on Rails by Optical Microscopy, Electron Microscopy, X-ray and Synchrotron X-ray Diffraction. Mat. Sci. Eng., A303 (2001), 150-157. [2] Grassie, S. L.: Short Wavelength Rail Corrugation: Field Trials and Measuring Technology. Wear, 191 (1996), 149-160. [3] Lojkowski, W.: Djahanbakhsh, M.: Bürkle, G.: Gierlotka, S.: Zielinski, W.: Fecht, H.-J.: Nanostructure Formation on the Surface of Railway Tracks. Mat. Sci. Eng., A303 (2001), 197-208.

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