24September2017

Materials and Electronics Engineering

Evolution of structure and properties of railhead fillet in long-term operation

V.E. Gromov1*, Yu.F. Ivanov2, 3, 4, O.A. Peregudov1, K.V. Morozov1, X.L. Wang5, W.B. Dai6, Yu.V. Ponomareva1, O.A. Semina1

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1Siberian State Industrial University
2Institute of high current electronics, Sib.D, RAS
3National research Tomsk state University
4National research Tomsk polytechnical University
5Research Institute, Northeastern University, Shenyang, China
6School of Materials and Metallurgy, Northeastern University, Shenyang, China

Materials and Electronics Engineering 20152:4

Publication Date (Web): October 10, 2015 (Article)

DOI:10.11605/mee-2-4

*Corresponding author. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

 

Abstract

 


Figure 5 Structure being formed in surface layer of rail steel as a result of service (1000 mln.t.). In (c) the particles of carbide phase located along the boundaries of fragments are designated by arrows.

By methods of modern physical materials science the nature of structure – phase changes in the fillet surface layers of rail in the process of long-term operation was established (passed tonnage 500 and 1000 mln tons brutto). A multifactor character of increase in microhardness of surface layers was analyzed.


 

Keywords

Rails, Surface, Phase composition, Defect substructure

 

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Introduction

    Service resistance of rails is the vital condition of the reliable work of railways in the modern conditions of high density of freight traffic and intensity of movement [1]. A long operation of rails is accompanied by the significant change of structure and properties of the surface layer of the material, the formation of defects resulting in the removal of rails [2, 3]. Up to 15% of all Russian rails removed in the order of the single replacement have intolerable wear and collapse norms [2]. The understanding of processes taking some course in steel is one of the essential conditions of controlling the state of material and allows the operation resources of rails to be forecast.

    The aim of the research is the study of physical nature of evolution of rail steel properties based on the analysis of phase composition, defect substructure and microhardness of ‘working rounding off’ of rails after a long service on railways.

Experiments

    Samples of rail steel P65 the properties and elemental composition of which are regulated by Russian State Standard P51685-2000 were used as a test material. The samples of rail steel were cut from the article in the initial state and after service on railway (passed tonnage 500 mln tons brutto and 1000 mln tons brutto). The steel structure of ‘working’ rounding off having an increased wear suggesting its primary interaction with rolling stock in the process of service and being the reason of rail removal was analyzed. The metal structure was studied by methods of transmission (method of thin foils) electron microscopy. The foils were preparedby method of plate thinning located on the surface of rounding off and at a distance of 2 mm and 10 mm from the surface of rounding off. The scheme of sample preparing is shown in Fig. 1. 

Figure 1 Scheme of preparing of rail sample in studying its structure by methods of electron diffraction microscopy. Solid line designates the direction on rounding off, dotted lines conditionally designate the regions of location of metal layers used for foil preparation.

Figure 1 Scheme of preparing of rail sample in studying its structure by methods of electron diffraction microscopy. Solid line designates the direction on rounding off, dotted lines conditionally designate the regions of location of metal layers used for foil preparation.

Results and discussion

    Strength properties of rail steel before and after service were characterized by the value of microhardness. Microhardness of rail metal was determined along the cross-section of head in the crosswise direction by Vickers method (load on indenter 2H) from surface along the radius of rounding off on the segment ≈ 10 mm similar to the situation shown in Fig 1. The results of microhardness profile construction are shown in Fig 2.

Figure 2 Dependence of microhardness HV on distance X from the surface of rail ‘working’ rounding off in the initial state (curve 0), after passed tonnage of rails 500 mln. t. (curve 1) and 1000 mln. t. (curve 2).

Figure 2 Dependence of microhardness HV on distance X from the surface of rail ‘working’ rounding off in the initial state (curve 0), after passed tonnage of rails 500 mln. t. (curve 1) and 1000 mln. t. (curve 2).

    When analyzing the results shown in Fig. 2, it can be noted that service is accompanied by the formation of strengthened surface layer not less than 2 mm thick (Fig. 2, curve 1) with 500 mln t. tonnage. Increase in passed tonnage of rails up to 1000 mln. t. results in the appreciable increase in surface layer microhardness: in reference to the initial state of steel by ≈ 1.7 times, in reference to the state being formed with 500 mln. t. tonnage by ≈ 1.3 times (Fig. 2, curve 2). 

    It is obvious that alteration of surface layer hardness of rail metal is due to evolution of its defect substructure in the service process. Consider this point in more detail. The studies carried out by methods of diffraction electron microscopy showed that structure of rail steel in the initial state (before service) was formed by pearlite grains of lamellar morphology (Fig. 3, a), ferrite grains containing the cementite particles of different shapes and sizes (subsequently referred to as – grains of ferrite – carbide mixture) (Fig. 3, b) and grains of ferrite free of cementite particles (subsequently referred to as – structurally free ferrite) (Fig. 3, c). It should be noted that cementite particles are practically always located along the boundaries of structurally free ferrite. The main morphological constituent of steel structure is pearlite grains the relative content of which is 65%, the relative content of grains of ferrite-carbide mixture – 20%, the remainder – grains of structurally free ferrite.

Figure 3 Structure of rail steel in the initial state (before operation) obtained by Transmission electron microscopy.

Figure 3 Structure of rail steel in the initial state (before operation) obtained by Transmission electron microscopy.

    In the volute of ferrite grains and in the ferrite constituent of pearlite colonies a dislocation substructure in the form of chaos and nonregular nets is revealed (Fig. 4, a). Scalar dislocation density in the material averages 4.5 × 1010 cm-2.

    Analysis of steel structure by method of thin foils permitted to reveal the presence of bend contours of extinction that unambiguously is indicative of curvature – torsion of crystal lattice of the material caused by internal stress fields (Fig. 4, b, contours are designated by arrows). 

    Using the procedure considered in detail in [4] the estimates of curvature – torsion value of crystal lattice of structural constituents of steel x were done. As expected the largest value of curvature – torsion of crystal lattice is registered in grains of structurally free ferrite i.e. the largest strong constituent of the material; the largest value – in grains of lamellar pearlite.

Figure 4 Dislocation substructure (a) and bend extinction contours (b) revealed in rail steel (initial state). Bend extinction contours are designated by arrows in (b).

Figure 4 Dislocation substructure (a) and bend extinction contours (b) revealed in rail steel (initial state). Bend extinction contours are designated by arrows in (b).

    Thus, in the initial state (the state before service on railway) rail steel is a multiphase, morphologically complex heterostrong material the structure of which is formed by grains of lamellar pearlite, grains of ferritecarbide mixture and grains of structurally free ferrite.

    Steel service on railway is accompanied by essential transformation of phase composition and defect substructure of the material and this fact in a regular way was reflected in alteration of hardness value (Fig. 2). The results of the quantitative analysis of steel structure after two modes of service on railway are shown in Table 1. As expected the most significant transformations of material structure are revealed on the surface of rails. First, service of rails results in fragmentation of steel structure i.e. division of grains into regions with small angular disorientation (Fig. 5, a). Sizes of fragments depend on both the degree of rail service and the distance of the analyzed layer from the surface of rounding off. For example, the average sizes of fragments reduce repeatedly (by more than 8 times) as the fright traffic increases from 500 mln. t. to 1000 mln. t. Concurrently with it the degree of fragment disorientation αa3 increases. On the boundaries of fragments the particles of carbide phase are observed (Fig. 5, c). The particles have a round shape and sizes of particles vary within 10-20 nm. Consequently, structure fragmentation is accompanied by transformation of carbide subsystem of steel, namely failure of initial extractions of cementite and their repeated formation on the boundaries of fragments.

Figure 5 Structure being formed in surface layer of rail steel as a result of service (1000 mln.t.). In (c) the particles of carbide phase located along the boundaries of fragments are designated by arrows.

Figure 5 Structure being formed in surface layer of rail steel as a result of service (1000 mln.t.). In (c) the particles of carbide phase located along the boundaries of fragments are designated by arrows.

    Second, rail service is accompanied by transformation of dislocation substructure: increase in scalar dislocation density (Table 1) is noted, and transformation from substructure of dislocation chaos to netlike substructure is observed. 

Table 1 Characteristics of rail steel structure after service.   

table1

    Third, curvature – torsion value of steel crystal lattice x increases repeatedly (by more than 6 times). The last fact is indicative of the increase in internal stress fields in steels in service.

    Fourth, the transformation of carbide subsystem of steel is revealed (as it was noted above): Namely, failure of initial particles of cementite and formation of new particles of nanosize range on boundaries of subgrains and dislocations is noted. The probable mechanism of this transformation is dissolution of cementite particles as a result of interaction with dislocations, transition of carbon atoms on dislocation. Taking into consideration a high energy of interaction of carbon atoms with the core of dislocations it can be expected the formation of Cottrell and/or Suzuki atmosphere which can limit appreciably the mobility of dislocations. 

    A set of the facts presented above allows to propose the following opinions about the mechanisms of hardness increase in surface layer of rail steel subjected to long (1000 mln. t.) service. First, substructural hardening caused by the formation of nanosize fragments the boundaries of which are stabilized by the particles of carbide phase. Second, hardening by particles of carbide phase located in the volume of fragments and on dislocations (dispersion hardening). Third, hardening caused by formation of Cottrel and Suzuki atmospheres by carbon atoms on dislocations. Fourth, solid solution hardening caused by the possible saturation of crystal lattice of α-phase by carbon atoms. Fifth, hardening contributed by internal stress fields being formed due to the incompatibility of deformation of neighbouringgrains, α-phase and particles of carbide phase located in it. 

Conclusions

    It was established that operation of rails was accompanied by hardening of surface layer not less than 2 mm thick in passed tonnage 500 mln. t. brutto, the increase in passed tonnage of rails to 1000 mln. t. brutto resulted in the substantial increase in strength of surface layer. It was shown that increase in hardness of surface layer of rail steel subjected to long (1000 mln. t.) operation has a multifactor character and is due to first, substructural hardening; second, hardening by particles of carbide phase; third, hardening which is caused by the formation of Cottrell and Suzuki atmospheres by carbon atoms on dislocations; fourth, solid solution hardening; fifth, hardening contributed by internal stress fields. 

References

  1. V.E. Gromov, A.B. Yuriev, K.V. Morozov, Yu.F. Ivanov, Microstructure of hardened rails (Novokuznetsk: Inter-Kuzbass, 213p 2014).
  2. E.A. Shur, Damages of rails (M.: Intext, 192p 2012).
  3. E. Sheiman, Wear of rails. Journal of Friction and wear. 33, 413 (2012). doi:10.3103/S1068366612040101
  4. Yu.F. Ivanov, E.V. Kornet, E.B. Kozlov, V.E. Gromov, Hardened structural steel: structure and mechanisms of hardening (Novokuznetsk: SibSIU, 174p 2010).

Acknowledgements

The research was financed by the grant of Russian scientific fund (project № 15-12-00010).

References

  1. V.E. Gromov, A.B. Yuriev, K.V. Morozov, Yu.F. Ivanov, Microstructure of hardened rails (Novokuznetsk: Inter-Kuzbass, 213p 2014). 
  2. E.A. Shur, Damages of rails (M.: Intext, 192p 2012). 
  3. E. Sheiman, Wear of rails. Journal of Friction and wear. 33, 413 (2012). doi:10.3103/S1068366612040101 
  4. Yu.F. Ivanov, E.V. Kornet, E.B. Kozlov, V.E. Gromov, Hardened structural steel: structure and mechanisms of hardening (Novokuznetsk: SibSIU, 174p 2010). 

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Additional Info

  • Type of Publishing: JOUR - Journal
  • Title:

    Evolution of structure and properties of railhead fillet in long-term operation

  • Author: V.E. Gromov, Yu.F. Ivanov, O.A. Peregudov, K.V. Morozov, X.L. Wang, W.B. Dai, Yu.V. Ponomareva, O.A. Semina
  • Year: 2015
  • Volume: 2
  • Issue: 4
  • Journal Name: Materials and Electronics Engineering
  • Publisher: Nicety Press Company Limited
  • ISSN: 2410-1648
  • URL: http://www.meej.org/volume-2/october-2015/item/374-evolution-of-structure-and-properties-of-railhead-fillet-in-long-term-operation
  • Abstract:

    By methods of modern physical materials science the nature of structure – phase changes in the fillet surface layers of rail in the process of long-term operation was established (passed tonnage 500 and 1000 mln tons brutto). A multifactor character of increase in microhardness of surface layers was analyzed.

  • Publish Date: Saturday, 10 October 2015
  • Start Page: 4
  • DOI: 10.11605/mee-2-4