Advanced pearlitic and bainitic high strength rails promise to improve rolling contact fatigue resistance

Gregor Girsch1, René Heyder2

1voestalpine Schienen GmbH, Leoben-Donawitz, Austria, 2Deutsche Bahn AG, DB Systemtechnik, Brandenburg-Kirchmöser, Germany

Abstract

Fatigue resistance of rails is nowadays more and more important for both railways and rail producers to extend rail service life and reduce life cycle costs. This paper starts with a description of the parameters influencing rolling contact fatigue (RCF). Advanced rail steels with both pearlitic and bainitic structure, developed from voestalpine Schienen, promise to be a contribution to diminish RCF defects. The latest results of the track tests of these advanced rail grades currently carried out in joint projects with German Railway (DB) are reported. Finally the procedure from developing advanced rail materials to a standardized application is described as a "circle of development".

Introduction

Due to enhancing productivity, railroads all over the world encounter constantly increasing loads, which result in excess damage of the wheel-rail system. Thereby rolling contact fatigue (RCF) is the major issue in nearly all types of railway systems, i.e. heavy haul, high speed, mixed traffic and light rail systems. As it concerns both the rail and the wheel, dealing with RCF-defects became a more and more important factor of cost. There are a lot of influencing parameters on RCF, the question is: on what screw shall we drive to diminish RCF defects in order to prolong rail and wheel service life, resulting finally in cost savings?

On which screw to drive to diminish RCF?

The most important parameters that influence RCF are [1]: - loading conditions - wheel-rail contact conditions - maintenance strategy and quality-level of maintenance activities - material parameters

The loading conditions are given by the axle loads, speed and train frequencies. The possibilities to influence this factor regarding RCF-defects in a positive way are very limited, because railways are increasing loads, speed and train frequencies constantly to meet customer requirements. In the wheel-rail system a lot of parameters, e.g. track alignment, superstructure, running behaviour of the rolling stock and tribological parameters influenc e the complexity of contact conditions between wheel and rail. The constraints in this matter make it very difficult to set contact parameters to diminish RCF-defects, e.g. Head Checks. More important is most of the time the running behaviour of the vehicles. However, reducing the forces between wheel and rail is one - if not the - most important contribution for reducing RCF [2]. Maintenance on rails, i.e. grinding and/or milling, is done when RCF-defects occur. When the damage is too great, rails must be replaced . Rail machining is an important and effective way to deal with RCF-defects to ensure availability of tracks with the required safety and increase rail service life time [3]. However, it is expensive and finally not the cure for the original source of the problem. Considering the complexity and the restrictions of the RCF-influencing factors mentioned above, we believe that the improvement of the wheel and rail materials is a very important contribution to solve the

RCF problem at its origin. In the following, advanced rail materials are presented that show to improve RCF resistance. With regards to developments for wheel materials it shall be referred to the literature [4].

Advanced rail steels

Looking at the historical development of rail steels, fig. 1, it is noticeable that the hardness of rail steels increased continuously. The aim was to improve wear resistance. The steps mark also different technological milestones, e.g. the introduction of the LD-Steelmaking process and the Head Hardening process [5].

standardgrade

wear resistantgrade R260

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R320Crhigh wearresistant grade

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Fig. 1: Trends in Development of rail steels Since RCF defects became a major issue, a focus was given to the development of advanced rail steels with resistance to both wear and RCF-defects. Thenceforward the strength of the head-hardened rails was increased continuously, reaching nowadays tensile strengths of 1300 MPa and more - corresponding to hardness’s beyond 410 BHN [6]. But: The picture is not as simple as it appears. The development of rail steels was focused on pearlitic steels for the past decades; about 10 to 15 years ago, the development and investigation of rail steels with bainitic structures started. The reason for having yet not reached the final solution might be the complexity of these bainitic structures. Contradictory to pearlitic structures, where an improvement of the material can be characterized by simply refining the Ferrite–Cementite aggregates, the structure of bainitic steels has already a great variation in the basic microstructures. There exists the classical upper and lower bainite (= Ferrite + Cementite) and the group of the so-called carbide-free (CF) - bainites. These are aggregates of Ferrite and Martensite-Austenite (M-A) constituents, where the carbon is not bonded in the cementite but in islands of Martensite and Austenite. An additional complication is that these aggregates can show all variations from coarse globular to very fine acicular structures of the Ferrite and the M-A constituents. To be not at the end of all variations: mixtures of cementite and M-A constituents are possible by changing chemical composition of the rail steel and sometimes material properties are modified by annealing. Therefore the directives for the choice of the “right” bainitic rail steel are not yet on the table, but track tests indicate very strongly the advantages of these high strength bainitic steels regarding RCF–resistance, sometimes showing not any indication of RCF–cracks. Track tests are the only possibility to prove advantages of advanced rail steels and therefore the advanced rail grades are tested under practical conditions together with railways in order to investigate their damage behavior and to proof their benefits in track performance.

Track testing

Voestalpine Schienen carries out track tests of advanced rail grades under specific loading conditions in joint projects together with various railways worldwide, fig. 2, and DB is testing various rail grades of different rail producers, too. In the following the actual joint track tests between DB and voestalpine Schienen with focus on RCF investigation are presented.

Fig. 2: Track test matrix Both high strength pearlitic and bainitic rail steels were installed 2004/05 in three test locations on mixed traffic lines, see table 1. The selected radii range from 500 m to 3300 m to cover the area, where RFC-damage is the predominant failure mode.

Table 1: Test locations of VAS-DB actual RCF track tests

The steel “type”, the chemical composition and material properties of the rail steels that were put to the test are listed in table 2.

Table 2: Rail steels and their properties

Track testing procedure

Specific track testing procedures were applied in order provide an objective comparison of the rail damage behavior of the rail grades. Different rail grades (test- and reference rails) were installed in an alternating order along a curve to avoid interference with the “position in curve effect”. Two measuring points per rail and at least three rails of the same grade per test makes six data points available for each rail grade and each track test. The measurements were carried out by experts from DB System Technology. Inspections and measurements are done in half-year intervals for duration of at least three years or more than 100 MGT’s, respectively. The material loss by wear was calculated from profile measurements done by a MiniProf Rail instrument [7]. The propagation of cracks, i.e. Head Checks, was investigated both visually and by magnetic particle inspection (MPI). Automated in-track eddy current testing (ET) was used to determine the length of the cracks extending into the rail head [8]. The real space depth of the cracks (vertical distance from the rail surface to the crack tip) can be calculated by the angle of crack growth. The in-track ET measuring-system opens the possibility to prove the crack propagation not only on selected positions but also throughout the entire length of the test sections. Metallographic investigation methods on cut rail samples are planned at the end of the track test in order to verify the angle of crack growth and depth of rail damage by cracks as measured by ET method. To complete the investigation, welds were inspected visually and the longitudinal profile was measured by the Straight-Edge Compact [9].

Test results

The presentation of the results will be focused on the test curve in Kerzell, because the accumulated load exceeds here already 60 MGT’s. The findings in Mering confirm the observations made in Kerzell, however RCF-damage is less because of the larger curve radius and due to speed restrictions because of construction activities on the parallel line. The test section in Hannover was installed only recently to complete the overview of the range of radii. First results will be presented at the Conference in June 2006. Due to the fact, that the rails of the selected test curves are damaged mostly by RCF-defects, wear is not the mayor issue. The results support this, because the wear figures (vertical and gauge corner wear) are even after 60 MGT’s in the range of approximately 0.1 mm for all rail grades. The Bainitic rail grades

rail grade hh micro- Rm,min A5,min Hardnessstructure C Si Mn Cr [MPa] [%] [BHN]

R220 pearlite 0,50-0,60 0,20-0,60 1,00-1,25 ≤ 0,15 770 12 220-260R260 pearlite 0,62-0,80 0,15-0,58 0,70-1,20 ≤ 0,15 880 10 260-300

R260Mn pearlite 0,55-0,75 0,15-0,60 1,30-1,70 ≤ 0,15 880 10 260-300R320Cr pearlite 0,60-0,80 0,50-1,10 0,80-1,20 0,80-1,20 1080 9 320-360R350HT x pearlite 0,72-0,80 0,15-0,58 0,70-1,20 ≤ 0,15 1175 9 350-390

R350LHT x pearlite 0,72-0,80 0,15-0,58 0,70-1,20 ≤ 0,30 1175 9 350-390370LHT x pearlite 0,70-0,82 0,40-1,00 0,70-1,10 0,40-0,70 1175 9 >370380UHC x pearlite 0,90-1,00 0,20-0,35 1,20-1,30 0,25-0,30 1200 9 > 380400UHC x pearlite 0,90-1,00 0,20-0,35 1,20-1,30 0,25-0,30 1240 9 > 380

DOBAIN 340 x bainite 0,76-0,84 0,20-0,35 0,80-0,90 0,40-0,55 1100 11 340-380DOBAIN 380 x bainite 0,76-0,84 0,20-0,35 0,80-0,90 0,40-0,55 1250 10 380-420DOBAIN 430 x bainite 0,76-0,84 0,20-0,35 0,80-0,90 0,40-0,55 1400 9 > 430

hh..head hardened / heat treated

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DOBAIN380 and DOBAIN430 show a similar wear resistance as the grade R350HT and 370LHT, respectively, only the standard grade R260 has twice wear figures as the other four rail grades. However, it shall be mentioned that these advanced rail grades showed in track tests with tight curves a two to three times better wear resistance compared to the standard grade R260 [10]. Figure 3, left hand side shows the results of the MPI-investigation visualizing the formation of head checks on the five different rail grades after a total load of 60 MGT’s. The corresponding data is summarized in table 3. It can be noticed clearly that the rail grade with the lowest hardness (R260) has the longest head checks with the largest distance to each other, whereas the hardest pearlitic rail grade in this test (370LHT) shows the shortest cracks with the smallest real space distance. Investigations at various other track tests confirm that head checks are becoming finer, shorter and have smaller spacing in-between them by increasing rail hardness [11]. The tendency to form finer head checks on harder rail steels was also confirmed in tests in Scandinavia, where the grade 370LHT showed far less head checking and no spalling compared to R350HT rails.

Fig. 3: MPI graphs (left side); ET measurement (right side)

Remarkable is the result of the bainitic steel grades. They show very light head checking, in which the DOBAIN380 grade compares to the R350HT grade and the DOBAIN430 to the 370LHT grade respectively, both for wear and for crack formation and propagation. On two other track tests with rather tight curve radii – one in Austria on a mountain track and one in Germany on a heavy haul line – similar results concerning crack formation on the surface were found. On a heavy haul track test in Germany, the R350HT rails showed head checks after a total load of approx. 30 MGT’s whereas the DOBAIN430 rails did not. Even after a total load of almost 70 MGT’s the DOBAIN430 rails did not show typical Head Checks, but more a kind of an “orange peel” structure, fig. 4.

Fig. 4: Rail surface of DOBAIN430 and R350HT rails

More important than the visible length of the head checks on the rail surface is of course the depth of the cracks, because this determines rail service life time and rail maintenance strategies, respectively. The length of the detected cracks by ET-measurements is demonstrated in fig . 3, right hand side. The corresponding data for the crack length and the calculated crack depth assuming an angle of crack growth of 25° [12] is summarized in table 3. The crack growth rates – approx. two to three times less for R350HT compared to R260 – correspond also to the averaged figures found in other track tests, see fig. 5 [10, 11].

Table 3: Analysis of crack formation and crack length

DOBAIN430 R350HT

Fig. 5: Depth of rail damage by head checks for different rail grades The results obtained so far can be summarized as follows. - Crack depth in curves with medium radii is at a higher level than at small and very large radii

respectively. This corresponds perfectly to the illustration in figure 2. - With increasing hardness of pearlitic rail grades, head checks become finer and shorter with a smaller

real space distance and they develop with a significant lower pace. - Bainitic rails show significant better RCF-resistance, because they show very fine head checking only

randomly compared to R350HT rails; in tight curves only a “structured” surface can be observed. - Pearlitic rails show higher wear resistance with increasing hardness, e.g. R350HT rails show a two to

three times better wear resistance compared to R260 rails. - Bainitic rails show a similar wear resistance than head hardened rails (DOBAIN380 corresponds to

R350HT and DOBAIN430 to 370LHT).

Circle of development

It is a common procedure for railways to test new or advanced rail steels that promise to show better track performance. An example for that is the head hardened rail grade R350HT. This steel grade was developed in the eighties aiming to increase wear resistance, which was proved in track tests all over the world [11]. As a consequence, the use of head hardened rails is recommended for tight curves where wear is a problem [13, 14]. Internal regulations of DB follow the UIC practice and specify the use of head hardened rails (R350HT) in curves below 700 m radius. Additionally joint track tests of DB and voestalpine done earlier have also shown a three times better resistance against Head Checks of R350HT rails compared to the standard rail grade R260 [12]. To evaluate the technological benefits that were observed in these track tests economically, life cycle cost (LCC) calculations have been done, too. The LCC analysis stated an average 35 % reduction in total costs when using R350HT rails in curves instead of standard carbon rails [15]. Utilizing the unequivocal results based on the track tests and the LCC calculations, DB System Technology will now recommend an extension of the application of head hardened rails. The recommendation will cover larger curve radii (500-1500 m), where RCF-defects are more common. The procedure described above can be drawn in the form of a circle. This circle, fig. 6, starts with the development of new or improved technologies, e.g. advanced rail steels with better damage behavior. In a first step, the presumed technological benefits have to be investigated and proved by practical tests, i.e. a technological validation has to be done. Then the validation shall be completed in terms of economical

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and safety aspects (LCC and RAMS analysis) [15, 16]. Finally, when the validation is positive in terms of technology, economy and safety, the last logical step has to be the implementation of the new technology in standards and specifications of railways. Unfortunately, it can be observed that this procedure is not applied completely. Even though head hardened rails have proved their technological benefits already for long time, the application of this rail type is only in some railways a regular procedure.

Fig. 6: Circle of development

Conclusion and Outlook

voestalpine Schienen developed pearlitic and bainitic high-strength rail steels with both improved wear and RCF-resistance, properties which are of equal importance to railways nowadays. To investigate the track performance of these advanced rail steels, track tests under different loading conditions are carried out with railways all over the world. Extensive track testing is done with DB on a high sophisticated level and the latest results are very promising. It was found that the RCF resistance of the rail grade 370LHT is approx. 20 % higher compared to the grade R350HT and 50 % higher compared to the standard grade R260. Regarding wear, similar results are obtained. The performance of the bainitic rail grade DOBAIN430 is similar to the pearlitic grade 370LHT. Ultra-high-carbon high strength rails 400UHC are used in tight curves of different highly stressed heavy haul lines regularly. Due to the fact that they also show a better RCF perfomance at these conditions, tests were started now at conventional European railway lines. This rail type was put to the test at the time of writing and results will be reported at a later stage. All ongoing tests are continued to proof the results at higher accumulated loads. Cost savings by using advanced rail steels combined with appropriate maintenance strategies are quantified by life cycle cost analysis. An average 35 % reduction in total costs can be obtained when using R350HT rails instead of standard carbon rails in RCF loaded curves. As a consequence, DB System Technology recommends to extend the application of head hardened rails for curves with radii up to 1500 m. LCC calculations are further carried out using the results evaluated from actual track tests. Simultaneously, the development of these new rail grades is continued to optimize their properties to meet future customer requirements. The circle of development leads from the development of new rail steels to the technical, economical and safety validation. The final step is the implementation of the positive approved product into the regulations of the railway authority. It will be a joint effort of the technicians from DB and voestalpine Schienen to ensure that this last step is done in the same effective way as the first ones.

Development of

advanced rail gradesRCF È and Wear È

Technical ValidationTrack Testing

Validation of

Economical Benefits

and SafetyLCC and RAMS Analyses

Regular ImplementationRegulations, Standards

References

[1] R. Heyder. “Stress analysis and metallographic studies as a contribution to the understanding of phenomena in wheel/rail-contact”, ZEVrail Glasers Annalen, 127 (2003) 11-12, pp. 578-588.

[2] J. Kristan, L. Allen, J. LoPresti: “Evaluation of advanced rail steels and improved welding techniques under 35,7 tonne (39 ton) axle loads at the facility for acclereated service testing (FAST)”, Proceedings of the 8th International Heavy Haul Conference, Rio de Janeiro, 2005.

[3] W. Schöch, R. Heyder. “Rail Surface Fatigue and Grinding: Exploring the Interaction”, Proceedings of the 6th International Conference on Contact Mechanics and Wear of Rail/Wheel Systems, Göteborg, Sweden, 2003, pp. 133-138.

[4] M. Bannasch, K. Mädler. „Materials used for Wheels on Rolling Stock“, 7th World Congress on Railway Research 2006, Montreal, 2006.

[5] A. Moser, P. Pointner, G. Prskawetz. “Herstellung von kopfgehärteten Schienen aus der Walzhitze”, Berg- und Hüttenmännische Monatshefte (BHM), 133 (1988) 7, pp. 321-326.

[6] N. Frank, P. Pointner. “High performance rails for heavy haul traffic”, Proceedings of the 7th International Heavy Haul Conference, Bisbane, 2001, pp. 467-475.

[7] C. Esveld. “MINIPROF wheel and rail profile measurement”, Proceedings of the 2nd Mini Conference on Contact Mechanics and Wear of Rail/Wheel Systems, Budapest, 1996, pp. 34-43.

[8] R. Krull, H. Hintze, H. M. Thomas, T. Heckel. “Zerstörungsfreie Prüfung an Schienen heute und in der Zukunft”, ZEVrail Glasers Annalen, 127 (2003) 6-7, pp. 286-296.

[9] Elektro-Thermit GmbH & Co. KG. “Straightness Measuring Device Straight-Edge Compact – SEC”, Manual, (2004) 9.

[10] G. Girsch, N. Frank. “New Rail Grades – a Technical Performance Overview”, Proceedings of the 8th International Heavy Haul Conference, Rio de Janeiro, 2005.

[11] R. Heyder, G. Girsch. “Testing of HSH-rails in High Speed Tracks to Minimise Rail Damage”, Wear, 258 (2005) 7-8, pp. 1014–1021.

[12] G. Girsch, R. Heyder. “Head-hardened rail put to the test”, Railway Gazette International (2004) 1, pp. 42-44.

[13] UIC L eaflet 721: Recommendations for the use of rail steel grades. UIC International Union of Railways, 2nd edition, March 2005.

[14] Technische Mitteilung zum oberbautechnischen Regelwerk RO 10/2005. “Ausrüstungsstandard für Gleise / Schotteroberbau, Anhang 1: Einsatzbereich kopfgehärteter Schienen”, DB Netz AG, April 2005.

[15] G. Girsch, R. Heyder, N. Kumpfmüller, R. Belz. “Comparing the life-cycle costs of standard and head-hardened rail”, Railway Gazette International (2005) 9, pp. 549-551.

[16] N. Frank. “Reliability, availability, maintainability and safety (RAMS) of head hardened rails”, Rail Engineering International, Edition 2005, No. 2.