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Materials used for Wheels on Rolling Stock

Katrin Mdler, Manfred BannaschDeutsche Bahn AG, Technical Centre, Brandenburg-Kirchmser, GERMANY

1 Introduction

In the contact area between the wheel and the rail, the materials are used in those components encounterthe greatest demands. Not only is there the pure rolling motion under a constant normal and lateral loadto be carried, which leads to high shear stresses both on and beneath the surface of the respective parts.Furthermore, relative motions between wheel and rail create slip in the contact zone, causing bothmechanical and thermal loading of the material. Consequently, the different characteristics of damage towheel treads and the running surfaces of rails can be described by a more or less pronounced combinedeffect of adhesive wear, plastic deformation, rolling contact fatigue and thermal fatigue.The presentation gives an overview of the usual concepts in material selection for rolling stock wheelsand the associated operational practice from the point of view of damage analysis. In addition, options arepresented for exploiting the full range of existing or new materials, without causing a detrimental increasein wear on the rail, thereby putting the overall wheel-rail system at risk. To this end, results ofconventional operational tests and of specially developed rig testing are presented.

2 Wheel materials

As an appropriate response to known mechanisms of damage, the materials employed in wheels andrails in Germany as in the rest of Europe were, and indeed still are, those steels whose predominantlypearlitic structures containing hard cementite lamellae guarantee high resistance to wear. At the sametime a pearlitic microstructure, formed by transformation close to the point of equilibrium, ensures higherresistance to transformation in operational use than, for instance, bainitic or martensitic structures.

Although UIC Leaflet 812-3 [1] for solid wheels lists seven types of steel, which mainly differ in carboncontent, heat treatment state and therefore strength, EN 13262 [2] contains only four types (Table 1).Because Grade R1 for freight wagon wheels is on the decline in favor of the standard R7 material, andGrades R2/R3 never established themselves in operational practice, this represents the current state oftechnology in Europe.R7 is by far the most commonly used grade. It is used for all freight wagon wheels and on mostpassenger coaches. Where wheels made from R7 are intended for use in vehicles with tread brakes, thefracture toughness requirements (KIC) must be fulfilled as well as the usual characteristic mechanicalvalues. Experience has shown that where carbon content exceeds 0.5%, the K IC values of 80MPa mcalled for in [1] and [2] can only be attained where comparatively small grain size (fine grain), high purityand high homogeneity are present in the microstructure throughout the circumference of the wheel. Thisof course places heavy demands on manufacturing quality. For this reason, these wheels are commonlysupplied with lower carbon contents (

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generally an option. Rails of this type are described as head-hardened and, to minimize wear in theouter rail, are generally used only on track where the curve radius is

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Fig. 1: Wheel damage in diesel traction unitsa) Cracks and spalling in the wheel tread, b) Microstructure in the cross-section

Where cracks resulting from rolling contact fatigue have their origins beneath the surface, these are dueto the superposition of the maximum sub-surface shear stress over local defects in the steel. Generally,such cracks occur in disk-braked passenger coaches where speeds exceed 160km/h and are initiated byhard non-metallic inclusions. For the DB these sub-surface initiated cracks do not represent a problembecause the wheel treads of all passenger coaches are subject to a regular ultrasonic check onmechanized test rigs. Should a test of this type be missed however, catastrophic failure of the wheel canresult if the crack extends to the face of the tread and very large areas of the tread break away. Hence,when qualifying new wheel manufacturers for DB, there is a special emphasis on their adherence to therequirements for the degree of material purity.

Periodic out of roundness [3] is a problem that occurs in high-speed traffic, but also in vehicles that reach200km/h. This is a regular occurrence of changes in radius distributed around the rim of the wheel. Thelatest opinion is that higher order polygons (>5) are a consequence of local wear processes caused bydynamic effects e.g. higher speeds combined with higher track stiffnesses and lead to a rise in wheeldynamics. Against this background, there is naturally the question of the extent to which a high materialquality (homogeneity) consistent all round the wheel rim, combined with higher wear resistance, can delaythe onset of out of roundness in the wheels.

Both spalling on the tread and wheel out of roundness cause vibrations in the vehicle body and thus havea negative effect on ride comfort. At the same time they lead to heavier dynamic loadings on the vehicleand track [4]. Furthermore, rail traffic noise increases considerably. Elaborate reprofiling of the wheel,during which a considerable part of the wheels diameter is machined away, is necessary not only on the

damaged wheel but on several wheels of the affected vehicle for reasons of diameter matching. It is clearthat along with rising maintenance costs, this also brings a reduction in the life of the wheels to a levelthat is sometimes not economically justifiable. In many cases artificial removal of material from thewheel, by turning, is considerably greater than natural wear resulting from rolling contact in service.

Against this background there is also the recurring quest for materials capable of sustained resistance tothe increased loads. For this reason, Deutsche Bahns Technical Center has initiated and followedthrough the testing of new wheel materials in recent years.

4 Testing of new wheel materials

4.1 Material concepts

For high-speed traffic, tests were conducted on a Japanese wheel steel that had already been in servicefor some years in Shinkansen vehicles. In common with the UIC steels, the type in question is a non-alloysteel; however its carbon content lies between 0.65 0.7% which is of a similar order to the usual steelused for rails. Its tensile strength of 1050N/mm makes this steel comparable with an R9 in the uppertolerance range. Values for elongation, notched impact and fracture toughness not only meet therequirements for R9 but surprisingly also in spite of its higher carbon content those for R7 and R8.First and foremost this appears to be a result of the materials high purity and its comparatively smallgrain size. A further striking feature was the extreme homogeneity of the structure throughout the wheelrim.

a) b)

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A range of material concepts were pursued for passenger services up to 160km/h. Particular emphasiswas given to the testing of bainitic Austempered Ductile Iron (ADI) for its suitability as a wheel material forrail vehicles [5]. ADI exhibits a combination of high wear resistance and high fatigue strength with highductility when compared with other types of cast iron. An important side-effect: the graphite nodulesincluded in the cast iron microstructure act as a lubricant at the contact surface between the two frictionpartners, thus reducing their wear. At the same time, the graphite inclusions bring about a damping effectsome three times greater than that of steel, which means that ADI wheels promise a decrease of railtraffic noise. Material composition and heat treatment were matched together in such a way that tensilestrengths of about 1000N/mm were achieved. Over the course of the project, wheels for testing purposeswere manufactured by a foundry and a wheelset manufacturer. Besides the usual laboratory testing forassessment of material and manufacturing quality of the wheels, special attention was paid to theevaluation of the ADI wheel in terms of fracture mechanics [6]. At the end it was found that the criticalcrack size determined by means of fracture mechanics calculations can be safely and reliably diagnosedby means of non-destructive testing procedures. Under DBs routine maintenance procedures and testingschedules ADI wheels would thus be deployable without problems.

A further emphasis was laid on the testing of wheel materials with improved resistance against thermalloading. The types under consideration were two low-alloy steels S2 and S3 whose carbon content of0.4 to 0.45% was somewhat lower by comparison with R7. Their strengths reach from the lower (S2) tothe upper tolerance limit for R7. The S2 fulfills the toughness requirements for R7, while the toughness of

the S3 corresponds to the lowest requirements of an R8. By the addition of alloying elements, thetendency towards transformation of the pearlitic-ferritic microstructure into austenite is lowered. Under theloads imposed by wheel-slide there should thus be a lower likelihood of brittle martensite forming in thetread [7].

Before their planned service deployment, all these materials were subjected to a special test on thewheel/rail system test rig in Kirchmser. This not only tested whether the new wheel had a lowertendency towards out of roundness and how it responded to the loads imposed by wheel-slide; rather, theinvestigations centered on its influence on rail wear compared with the conventional R7 steel.

4.2 Rig testing

On the DB wheel-rail system test rig, rolling contact between wheel and rail is simulated at full-scale. It is

a wheel on roller test rig (Fig. 2). The full-size wheelset under investigation rolls on a driven rail rollerconsisting of two rail tyres measuring approx. 2100mm in diameter formed from standard 900A grade railsteel. The profiles of both the wheels and the rail tyres correspond to DBs normal matched wear profiles.The wheelset is mounted on a single-axle auxiliary rotary frame and can be loaded with axle loads of upto 30 tonnes at speeds of up to 300km/h. Curving conditions with contact of the gauge corner at anadjustable angle of attack, also like straight track conditions, can be controlled dynamically [8]. Loadingscenarios must approach the loads imposed on the wheelset during vehicle service as closely aspossible. They are based on the vehicle parameters and a combination of travel through curves and in astraight line, varying speeds, braking maneuvers and weathering conditions. In order to achievereproducible results within short test time frames, critical service conditions such as tight curves andemergency braking were emphasized out of normal proportion.

Initial results of longitudinal and cross-section measurements and of non-destructive crack testing of the

wheel tread are generally available after 6,000 8,000km, which corresponds to a test rig operatingduration of about four days. Testing can thus be carried out more economically and more quickly usingthe test rig. Furthermore, only a trial on the wheel-rail system test rig enables an assessment of the extentto which the new, more wear-resistant wheel material leads to greater wear on the rail. In tests on actualtrack, these assertions can only be made with limited certainty.

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In the case of the wheel materials described at 4.1, the following results were achieved on the test rigcompared with the R7 wheel material.

4.2.1 Comparison of Shinkansen material with R7

Test conditions:- Mean axle load: 13 tonnes- Straight ahead and curving with radii R = 600 to 1,800m- Speed: 110 - 190km/h

- No rail lubrication- End of test run if lateral force >30kN and bearing acceleration >250m/s as the limit for wheel out

of roundness- Wheels were reprofiled between the first and second tests

Results:

It was found that the development of wheelout of roundness is heavily dependent onthe material. In the first test run, the wheelsmade of Shinkansen steel alreadymanaged two and a half times the running

performance of the R7 wheels before theappearance of comparable out ofroundness.

Fig. 3: Running distances for getting wheels out of roundness

Fig. 2: Wheel-rail test rig, schematic

Rail roller

Wheelset

Force/displacementcylinders

Driving

31 500 km

10 000 km

9 500 km

23 300 km

0 km 20 000 km 40 000 km

Running distance

2nd test

1st test

Shinkansen steel

R7 steel

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After the wheels had been reprofiled to a depth of approx. 2.5mm, the second test run with theShinkansen steel wheels was terminated after 31,500km, even though the limit values for lateral forceand bearing acceleration had not been reached compared to R7 wheels.This confirmed the expectation that the steel with the higher carbon content and strength level exhibits abetter wear behaviour and that wheel out of roundness can be considerably delayed. One reason for thisappears to lie in the absence of free ferrite from the wheel tread.

After the test, only the R7 wheels showed small rolling contact fatigue cracks in the tread (Treadchecks).In parallel with the wheel investigation, changes in profile to the head of the rails were recorded andevaluated. In this way the potential for heavier damage to track due to the new wheel material can beidentified and assessed in a timely manner. The wear ratio of the wheel and rail was determined beforeand after the test from the cross-section of the wheel tread and rail head and the ratio of thecircumferences of the wheel and rail tyres. This coefficient indicates the ratio in which wheel and railmaterial are consumed on each rolling revolution, i.e. the extent to which a harder wheel causes greaterwear on the rail, and vice versa. Accordingly, a ratio of 1 means that the wheel and the rail wear to asimilar extent. In the previous case, a wheel-rail wear ratio of 0.86 was determined for the Shinkansensteel. In other words there is a slight tendency for wear to be transferred more to the rail. It is not possibleto make a conclusive assessment at present for the wheelset fitted with R7 wheels because this valuewas subject to far greater variability. Further test rig investigations should create clarity on this point.

4.2.2 Comparison of ADI with R7

Test conditions: as for Shinkansen steel/ R7

Results:

The wheels made from ADI material represent a special case within the test sequence: because of thelubricating effect of the spheroidal graphite contained within the material structure which is released undercontact conditions at the wheel tread, the coefficient of friction was reduced and wear on the wheel andrail considerably diminished as a result. Thus the wheelset on the test rig managed a runningperformance in excess of 50,000km without reaching the termination criteria for either bearingacceleration or lateral force. However, in moderation, it should be remarked that the lubricating effect on

actual rails in service would naturally drop to a lower level than that on the test rig whose rail tyres inprinciple measure just 6.6m in length. After the test by contrast with the R7 wheels the ADI wheelsshowed no signs of tread checks.Both in the cross-section of the ADI wheel and the cross-section of the rail tyress there was virtually noevidence of wear.

After the positive test rig results, trials were planned on double-deck coaches in regional passengerservice at speeds of up to 160km/h. However, during ultrasonic testing on the mechanized test rig, thewheels manufactured for this purpose showed unpermitted indications in the wheel tread and web so thatthe use of these wheels in service was abandoned.

4.2.3 Comparison of S2 and S3 with R7

Test conditions:

1. First test run: Curving (6,000km) = check of tendency to out of roundness- Mean axle load: 116kN- Speed: 50 - 120km/h- Curving with radii: 300 1,200m- Effect of wet conditions

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2. Second test run: Straight line/braking (1,900km) = check of tendency to hardening underdemands imposed by wheel-slide- Loads and speeds as above- 3 x braking events from 200km/h to rest- Hunting- Effect of wet conditions

Results:

After the first test run, both types ofsteel exhibited greater wear in thecross-section compared to the R7 aswell as a heavier loss of diameter(Fig. 4). In addition, the wheels madefrom S2 steel showed signs ofpronounced out of roundness evidently as a result of theircomparatively low strength andhigher free ferrite content in themicrostructure whilst for the R7 and

S3, out of roundness wascomparable with the R7 wheels listedin 4.2.1.

Fig. 4: Wear behavior of S2, S3 and R7 wheels after first test run

After the second test run the wheels made from S2 and S3 steels with their higher resistance againstthermal loadings exhibited little wear, in line with expectations. However, the R7 wheels on this testexhibited also little wear.Because the rig tests with both the S2 and the S3 steels were unable to identify any improvementcompared with the conventional R7 steel, in-service testing with the more expensive wheels (due to

alloying elements) was not pursued.

4.3 In-service testing

Since 2003, 45 wheelsets with the Shinkansen wheel material have been under trial in ICE trailervehicles. The experience under this regime shows that, compared with wheels made from R7, thesewheels have required less frequent reprofiling. The relevant assessments for 16 wheelsets in each of thewheel materials are presented in fig. 5.

Fig. 5: Running distances betweenreprofiling of wheels made of R7 andShinkansen steel

R7

S3

S2

0

0,02

0,04

0,06

0,08

0,1

0,12

Loss

oftreaddiameter(mm/1000km)

Running distance between reprofiling

(planned and not planned)

0

510

15

20

2530

3540

1-50

51-100

101-150

151-200

201-250

251-300

301-350

Running distance classes, Tkm

Percentageofre-profilingevents,

%R7Shinkansen

material

Running distance between reprofiling

(planned and not planned)

0

510

15

20

2530

3540

1-50

51-100

101-150

151-200

201-250

251-300

301-350

Running distance classes, Tkm

Percentageofre-profilingevents,

%R7Shinkansen

material

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It shows that because of unplanned events (profile deviations, out of roundness, tread damage, etc.) acertain number of the R7 wheels required reprofiling long before they reached the ultrasonic testscheduled for 240 Tkm, whilst the wheels made from Shinkansen steel largely reached this scheduledtest without intermediate reprofiling.This confirms the results of the rig tests. To underpin this result statistically, a further large-scale trial isplanned.

5 Conclusions

With the introduction of high-performance rail traffic, the demands on wheel to rail contact have risenonce more. There is a link between this and the typical modes of wear in wheels which arise and limit thewheels life. The wheel material employed and the manufacturing quality of the wheel have a considerableinfluence on the formation of out of roundness and tread damage. To test new wheel materials for railwayservice, both the usual laboratory tests and also elaborate in-service trials are carried out. The DeutscheBahns wheel-rail system test rig offers an opportunity for pre-testing wheelsets with new wheel materialsunder real loading conditions. The tendency for wheel out of roundness and tread damage to occur canbe examined as can the question of wear on the rail.Tests conducted in recent years have shown that the microstructure and the homogeneity of mechanicalproperties around the wheel rim influence the occurrence of out of roundness. There are indeedalternatives to those wheel materials that have been used hitherto. Furthermore, particular attention mustbe paid to manufacturing quality, as the experiences with wheels made from Shinkansen steel and thebainitic cast iron ADI have demonstrated.

References

[1] UIC 812-3 Technical specification for the supply of rolled solid wheels of non alloy steel fortraction and rolling stock. 5th Issue. 01/1984

[2] EN 13262 Railway applications Wheelsets and bogies Wheels: Product requirements. Issue01/2006

[3] Cassidy, P.: Non-periodic wheel out of roundness and microstructural inhomogeneity. Proc. of 6thInt. Conf. on Contact Mechanics and Wear of Rail/ Wheel Systems, Gothenburg, Sweden, June

10-13, 2003, p.[4] Madeyski v., T.: Zusammenwirken Fahrzeug/ Fahrweg und Manahmen zur gegenseitigen

Senkung der Beanspruchung. ZEV + DET Glas. Ann. 122 (1998) 9/10, p. 613-619[5] Mdler, K.: Suitability of ADI as an alternative material for railcar wheels. http://www.ductile.org/

magazine/2000_2/railcar.htm[6] Kuna, M., Mdler, K., Hbner, P. and G. Pusch: Anwendung bruchmechanischer Konzepte bei

der Entwicklung von Eisenbahnrdern aus bainitischem Gusseisen. konstruieren & giessen 27(2002) 3, S.

[7] Poschmann, I. and C. Heermant: Werkstoffe fr rollendes Eisenbahnmaterial. EI -Eisenbahningenieur 53 (2002) 8, p. 47-51

[8] Mdler, K., Ullrich, D. and M. Luke:Rolling Contact Phenomena at Wheels and Rails observed atDBS Full-Scale Simulation Test Rig. Proc. of 6th International Conference on Contact Mechanicsand Wear of Rail/Wheel Systems (CM2003) in Gothenburg, Sweden June 1013, 2003, p. 17-21