1

Fig.2 Photograph of tensile test sample before fracture

Fig.1 Positions of impact and tensile testspecimens (reproduced from reference [3])

Tensile testImpact test specimens

Wheel circum-ferential direction

15th. Annual (International) Conference on Mechanical Engineering-ISME2007 May 15-17, 2007, Amirkabir University of Technology, Tehran, Iran

ISME2803

A Comparative Study Of Monotonic Tests For Various Steel Wheels Of Rail Vehicle

A. Asadi Lari S. H. Hashemi

Iran University of Science and Technology The University of Birjand School of Railway Engineering, Tehran, Iran Department of Mechanical Engineering, Birjand, Iran

asadi_l@iust.ac.ir shhashemi@birjand.ac.ir

Abstract The authors of this paper report output results of a testprogramme conducted on two sets of tensile test specimensmade from grade B3N and R7T pearlitic steel wheels. Thetested steel wheels are the main materials currently used inIranian Railways for rail vehicles. The mechanical propertieswere measured in the wheel circumferential direction usinground bar tensile specimens at low strain rate. Theexperimental results almost fulfilled the target values set bytest regulations of International Union of Railways (UIC). Theobtained data were compared then with similar results fromtoughness and tensile testing of B2N and R7 pearlitic steelspecimens, conducted recently by the authors of this paper. Keywords: B3N Tyred Wheel, R7T Monobloc Wheel,Monotonic Tests Introduction The B2N and B3N tyred and R7T monobloc steelwheels are currently used in Iranian Railways inpassenger fleet. Severe wear has been observed in thewheels made from the B2N tyred type even with aservice life of less than 20,000km. Furthermore, anotherwheel failure mode, that is, rolling contact fatigue (rcf)has been observed on the contact surface of the B3Ngrade steel. The recent replacement of B2N with B3Ngrade steel has shown no significant change to reducethe wear problem in railway wheels. It is believed thatthe main parameters involved in the wear of the steelwheels are the geometry of the wheel, its loadingconfiguration and the mechanical characteristics of thematerial [1-2]. In this paper particular attention is givento the latter parameter (i.e. mechanical features). Forthis purpose an experimental study in two stages wasconducted to determine static mechanical properties ofthe B3N and R7T steel wheels in addition to theprevious study for dynamic mechanical properties of the B2N and R7 grades. The results of this research arepresented in the following sections. Test specimens preparation To measure the mechanical properties of the steelwheels, round tensile bars and standard Charpyspecimens were taken from the test wheels (accordingto the BS589 standard [3-4]) as shown in Fig.1. To prepare the laboratory samples, the test wheels werefirst cut up by flame cutting and then machined to flatstrips and to round test specimens. The Charpy testspecimens were extracted from these flat strips far fromthe heat affected zone (HAZ) by machining process.Similarly, tensile samples were prepared as shown inFig.2.

Material Characterisation Fig.3 shows the experimental set up used in tensile experiment of tested steel wheels. The measurement of the material tensile properties was carried out using round tensile bars. The tensile specimens had 50 mm gauge length and 7mm gauge diameter. Three tensile specimens were made from each wheel (B3N and R7T) and tested on a 600kN Zwick test machine under displacement control at a ram rate of 0.01 mm/s. An extensometer was used in each test to monitor the axial strains in tensile samples during the tests. The load and displacement data were recorded in each test by a computer and later used for tensile properties determination (see Table 1).

www.me-en.com mechanical engineering mechanical engineering

2

Fig.3 Photograph of tensile test set up

Table 1. Measured tensile properties of steel wheels

Steel grade

E-Modulus

GPa

Proof at

0.1% MPa

Rm MPa

Strain at max

load %

Strain at

fracture %

R7T 184-207 430-466

785-813

7.8-9.3 12.9-15.0

B3N 216-222 395-407

774-802

9.7-10.2 11.9-13.1

B2N 210 364 772 23 R7 200 625 950 22.5 Note: a number of tests for each steel grade were carried out and a range of the results is presented above.

Toughness properties Three full-size U-notch Charpy samples made from eachsteel wheel (B2N and R7) were tested on standard 300JCharpy impact machine. Fig.4 shows a test sample beforeand after experiment.

The results of the toughness measurement for these twosteel grades are presented in Table 2.

Grades

Measured values

Standard values

B2N 12 J Min 12J R7 21 J Min 20J

Fractography A SEM Fractography from the fracture areas was carried out to accomplish our experiments. The SEM specifications are briefly outlined below. Type: Cambridge Model: S360 Magnification size: (currently) ×50,000 All cross sections of the test specimens after carrying out the tensile test were observed with a SEM at Iran University of Science and Technology. Figure 5 demo-nstrates such images from both the B3N and R7T grades cross sections.

Discussion It can be seen that the measured values for the ultimate tensile stresses of the B3N grade have a good agreement with the figure given by the standard, whereas none of these values for the R7T grade lies within the related range in the standard. Comparing the average of this ultimate tensile strength for both the R7T and B3N grades shows a 1% difference, while this magnitude is about 8 percent as given in the standard. Although yield strength is not given in the standard, it was measured within the tensile tests. A difference of about 10 percent between the two grades is observed in Iran, whereas this difference is 24% according to a test report delivered by Bonotranz Co, one of the main wheel suppliers from Czech republic for RAJA Co.

Fig.4 Photograph of a Charpy test specimen before and after fracture

Table 2 Charpy toughness of different grades tested previously

Fig.5 SEM of tensile specimens, ×500, (a) B3N, (b) R7T grade steel

www.me-en.com mechanical engineering mechanical engineering

3

The dynamic toughness of steel wheels tested (the totalabsorbed fracture energy, i.e., the area under the load-displacement diagram) was estimated using a computersoftware. Both rail vehicle monobloc wheel materials hadhigh toughness levels, showing good loading bearingcapability as well as durability when facing their matingsurface, i.e., rail. However, it was expected that the R7Tgrade could have had significant higher toughness levels,resulting in more safe life time. As a result, the R7T grade steel, which has generally been regarded as a possiblecandidate for material replacement in rail vehicles, shows no significant improvement in bearing the contact forces. On the other hand, no considerable difference between theyield point strengths of the two steel grades (i.e., B2N andB3N) with the nominal contact stresses within thewheel/rail interface causes a fast conversion into plasticmode at the first contact. Also, relatively low elongationpercentage of the B3N grade results in a reduction in thematerial resistance against both types of wheel failure, i.e.,wear and rolling contact fatigue. Similarly, a lowmagnitude of elongation percentage of the R7T grade mightcause a decreased resistance to reduce the surface materialductility. This reduction of the material ductility results inthe creation of wear particles within the contact surface ofsteel wheels. Conclusions According to the test results a variety of mechanicalcharacteristics for a variety of wheel grades exist. Hence, it is predicted that these steel wheels show different behaviour during their operation. The dynamic toughness of steel wheels tested (the totalabsorbed fracture energy, i.e., the area under the load-displacement diagram) was estimated using a computersoftware tool. Both rail vehicle monobloc wheel materialshad high toughness levels, showing good loading bearingcapability as well as durability when facing their mating surface, i.e., rail. However, it was expected that the R7Tgrade could have had significant higher toughness levels,resulting in more safe life time. As a result, the R7T grade steel, which has generally been regarded as a possiblecandidate for material replacement in rail vehicles, showsno significant improvement in bearing the contact forces. Inother words, no considerable difference between the yieldpoint stresses of the two steel grades (i.e., R7T and B3N)causes a fast conversion into plastic mode at the firstcontact. Consequently, despite higher prices paid forprocurement of the R7T grade wheels, the objectives wereless likely achieved for a long-term operation. This finding, though, cannot be extended for all monobloc wheels whichwere recently supplied. Also, relatively low elongation percentage results in areduction in the material resistance against both types ofwheel failure, i.e., wear and rolling contact fatigue. Acknowledgement The authors gratefully acknowledge the cooperation ofRAJA Co (the largest subsidiary company of IranianRailways to operate passenger trains) for providing steelwheels and to fund this study.

References

[1] Asadi Lari, A., Hashemi, S. H., Schmid, F., and Kapoor, A., “A Study of Mechanical Properties and the Fracture Micromechanism of Pearlitic Steel for Railway Wheels of Grade B2N”, CD Proceedings of the 7th International Conference on Railway Engineering, (5-7 July, 2004, London, UK), Edited by M. C. Forde, ISBN: 0-947644-55-5.

[2] Asadi Lari, A, Hashemi, S.H, Experimental Investigation of Charpy Impact Resistance In Rail Vehicle Steel Wheels, Proceedings of ESDA2006, Torino, Italy.

[3] British Standard, BS 5892-4:1992 "Railway Rolling Stock Materials - Part 4: Specification for Forged and Rolled Tyres".

[4] British Standard, BS 5892-3:1992 "Railway Rolling Stock Materials - Part 3: Specification for monobloc wheels for traction and trailing stock".

www.me-en.com mechanical engineering mechanical engineering