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A pin-on-disc study on the tribology of cast iron, sinter and composite railway brake blocks at low temperatures
Author’s Accepted Manuscript A pin-on-disc study on the tribology of cast iron, sinter and composite railway brake blocks at low temperatures Yezhe Lyu, Ellen Bergseth, Jens Wahlström, Ulf Olofsson www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(18)31393-0 https://doi.org/10.1016/j.wear.2019.01.110 WEA102784
To appear in: Wear Received date: 6 November 2018 Revised date: 17 January 2019 Accepted date: 17 January 2019 Cite this article as: Yezhe Lyu, Ellen Bergseth, Jens Wahlström and Ulf Olofsson, A pin-on-disc study on the tribology of cast iron, sinter and composite railway brake blocks at low temperatures, Wear, https://doi.org/10.1016/j.wear.2019.01.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A pin-on-disc study on the tribology of cast iron, sinter and composite railway brake blocks at low temperatures
Yezhe Lyu*, Ellen Bergseth, Jens Wahlström, Ulf Olofsson
Department of Machine Design, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden
*
Corresponding author. Tel.: +46 8 790 67 66; fax: + 46 8 20 22 87. [email protected] (Y. Lyu).
Abstract Most freight wagons in the EU use cast iron brake blocks. Cast iron brake blocks have a stable braking capability in different environmental conditions, but wear down the wheel tread quickly. Therefore, there is a need to understand the tribology of other brake block materials. A pin-on-disc tribometer placed in a temperaturecontrolled chamber is used to investigate the tribology of cast iron, sinter and composite railway brake blocks at low ambient temperatures. Pins made from different brake blocks are tested with discs made from steel wheels. Both friction coefficient and wear are evaluated at five different temperatures from +10 to -30 °C. The cast iron block demonstrated the greatest wear at -10
and -20 °C, due to the ductile-to-brittle transition at low
temperatures. The worn graphite from cast iron is likely to become a solid lubricant, reducing the friction at -10 and -20 °C. For the composite brake block, a gradual decrease in friction with decreasing temperature was found. The sinter brake block was not sensitive to changes in ambient temperature. The sliding speed in the current study is relatively low and further study at higher speed is suggested in order to evaluate the tribological performance of different brake blocks.
Keywords: Railway; Brake blocks; Temperature; Pin-on-disc; Tribology; Composite
1. Introduction Railway transport is an open tribological system for many mechanical components, and the mechanical brakes is one of them. For cold regions, this can be a challenge for the operators in terms of wear of the contact surfaces, but also a safety issue due to a varying coefficient of friction that prolongs the stopping distance. In the winter of
2017, several incidents regarding the stopping distance of freight wagons equipped with composite brake blocks occurred in the north of Sweden at temperatures down to -10 °C [1]. It seems as if the friction coefficient of composite brake blocks decreases considerably at low temperatures, resulting in a prolonged braking distance and near misses. A similar scenario has also been noticed in Finland [2]. The brake system on railway vehicles is different from that on automotive vehicles. Briefly, the driver of automotive vehicles can reduce the braking distance by pushing the brake pedal harder. On the other hand, the compressed-air brake system on railway vehicles only allows limited pressure between the brake block and wheel tread. Under such circumstances, the driver is not able to dynamically adjust the pressure between the brake block and wheel tread. A high friction coefficient between the brake blocks and wheel treads should always be maintained. Almost all freight wagons in the EU use block braking as the mechanical braking system due to its great braking capability and consistent braking behaviour. Cast iron with a high thermal conductivity has for decades been the most commonly used brake block material [3]. However, cast iron blocks have drawbacks. They wear down the wheel tread very quickly, resulting in a high level of roughness (waviness and corrugation) on wheel treads [4]. This irregularity on the wheel tread is prone to generate a loud rolling noise. In addition, it has been reported in the literature that a large portion (20% to 50%) of the worn materials during braking become airborne particles [5-7]. Since both cast iron brake block and steel wheel are iron-based materials, the airborne particles generated during the braking of railway vehicles contain a large amount of iron oxides which are highly toxic and induce oxidative stress in human lung cells [8]. In Europe, sinter and composite brake blocks are gradually replacing cast iron since they generate less rolling noise and airborne particles. Researchers have evaluated the tribology and airborne particle emissions of sinter and composite brake blocks in the laboratory. Ferrer et al. [9] tested the tribology of cast iron and Fe-C-Crgraphite sinter brake blocks using a pin-on-disc tribometer. Olofsson et al. systematically investigated the tribological performance and airborne particle emissions [10] of cast iron, sinter and composite brake blocks at elevated temperatures [11] and with snow addition [12]. The effect of humidity on tribology and airborne particle emissions of different brake block materials has also been studied [3]. Ma et al. also studied the wear performance at wheel rail contact in low ambient temperatures [13]. Vakkalagadda et al. analysed the performance of brake blocks (both cast iron and composite) used in the Indian railway system [14]. Since sinter and composite materials are rather new as railway brake blocks, their tribological behaviour and airborne particle emissions are not fully understood. The purpose of this study is to address the difference in friction and wear
from cast iron, sinter and composite railway brake blocks at low ambient temperatures. This topic is of great importance, especially to the Nordic countries where the temperatures are quite low in winter.
2. Experimental set-up 2.1 Materials Three different railway brake block materials, i.e. cast iron, sinter and composite, were tested in this study. All three kinds of brake block are commercial products used in Swedish freight wagons as well as Stockholm Metro trains. The counterpart is a standard R7 railway wheel used in both Swedish freight wagons and passenger trains. Chemical compositions of cast iron, sinter, composite brake blocks and R7 wheel steel are shown in Table 1. Their microstructure has been investigated previously and can be found in [3]. Cylindrical pin samples (10 mm in diameter and 20 mm in height) featured a flat testing surface were extracted from brake blocks. Disc samples (100 mm in diameter and 12 mm thick) were manufactured from wheels. The initial arithmetic surface roughness (Ra) of the pin and disc samples were manufactured to 2.00 ± 0.20 μm and 0.45 ± 0.02 μm, respectively. Both the roughness values of the pin and disc samples correspond to the surface conditions of newly manufactured brake blocks and newly grinded wheel tread.
Table 1 Chemical composition of the sinter, composite brake blocks and R7 railway wheel as obtained from elemental analysis EDX (wt. %). Chemical composition of cast iron was provided by the supplier. Element
C
Si
Mn
S
P
Cast iron
3.5
1.0-1.8
0.8-1.4
<0.08
Sinter
-
6.08
7.93
4.43
-
Composite
-
17.95
-
-
R7 wheel
0.52
0.4
0.8
<0.035
Fe
O
Mg
Al
Ca
Cu
Sn
-
-
-
-
-
-
18.12
7.19
-
-
3.25
47.60
5.40
-
2.29
47.70
4.06
8.05
18.19
-
-
-
balance
-
-
-
-
-
-
<0.04 balance
2.2 Testing procedure Tests were conducted in a temperature-controlled climate chamber, which had previously been used to test tribology of wheel-rail contact at low temperatures and humid atmosphere [15-17]. Fig. 1 shows the schematic of the configuration. The main part of this configuration is a pin-on-disc tribometer, which contains a horizontally
rotating disc (G) and a dead-loaded pin (F). This pin-on-disc tribometer was used to imitate the sliding contact between the brake block and wheel tread. The normal force and rotating speed are adjustable. The whole rig is set inside a temperature-controlled box (B) in which temperatures can be controlled from –50 °C to 20 °C. A seal cover made of thermal insulation material is placed on the test bed to ensure that the pin-on-disc machine is isolated from external environment. A flexible plastic tube (C) containing a uni-directional fan (L) connects the climate chamber (B) and an air conditioner (D). The working range of the air conditioner is from –50 °C to 20 °C is. A sensitive transducer (J) is fixed in the temperature-controlled box (B) and linked to an automatic data logger and controller (K). When the experiment starts, the fan (L) will operate, pumping cold air into the box (B). Once the temperature reaches the desired level, the controller (K) will give a signal and stop the fan (L). Meanwhile, the air conditioner will also stop working. Consequently, a constant environment in terms of temperature can be obtained in the temperature-controlled box (B).
Fig. 1 Schematic diagram of the test configuration
Since the train speed close to a platform is usually low, a relatively low sliding speed was chosen in the current study. Every test lasted for 1200 s under a contact condition of 0.8 MPa and 0.45 m/s. The current contact pressure 0.8 MPa is a typical contact pressure level at the railway brake block contact [9, 10]. During the test, the tangential force between the pin and disc was measured by an HBM® Z6FC3/20 kg load cell mounted on the pin holder. The friction coefficient was calculated by dividing the tangential force by the normal load. A Sartorius® ME614S analytical balance (accuracy 0.1 mg) was used to measure the wear of the pin and disc samples. Each brake block material was tested three times at five different temperature levels: 10 °C, 3 °C, 10 °C, -20°C and -30 °C. A Hitachi® S-3700N scanning electron microscope (SEM) equipped with Bruker®
Xflash 6-10 energy-dispersive X-ray spectroscopy (EDX) was employed to characterize the worn surfaces of some tested samples.
3. Results Fig. 2 shows the time history of the friction coefficient from representative tests of three brake block materials at -30 °C and 10 °C. It should be noted that other tests have also a similar behaviour. For all three brake block materials, there is a running-in period at the beginning of testing, after which the friction coefficient reached a steady state. The data in the steady state is of great importance, since the contacting brake block and wheel tread are totally run-in with each other in this period. An average value was calculated with the data in the steady state, i.e. time interval from 600 s to 1200 s of each test. Using this average value of three repetitions for each test condition, the mean value and standard deviation of the friction coefficient as a function of temperature can be obtained (Fig. 3). Fig. 3 shows that the composite brake block has the lowest friction coefficient at all five temperatures. It may also be noted that the friction coefficient of composite brake block decreases rapidly in subzero temperatures. The cast iron brake block has a transition in friction coefficient with the decreasing temperature from 10 °C to -30 °C. At -10 °C and -20 °C, the friction coefficient of a cast iron brake block is much lower than at the other three temperatures. The friction coefficient of a sinter brake block remains stable at all five temperatures and seems not to be sensitive to the change of temperature. Note that the sinter and composite combinations show the same friction level at 10 °C, but the composite will significantly reduce brake performance at -30 °C.
Fig. 2 Time history records of friction coefficient of three brake block materials at 10 °C and -30 °C
The flash temperature at the pin-disc contact is not measured. In previous study on the same materials and test conditions in room temperature, the flashing temperature between the brake block and wheel materials in the steady state are about are approximately 55 °C (organic composite), 73 °C (sintered) and 84 °C (cast iron), respectively [3]. One could expect lower flash temperature in the current study due to the lower ambient temperature.
Fig. 3 Friction coefficient (mean value and standard deviation) of three brake blocks as a function of temperature
The wear of the pin and disc samples corresponds to the entire test duration rather than the steady state. The mean values and standard deviations of wear at different temperatures are summarized in Table 2. It should be noted that some exceptional values are underlined in bold and italic. At -10 °C and -20 °C, the cast iron pins and mating discs yielded extremely high wear (one magnitude greater than at other temperatures), showing an opposite behaviour to the friction coefficient (at -10 °C and -20 °C, cast iron has a lower friction coefficient than at the other three temperatures). The sinter brake block and its mating disc shows relatively stable wear at different temperatures, similarly to its performance in the friction coefficient. The composite brake block demonstrates very minor wear at all five temperatures. At -30 °C, the disc sample tested against composite brake block has a negative wear, indicating that material was transferred to the disc surface rather than being worn off.
Table 2 Wear (mean value and standard deviation) of three brake block pins and the mating discs tested at different temperatures. (Exceptional values are underlined in bold and italic) Materials
Cast iron
Sinter
Composite
Temperature (°C)
Pin wear (mg)
Disc wear (mg)
Mean
Std.
Mean
Std.
10
16.5
1.8
15.9
1.8
3
20.4
4.1
15.05
0.85
-10
500.8
3.8
317.85
6.65
-20
731.05
6.45
269.9
5.6
-30
10.15
3.35
9.3
2.5
10
1.85
0.85
0.65
0.05
3
5.35
0.35
1.75
0.35
-10
7.65
0.55
1.85
0.55
-20
8.65
0.05
1.65
0.24
-30
5.45
0.47
0.85
0.35
10
0.3
0.1
1.3
0.11
3
0.15
0.05
1.4
0.13
-10
0.6
0.1
2.7
0.2
-20
0. 5
0.07
2.25
0.03
-30
0.45
0.15
-0.5
0.1
4. Discussion The purpose of block brakes is to stop the train in a limited distance. A high enough friction coefficient between the brake block and wheel tread should be maintained in varied environmental conditions. In the current study, the cast iron brake block yielded a friction coefficient of between 0.5 and 0.9 at different temperatures (black dash dotted line in Fig. 3). The sinter brake block also gives a friction coefficient of greater than 0.5 at all five temperatures (red dashed line in Fig. 3). But the composite brake block only has a friction coefficient of around 0.5 at 10 °C, which decreases down to 0.3 with temperatures decreasing to -30 °C (blue solid line in Fig. 3). It should be noted that the sliding speed in the current study is relatively low. Usually a further decline in friction coefficient can be expected with an increase of sliding speed [18, 19]. Recently, the Swedish Transport Agency has recorded some incidents in winter with freight wagons equipped with composite brake blocks [1]. The friction of composite brake blocks seems to decrease at low temperatures, resulting in a prolonged braking distance. One incident recorded a train 100% equipped with composite brake blocks. The train was running with a speed of 18 km/h and braking was applied 200 m in advance of the STOPsignal. Finally, the train was stopped 20 m after the signal by using emergency braking. Another freight train
operated at -2 °C and an ice layer has built-up on the composite brake blocks, impeding the braking capability. Olofsson et al. tested the same composite brake block at -2 °C and demonstrated a decline of friction coefficient by 0.2 by applying snow particles into the sliding contact [12]. A similar scenario was seen at wheel-rail contact tested at sub-zero temperatures, where the friction and wear decreased dramatically [15]. The composite brake block contains a large proportion of phenolic resin, which is prone to adsorb water vapour. The adsorbed water vapour forms an ice condensation layer on the composite brake block, which will act as a lubricant and reduce the friction coefficient. At -30 °C, the composite brake block has another exceptional behaviour, negative (-0.5 mg) wear on the mating disc (Table 2). The negative wear indicates that some worn materials or materials from the environment were added onto the disc surface. SEM was used to observe the disc surface tested with a composite brake block at -30 °C, as shown in Fig. 4. The figure illustrates a black phase in the path of a wear scar and its chemical composition achieved by EDX. The chemical analysis suggests a complex constitution of many elements, which are likely to come from the composite brake block. This transferred phase increased the weight of the disc sample, resulting in the negative wear at -30 °C.
Fig. 4 SEM photograph and EDX analysis of a black phase in the wear scar of the disc sample tested with a composite brake block at -30 °C
The cast iron brake block shows low friction (Fig. 3) and high wear (Table 2) at -10 °C and -20 °C. SEM analysis of the worn surfaces was conducted on both cast iron pin and wheel steel disc samples tested at -20 °C, as shown in Fig. 5. The cast iron brake block tested at -20 °C (Fig. 5a) shows a severe wear scenario with a large area of delamination and a great deal of wear debris, corresponding to the large amount of wear in Table 2. The
worn surface of the mating disc tested with cast iron at -20 °C also contains a large amount of wear debris (Fig. 5b). The large amount of wear of iron-based materials (both cast iron and steel) at low temperatures is attributed to the ductile-to-brittle transition [20-23]. The body-centred cubic iron-based materials have a gradual decrease of toughness with decreasing temperature and one ductile-to-brittle transition usually occurs from -10 °C to 20 °C. At such temperatures, cracks tend to generate between the ferrite-pearlite phase and graphite-ferrite phase boundaries (Fig. 6). These cracks are prone to extend during relative sliding and result in materials removal from the samples. Sometimes, wear is not in accordance with the friction. The wear of tests with cast iron brake block at -10 and -20 °C are much higher than at other three temperatures (Table 2), but the friction coefficients at these two temperatures have lower values (Fig. 3). One possible explanation is due to the graphite removed from the cast iron brake block into the interface. EDX analysis of the disc surface tested with cast iron at -20 °C reveals that a large amount of graphite transferred from the cast iron onto the steel disc surface. The transferred graphite was likely to act as a solid lubricant and somewhat relieve the friction between the pin and disc. The transferred graphite is only observed at -10 °C and -20 °C, but not at -30 °C. With a further decrease of temperature, there is a pronounced tendency to have an ice condensation layer on the metal surface [24]. This condensed ice layer encourages the generation of oxide layers, protecting the contacting bodies from severe wear [25, 26]. Fig. 7 shows the SEM micrograph and EDX analysis of the oxide flakes on the cast iron pin and wheel steel disc tested at -30 °C. The iron oxides on the cast iron brake block and steel wheel are considered to be Fe 2O3 and FeO, respectively. The sinter brake block seems to behave in a stable way as regards both friction coefficient and wear under current test conditions. The current study only tested a relatively slow sliding speed that corresponds to a situation in which a train is going to stop at a platform. More tests with higher sliding speeds are suggested in order to evaluate the friction and wear performance of a sinter brake block.
Fig. 5 Worn surfaces of a) cast iron pin and b) the mating disc samples tested at -20 °C. Sliding direction from left to right. Green lines indicate the possible transfers of graphite from cast iron pin to wheel steel disc surface
Fig. 6 Microstructure of cast iron brake block is composed of black graphite surrounded by grey ferrite with lamellar pearlite. Graphite, ferrite and pearlite have clear phase boundaries with each other.
Fig. 7 Worn surfaces of a) cast iron pin and b) the mating disc samples tested at -30 °C. Sliding direction from left to right. EDX analysis suggests Fe2O3 on cast iron brake block surface and FeO on the steel wheel surface.
Conclusions A pin-on-disc tribometer has been used to investigate the tribology of cast iron, sinter and composite railway brake blocks at low ambient temperatures. The following conclusions can be drawn from the present study:
The composite brake block has a gradual decrease in friction coefficient with decreasing ambient temperature which could be explained by the water adsorption of phenolic resin. It is, therefore, not advisable only to use composite brake blocks on trains in winter. A combination of different types of brake blocks is more appropriate, which is in line with the incidents involving Swedish freight wagons.
The disc surface tested with a composite brake block at -30 °C shows a negative wear, indicating material transfer from the composite brake block to the steel wheel surface.
The cast iron brake block and its mating wheel steel yield very high wear at -10 °C and -20 °C due to the low toughness of iron-based materials at low temperatures. The worn graphite from cast iron becomes a solid lubricant, reducing the friction coefficient at these two temperatures.
At low sliding speed, the tribological performance for the sinter brake block is not sensitive to the changes of ambient temperature. Additional tests at higher sliding speeds are suggested in order to evaluate the advantages of the sinter brake block.
References [1] M. Fahlen, Safety problems with composite brake blocks, Swedish Transport Agency, Borlänge, 2017. [2] T. Lähdetluoma, Rail Noise in Finland, VR Group, 2018. [3] Y. Lyu, E. Bergseth, M. Tu, U. Olofsson, Effect of humidity on the tribological behaviour and airborne particle emissions of railway brake block materials, Tribol Int, 118 (2018) 360-367. [4] T. Vernersson, Thermally induced roughness of tread-braked railway wheels: Part 1: brake rig experiments, Wear, 236 (1999) 96-105. [5] B.D. Garg, S.H. Cadle, P.A. Mulawa, P.J. Groblicki, C. Laroo, G.A. Parr, Brake wear particulate matter emissions, Environ Sci Technol, 34 (2000) 4463-4469. [6] G. Perricone, M. Alemani, I. Metinöz, V. Matějka, J. Wahlström, U. Olofsson, Towards the ranking of airborne particle emissions from car brakes–a system approach, Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 231 (2017) 781-797. [7] N. Bukowiecki, P. Lienemann, M. Hill, M. Furger, A. Richard, F. Amato, A. Prévôt, U. Baltensperger, B. Buchmann, R. Gehrig, PM10 emission factors for non-exhaust particles generated by road traffic in an urban street canyon and along a freeway in Switzerland, Atmospheric Environment, 44 (2010) 2330-2340. [8] H.L. Karlsson, L. Nilsson, L. Möller, Subway particles are more genotoxic than street particles and induce oxidative stress in cultured human lung cells, Chemical research in toxicology, 18 (2005) 19-23. [9] C. Ferrer, M. Pascual, D. Busquets, E. Rayón, Tribological study of Fe–Cu–Cr–graphite alloy and cast iron railway brake shoes by pin-on-disc technique, Wear, 268 (2010) 784-789. [10] U. Olofsson, A study of airborne wear particles generated from the train traffic—Block braking simulation in a pin-on-disc machine, Wear, 271 (2011) 86-91.
[11] T. Vernersson, R. Lunden, S. Abbasi, U. Olofsson, Wear of railway brake block materials at elevated temperatures: Pin-on-disc experiments, Eurobrake 2012, 2012. [12] U. Olofsson, J. Sundh, U. Bik, R. Nilsson, The influence of snow on the tread braking performance of a train: A pin-on-disc simulation performed in a climate chamber, Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit, 230 (2016) 1521-1530. [13] L. Ma, L.B. Shi, J. Guo, Q.Y. Liu, W.J. Wang, On the wear and damage characteristics of rail material under low temperature environment condition, Wear, 394-395 (2018) 149-158. [14] M.R.K. Vakkalagadda, D.K. Srivastava, A. Mishra, V. Racherla, Performance analyses of brake blocks used by Indian Railways, Wear, 328-329 (2015) 64-76. [15] Y. Lyu, E. Bergseth, U. Olofsson, Open System Tribology and Influence of Weather Condition, Sci Rep, 6 (2016) 32455. [16] Y. Lyu, Y. Zhu, U. Olofsson, Wear between wheel and rail: A pin-on-disc study of environmental conditions and iron oxides, Wear, 328-329 (2015) 277-285. [17] Y. Zhu, Y. Lyu, U. Olofsson, Mapping the friction between railway wheels and rails focusing on environmental conditions, Wear, 324-325 (2015) 122-128. [18] K.M. Shorowordi, A.S.M.A. Haseeb, J.P. Celis, Velocity effects on the wear, friction and tribochemistry of aluminum MMC sliding against phenolic brake pad, Wear, 256 (2004) 1176-1181. [19] P.J. Blau, J.C. McLaughlin, Effects of water films and sliding speed on the frictional behavior of truck disc brake materials, Tribol Int, 36 (2003) 709-715. [20] V.E. Panin, L.S. Derevyagina, N.M. Lemeshev, A.V. Korznikov, A.V. Panin, M.S. Kazachenok, On the Nature of Low-Temperature Brittleness of BCC Steels, Phys Mesomech, 17 (2014) 89-96. [21] A.K. Shoemaker, S.T. Rolfe, The static and dynamic low-temperature crack-toughness performance of seven structural steels, Engineering Fracture Mechanics, 2 (1971) 319-339. [22] Y.Y. Song, D.H. Ping, F.X. Yin, X.Y. Li, Y.Y. Li, Microstructural evolution and low temperature impact toughness of a Fe–13%Cr–4%Ni–Mo martensitic stainless steel, Materials Science and Engineering: A, 527 (2010) 614-618. [23] K. YM, Effect of microstructure on the yield ratio and low temperature toughness of linepipe steels, ISIJ international, 42 (2002) 1571-1577. [24] A. Wexler, Vapor pressure formulation for water in range 0 to 100 C. A revision, J. Res. Natl. Bur. Stand. A, 80 (1976) 775-785. [25] T. Quinn, Oxidational wear, Wear, 18 (1971) 413-419. [26] R.M. Cornell, U. Schwertmann, The iron oxides: structure, properties, reactions, occurrences and uses, John Wiley & Sons, 2003. Highlights Tribology of different type railway brake blocks at low temperatures is studied.
Cast iron, sinter and composite brake blocks are tested in a pin-on-disc machine.
Cast iron block has the greatest wear and reduced friction at -10 °C and -20 °C.
A decrease in friction with decreasing temperature is found for composite block.
Sinter block is not sensitive to changes of temperature in the current study.
PII: DOI: Reference:
S0043-1648(18)31393-0 https://doi.org/10.1016/j.wear.2019.01.110 WEA102784
To appear in: Wear Received date: 6 November 2018 Revised date: 17 January 2019 Accepted date: 17 January 2019 Cite this article as: Yezhe Lyu, Ellen Bergseth, Jens Wahlström and Ulf Olofsson, A pin-on-disc study on the tribology of cast iron, sinter and composite railway brake blocks at low temperatures, Wear, https://doi.org/10.1016/j.wear.2019.01.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A pin-on-disc study on the tribology of cast iron, sinter and composite railway brake blocks at low temperatures
Yezhe Lyu*, Ellen Bergseth, Jens Wahlström, Ulf Olofsson
Department of Machine Design, KTH Royal Institute of Technology, SE-100 44, Stockholm, Sweden
*
Corresponding author. Tel.: +46 8 790 67 66; fax: + 46 8 20 22 87. [email protected] (Y. Lyu).
Abstract Most freight wagons in the EU use cast iron brake blocks. Cast iron brake blocks have a stable braking capability in different environmental conditions, but wear down the wheel tread quickly. Therefore, there is a need to understand the tribology of other brake block materials. A pin-on-disc tribometer placed in a temperaturecontrolled chamber is used to investigate the tribology of cast iron, sinter and composite railway brake blocks at low ambient temperatures. Pins made from different brake blocks are tested with discs made from steel wheels. Both friction coefficient and wear are evaluated at five different temperatures from +10 to -30 °C. The cast iron block demonstrated the greatest wear at -10
and -20 °C, due to the ductile-to-brittle transition at low
temperatures. The worn graphite from cast iron is likely to become a solid lubricant, reducing the friction at -10 and -20 °C. For the composite brake block, a gradual decrease in friction with decreasing temperature was found. The sinter brake block was not sensitive to changes in ambient temperature. The sliding speed in the current study is relatively low and further study at higher speed is suggested in order to evaluate the tribological performance of different brake blocks.
Keywords: Railway; Brake blocks; Temperature; Pin-on-disc; Tribology; Composite
1. Introduction Railway transport is an open tribological system for many mechanical components, and the mechanical brakes is one of them. For cold regions, this can be a challenge for the operators in terms of wear of the contact surfaces, but also a safety issue due to a varying coefficient of friction that prolongs the stopping distance. In the winter of
2017, several incidents regarding the stopping distance of freight wagons equipped with composite brake blocks occurred in the north of Sweden at temperatures down to -10 °C [1]. It seems as if the friction coefficient of composite brake blocks decreases considerably at low temperatures, resulting in a prolonged braking distance and near misses. A similar scenario has also been noticed in Finland [2]. The brake system on railway vehicles is different from that on automotive vehicles. Briefly, the driver of automotive vehicles can reduce the braking distance by pushing the brake pedal harder. On the other hand, the compressed-air brake system on railway vehicles only allows limited pressure between the brake block and wheel tread. Under such circumstances, the driver is not able to dynamically adjust the pressure between the brake block and wheel tread. A high friction coefficient between the brake blocks and wheel treads should always be maintained. Almost all freight wagons in the EU use block braking as the mechanical braking system due to its great braking capability and consistent braking behaviour. Cast iron with a high thermal conductivity has for decades been the most commonly used brake block material [3]. However, cast iron blocks have drawbacks. They wear down the wheel tread very quickly, resulting in a high level of roughness (waviness and corrugation) on wheel treads [4]. This irregularity on the wheel tread is prone to generate a loud rolling noise. In addition, it has been reported in the literature that a large portion (20% to 50%) of the worn materials during braking become airborne particles [5-7]. Since both cast iron brake block and steel wheel are iron-based materials, the airborne particles generated during the braking of railway vehicles contain a large amount of iron oxides which are highly toxic and induce oxidative stress in human lung cells [8]. In Europe, sinter and composite brake blocks are gradually replacing cast iron since they generate less rolling noise and airborne particles. Researchers have evaluated the tribology and airborne particle emissions of sinter and composite brake blocks in the laboratory. Ferrer et al. [9] tested the tribology of cast iron and Fe-C-Crgraphite sinter brake blocks using a pin-on-disc tribometer. Olofsson et al. systematically investigated the tribological performance and airborne particle emissions [10] of cast iron, sinter and composite brake blocks at elevated temperatures [11] and with snow addition [12]. The effect of humidity on tribology and airborne particle emissions of different brake block materials has also been studied [3]. Ma et al. also studied the wear performance at wheel rail contact in low ambient temperatures [13]. Vakkalagadda et al. analysed the performance of brake blocks (both cast iron and composite) used in the Indian railway system [14]. Since sinter and composite materials are rather new as railway brake blocks, their tribological behaviour and airborne particle emissions are not fully understood. The purpose of this study is to address the difference in friction and wear
from cast iron, sinter and composite railway brake blocks at low ambient temperatures. This topic is of great importance, especially to the Nordic countries where the temperatures are quite low in winter.
2. Experimental set-up 2.1 Materials Three different railway brake block materials, i.e. cast iron, sinter and composite, were tested in this study. All three kinds of brake block are commercial products used in Swedish freight wagons as well as Stockholm Metro trains. The counterpart is a standard R7 railway wheel used in both Swedish freight wagons and passenger trains. Chemical compositions of cast iron, sinter, composite brake blocks and R7 wheel steel are shown in Table 1. Their microstructure has been investigated previously and can be found in [3]. Cylindrical pin samples (10 mm in diameter and 20 mm in height) featured a flat testing surface were extracted from brake blocks. Disc samples (100 mm in diameter and 12 mm thick) were manufactured from wheels. The initial arithmetic surface roughness (Ra) of the pin and disc samples were manufactured to 2.00 ± 0.20 μm and 0.45 ± 0.02 μm, respectively. Both the roughness values of the pin and disc samples correspond to the surface conditions of newly manufactured brake blocks and newly grinded wheel tread.
Table 1 Chemical composition of the sinter, composite brake blocks and R7 railway wheel as obtained from elemental analysis EDX (wt. %). Chemical composition of cast iron was provided by the supplier. Element
C
Si
Mn
S
P
Cast iron
3.5
1.0-1.8
0.8-1.4
<0.08
Sinter
-
6.08
7.93
4.43
-
Composite
-
17.95
-
-
R7 wheel
0.52
0.4
0.8
<0.035
Fe
O
Mg
Al
Ca
Cu
Sn
-
-
-
-
-
-
18.12
7.19
-
-
3.25
47.60
5.40
-
2.29
47.70
4.06
8.05
18.19
-
-
-
balance
-
-
-
-
-
-
<0.04 balance
2.2 Testing procedure Tests were conducted in a temperature-controlled climate chamber, which had previously been used to test tribology of wheel-rail contact at low temperatures and humid atmosphere [15-17]. Fig. 1 shows the schematic of the configuration. The main part of this configuration is a pin-on-disc tribometer, which contains a horizontally
rotating disc (G) and a dead-loaded pin (F). This pin-on-disc tribometer was used to imitate the sliding contact between the brake block and wheel tread. The normal force and rotating speed are adjustable. The whole rig is set inside a temperature-controlled box (B) in which temperatures can be controlled from –50 °C to 20 °C. A seal cover made of thermal insulation material is placed on the test bed to ensure that the pin-on-disc machine is isolated from external environment. A flexible plastic tube (C) containing a uni-directional fan (L) connects the climate chamber (B) and an air conditioner (D). The working range of the air conditioner is from –50 °C to 20 °C is. A sensitive transducer (J) is fixed in the temperature-controlled box (B) and linked to an automatic data logger and controller (K). When the experiment starts, the fan (L) will operate, pumping cold air into the box (B). Once the temperature reaches the desired level, the controller (K) will give a signal and stop the fan (L). Meanwhile, the air conditioner will also stop working. Consequently, a constant environment in terms of temperature can be obtained in the temperature-controlled box (B).
Fig. 1 Schematic diagram of the test configuration
Since the train speed close to a platform is usually low, a relatively low sliding speed was chosen in the current study. Every test lasted for 1200 s under a contact condition of 0.8 MPa and 0.45 m/s. The current contact pressure 0.8 MPa is a typical contact pressure level at the railway brake block contact [9, 10]. During the test, the tangential force between the pin and disc was measured by an HBM® Z6FC3/20 kg load cell mounted on the pin holder. The friction coefficient was calculated by dividing the tangential force by the normal load. A Sartorius® ME614S analytical balance (accuracy 0.1 mg) was used to measure the wear of the pin and disc samples. Each brake block material was tested three times at five different temperature levels: 10 °C, 3 °C, 10 °C, -20°C and -30 °C. A Hitachi® S-3700N scanning electron microscope (SEM) equipped with Bruker®
Xflash 6-10 energy-dispersive X-ray spectroscopy (EDX) was employed to characterize the worn surfaces of some tested samples.
3. Results Fig. 2 shows the time history of the friction coefficient from representative tests of three brake block materials at -30 °C and 10 °C. It should be noted that other tests have also a similar behaviour. For all three brake block materials, there is a running-in period at the beginning of testing, after which the friction coefficient reached a steady state. The data in the steady state is of great importance, since the contacting brake block and wheel tread are totally run-in with each other in this period. An average value was calculated with the data in the steady state, i.e. time interval from 600 s to 1200 s of each test. Using this average value of three repetitions for each test condition, the mean value and standard deviation of the friction coefficient as a function of temperature can be obtained (Fig. 3). Fig. 3 shows that the composite brake block has the lowest friction coefficient at all five temperatures. It may also be noted that the friction coefficient of composite brake block decreases rapidly in subzero temperatures. The cast iron brake block has a transition in friction coefficient with the decreasing temperature from 10 °C to -30 °C. At -10 °C and -20 °C, the friction coefficient of a cast iron brake block is much lower than at the other three temperatures. The friction coefficient of a sinter brake block remains stable at all five temperatures and seems not to be sensitive to the change of temperature. Note that the sinter and composite combinations show the same friction level at 10 °C, but the composite will significantly reduce brake performance at -30 °C.
Fig. 2 Time history records of friction coefficient of three brake block materials at 10 °C and -30 °C
The flash temperature at the pin-disc contact is not measured. In previous study on the same materials and test conditions in room temperature, the flashing temperature between the brake block and wheel materials in the steady state are about are approximately 55 °C (organic composite), 73 °C (sintered) and 84 °C (cast iron), respectively [3]. One could expect lower flash temperature in the current study due to the lower ambient temperature.
Fig. 3 Friction coefficient (mean value and standard deviation) of three brake blocks as a function of temperature
The wear of the pin and disc samples corresponds to the entire test duration rather than the steady state. The mean values and standard deviations of wear at different temperatures are summarized in Table 2. It should be noted that some exceptional values are underlined in bold and italic. At -10 °C and -20 °C, the cast iron pins and mating discs yielded extremely high wear (one magnitude greater than at other temperatures), showing an opposite behaviour to the friction coefficient (at -10 °C and -20 °C, cast iron has a lower friction coefficient than at the other three temperatures). The sinter brake block and its mating disc shows relatively stable wear at different temperatures, similarly to its performance in the friction coefficient. The composite brake block demonstrates very minor wear at all five temperatures. At -30 °C, the disc sample tested against composite brake block has a negative wear, indicating that material was transferred to the disc surface rather than being worn off.
Table 2 Wear (mean value and standard deviation) of three brake block pins and the mating discs tested at different temperatures. (Exceptional values are underlined in bold and italic) Materials
Cast iron
Sinter
Composite
Temperature (°C)
Pin wear (mg)
Disc wear (mg)
Mean
Std.
Mean
Std.
10
16.5
1.8
15.9
1.8
3
20.4
4.1
15.05
0.85
-10
500.8
3.8
317.85
6.65
-20
731.05
6.45
269.9
5.6
-30
10.15
3.35
9.3
2.5
10
1.85
0.85
0.65
0.05
3
5.35
0.35
1.75
0.35
-10
7.65
0.55
1.85
0.55
-20
8.65
0.05
1.65
0.24
-30
5.45
0.47
0.85
0.35
10
0.3
0.1
1.3
0.11
3
0.15
0.05
1.4
0.13
-10
0.6
0.1
2.7
0.2
-20
0. 5
0.07
2.25
0.03
-30
0.45
0.15
-0.5
0.1
4. Discussion The purpose of block brakes is to stop the train in a limited distance. A high enough friction coefficient between the brake block and wheel tread should be maintained in varied environmental conditions. In the current study, the cast iron brake block yielded a friction coefficient of between 0.5 and 0.9 at different temperatures (black dash dotted line in Fig. 3). The sinter brake block also gives a friction coefficient of greater than 0.5 at all five temperatures (red dashed line in Fig. 3). But the composite brake block only has a friction coefficient of around 0.5 at 10 °C, which decreases down to 0.3 with temperatures decreasing to -30 °C (blue solid line in Fig. 3). It should be noted that the sliding speed in the current study is relatively low. Usually a further decline in friction coefficient can be expected with an increase of sliding speed [18, 19]. Recently, the Swedish Transport Agency has recorded some incidents in winter with freight wagons equipped with composite brake blocks [1]. The friction of composite brake blocks seems to decrease at low temperatures, resulting in a prolonged braking distance. One incident recorded a train 100% equipped with composite brake blocks. The train was running with a speed of 18 km/h and braking was applied 200 m in advance of the STOPsignal. Finally, the train was stopped 20 m after the signal by using emergency braking. Another freight train
operated at -2 °C and an ice layer has built-up on the composite brake blocks, impeding the braking capability. Olofsson et al. tested the same composite brake block at -2 °C and demonstrated a decline of friction coefficient by 0.2 by applying snow particles into the sliding contact [12]. A similar scenario was seen at wheel-rail contact tested at sub-zero temperatures, where the friction and wear decreased dramatically [15]. The composite brake block contains a large proportion of phenolic resin, which is prone to adsorb water vapour. The adsorbed water vapour forms an ice condensation layer on the composite brake block, which will act as a lubricant and reduce the friction coefficient. At -30 °C, the composite brake block has another exceptional behaviour, negative (-0.5 mg) wear on the mating disc (Table 2). The negative wear indicates that some worn materials or materials from the environment were added onto the disc surface. SEM was used to observe the disc surface tested with a composite brake block at -30 °C, as shown in Fig. 4. The figure illustrates a black phase in the path of a wear scar and its chemical composition achieved by EDX. The chemical analysis suggests a complex constitution of many elements, which are likely to come from the composite brake block. This transferred phase increased the weight of the disc sample, resulting in the negative wear at -30 °C.
Fig. 4 SEM photograph and EDX analysis of a black phase in the wear scar of the disc sample tested with a composite brake block at -30 °C
The cast iron brake block shows low friction (Fig. 3) and high wear (Table 2) at -10 °C and -20 °C. SEM analysis of the worn surfaces was conducted on both cast iron pin and wheel steel disc samples tested at -20 °C, as shown in Fig. 5. The cast iron brake block tested at -20 °C (Fig. 5a) shows a severe wear scenario with a large area of delamination and a great deal of wear debris, corresponding to the large amount of wear in Table 2. The
worn surface of the mating disc tested with cast iron at -20 °C also contains a large amount of wear debris (Fig. 5b). The large amount of wear of iron-based materials (both cast iron and steel) at low temperatures is attributed to the ductile-to-brittle transition [20-23]. The body-centred cubic iron-based materials have a gradual decrease of toughness with decreasing temperature and one ductile-to-brittle transition usually occurs from -10 °C to 20 °C. At such temperatures, cracks tend to generate between the ferrite-pearlite phase and graphite-ferrite phase boundaries (Fig. 6). These cracks are prone to extend during relative sliding and result in materials removal from the samples. Sometimes, wear is not in accordance with the friction. The wear of tests with cast iron brake block at -10 and -20 °C are much higher than at other three temperatures (Table 2), but the friction coefficients at these two temperatures have lower values (Fig. 3). One possible explanation is due to the graphite removed from the cast iron brake block into the interface. EDX analysis of the disc surface tested with cast iron at -20 °C reveals that a large amount of graphite transferred from the cast iron onto the steel disc surface. The transferred graphite was likely to act as a solid lubricant and somewhat relieve the friction between the pin and disc. The transferred graphite is only observed at -10 °C and -20 °C, but not at -30 °C. With a further decrease of temperature, there is a pronounced tendency to have an ice condensation layer on the metal surface [24]. This condensed ice layer encourages the generation of oxide layers, protecting the contacting bodies from severe wear [25, 26]. Fig. 7 shows the SEM micrograph and EDX analysis of the oxide flakes on the cast iron pin and wheel steel disc tested at -30 °C. The iron oxides on the cast iron brake block and steel wheel are considered to be Fe 2O3 and FeO, respectively. The sinter brake block seems to behave in a stable way as regards both friction coefficient and wear under current test conditions. The current study only tested a relatively slow sliding speed that corresponds to a situation in which a train is going to stop at a platform. More tests with higher sliding speeds are suggested in order to evaluate the friction and wear performance of a sinter brake block.
Fig. 5 Worn surfaces of a) cast iron pin and b) the mating disc samples tested at -20 °C. Sliding direction from left to right. Green lines indicate the possible transfers of graphite from cast iron pin to wheel steel disc surface
Fig. 6 Microstructure of cast iron brake block is composed of black graphite surrounded by grey ferrite with lamellar pearlite. Graphite, ferrite and pearlite have clear phase boundaries with each other.
Fig. 7 Worn surfaces of a) cast iron pin and b) the mating disc samples tested at -30 °C. Sliding direction from left to right. EDX analysis suggests Fe2O3 on cast iron brake block surface and FeO on the steel wheel surface.
Conclusions A pin-on-disc tribometer has been used to investigate the tribology of cast iron, sinter and composite railway brake blocks at low ambient temperatures. The following conclusions can be drawn from the present study:
The composite brake block has a gradual decrease in friction coefficient with decreasing ambient temperature which could be explained by the water adsorption of phenolic resin. It is, therefore, not advisable only to use composite brake blocks on trains in winter. A combination of different types of brake blocks is more appropriate, which is in line with the incidents involving Swedish freight wagons.
The disc surface tested with a composite brake block at -30 °C shows a negative wear, indicating material transfer from the composite brake block to the steel wheel surface.
The cast iron brake block and its mating wheel steel yield very high wear at -10 °C and -20 °C due to the low toughness of iron-based materials at low temperatures. The worn graphite from cast iron becomes a solid lubricant, reducing the friction coefficient at these two temperatures.
At low sliding speed, the tribological performance for the sinter brake block is not sensitive to the changes of ambient temperature. Additional tests at higher sliding speeds are suggested in order to evaluate the advantages of the sinter brake block.
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Cast iron, sinter and composite brake blocks are tested in a pin-on-disc machine.
Cast iron block has the greatest wear and reduced friction at -10 °C and -20 °C.
A decrease in friction with decreasing temperature is found for composite block.
Sinter block is not sensitive to changes of temperature in the current study.