demi̇ryolu uygulamalari i̇çi̇n sertleşti̇ri̇lmi̇ş i̇ki̇ çeşi̇t

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DEMİRYOLU UYGULAMALARI İÇİN SERTLEŞTİRİLMİŞ İKİ ÇEŞİT
BORLU ÇELİĞİN TRİBOLOJİK İNCELENMESİ*
Fatih Hayati Çakır1, Fatih Bozkurt2, Osman Nuri Çelik3, Ümit Er4
Özet
Güvenlik ve ray bakımının sağlanması açısından ray aşınması önemli bir konudur. Ray
aşınmasının periyodik kontrolü, yağlama, ray yüzeyinin iyileştirilmesi, ray metalürjisi ve
mikroyapı çalışmaları ray aşınması için uygulanan basit çözümlerdir. Raylar Brinell sertlik
değerlerine göre sınıflandırılmaktadır. “Premium” raylar özel ısıl işlem uygulanmış ve
yüksek aşınma direncine sahiptir. Bu kalite kontrollü soğuma oranı uygulanarak rayda daha
sert perlitik yapı elde edilerek sağlanır. Bu çalışmada ise yüksek hızlı tren raylarında
kullanılan perlitik yapı, su verilmiş ray numunesi ve iki farklı kalitedeki borlu çelik tribolojik
olarak incelenmiştir. Tribolojik araştırmalar göstermektedir ki yüksek karbonlu borlu çelikler
en iyi aşınma direncini göstermektedir.
Anahtar Kelimeler: Ray aşınması, borlu çelik, aşınma direnci
TRIBOLOGICAL EXAMINATION OF HARDENED TWO TYPES
BORON STEEL FOR RAILWAY APPLICATIONS
Abstract
Wear of railway infrastructure is an important issue to provide safety and railway
maintenance. Applied basic solutions to deal with rail wear are periodic rail control,
lubrication, surface treatment for the rail surface and studies on rail metallurgy and
microstructure. Rails are classified with their Brinell hardness values. Premium rails are
specially heat treated and more wear resistant. This quality is obtained by controlling cooling
rate to obtain harder pearlitic structure in rail structure. In this study, a high-speed-train
railway material which is pearlitic, quenched rail samples and two kinds of quenched boron
steel are investigated tribologically. Four different structures were obtained and their results
were compared. Result from the tribological examination showed that boron steel with higher
carbon ratio posses best wear resistance.
Keywords: Rail wear, boron steel, wear resistance
* Bu çalışma IWCEA 2015’de sunulmuştur.
1
Arş. Gör.,Eskisehir Osmangazi Üniversitesi, [email protected]
2
Ing., Pardubice Üniversitesi, [email protected]
3
Doç. Dr.,Eskisehir Osmangazi Üniversitesi, [email protected]
4
Yrd. Doç. Dr.,Eskisehir Osmangazi Üniversitesi, [email protected]
DEMİRYOLU UYGULAMALARI İÇİN SERTLEŞTİRİLMİŞ İKİ ÇEŞİT BORLU ÇELİĞİN TRİBOLOJİK
İNCELENMESİ
Introduction
Rail and wheel form were developed in mid 1700s and have not significantly changed
since then (Kapoor 2000 and Bhushan 2001). The main purpose of development of new rail
material is improving wear performance and rolling contact fatigue (RCF) through higher
hardness (J.H. Beynon 1996). Increasing hardness can be done by adding alloying elements
to steel (mainly carbon) and applying heat treatments. But there is theoretical limit for
hardness that can be achieved in pearlitic steels and current rail is approaching the limits (J.
Kristan 2003 and K. Sawley 2003). To prolong the service life of rail and wheels, it is
essential to increase the wear resistance of the contacting bodies. By increasing the wear
resistance of the rail head, the transverse profile of the rail will be kept within tolerances of
optimal running conditions and the wheel wear will be reduced. A more stable transverse rail
profile will also improve the steering capability of the trains. The prolonged service life will
lead to longer maintenance intervals and decreased overall operating costs (Kassfeldt 2009).
For rail materials preferred microstructure is the pearlite phase, which contains iron and
cementite that are arranged in lamellar form (ASM Handbook Committee 1991). Lamellar
spacing determines the hardness of the structure. Air cooling of rails results in pearlitic
structure, the hardness is approximately 300 HB. Heat-treated rails are cooled with the help
of compressed air and pressurized water. Higher cooling rates still preserves pearlitic
structure. Lamells in pearlitic structure became closer and hardness can be as high as 340 –
400 HB. The hardness of modern HT pearlitic rail can be increased to 350 – 400 HB without
alloying. UIC recommends the usage of different rail grades in different loading conditions.
The main criterion is the curvature radius. At plain areas, the standard rail is used, whereas at
curvature below 500 meters, premium rail is strongly recommended (Innotrack 2006).
Premium rails have higher hardness values. Higher hardness is less prone to wear under
extreme working conditions. Standard pearlitic rail's hardness cannot be increased without
alloying so further research is focused on new materials and microstructures.
The adhesion of the wheel-rail interface is important factor in railway transportation, as
it determines the acceleration and braking capabilities of train. Therefore, the loss of adhesion
coefficient of wheel-rail has vital parameter on both traction and braking. Low adhesion of
the wheel-rail interface leads to wheel sliding on the rail surface during the traction process
and accelerates the surface damage of wheel-rail materials, such as skidding marks of rail
surface and scratch damage of wheel tread. On the other hand, poor adhesion may lead to
extended and unpredictable stopping distance. Therefore, maintaining correct levels of the
adhesion coefficient in railway transportation is essential (Q.Y. Liu 2013).
Wheel-rail adhesion is affected by many factors such as water, leaves, lubrication oil,
wear debris, axle load, surface roughness and so on. In order to understand the mechanism of
the adhesion under various conditions, some experiments and numerical calculation have
been carried out. Sanding as a friction modifier can increase the adhesion coefficient of
wheel-rail under wet and oil conditions. Number of methods have been used to assess the
adhesion characteristics of wheel-rail. Laboratory bench techniques have been used including
pin-on-disc, disc-on-flat and twin disc testing. Field measurement of adhesion behavior has
been taken using track mounted tribometer(Q.Y. Liu 2013).
Boron steel is known as a wear resistant material and is today used for wear protection
in applications such as rock and ore handling. High strength boron steel is today not used as
an engineering material heavily loaded contacts with relative motion. With increased
knowledge of the tribological performance of the boron steel, new areas will be opened up
and new improved products and applications can be found. Boron steel tribology was
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EJOIR – ARALIK 2015 IWCEA ÖZEL SAYISI CİLT 3
İNCELENMESİ
investigated by other Kassfedlt at that study hardened boron steel (525 HV approximately
499 HB) was investigated. Two disc tribometer was used at wear tests. Boron steel was
analyzed in two categories smooth surface and rough surface. At that study boron steel was
compared with UIC 1100 steel, that study shows that boron steel posses better wear
resistance than UIC 1100 steel friction coefficient of boron steel against wheel material was
more stable than UIC 1100 steel (Kassfeldt 2009). In other study bainitic Mo-B steels are
investigated. Five different chemical composition of steels are heat treated. Three groups (as
rolled, air cooled, water quenched) of five different steels were tested. Rolling/sliding wear
test showed that % 0.18 C %1 Si % 0.002 B as water quenched form had highest wear
resistance (Jin 1997).
This work was divided into 5 steps: determining the properties of the reference rail
material and boron steels, heat treatment, evaluating the microstructures, performing hardness
test and conducting the wear test on the samples. Rail material which conforms to EN 13674
standard supplied and verified. First group of samples are tested as received condition.
Second group of samples are quenched and tempered rail material. Third group of material
was quenched and tempered boron steel with % 0.63 C quenched and tempered. Fourth group
of material was boron steel with % 0.32 C. Hardness test and micro structural analysis was
performed to verify heat treatment success. Finally wear tests was performed for all groups.
Experimental Work
At the beginning of the experimental work, the supplied rail material and boron steel
was investigated and verified. The chemical composition of the rail material and boron steels
are listed in the following Table 1 – 3. Hardness test performed to rail section in order to
verify rail quality.
Table 1: Reference Rail Spectral Analysis Results
C
Si
Mn
P
S
Cr
Al Max V Max
0.86075 0.3275 0.99025 0.0135 0.023 0.06175 <0.001 <0.001
Mo
Co
Cu
Nb
Ti
V
W
Fe
0.002175 0.001875 0.02025 <0.001 0.002325 <0.001 <0.004 Balance
Table 2: Erdemir 5630 Boron Steel Spectral Analysis Results
C
Si Mn
P
S
Cr
Al
V
0.32 0.29 1.33 0.012 0.002 0.14 0.049 0.003
Mo Ni Cu
B
Ti
N
W
Fe
0.006 0.05 0.05 0.004 0.035 0.0057 <0.004 Balance
Table 3: 51B60H Boron Steel Spectral Analysis Results
C
Si Mn
P
S
Cr
Al
V Max
0.63 0.24 0.91 0.009 0.004 0.87 0.008 <0.001
Mo Ni Cu Sn
B
Ti
W
Fe
0.01 0.1 0.08 0.005 0.0012 0.026 <0.004 Balance
The supplied train rail was cut into pieces with a hand saw, and parts of the pieces were
machined using a milling machine. The top of the rail curvature was removed using the
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milling machine to obtain the required surface finish and flatness. Pieces were taken from
head of rail as shown Figure 1. First sample was used as supplied, second sample was
quenched and tempered. Austenization temperature was 860°C. Oil quenching applied and 1
hour temper was applied at 200°C.In addition two kinds of boron steel with different
chemical composition was supplied as blocks. Pieces were taken at rolling direction. Both of
boron steels were austenitized at 850°C for 30 minutes and quenched room temperature in
water. After quenching both boron steels were tempered at 200°C for 1 hour.
Figure 1: Sampling position of the rail
Hardness Test
Average hardness of the reference rail samples is 33 HRC, which is approximately 350
HB. Hardness is a main criteria which determines rail quality. Hardness measurements were
taken from all samples repeatedly. Three samples were used at each group. Hardness values
of the samples are listed in Table 4.
Table 4: Hardness of Samples
Reference
Tempered 51B60HBoron Steel Erdemir 5630 Boron Steel
Martensite
Sample No
1 2
3
1 2
3
1
2
3
1
2
3
Hardness HRC 33 34 34.33 59 60 59.5 61.5
61.5 61
54.5
55.5
55
Reference
(Pearlitic)
Microstructure
The samples were mounted on polymerat approximately 180°C for 3 minutes. The
surfaces of the samples were grinded using a StruersTegraforce automatic grinding machine
at three stages mesh number of 220, 500 and 700. The grinding load for each sample was 30
N for 10 minutes. After grinding, the samples were polished using the same machine for 3
minutes with 3 µm diamond solution. The samples were etched with 2% nital. The surface
roughness value of the polished samples was produced below Ra= 0.02 µm. Microstructures
of the specimens of pearlite, tempered martensite, 51B60H and Erdemir 5630 boron steels are
shown in Figure 2.
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Reference(Pearlite)
Reference Tempered Martensite
51B60H Quenched and Tempered
Erdemir 5630 Quenched and Tempered
Figure 2: Microstructure of samples 500X
Wear Tests
The wear test samples were ground, and the polished – surface roughness of the
samples was between 0.010 – 0.020 µm. To simulate the wheel/rail contact in extreme wear
conditions, CSM wear test module was used as shown in Figure 3. The wear test parameters
were 10N load, 2.5 mm wear radius and 50m distance 100 RPM speed. Certificated and
unused WC ball was used in the every wear tests. The wear tests were performed on a ball-on
disk geometry according to the DIN 50324 standard. The schematic illustration of the test
condition is shown in Figure 4. In the experiments, the counterpart is Ø 3 mm with a WC –
6% Co ball, which sphericity and compositions were certified. The hardness of the ball was
91.6 HRA, and its modulus of elasticity is 690 GPa, Poisson ratio is 0.22. It was assumed that
only the rail material was worn at the end of the experiment because the material that was
used was too hard compared to the samples. The calculated Hertz contact stresses during the
tests are about 2.903 GPa. All wear test samples were cleaned with alcohol prior to testing.
After each wear test, profile measurements were performed to determine the worn area of the
wear section. The worn sections were evaluated with using OriginPro®. The worn sections
are shown in Figure 5. The load was maintained constant at 10 N during all tests. The
tribometer can record in situ friction force and coefficient. The test conditions were same for
all tests. Friction data were stored in a computer during the tests. Similar works performed
these tests in the same manner (Hiratsuka 2011 and Hernandez 2007).
Results of laboratory test shows correlation of real wheel rail wear regime. Hernandez
et al. (Hernandez 2011) is also shows this correlation. Hernandez made a comparison of
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İNCELENMESİ
samples with full scale test and ball on disk test. Hernandez concluded that if right algorithm
is used rail wear performance can be assessed by using ball on disk tests. According to
Hernandez ball on disk method is a reliable tool and can be used as a pre-screening rail
performance method. Worn section assessment of this work was performed similarly to
Herndandez's ball on disk study. Figure 5 shows the measured worn sections. Wear rate was
calculated to exclude results from the effect of load and duration. The values of worn area
and wear rate are listed in Table 5.
Figure 3: Wear test module CSM tribometer
Figure 4: Wear test conditions
Table 5: Average wear area and wear rate of samples
Wear
Rate
(mm3/N/
2
WornArea µm m)
Reference(Pearlitic)
140.63
4.43E-06
ReferenceTemperedMartensite 41.33
1.30E-06
51B60H Boron Steel
31.56
1.01E-06
Erdemir 5630 Boron Steel
45.48
1.44E-06
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İNCELENMESİ
Figure 5: 50m Wear Test worn profiles
A study of the worn surfaces makes it possible to understand which wear mechanism is
dominant of the wear process. In Figure 6 SEM images of the worn surfaces are shown in
Fig.6. At the reference pearlite sample some small valleys can be seen. The scratch lines are
existed in worn surfaces of tempered martensite sample which means abrasive wear
mechanism more dominant. 51B60H and Erdemir 5630 boron steel samples have also scratch
lines and same surface topography. After the investigation of worn area; reference pearlitic
rail materials wear mechanism is mainly adhesive because there were not significant deep
groove marks. Tempered martensites main wear mechanism was both adhesive and abrasive.
Ploughing marks and smeared particles were both seen.
The coefficient of friction (COF) of as a function of sliding distance for the samples
sliding against on WC ball is presented in Figure 7. The mean COF of the pearlite, tempered
martensite, 51B60H and Erdemir 5630 boron steels are 0.34, 0.4, 1.1 and 0.8 respectively.
Approximately 6 – 7 meter after the start of the experiment, pearlite and tempered martensite
samples show more stable character than boron steel samples. Erdemir 5630 boron steel wear
mechanism was mainly adhesive while 51B60H showed similar type of wear to tempered
martensite. According to wear test the most wear resistant group is 51B60H boron steel.
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İNCELENMESİ
Reference(Pearlite)
Tempered Martensite
51B60H Quenched and Tempered
Erdemir 5630 Quenched and Tempered
Figure 6: Images of worn area
Figure 7: Friction coefficients for 50 m wear test
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İNCELENMESİ
Discussion
The investigated samples show that the designed heat treatment procedures provide the
desired microstructures. According to the hardness tests, the measured hardness values of the
pearlitic steel, tempered martensite, 51B60H boron steel,Erdemir5630boron steel samples are
34 HRC, 59 HRC and 61 HRC, 55 HRC respectively. Quenching processes increase the
hardness.
The results of the wear tests show quenching improves the wear properties of rails,
additionally higher carbon boron steel improve the wear properties. The wear values of
51B60H boron steel and tempered martensite samples are close to each other. Significant
wear resistance is obtained in all quenched samples.
Regarding the friction coefficient of the investigated ball on disk system, the reference
sample has the lowest friction force (approximately 0.34). Tempered martensite proposes
higher but reasonable friction coefficient (approximately 0.4) but boron steels shows
significantly high friction coefficients. 51B60H boron steel's friction coefficient is quite high
approximately 1.1. This can be explained of abrasive wear character of boron steels. The
highest coefficient of friction was measured at the 51B60H boron steel which was the most
wear resistant material of tests. Erdemir 5630 boron steel showed lower friction coefficient
than 51B60H but its wear rate was pretty close to tempered martensite. Pearlitic structure
wears and worn particles lowers the friction. Worn particles act like small balls and rolling of
this particles lowers the friction while boron steels show less wear but significant friction. It
can be said that friction results could severely change if counterpart was more prone to wear
(Like wheel material or 52100 ball).
Conclusion
The wear results of the ball-on-disk tests show that the boron steels has better wear
resistance than the pearlitic steels. The wear resistance of the higher carbon boron steel is
lower than lower carbon boron steel. Boron steels shows significantly higher friction
coefficients against WC ball. This paper shows the potential of implementing the boron steels
on railway usage because of their better wear performance. In future work, boron steels
mechanical properties should be investigated.
References
ASM International and Handbook Committee, ASM handbook 4, 4,. [Metals Park, Ohio]:
ASM International, 1991.
Beynon, J.H., Garnham, J.E., Sawley, K.J. (1996). Rolling contact fatigue of three pearlitic
rail steels. Wear, 192 (1–2), 94–111.
Bhushan, B. (2001). Modern tribology handbook. Boca Raton, FL: CRC Press.
Hernández F.C. R., Demas N. G., Davis, D. D., Polycarpou, A. A., ve Maal, L. (2007).
Mechanical properties and wear performance of premium rail steels. Wear, 263(1-6),
766-772
Hernández F.C. R, Demas N.G., Gonzales K., ve Polycarpou A.A. (2011). Correlation
between laboratory ball-on-disk and full-scale rail performance tests. Wear, 270(7-8),
479-491.
Hiratsuka K. and Yoshida T. (1997). The twin-ring tribometer - Characterizing sliding wear
of metals excluding the effect of contact configurations. Wear, 270(11-12), 742-750.
153
İNCELENMESİ
INNOTRACK. (2006).Definitive guidelines on the use of different rail grades. s.l.
Jin N. ve Clayton P. (1997). Effect of microstructure on rolling/sliding wear of low carbon
bainitic steels. Wear, 202(2), 202–207.
Kapoor A., Fletcher, D. I., Schmid, F., Sawley K. J., and Ishida M. (2000). Tribology of Rail
Transport. in MODERN TRIBOLOGY HANDBOOK. 2(2) vols., FL: CRC Press.
Kassfeldt, E., ve Lundmark, J. (2009). Tribological properties of hardened high strength
Boron steel at combined rolling and sliding condition. Wear, 267, 2287-2293.
Kristan, J., Sawley K., Canadinc, D., Lee, K.M., Polycarpou, A.A., ve Sehitoglu, H.
(2003).Wear and rolling contact fatigue in bainitic steel microstructures, in: 6th
International Conference. Contact Mech. and Wear of Rail/Wheel Systems.
Liu, Q.Y., Wang, W.J., Wang, H., Wang, H.Y., Guo, J., Zhu, M.H., ve Jin, X.S. (2013). Subscale simulation and measurement of railroad wheel/rail adhesion under dry and wet
conditions. Wear, 302, 1461-1467.
Oscar A.C, Zili, L., Roger, L. (2011). A laboratory investigation on the
influence of the particle size and slip during sanding on the adhesion and
wear in the wheel-rail contact. Wear, 271, 14–24.
Sawley, K., Kristan, J. (2003). Development of bainitic rail steels with potential resistance to
rolling contact fatigue, Fatigue and Fracture of Engineering Materials and
Structurees. 26, 1019–1029.
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