On the development of bainitic alloys for railway wheel applications

On the development of bainitic alloys for railway wheel applications
On the development of bainitic alloys
for railway wheel applications
by A. Kapito*, W. Stumpf†, and M.J. Papo*
The ferrite-pearlite microstructure is the most popular microstructure
for alloys used in structural applications, including railway wagon
wheels. These alloys have been designed through alloying and thermomechanical processing to have a refined microstructure. Ferritepearlite alloys are low cost, weldable, have good fabricability, and are
reliable under extreme conditions. Given these performance attributes,
it seems unlikely then that their dominant position as structural steels
would ever be challenged by alternative microstructures. One major
achievement in the development of ferrite-pearlite steels has been in
the refinement of their interlamellar spacing to very fine distances of
the order of < 0.3 µm. A refined microstructure increases the hardness
of the alloy, thus increasing its life under wear conditions. The
interlamellar spacing in pearlitic steels has, however, been refined
almost to its theoretical limit. The increasing demand for speed and
increased axle loading on railway wagons requires the use of stronger,
tougher, and more durable materials. This has opened the window for
the development of novel bainitic steels.
Bainitic alloys have a higher level of microstructural refinement
than pearlitic ones. They have shown to have good wear resistance
and rolling-contact fatigue resistance, and high toughness. This paper
will discuss the progress to date on the development of bainitic railway
wheel alloys. Four alloy chemistries have been chosen for possible
further development.
Keywords
Bainite, railway wheel, Class B, ferrite-pearlite, Brinell hardness.
Introduction
Railway wagon wheels experience a
tremendous amount of wear and fatigue
cracking1–4. The current ferrite-pearlite alloys
are the dominant choice for railway wheel
applications. In South Africa, railway wheels
are made from medium-high carbon grade
steels with 0.57-0.77 wt% carbon, and have a
ferrite-pearlite microstructure. These steels,
however, have a low yield strength (~680
MPa) and resistance to rolling contact fatigue
(RCF). Typical tensile strengths of these alloys
range between 900–1 072 MPa, depending on
alloy chemistry, with elongations in the range
of 7–17 percent. Typical hardness values are
255–363 BHN.
Attempts to improve mechanical properties
of ferrite-pearlite steels have focused on
increasing the hardness to increase wear
resistance and service life. Increased hardness
The Journal of The Southern African Institute of Mining and Metallurgy
* Advanced Materials Division, Mintek.
† Department of Materials Science and Metallurgical
Engineering, University of Pretoria.
© The Southern African Institute of Mining and
Metallurgy, 2012. SA ISSN 2225-6253. This paper
was first presented at the ZrTa2011 New Metals
Development Network Conference, 12–14 October
2011, Mount Grace Country House & Spa,
Magaliesburg.
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Synopsis
has been achieved through micro-alloying with
vanadium (V) and niobium (Nb), production of
cleaner steels, and the refinement of the
microstructure through the reduction of the
interlamellar spacing to values ~0.3 µm3–6.
Premium Rail steels have been developed for
high axle loads, with tensile strengths of
~1 300–1 400 MPa and hardness range of
340–390 BHN.
Despite these developments, the ferritepearlite steels are limited in that there is a
threshold limit to the hardness that can be
achieved, making it difficult to push their wear
performance further, owing to the fact that the
lamellar spacing has been refined close to its
limit4,7. In an attempt to develop rail steels
with higher hardness, alternative
microstructures such as bainite are being
explored8. Bainitic rail steels are attractive
because of their lower carbon contents and
finer microstructure, resulting in higher
strengths, hardness, toughness, and better
weldability5. Bainite, like pearlite is a mixture
of ferrite and cementite but in a non-lamellar
morphology. There are generally two types of
bainite: upper and lower5. Upper bainite forms
at temperatures between 400–500°C and is
made up of ferrite laths with carbides between
the laths. Lower bainite forms at temperatures
between 250–400°C and is also made up of
ferrite laths, but the carbides form within the
laths.
It is the aim of this ongoing project to
develop a bainitic alloy that has better
mechanical and wear properties than the
current ferrite-pearlite alloys used to make
railway wagon wheels. The alloy must be costeffective and produced locally using existing
technology. The development of a new bainitic
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On the development of bainitic alloys for railway wheel applications
rail wheel steel needs to be considered in the context of the
existing bainitic alloys, and Table I lists the chemical
composition and mechanical properties of a number of
bainitic steels that have been developed for rail applications.
Both upper and lower bainite have potential for railway
applications; however, focus has been on the upper bainitic
microstructure, particularly on the development of carbidefree alloys. The presence of brittle carbides in the
microstructure of bainitic alloys has, to a large extent,
hindered their commercialization potential1,7. The addition of
silicon (~2 wt%) to the steel chemistry suppresses the
formation of carbides in upper bainite1,5. This results in a
microstructure that comprises bainitic ferrite, ductile highcarbon retained austenite (γ), and possibly some martensite
(α’). The carbide-free bainitic alloys can attain hardness
values of >400 BHN, ultimate tensile strength (UTS) ~1215
MPa, and yield strength ~860 MPa with elongations of up to
15 percent. All the bainitic alloys listed in Table I, with the
exception of Micralos, were found to have higher hardness
values and better RCF resistance than ferrite-pearlite alloys;
however, their wear resistance was poorer9–12. In general,
therefore, the currently developed bainitic alloys have better
RCF resistance but their wear resistance seems questionable.
alloy and bainitic V1 and V2 alloys, were melted in an
induction furnace open to the atmosphere, with a target
composition as listed in Table II.
The critical heat treatment temperatures for the alloys
were calculated using Equations [1–5] as listed below13.
Ac1 = 723 - 20.7Mn - 16.9Ni + 29.1Si + 16.9Cr +
290As + 6.38 W
[1]
Ac3 = 910 - 203√C - 15.2Ni + 44.7Si + 104V +
31.5Mo + 13.1W
[2]
Ms = 539 - 423C - 30.4Mn - 17.7Ni - 12.1Cr 11.0Si - 7.0Mo
[3]
Bs = 630 - 45Mn - 40V - 35Si - 30Cr - 25Mo 20Ni - 15W.
[4]
[5]
The critical heat treatment temperatures are the temperatures at which ferrite begins to transforms to austenite
(Ac1), the temperature at which the steel becomes fully
austenitic (Ac3), the martensite start temperature (Ms), and
the bainite start temperature (Bs). These temperatures were
used to choose the annealing, tempering, homogenizing, and
the salt bath temperatures.
After casting, the Class B alloy was solution annealed at
870°C for two hours and then quenched in a spray of water.
It was then allowed to cool in air before tempering at 580°C.
Experimental procedure
Production and heat treatment of experimental alloys
Three alloys, namely a conventional Class B ferrite-pearlite
Table I
Commercial bainitic rail alloys
Name
Chemical composition (wt %)
J6
LB
CF-B
Micralos
Low-medium carbon Si-Mn-Mo-V steel
C
Mn
Si
0.3
0.4
0.2
2
1
2
1.8
-
Name
1.4
Ni
Cr
0.5
0.2
1.9
0.4
0.1
Mechanical Properties
J6
LB
CF-B
Micralos
Low-Medium Carbon Si-Mn-Mo-V steel
Hardness
UTS (MPa)
Yield (MPa)
Elongation (%)
42-46HRC
>430BHN
>400BHN
386BHN
310BHN
1215
1090
1135
860
9
9
15
15
Table II
Target composition of experimental alloys
Sample
Class B
V1
V2
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Element (wt %)
C
Mn
Si
Al
Cr
B
0.57-0.67
0.3-0.4
0.3-0.4
0.60-0.90
1.5-2
1.5-2
0.15 min
2-3
0.5-1
0.08 max
0.02-0.03
0.02-0.03
0.3 max
0.5-0.6
0.5-0.6
0.0015-0.003
0.0015-0.002
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On the development of bainitic alloys for railway wheel applications
Table III
Actual chemical compositions of experimental alloys
Sample
Class B
V1
V2
Element (wt %)
C
Mn
Si
Al
Cr
B
0.59
0.41
0.40
0.72
1.99
1.79
0.4
2.8
0.8
0.07
0.02
0.02
0.3
0.5
0.5
0.003
0.002
Alloys V1 and V2, after casting, were homogenized for 2
hours at 1 000°C and air-cooled. These alloys were then
forged and hot-rolled at 1 000°C into plates with dimensions
of 300 mm × 70 mm × 15 mm. The plates were then austenitized (Ac3 +50°C) and immediately quenched in a salt bath at
400°C for 1 hour to allow for bainite transformation.
Characterization and mechanical testing of
experimental alloys
Cross-sections of the as-cast, homogenized, and heat-treated
alloys were cut and tested for composition using spark
emission spectroscopy. Additional cross-sections were
ground, polished to a 1 µm finish, and etched with 2% Nital
to reveal the microstructure. A Nikon optical microscope was
used to study the microstructures. The scanning electron
(SEM) microscope was also used to study the microstructure
in the backscatter (BSE) mode.
Brinell hardness measurements using a load of 750 kg
were performed on cross-sections of the as-cast,
homogenized, and heat treated ingots.
Tensile specimens with 6 mm diameter and 25 mm gauge
length were used for tensile tests. The tests were carried out
in accordance with the requirements of ASTM E8-99. In this
test, the specimens were subjected to a continually increasing
uniaxial tensile force using a tensile test machine while
measurements of the elongation of the specimen were
simultaneously recorded.
Charpy impact samples were machined from the heattreated ingots. These were tested at room temperature in
accordance with ASTM E23-72. The machined specimens had
dimensions of 10 mm × 10 mm × 55 mm and contained a 45°
V notch, 2 mm deep with a 0.25 mm root radius along the
base. The energy absorbed during impact was measured.
Results and discussion
Chemical composition
Table III lists the actual chemical compositions of the experimental alloys.
Alloys V1 and V2 had varying silicon (Si) contents to
determine its effects on the formation of bainite. Manganese
(Mn) and chromium (Cr) were added for hardenability. Boron
(B) was added to suppress the formation of allotriomorphic
ferrite and to increase the hardenability of bainite.
Metallography
The microstructures for the alloys produced in the as-cast,
homogenized, and heat-treated conditions are shown in
Figures 1 to 3. Figure 1 shows the microstructures for the
Class B alloy, and Figures 2 and 3 for V1 and V2 alloys,
respectively.
The microstructures for the Class B alloy in both the ascast and solution-treated condition were ferrite-pearlite (see
Figure 1). The solution annealing and tempering refined the
pearlitic microstructure.
The microstructure of V1 in the as-cast condition is
ferrite-pearlite (Figure 2a). After homogenization, the pearlite
is refined as shown in Figure 2b. The final microstructure
after heat treatment is that of upper bainite and untempered
martensite. The martensite is formed during water quenching
after the salt bath quench, which implies that the
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Figure 1—The microstructure of the Class B alloy showing (a) ferrite-pearlite in the as-cast and (b) the solution-treated and tempered condition
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On the development of bainitic alloys for railway wheel applications
Figure 2—The microstructure of alloy V1 showing (a) pearlite in the as-cast condition, (b) refined pearlite in the homogenized condition, and (c) upper
bainite and martensite in the heat-treated condition
Figure 3—The microstructure of alloy V2 showing (a) pearlite in the as-cast condition, (b) refined pearlite in the homogenized condition, and (c) upper
bainite in the heat-treated condition
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microstructure after the salt bath treatment was bainite and
retained austenite. Alloy V1 has a high Si content of ~3 wt%.
Silicon enriches the austenite (γ) with carbon and stabilizes it
to lower temperatures.
Similarly, the microstructures of alloy V2 in the as-cast
and homogenized conditions were pearlitic, with a finer
microstructure after homogenization. The amount of upper
bainite formed during the salt bath quench, however, was
greater than that formed in V1, due to the lower silicon content
of V2. It is ideal to increase the amount of bainite formed when
developing alloys for railway wheel applications. This is
because the transformation of retained austenite into
martensite during operation embrittles the railway wheel and
causes cracking. A silicon content of <3 wt% is thus ideal for
the production of the experimental bainitic alloys; in fact, 2
wt% is usually used to produce carbide-free bainite1,5,7,12.
higher than Class B. A higher hardness translates into a
higher wear resistance.
Tensile tests
The tensile tests results for the alloys as listed in Table V
showed that V1 was very brittle, with an elongation of only
0.8%. This is because of the martensite present in its
microstructure. Testing this alloy in the tempered condition
could improve the results. Alloy V2 had a tensile (1131 MPa)
and yield (700 MPa) strength higher than Class B, but with
good ductility of ~15%. Compared to alloy V1 it had better
ductility and higher strengths. The higher yield strengths of
alloys V1 and V2, compared to the Class B alloy, implies that
these alloys should have superior resistance to RCF.
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Scanning Electron Microscopy (SEM)
Figure 4 shows the SEM micrographs of the bainitic alloys.
SEM imaging was used to study the features of the
microstructure at higher magnifications than those possible
with optical microscopy. SEM imaging revealed the presence
of martensite in alloy V2, whereas this was not clear with
optical microscopy (Figure 4a). Both alloys V1 and V2 were
composed of bainite ferrite and martensite. There seems to be
no evidence of carbides in the microstructure. However, to
verify this result, transmission electron microscopy (TEM)
will need to be utilized as this has an even higher magnification capability than the SEM. The SEM was also unable to
resolve the individual bainite-ferrite laths. The maximum
resolution that could be achieved with the SEM without
affecting the clarity of the image was 0.5 µm. With TEM,
resolutions of 0.2 µm can be achieved.
Hardness
The hardness values of the alloys are listed in Table IV. Class
B had a typical hardness value of 255–269 BHN in the ascast and tempered condition. V1 has a high hardness of 490
BHN, probably due to the martensite in its microstructure. V2
in the heat-treated condition had a hardness of 363 BHN,
Table IV
Brinell hardens results for the experimental alloys
Condition
Brinell (BHN)
Class B
V1
V2
255
269
397
279
490
304
263
363
As-cast
Homogenized
Heat-treated
Table V
Tensile test results for the experimental alloys
Sample identity
Class B
V1
V2
UTS
(MPa)
Yield Stress
(MPa)
Elongation
(%)
730
925
1131
463
862
700
23.0
0.8
14.9
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Figure 4—SEM micrographs of (a) V1 and (b) V2 bainitic alloys showing martensite (M) and bainite (B)
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Table VI
Charpy V-notch results for the experimental alloys
Sample identity
Impact energy (J)
Class B
V1
V2
21.0
3.4
4.1
microstructure and avoid martensite and ferrite
formation.
Acknowledgement
We would like to thank Mintek and AMI-FMDN for their
financial support and permission to publish this work. We
also thank the University of Pretoria and Professor Waldo
Stumpf for his technical support.
Charpy V-notch tests
References
The conventional Class B alloys are expected to have a
minimum impact energy of 16 J. From the Charpy impact test
results Class B was found to have an impact energy of 21 J,
which is within specification. High-silicon bainitic steels with
retained austenite have been found to have an impact energy
of ~25 J. However, the presence of martensite in the experimental is detrimental to their impact toughness. Alloys V1
and V2 had very low impact toughness values of 3–4 J.
Although alloy V2 had good ductility results from the tensile
test, this did not translate to good impact toughness.
1. BHADESHIA, H.K.D.H. Improvements in and relating to carbide-free bainitic
steels and methods of producing such steels. International Publication
Number: WO 96/22396, 11 January 1996. International Patent
Classification: C21D 9/04, 1/20, 1/02, C22C 38/00.
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rails with high wear and damage resistance for heavy haul railways.
Nippon Steel Technical Report No. 85, January 2002. pp. 167–172.
3. LEE, K.M. and POLYCARPOU, A.A. Wear of Conventional pearlitic and
improved bainitic rail steels. Wear, vol. 259, 2005. pp. 391–399.
Conclusions
According to previous studies5, the mixed microstructures
consisting of bainite and martensite have been found to be a
consequence of inadequate heat-treatment or the use of steels
with inadequate hardenability. Early research has indicated
that bainite-martensite microstructures have poor ductility,
toughness, and strength. As the amount of bainite increases
the mechanical properties should improve. It is therefore vital
to determine the correct heat treatment for the steel alloys to
avoid martensite formation and maximize bainite transformation.
From the preliminary results, the following conclusions
can be drawn:
® A high silicon content of ~3 wt% stabilizes the
austenite during transformation and decreases the
amount of bainite transformation significantly
® Bainite has higher tensile and yield strengths than
pearlite, but with good ductility
® The toughness of the experimental bainitic alloys was
very low compared to that of pearlitic alloy. This is
believed to be due to the presence of martensite in the
microstructure. Tests with retained austenite or
tempered martensite should show better toughness
results.
wheels manufactured with low-medium carbon Si-Mn-Mo-V steel. Journal
of University of Science and Technology Beijing, vol. 15, no. 2, April
2008. pp. 125.
5. BHADESHIA, H.K.D.H. Bainite in Steels, 2nd edn. London, IOM
Communications Ltd. 2001. pp. 347.
6. YATES, J.K. Innovation in Rail Steels. www.msm.cam.ac.uk/phasetrans/parliament,html. Accessed May 2010.
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Science: Science and Technology. Pergamon Press, Elsevier Science, 2002.
pp. 1–7.
8. LEE, K.M. and POLYCARPOU, A.A. Wear of Conventional pearlitic and
improved bainitic rail steels. Wear, vol. 259, 2005. pp. 391–399.
9. CLARKE, M. Wheel rolling contact fatigue (RCF) and rim defects investigation. Wheel Steels Handbook. Research Programme Engineering Rail
Safety and Standards Board. pp.1–20.
10. STOCK, R. and PIPPAN, R. RCF and wear in theory and practice—the
influence of rail grade on wear and RCF. Wear, vol. 271, no. 1–2, 18 May
2011. pp. 125–133.
11. GIANNI, A., GHIDINI, A., KARLSSON, T., and EKBERG, A. Bainitic steel grade for
Future work
In the near future the following aspects need to be addressed:
® TEM analysis of the bainitic alloys to determine the
presence/absence of carbides in the microstructure
® Wear testing of the produced alloys will benchmark the
bainitic alloys V1 and V2 against the conventional
ferrite-pearlite Class B rail grade. Wear is vital in
determining the life of railway wheels
® Dilatometry testing to determine the correct heat
treatment for the alloys so that continuous cooling and
transformation (CCT) diagrams can be determined. CCT
diagrams will allow for the selection of correct cooling
temperatures and rates to achieve a bainitic
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4. ZHANG, M. and GU, H. Microstructure and mechanical properties of railway
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solid wheels: metallurgical, mechanical, and in-service testing.
Proceedings of the Institution of Mechanical Engineers, Part F: Journal of
Rail and Rapid Transit March 1, 2009. p. 223. Special Issue Paper, vol.
163. pp. 163–171.
12. ZHANG, M. and GU, H. Microstructure and mechanical properties of railway
wheels manufactured with low-medium carbon Si-Mn-Mo-V steel. Journal
of University of Science and Technology Beijing, vol. 15, no. 2, April
2008. pp. 125.
13. STUMPF, W. University of Pretoria, heat treatment of steels. Part of the
postgraduate course NHB 700, University of Pretoria, August-September
2010.
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