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Lecture 4
Pearlitic Steels
Dr. Javad Mola
Institute of Iron and Steel Technology (IEST)
Tel: 03731 39 2407
E-mail: [email protected]
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Applications of Pearlitic Steels
Pearlite is the base
microstructure for rail and the
starting microstructure for
high-strength wire applications.
The semi-finished products
used to make rails and wires
are blooms and billets
respectively.
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Wheels on Rails
Head
Web
FootHead
Wheel
Rolling/sliding wear
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Wheels on Rails
Wheels are made conical so that if the wheelset is displaced laterally, a centering
force is exerted upon it. Rails are put into track at a slight angle to keep the contact
point of the conical wheel in the center of the rail head. Therefore, in a straight track,
the wheel flange should not touch the rail at all. This design reduces the risk of
derailment in tight curves where lateral displacement could be much greater.
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Rail Damage
Wear and damage to curved rails
under heavy traffic conditions
Typical outer (high
side) rail wearTypical inner (low side)
rail wear
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Rail Wear
Rails are subject to heavy contact cyclic loading and require
a high wear resistance.
Stages of rail wear:
Severe plastic deformation in a thin surface layer of the
rail, of the order of 0.1 mm in depth, which becomes
shallower as the hardness increases.
Development of subsurface cracks in the severely
deformed layer, generally at the interface of the deformed
layer and the undeformed microstructure
Propagation of cracks to the surface of the rail and the
associated spalling off of small slivers or flakes of the rail
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Hot Rolling of Rails
Rails are produced by hot rolling of BOF
steels with low contents of Cu and Sn
because these elements increase the
likelihood of hot shortness. H can also cause
embrittlement. An early type of rail failure was
associated with entrapped hydrogen that
produced shatter crack or flakes in heavy rail
sections, but that difficulty has been
effectively overcome by controlled cooling and
by vacuum degassing of liquid steel.
Continuously-cast blooms with a rectangular
section are hot rolled in reversing or universal
mills. The rail cooling is done in a water
cooling section followed by a slow cooling
bed. The cooling rate after hot rolling is a
critical parameter controlling the
microstructure and therefore the hardness of
rails. Due to their asymmetrical shape, rails
tend to bend during cooling. This can be partly
compensated by applying an initial pre-
bending prior to cooling. The final rail
strengthening is carried out in a post-cooling
rail straightening line.
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Microstructure Control
Steel No. %C %Si %Mn %P %S %Cr %N %Al tot.%Al as solute
1 0.71 0.32 1.00 0.016 0.019 0.06 0.006 0.009 -
2 0.64 0.35 1.50 0.020 0.020 0.02 0.006 0.003 -
3 0.71 0.47 0.98 0.018 0.022 1.00 0.003 0.003 0.003
Time, sec
Steel
Tem
per
atu
re, °
C
Austenitization temperature: 1000 °CSoaking time: 15 min
A pearlitic microstructure is
obtained by controlled cooling. To
obtain pearlite with a fine
interlamellar spacing, the
transformation to pearlite must take
place at a temperature close to the
pearlite nose at about 550 °C.
Elements such as Mn which
depress the transformation
temperature lead to finer pearlite
during continuous cooling.
Werkstoffkunde STAHL - Band 2: Anwendung | Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.
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Chemical Composition and Strength
Typical chemical compositions and strengths for rail steels
UIC = Union Internationale des Chemins de Fer (international union of railways)
Grade
R200
Grade
R220
R260
R260Mn
R320Cr
R350HT
R350HTL
R0700
R0900A
___
R0900B
R1100
___
___
B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.
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Hardness vs. Wear Rate
Improved rail wear resistance correlates with
increased hardness
Contact pressure:
Higher hardness
Lower wear rate
1220 MPa
700 MPa
Hardness, BHN
Wea
r R
ate
, µg
per
met
er r
olle
d x
10
-3
P. Clayton, D. Danks, Effect of interlamellar spacing on the wear resistance of eutectoid steels under rolling-sliding conditions, Wear. 135 (1990) 369–389.
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Wear Rate
Improved rail wear resistance correlates with fine interlamellar ferrite/cementite spacing of pearlitic microstructures which increases hardness and strength.
(Contact pressure)
1220 MPa900 MPa
Wea
r R
ate
, µg
per
met
er r
olle
d x
10
-3
Pearlite Interlamellar Spacing, nmP. Clayton, D. Danks, Effect of interlamellar spacing on the wear resistance of eutectoid steels under rolling-sliding conditions, Wear. 135 (1990) 369–389.
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Strength
Yield stress and hardness of pearlite increase as the pearlite interlamellar spacing decreases
Pearlite Interlamellar Spacing-0.5, Å-0.5
Pearlite Interlamellar Spacing, Å
YS (
ksi)
Ha
rdn
ess
(HR
C)
YS (
MPa
)
grain size
FineMediumCoarse
J.M. Hyzak, I.M. Bernstein, The role of microstructure on the strength and toughness of fully pearlitic steels, Metall. Trans. A. 7 (1976) 1217–1224.
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Fatigue Strength
Fati
gu
e St
ren
gth
, MPa
Tensile Strength, MPa
Tensile
specimens
Bending fatigue
specimens
The fatigue strength improves as the tensile strength increases.
600 700 800 900 1000 1100 1200± 100
± 200
± 300
± 400
± 500
± 600
Werkstoffkunde STAHL - Band 2: Anwendung | Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.
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Control of Pearlite Interlamellar SpacingLo
g
in Å
2.8
4.0
3.8
3.6
3.4
3.2
3.0
TE –T , °C
20 40 60 80 100 120 140
=1 m
=0.1 m
Average true interlamellar spacing of pearlite as a function of undercooling below Ae1 for some carbon and low-alloy steels
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Rail Toughness vs. Pearlite Interlamellar Spacing
Another beneficial effect of pearlite lamellae refinement is the reduced DBTT (ductile-brittle transition temperature).
Cementite lamella thickness, nm
Tra
nsi
tio
n t
emp
era
ture
, °C
FerriteCementite
Werkstoffkunde STAHL - Band 2: Anwendung | Springer, Verein Deutscher Eisenhüttenleute (Hrsg.), Düsseldorf, 1985.
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Tra
nsi
tio
n t
emp
era
ture
, K
Rail Toughness vs. Austenite Grain Size
The finer the grain size of prior austenite, the higher the toughness. Pearlite colony size has a less important influence on toughness. In fact, it is the size of the microstructural unit of pearlite with the same crystal orientation of ferrite (may consist of several colonies) which controls the toughness. The size of this unit is controlled by, but not equal to, the prior austenite grain size.
Test temperature, °F
Test temperature, K
Imp
act
en
erg
y, 1
03
J/m
2
Austenite grain size (d), 10 µm
Austenite grain size (d-0.5), cm-0.5
J.M. Hyzak, I.M. Bernstein, The role of microstructure on the strength and toughness of fully pearlitic steels, Metall. Trans. A. 7 (1976) 1217–1224.
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Head Hardening
The strong correlation of improved rail wear resistance with fine pearlite interlamellar spacing and high pearlite hardness has led to processing and alloying approaches to produce fine pearlite. An effective processing approach has been to produce pearlite of fine interlamellar spacing and high hardness on the surface of rails by head hardening heat treatments, applied by accelerated cooling with forced air, water sprays, or oil or aqueous polymer quenching either online while the steel is still austenitic immediately after hot rolling or by offline reheating of as-rolled rails.
Brinell hardness numbers in the transverse section of a rail subjected to offline head hardening heat treatment
G. Krauss, Steels: Processing, Structure, and Performance, Second Edition - ASM International, ASM International, Materials Park, Ohio, 2005
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Corrosion of Rails
In a non-polluted environment with no salt in the air, the corrosion rate will be approximately 0.05 mm/year. It will therefore take 20 years for 1 mm of steel to be compromised by rust from each side, or 2 mm from both sides. Steel rail is about 15 mm wide at its narrowest point so after 10 years, only about 7% of its section will be compromised.
Head
Web
Foot
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Jointed Rails
Rails are produced in fixed lengths and need to be joined end-to-end to make a continuous surface on which trains may run.The traditional method of joining the rails is to bolt them together using metal fishplates, producing jointed track. Jointed track does not have the ride quality of welded rail and is less desirable for high-speed trains. Furthermore, areas around the bolt holes are susceptible to cracking, which can lead to breaking of the rail head (the running surface).
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Continuous Welded Rails
Most modern railways use continuous welded rail (CWR), sometimes referred to as ribbon rails. In this form of track, the rails are welded together by flash butt welding to form one continuous rail that may be several kilometers long, or thermite welding to repair or join together existing CWR segments. Because there are few joints, this form of track is very strong, gives a smooth ride, and needs less maintenance; trains can travel on it at higher speeds and with less friction. Welded rails are more expensive to lay than jointed tracks, but have much lower maintenance costs.
Flash butt welding is the preferred process which involves an automated track-laying machine running a strong electrical current through the touching ends of the rail ends. The ends become white hot due to electrical resistance and are then pressed together forming a strong weld. Thermite welding is a manual process requiring a reaction crucible to contain the molten iron. Thermite-bonded joints are seen as less reliable and more prone to fracture or break.
Flash Butt Welding
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Sun Kinks (Buckling) and Pull-Aparts
If not restrained, rails would lengthen in hot weather and shrink in cold weather. To provide this restraint and to prevent the build-up of stresses due to dimensional changes in a long section of the railway, the rail is prevented from moving in relation to the sleeper by the use of clips which resist the longitudinal movement of the rail. There is no theoretical limit to how long a welded rail can be. However, if longitudinal and lateral restraint are insufficient, the track could become distorted in hot weather and cause a derailment. Distortion due to heat expansion is known as sun kink or buckling. A rail broken due to cold-related contraction is known as a pull-apart.
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Bainitic Rail Steels?
Chemical compositions (mass-%) of typical bainitic and pearlitic rail steels
H.K.D.H. Bhadeshia, Bainite in Steels : Theory and Practice, 3rd Edition, Maney Publishing, Leeds, 2015.
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Bainitic Rail Steels?
Early researches had indicated poorer performance of bainitic steels compared to the higher C pearlitic steels of the same hardness level. More recent results, however, suggest that low C bainitic steels outperform pearlitic steels in terms of the wear resistance. Confirmation of this claim by more sophisticated wear tests designed to truly simulate the service conditions of rails can lead to the increased use of low C bainitic steels for rails. The lower C content of bainitic steels can be beneficial to toughness, ductility, and weldability.
Pin
-rin
g w
ea
r ra
te
H.K.D.H. Bhadeshia, Bainite in Steels : Theory and Practice, 3rd Edition, Maney Publishing, Leeds, 2015.
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Pearlitic Wires
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Pearlitic Wires
B. Verlinden, J. Driver, I. Samajdar, R. D. Doherty, Thermo-Mechanical Processing of Metallic Materials, Elsevier, 2007.
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Wire Drawn Iron
Wire drawing of Fe to very large strains
D. Kuhlmann-Wilsdorf, N. Hansen, Theory of work-hardening applied to stages III and IV, Metall. Trans. A. 20 (1989) 2393–2397.
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IV : Unlimited cross slip
Stage IV of Work Hardening during Wire Drawing
Stage IV: Linear hardening similar to stage II but with a smaller slope. This stage is only observed under special straining conditions such as wire drawing, torsion, and rolling.
II : Cross slip difficult
D. Kuhlmann-Wilsdorf, N. Hansen, Theory of work-hardening applied to stages III and IV, Metall. Trans. A. 20 (1989) 2393–2397.
D. Kuhlmann-Wilsdorf, Questions you always wanted (or should have wanted) to ask about workhardening, Mater. Res. Innov. 1 (1998) 265–297
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Wire Drawn Iron
Late stage III Stage IV
Stage IVStage IV
(true strain)=0.22 =0.89
=2.01 =6.02
D. Kuhlmann-Wilsdorf, Questions you always wanted (or should have wanted) to ask about workhardening, Mater. Res. Innov. 1 (1998) 265–297
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Strengthening of Pearlite by Wire Drawing
B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.
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Strengthening of Fe-C Steels by Wire Drawing
UTS vs. C content of steel wire after 95-99% cold reduction.
Tire cord steels typically have eutectoid C contents
B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, Association for Iron and Steel Technology, Warrendale, 2011.
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Patenting Heat Treatment
Patenting consists of heating to austenite and continuous cooling or isothermal holding to produce a uniform fine pearlite microstructure. Bainitic microstructures were found to be sensitive to delamination after drawing and, therefore, fine pearlite with a tensile strength of 1500 MPa (220 ksi) was found to be the most suitable starting microstructure for wire drawing.
800
600
400
200
700
500
300Te
mp
era
ture
, °C
10-1 10 102 103 104 105
Time, sec
1
Fine pearlite
Coarse pearlite
Fine pearlite
and upper
bainite
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Microstructural Changes in Wire Drawn Pearlite
Atom probe tomography (APT) of a Fe-0.81C-0.49Mn-0.20Si (mass-%) steel. For clarity, only 2% of Fe atoms and 20% of carbon atoms are shown.
Y.J. Li et al., Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite, Acta Mater. 59 (2011) 3965–3977.
True strain by
wire drawing
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Microstructural Changes in Wire Drawn Pearlite
Atom probe tomography (APT) of a Fe-0.81C-0.49Mn-0.20Si (mass-%) steel.
Severe cold straining leads to the dissolution of cementite and the carbon enrichment of ferrite.
Y.J. Li et al., Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite, Acta Mater. 59 (2011) 3965–3977.
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