M. HNIZDIL, P. KOTRBACEK: HEAT TREATMENT OF RAILS
329–332
HEAT TREATMENT OF RAILS
TOPLOTNA OBDELAVA TIRNIC
Milan Hnizdil, Petr Kotrbacek
Brno University of Technology, Faculty of Mechanical Engineering, Heat transfer and fluid flow laboratory,
Technicka 2896/2, 616 69, Brno, Czech Republic
hnizdil@fme.vutbr.cz
Prejem rokopisa – received: 2015-12-23; sprejem za objavo – accepted for publication: 2016-03-31
doi:10.17222/mit.2015.357
Heat treatment is increasingly used in the heavy industry. The main advantage of this method is the achievement of the required
material and mechanical properties. Heat treatment allows for a manufacturing process, which can improve product performance
by increasing the steel strength, hardness and other desirable characteristics. The microstructure, grain size and chemical
composition of steel affect its overall mechanical behavior. Heat treatment is an efficient way to manipulate the properties of a
steel product by controlling the cooling rate. It can be expressed using the heat-transfer coefficient (HTC). The controllability of
the cooling process is very important. Mist and water nozzles may provide good controllability of the HTC. An experimental
stand was designed and built. The stand consists of a movable trolley with a test sample, which moves under a spray at a given
velocity. Sensors record the temperature history of the tested material. This experimental stand enables simulations of a variety
of cooling regimes and evaluations of the final structures of tested samples. The same experimental stand is also used for
designing cooling sections in order to determine the required heat-treatment procedures and the final structures. This paper
describes a cooling-section design procedure for obtaining the required structure and mechanical properties of rails.
Keywords: heat transfer, heat treatment, cooling, heat-transfer coefficient, spray cooling
Uporaba toplotne obdelave se v te`ki industriji pove~uje. Glavna prednost te metode je, da se dose`e zahtevane mehanske
lastnosti materiala. Toplotna obdelava omogo~a postopke izdelave, ki lahko izbolj{ajo lastnosti proizvodov s tem, da pove~ajo
trdnost jekla, trdoto in druge za`eljene zna~ilnosti. Mikrostruktura, velikost zrn in kemijska sestava jekla vplivajo na mehanske
lastnosti. Toplotna obdelava je u~inkovita pot za vplivanje na lastnosti jeklenega proizvoda s kontroliranjem hitrosti ohlajanja.
Lahko se jo izrazi z uporabo koeficienta prenosa toplote. Mo`nost kontrole postopka ohlajanja je zelo pomembna. Obvladanje
procesa ohlajanja je zelo pomembno. Vodna para in vodne {obe omogo~ajo dobro kontrolo koeficienta prenosa toplote (angl.
HTC). Na~rtovano in postavljeno je bilo eksperimentalno stojalo. Stojalo sestoji iz vozi~ka z vzorcem, ki se pomika pod {obe z
dano hitrostjo. Senzorji bele`ijo temperaturno zgodovino vzorca. Eksperimentalno stojalo omogo~a simulacijo razli~nih
re`imov ohlajanja in oceno kon~ne mikrostrukture preizku{enega vzorca. Isto stojalo je uporabno tudi kot orodje pri na~rtovanju
hladilnih odsekov za dolo~anje postopka toplotne obdelave in kon~ne mikrostrukture. ^lanek opisuje postopek na~rtovanja
odseka za izvajanje hlajenja, za zagotavljanje `eljene mikrostrukture in mehanskih lastnosti `elezni{kih tirnic.
Klju~ne besede: prenos toplote, toplotna obdelava, ohlajanje, koeficient prenosa toplote, ohlajanje s pr{enjem
1 INTRODUCTION
Heat treatment of rolled materials by hot rolling
plants has become frequent. Alloying elements are typi-
cally used to improve material properties. Heat treatment
is a different approach applied to achieve the required
material properties using fewer alloys in the steel. Heat
treatment enables the manufacture of modern steels with
a higher ratio of yield strength and elongation. The con-
trollability of the cooling process is the most important
aspect for achieving the required mechanical properties.
An appropriate cooling intensity and its duration are
chosen with respect to the continuous cooling transfor-
mation diagram (CCT) for the selected material.
Numerical simulation of the cooling follows. One task is
to determine the boundary condition (HTC – heat trans-
fer coefficient) for the simulation because various para-
meters such as nozzle type, spray distance, water
impingement density, nozzle position, nozzle overlap,
movement velocity and scales have significant influences
on the cooling intensity.1–3 Additionally, accurate ther-
mo-physical material properties are needed for simu-
lations.4 The Heat Transfer and Fluid Flow Laboratory
developed a methodology for predicting the temperature
field of heat-treated rails. This methodology is described
in this article.
2 DESIGN STRATEGY FOR THE COOLING
SECTION
Three different types of experiments were done to
predict the required cooling regime defined by the CCT
diagram. A special hardening-capacity test bench (Fig-
ure 1) was developed to find the limits of a quenched
rail. The test bench consists of a heater, a trolley with a
tested sample, a water nozzle holder and a pneumatically
driven deflector.
Each test starts with heating a rail-head sample to the
initial temperature. This temperature is held for more
than 10 min to attain the austenite structure of the entire
body. The sample is protected with an inert atmosphere
in the furnace to prevent the development of scales.
Next, the sample is moved from the heater to a position
Materiali in tehnologije / Materials and technology 51 (2017) 2, 329–332 329
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.78:66.017:669.14.018.298 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 51(2)329(2017)
under the nozzle. The pneumatically driven deflector,
positioned between the nozzle and the sample, is moved
and the water sprays the top of the rail head. Each
sample is equipped with thermocouples under the surface
to detect the temperature gradient in the material. The
material hardness and its structure are observed. The
hardness values of the original (base) material and the
heat-treated material are compared because the heat-
treatment process is dependent on the quality of the
steel-making process (chemical composition, enclosures,
casting speed, etc.).5–8
If these tests are sufficient, heat-transfer tests are then
performed. A special testing bench called a linear stand
(Figure 2) was developed by the Heat Transfer and Fluid
Flow Laboratory. This bench is a six-meter long girder
with a trolley which can move the tested rail sample,
plate, etc., through the cooling section. A 25 mm thick,
flat austenitic steel plate is used for the heat-transfer
tests. It is embedded with four thermocouples positioned
0.5 mm under the sprayed surface. This plate is moved
through the spray-cooling system (2 m long) in two
directions, forward and backward.
The dependences of the heat-transfer coefficient on
the surface temperature are evaluated for various cooling
parameters (spray distance, type of nozzle, water im-
pingement density, etc.). The obtained boundary condi-
tions are used for simulations to predict the temperature
field in the rail head. The shape of the rail also has a
significant influence on the cooling intensity. Therefore,
an authentically shaped austenitic steel sample is made
and embedded with several thermocouples, positioned 2
mm under the rail surface. The length of this sample is
around 300 mm. Simulations of the rail cooling are
compared with the temperatures measured during the
austenitic rail cooling. The boundary conditions obtained
from the flat austenitic steel plate are adjusted and the
model is verified with measurements. The last step is the
verification including a full-scale, carbon-rail sample. A
sample is fixed on the trolley (the linear stand) and
moved through the cooling section. The measured and
simulated temperatures are compared and the cooling
model is verified. Finally, the hardness is measured
again. This is the most important result that shows if the
cooling regime works optimally.
3 RESULTS
The cooling strategy was described in Section 2. The
experimental steps began with the study of CCT
diagrams. Material R260 was chosen for the heat-treat-
ment tests. The hardness-capacity tests were performed
first to find the limit of the quenched material and to
choose the appropriate nozzle size with respect to the
required cooling regime. All the tested samples were
embedded with thermocouples to verify the cooling
regime. These samples were sawed after quenching and
hardness was measured along the center line of the rail
head (from the top of the surface down to the center – the
red line). An example of the cooling regime (for a
successful test) is shown in Figure 3.
The measured hardness for this sample was around
400 HV0.3 (Figure 4). The required fine pearlite struc-
ture was found using a microstructure analysis (Figure 5).
M. HNIZDIL, P. KOTRBACEK: HEAT TREATMENT OF RAILS
330 Materiali in tehnologije / Materials and technology 51 (2017) 2, 329–332
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Hardening-capacity test bench
Slika 1: Preizku{anje zmogljivosti utrjevanja
Figure 3: Temperature record for the rail head during a successful
static-hardness-capacity test
Slika 3: Zapis temperature glave tirnice med uspe{nim preizkusom
zmogljivosti trdote
Figure 2: Linear-stand scheme
Slika 2: Shema linearnega stojala
An appropriate choice of the nozzle and the verifi-
cation of the cooling regime using the CCT diagram
were confirmed with a static-hardening-capacity test.
The next step was to find the cooling parameters for the
moving samples (transient boundary conditions). The
linear stand was used for these tests. A 25 mm thick, flat
austenitic steel plate was used and several parameters
such as water pressure, spray distance, spray angle and
movement velocity were tested. The boundary conditions
obtained from the experiment were used to simulate the
cooling regime for a real moving rail. A full-scale rail
sample (material R260) was built and embedded with six
thermocouples positioned 2 mm under the surface (Fig-
ure 6).
M. HNIZDIL, P. KOTRBACEK: HEAT TREATMENT OF RAILS
Materiali in tehnologije / Materials and technology 51 (2017) 2, 329–332 331
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Measured hardness of the rail head after the static-hard-
ness-capacity test
Slika 4: Izmerjena trdota v glavi tirnice med stati~nim preizkusom
trdote
Figure 7: Temperature record for the heat-treated full-scale sample
Slika 7: Temperatura zabele`ena na toplotno obdelanem realnem
vzorcu
Figure 5: Microstructure – close to the sprayed surface – center of the
rail-head surface
Slika 5: Mikrostruktura – blizu po{kropljene povr{ine – sredina po-
vr{ine glave tirnice
Figure 8: Measured hardness at the center line of the rail head
Slika 8: Trdota izmerjena na sredini glave tirnice
Figure 6: Full-scale carbon sample heated to the initial temperature
before entering the cooling section
Slika 6: Realen vzorec ogret do za~etne temperature pred vstopom v
podro~je ohlajanja
The first full-scale test showed that the rail shape has
a significant influence on the cooling intensity, so
additional experiments with a full-scale austenitic sam-
ple and temperature-field prediction-model tuning were
necessary to obtain the required material structure.
Finally, a full-scale, carbon-rail sample was made and it
too was embedded with thermocouples. Simulated tem-
peratures were compared to measured temperatures and
the model was verified. An example of the cooling re-
gime of a successful full-scale test is shown in Figure 7.
The measured hardness of this sample was around
400 HV0.3 (Figure 8). This corresponded to the results
from the static-hardening-capacity test.
4 CONCLUSION
The goal of this article was to illustrate a verified
methodology for rail heat treatment. The first step was to
compute a CCT diagram and determine the settings for
the optimum cooling regime to achieve the required
material structure. The accuracy of the CCT diagram was
verified with a hardening-capacity test (Jominy test). The
next step was to measure the dependence of the heat-
transfer coefficient on the surface temperature using a
flat austenitic steel plate with thermal sensors. Various
cooling parameters were tested: water pressure, spray
distance, spray angle, movement velocity and others.
These boundary conditions were used to predict the tem-
perature-field evolution in the rail. It was found that the
rail shape has a significant influence on the cooling in-
tensity. A full-scale austenitic steel rail sample (300 mm
long) was made and the cooling model was tuned using
simulation data and the temperatures measured during
the experiments with a full-scale sample. The final
design of the cooling section was made and the cooling
model was verified by measuring an authentic full-scale
carbon sample in the laboratory. The final hardness of
the heat-treated rail sample was measured and compared
with the data obtained during the hardening-capacity
test. Both of these results were around 400 HV0.3. A
fine perlite structure was found along the center line of
the heat-treated rail-head sample.
Acknowledgement
The research leading to these results received funding
from the MEYS under the National Sustainability
Programme I (Project LO1202).
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M. HNIZDIL, P. KOTRBACEK: HEAT TREATMENT OF RAILS
332 Materiali in tehnologije / Materials and technology 51 (2017) 2, 329–332
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS