Note: Descriptions are shown in the official language in which they were submitted.
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ELEKTRO-THERMIT GmbH
Method for heat treatment of a rail joint which is
produced by fusion welding, and a burner arrangement
which is intended for use in such a method
The invention relates to a method according to the pre-
characterizing clause of claim 1.
The invention furthermore relates to a burner
arrangement which is intended for use in the course of
such a method.
The~use of thermit welding methods to produce a welded
joint between two rail ends leads, as is known, to a
cast steel structure in the joint region, which
structure is composed of thermit-produced steel and
rail steel dissolved therein, there being heated zones
on both sides of this fused structure having a
characteristic structural formation, which formations
in each case merge continuously into the rail regions
which have not been influenced by the welding process.
The structural formation takes place mainly as a
function of the time profile of the temperature field
which is formed in the rail body as a consequence of
the welding process, in particular the locally
different heating rates, temperatures and cooling
rates. Thus, for example, a structure is located in the
immediate vicinity of that region of the heat affected
zones which are melted as a consequence of the welding
process, which structure is coarse-grained, hard and
pearlitic as a result of the severe overheating, this
region being adjoined - looking in the direction away
from the welded joint within the heated zone - by
regions of decreasing hardness, to be precise as far as
- those regions in which minimal hardness formation
arises as a result of the perlite spheroidizing and the
rail regions which are not influenced by the welding
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process only following on after these virtually soft-
annealed regions. The locally reduced hardness and thus
wear resistance of the rail joint makes it necessary to
carry out heat treatment after the welding process,
depending on the extent of the hardness differences and
the respectively used rail steel.
For example, self-hardening steels having a minimum
strength of 900N/mm2 and a hardness of 260 HV to 280 HV
are used as the rail steel, their alloying components
being composed mainly of carbon and manganese. Special
quality classes of these self-hardening steels, formed
by the addition of other alloying components such as
chromium and vanadium, are also used, these steels
having a minimum strength of llOON/mm2 and a hardness of
310 HV to 330 HV. Furthermore, self-hardening steels
whose strength and hardness have been improved further
by special heat treatment methods are also used. These
comprise finely pearlitized steels whose hardness has
been increased either only in the regions of the rail
head which are severely stressed in track, or entirely,
up to values of 350 HV to 400 HV. The structural change
mentioned initially as a consequence of the welding
process now has a different effect, to be precise
depending on the specific rail steel used.
DE 43 19 417 C1 discloses a method for the fusion
welding of head-hardened fine-pearlitic rails, in
which, once steel produced by thermit welding has been
included in the casting mold which surrounds the end
faces of the rail ends to be joined, to be precise
after the welded material has cooled down, the running
surface of the weld and the adjacent heat affected
zones are heated from above by means of a burner system
for a short time period of 50 s to 150 s, the burning
area of the burner system used being positioned at a
distance from the running surface of the weld, which
distance is a function of the weight per meter of the
rail. The aim is to achieve fine pearlitization of the
-
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welding material and of the adjacent heat affected
zones, and to make the hardness profile more uniform
between the welding material and those regions of the
rails which have not been influenced by -the welding
process. As a result of the fact that the heat required
for heat treatment is transferred extremely quickly,
inter alia during a predetermined time interval, via
the running surface of the rail to be treated, a
defined temperature gradient is achieved in the
vertical direction, so that the depth of that region
where the steel is austenitic and which extends from
the top surface of the rail is defined by the
parameters which characterize the heat transfer.
Dispensing with the use of an external cooling means,
cooling takes place directIy through the cold regions
of the rail body itself. A fine-pearlitic structure is
formed and, in particular, the hardness profile and
thus the wear resistance are made uniform, in the
surface region of the rail head - extending beyond the
welded joint in the longitudinal direction of the
rails.
Finally, the documents US 5 306 361 and 5 377 959
~ disclose a method and an apparatus for heat treatment
of such welded joints after the welding has been
carried out, thermit being inserted in a mold which
completely surrounds the welded joint, and the amount
of heat which is to be introduced into the rail profile
being controlled such that the steel is made
austenitic, quenching subsequently being carried out
using compressed air. This method thus leads to
complete normalization of the rail profile at least in
the regions close to the surface.
The aim of the above heat treatment methods is to make
the material characteristics uniform beyond the welding
joint, in particular to match the material
characteristics to those of the rail material outside
the welded joint. This refers in particular to
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characteristics such as tensile strength and hardness.
The essential feature of the known heat treatment
methods is that the welded joint is first of all cooled
virtually down to ambient temperature, the structural
regions to be treated subsequently being heated and
cooled in accordance with a temperature/time function.
This means that the heat which is still contained in
the welding material and the heat affected zones as a
consequence of the welding process remains essentially
unused. In addition to an increased time requirement as
a result of the cooling process, this method of
operation is also characterized by increased fuel
consumption because of the renewed heating to above the
temperature at which the steel becomes austenitic.
The stresses on the rail joint in the region of the
rail foot, which result from the track use, comprise a
bending oscillating stress with correspondingly
alternating tensile stresses.
The object of the invention is to design a method of
the type mentioned initially which leads to a welded
joint which is appropriate for the stresses and is
characterized, in comparison with the described prior
art, by a shorter time requirement and improved heat
efficiency. This object is achieved in the case of such
a method by the features of the characterizing part of
claim 1.
According to this, the essential feature of the
invention is that the heat treatment starts immediately
after the thermit welding process, that is to say after
a mold head has been removed, the risers have been cut
off and, if necessary, after carrying out the grinding
work, that is to say at a time at which the welded
joint region still contains a considerable amount of
residual heat, but is at a temperature that is less
than the temperature at which the steel becomes
austenitic. The introduction of heat in order to make
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the steel austenitic again, specifically with the aim
of achieving a fine-grained austenitic structure, can
in this way be limited quantitatively to the use of the
said residual heat. The reduced fuel consumption in the
course of this heat treatment and the improved
utilization of the heat which has to be introduced into
the joint region in order to carry out the welding
process lead to considerably improved heat efficiency
of the overall welding method, to reduced fuel
consumption and, accordingly, to reduced costs. The
heat treatment, namely normalization, is carried out at
least in the region of the rail foot in order to
eliminate coarse-grained elements which are present
here both in the region of the fused structure and in
the heat affected zones which are adjacent on both
sides, and to replace them by a fine-grained normalized
structure which can withstand the stresses of the track
use, particularly with respect to its ductility
characteristics.
This heat treatment of the joint region is followed by
cooling to ambient temperature, final grinding and
removal of the foot risers and the remains of the mold.
The unfavorable residual stress distribution which is
produced as a result of the welding process can
furthermore be at least partially overcome by a heat
treatment method according to the invention.
The features of claim 2 are directed at extending the
region to be normalized, to be precise to the region of
the rail foot and of the rail head. The heat treatment
of the flanks of the rail head in practice results in
normalization of the running surface, however, because
of the fast propagation of the temperature field which
is produced.
The features of claims 3 and 4 are directed at
different forms of cooling the heated austenitic joint
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region. In principle, the cooling can be carried out in
stationary ambient air - but a quenching treatment, for
example using compressed air, is also feasible
depending on the achievable cooling rates and the
desired surface hardnesses.
The features of claims 5 and 6 are directed at further
detailing a particularly advantageous variant of the
heat treatment method. The aim of this type of method
is also to control locally, in terms of its variation
over time, the introduction of heat into the fused
region, including the adjacent heat affected zones, as
a function of the geometry of the rail profile, of the
extent of the heat affected zones including the fused
region, and of the temperature field which occurs
within the rail body as a result of the heat conduction
and heat propagation conditions in such a way as to
produce structural conversions which give the rail
joint region the macroscopically desired
characteristics, the main aspects of which are adequate
ductility and, in the region of the rail head, in
particular the running surface, sufficient hardness and
resistance to fracture propagation. The three-
dimensional distribution of the introduction of heat in
the region of the rail joint, the amount of heat
introduced locally and the time for which the heat is
introduced are available as parameters for controlling
the temperature field, the last-mentioned parameter
being used in particular to control the depth to which
it is intended to bring about a structural conversion.
The cooling can take place via the remaining "cold"
rail body, stationary ambient air and, if required, via
a quenching treatment which acts at least locally. The
aim of the treatment in each case is to replace a
coarse-grained structure produced by the welding
process, at least at the points envisaged for this
purpose, by a fine-grained normalized structure, whose
strength and ductility characteristics have in
consequence been improved. The basic method steps which
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are associated with this normalization, namely rapid
heating to a temperature slightly above the temperature
at which the steel becomes austenitic, and maintaining
this temperature until a fine-grained austenitic
structure is formed, with subsequent rapid cooling, are
known in principle and will not be described in more
detail here as well.
It is furthermore the object of the invention to design
an apparatus which is suitable for use for the purposes
of the method according to the invention and which can
be handled easily, particularly at the installation
site. This object is achieved in the case of such an
apparatus by the features of the characterizing part of
claim 7.
The essential feature of the invention according to
this is that the burners are distributed symmetrically
with respect to two mutually perpendicular planes and
that they are connected to a distributor tube which is
used to convey a combustible gas mixture. This
apparatus is used in such a manner that the apparatus
is positioned such that the transverse center plane
coincides with the corresponding center plane of the
fused region, which ensures the required symmetry
characteristics for introducing heat into the rail
joint region, because of the burner arrangement. The
apparatus is preferably intended to be placed onto
parts of the rail profile, since this ensures that
defined distances are reproducibly set between the
nozzle arrangements of the burners and the surface
sections of the rail profile which are to be heated.
The features of claims 8 to 11 are directed at the
three-dimensional distribution of the burners, the
fused region being heated by means of two burners which
act on the head flanks of the rail head, and the
heating of the rail foot region being brought about by
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means of burners which are respectively directed at the
heated zones on either side of the fused region.
The supporting elements according to the features of
claim 12 as well as the spacers according to the
features of claim 13, in conjunction with a rail
profile, result in heat transfer conditions which are
always reproducible in the region of the rail joint,
and thus make it easy to carry out on-site work.
A burner arrangement which is suitable for use for the
purposes of the mèthod according to the invention will
be described in more detail in the following text with
reference to the exemplary embodiment shown in the
drawing, in which:
Fig. 1 shows an end view of a burner arrangement
according to the invention placed onto a rail profile;
Fig. 2 shows a plan view of the burner arrangement
according to arrow II in Fig. 1;
Fig. 3 shows a side view of the burner arrangement
according to arrow III in Fig. 2.
1 designates the rail profile to be treated, in
particular the joint region between two rail ends,
whose respective rail foot 2 rests, in a manner not
illustrated in the drawing, on the track bed via ties.
This thus comprises a laid track, on which the burner
arrangement can be used.
The burner arrangement comprises a distributor tube 3,
which is designed to carry a combustible gas mixture,
composed, for example, of propane and oxygen, on one
end 4 of which distributor tube 3 a gas connection 5 is
fitted, and whose end 6 opposite the end 4 is of closed
design. The distributor tube 3 extends horizontally at
a distance above the rail head 7 and is supported with
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g
respect to the rail profile in a manner which will be
explained in the following text.
Two branching tubes, 8, 9 are connected to the
distributor tube 3 and each lead to burners 10, 11
whose outlet openings are assigned to the head flanks
12, 13 of the rail head 7 and are held at a defined
distance from said flanks. The branching tubes 8, 9 as
well as the burners 10, 11 extend essentially on a
transverse center plane 14, which at the same time
forms the plane of symmetry of the fused region 15,
adjoined on both sides by the heated zones of the weld
- not shown in the drawing. Further branching tubes 18,
19, at whose ends burners 20, 21 are in turn arranged,
extend in each case on both sides of the transverse
center plane 14, to be precise symmetrically with
respect to it, in planes 16, 17. The arrangement of the
branching tubes 8, 9 as well as 18, 19 alongside the
associated burners 20, 21 is of symmetrical design with
respect to a vertical longitudinal center plane 22 of
the rail profile 1.
The burners 20, 21 are furthermore arranged such that
their respective outlet directions point directly at
the heated zones on both sides of the fused region 15,
to be precise in a region adjacent to the rail foot 2.
The burners 20, 21 furthermore have two burner surfaces
which extend at an angle to one another, are in each
case provided with outlet nozzles, and whose
orientation is essentially matched to the orientation
of the opposing surfaces of the rail foot and of the
web region directly adjacent to the rail foot 2.
23 designates supporting elements which are in each
case fitted to the housings of the burners 21 and are
intended to stand on facing surfaces of the rail foot
2. Identical supporting elements 24 are arranged on the
opposite burners 21. The totality of these supporting
elements 23, 24 forms a support for the burner
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arrangement according to the invention, which support
is symmetrical with respect to the planes 14, 22.
Furthermore, 25, 26 designate web-like spaces which
extend originating from the sides facing the rail
profile 1 of all the branching tubes 18, 19 and are
intended to rest on the head flanks 12, 13. A
compensating mass 27 which is fitted to the end 6 of
the distributor tube 3 is used to further ensure the
stability of the burner arrangement.
The distribution and arrangement of the burners which
are used on different surface regions of the welded
joint are - as can be seen from the above statements -
symmetrical with respect to the two planes 22, 14. Thedistance between the surfaces of the burner nozzles and
the opposing surfaces of the rail profile, the amount
of the gas mixture which can be burnt to carry out the
heat treatment method, in this case the normalization,
and which is passed via the distributor tube 3, and
thus the heat to be transferred as well as the time
duration of the heat transfer are controlled as a
function of the residual heat in the rail profile as a
result of the preceding welding process.
