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* Corresponding author: kuzin.nick81@gmail.com
Improving the performance parameters of railway wheels with
the help of optimal design technologies of their electric pulse
processing
Igor Vakulenko1, Nickolay Kuzin2,3,*, Leonid Vakulenko4, Sergey Raksha1, and Svetlana Proidak1
1 DNURT, Department of Applied Mechanics and Materials Science,49010,Dnipro,Lazaryan St., 2, Ukraine
2 DNURT, Lviv Branch, 79052, Lviv, I.Blazhkevych Str., 12-a, Ukraine
3 Lviv Research Institute of Forensic Science, 79000 Lviv, Ukraine
4 Head Department of the Pridniprovk Railway, 49600, Dnipro, D.Yavornizkogo Av., 108, Ukraine
Abstract. The processing by pulses of electric current (PEC) of a fragment of the rim of a railway wheel
was carried out on the DS10D equipment. When the density of the electric current is from 3 to 17 A/mm2,
the processing cycle consisted of the action of a pulse with a duration of 2.5-3 s and a pause between
pulses of 4 s. After 150 cycles of PEC, the hardness of cold-formed metal was reduced from 10 to 20%.
Studies of the parameters of the metal structure of the wheels have determined that the processing of PEC
leads to a decrease in the number of defects in the internal structure, which are accumulated as a result of
cold deformation of the rim along the rolling surface of the railway wheel. It has been established that,
according to the nature of the influence on the hardness distribution of carbon steel, the PEC treatment
corresponds to changes during tempering in the average temperature range and allows to significantly
increase the operating parameters of the wheels of railway equipment.
1. Introduction
Compared to the thermal softening technology of
hardened metal, processes that are based on the use of
other physical influences such as magnetic field
processing [1] or electric current pulses [2] have
received sufficient propagation. Combines such
processing with the absence of significant thermal
effects on the development of processes structural
transformation.
Known effects of plasticization from the action of
electric current pulses are associated with the influence
on the motion cloud free electrons in the crystalline
lattice of a metal material [3]. As the energy of moving
electrons increases as a result of the electric current, their
impulse is partially transmitted to the dislocations,
resulting in a lowering of the activation energy for the
start of displacement [4]. However, according to [5], the
effect plasticization of metal materials is considered not
be related with activation energy, but depends on the
degree of divergence between the direction of action
electric impulse and the propagation plastic deformation
of the metal.
Taking into account that in the nodes of crystal
lattices there are non-neutral metal atoms, and ions, a
local violation in their arrangement, for example, in the
formation of a dislocation, leads to the idea that the
dislocation must also have a certain electric charge.
Thus, the interaction of the dislocation with the
purposeful movement of the cloud moving electrons
should be based on the influence of electric pulse on
effect plasticization of the metal material.
2. Status of the problem
The origin and distribution of the plastic flow in metals
and alloys is accompanied by the movement of
dislocations with different signs, which are characterized
by a different reaction to the action of electric pulse [6].
Under these conditions, with a predetermined direction
in relation to the active stress at the sample load, the
summation of the pulses electric current will lead to
acceleration of the dislocations one sign and complicate
the motion of dislocations opposite sign. For
dislocations, which is characterized by a relatively low
resistance level by the obstacle, the pulse of electric
energy will be sufficient to start their movement. As a
result, a local acceleration of a number dislocations of
such a sign will occur, which in turn will lead to
decrease in the operating syress to ensure the conditions
for the continuous propagation of plastic deformation
[7]. For dislocations of the opposite sign, the separation
from the blocking places can only be due to the
occurrence of thermal fluctuations of a certain level.
This is due to fact that the nature of effect pulse action
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0
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MATEC Web of Conferences 294, 05004 (2019) https://doi.org/10.1051/matecconf/201929405004
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electric current is an obstacle to the beginning of motion
such dislocations. The ratio between the quantities of
indicated dislocations and causes achievement the effect
of plasticization metal on the action of pulses electric
current. At the same time, the sensitivity of metal
material to the direction of action of the pulses should be
associated with the recombination of the dislocation
structure, which will lead to an appropriate directional
plastic deformation.
Taking into account that the first attempts to use the
pulse action of electric current were aimed at increasing
the plastic propertiesmonocrystalsof metals during
plastic deformation, the electric current density in the
pulse was equal to 10³ - 2·10³ A / mm². Approximately
similar regimes were used in the treatment of cold drawn
wire for increase its plastic properties [2-4]. The use of
this technology was not limited only to the increased
plasticity of cold drawn metal, it was found to be used
with special surface finishes, coatings, etc. [8-10]. The
high density of electric current in a pulse restricts the use
of such technology, primarily the power of equipment.
Indeed, according to [5,6], the intersection of single
crystals and cold drawn wire basically did no more 1mm.
The use of processing metal layers on the rim railway
wheel, based on the action of electric current with such a
high density in the pulse is limited to the equipment
required power.
3.Material and methodology of research
The material for research was the fragment of rim
railway wheel after operation with a carbon content of
0.61% and concentration of other chemical elements
complied with the requirements of the Standard of
Ukraine. The processing of electric current pulses of a
fragment rim railway wheel was carried out on the
equipment DS10D, using an alternating current.
The calculation of the pulse frequency was carried
out under conditions where the thickness of the metal
layer is limited to the appearance of the "skin effect" by
the ratio:
21
0
()
k
f
πμ μγδ
−
=⋅⋅⋅⋅
(1)
where
k
f
- the critical frequency (Hz) above which the
"skin effect" occurs,
γ
- the specific electrical
conductivity (Ohm / m),
0
μ
,
μ
- the magnetic
permeability of the vacuum and the test metal,
respectively (Gn / m),
δ
- the thickness of the metal layer
that is required process (m)
In developing the processing technology of electric
current pulses fragment rim railway wheel determined
the optimal shape of the pulse electric current, which
looked like a symmetric trapezoid. The time interval
when increasing and decreasing the amplitude value of
the electric current was 0.5-1s. For the treatment of a
layer metal in the thickness of 10-15mm there was
enough current density to 17 A/mm², the processing
cycle consisted of a pulse duration of 2.5-3s and a pause
between them up to 4s.
The hardness used was measured using the Vickers
method (
Hv
) as a strength characteristic. The evaluation
of the parameters thin crystalline structure was carried
out using X-ray structural analysis techniques. The
microstructure of the metal was studied under a light
microscope used methods of quantitative metallography.
4. Results and discussions
The metal on the rolling surface rim of the railway wheel
during operation is subject to a very high degree of cold
plastic deformation with considerable heterogeneity in its
distribution. For ease of research, the rolling surface of the
railway wheel was divided into three sections, with varying
degrees of cold plastic deformation. The plot (I) - part of the
rim near the crest, (II) – middle part of the rolling surface
and (III) - near the lateral surface of the wheel.
Measurement of hardness on the rolling surface confirmed
the existence of a significant difference in the degree
deformation of the metal rim railway wheel after operation.
For regions (I) and (III), the hardness was in the range of 5 -
5,5 GPa, and for (II) it was 4 - 4,9 GPa. The lower level of
hardness on the site (II) is due to the acceleration process of
softening cold-deformed metal from heating during the
braking stages of the rolling stock. Indeed, if we assume
that the area (II) is characterized by a maximum wear metal,
then the degree of plastic deformation should be higher
compared with other sections of the rim. The results of
studies [11] determined that in the case of heating to certain
temperatures, the degree of softening metal will be
proportional to the value of cold plastic deformation.
Given that the process of electric current pulses is
accompanied by changes in the geometric dimensions of
the sample (Fig.1) the nature of the influence on the
hardness metal may be similar to that of a reversible load
[4,5].
Fig. 1. The sample size change from the cycle number during
the processing by pulses of electric current [12].
Indeed, regardless of the nature metal materials, the
development of softening processes is observed for a
wide range values of plastic deformation, in the event of
divergence in the direction or form subsequent loading
of the previous one. On the basis of this it can be
assumed that in addition to the influence of electric
current pulse on the acceleration of motion of cloud free
electrons, one should expect the emergence of additional
lever in the form of a mechanical component of the
process. On the basis of the analysis obtained results, it
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was determined that as a result of the electric current
pulses, a certain level of metal softening was achieved.
Reduction of the hardness level for the region (I) was 20;
for (ІІ) 9 and for (ІІІ) 11% relative to the state after a
cold plastic deformation [12]. Taking into account that in
electric pulse processing the temperature of the metal did
not exceed 50
С, the achieved level of softening can’t
be related to the thermal nature of the effect. More it is
known that heating up to temperatures of 200-250
С,
due to the development of processes of deformation
aging, should lead to an increase in the hardness of cold-
deformed carbon steels [11]. The obtained of equal
effect softening metal on rolling surface railway wheel
after electric current pulses is confirmed by the
appearance of qualitative changes in the internal
structure of the metal.
Figure 2 shows the microstructure of metal volumes
near the rolling surface railway wheel after operation.
The structure corresponds to the metal after a high
degree of cold plastic deformation.
In the background of high turbulence structure
colonies of perlite (Fig. 2a,b), the presence sites of
structurally free ferrites (Fig. 2c) can be regarded as
evidence of heating metal near the rolling surface to
temperatures higher than the beginning of phase
transformations. On the other hand, the absence in this
section signs formation of additional boundary sections,
which are characteristic of the development processes of
recrystallization [11], indicates a very short period of
exposure metal at these temperatures [12,13].
After electric pulses treatment fragment of the rim
railway wheel, reducing the level of hardness is
accompanied by quite logical changes in the internal
structure of the metal (Fig. 3). From the comparative
analysis of the structural components, according to
external features, the effect of electric pulses should be
attributed to the thermal influence on metal after plastic
deformation (Fig. 3a). Indeed, in comparison with the
structural state metal wheels after operation (Fig. 2a,b),
electric pulse treatment results not only in the coarsening
of structural components (Fig. 3a,b), but also in some
changes in their distribution.
Comparative analysis of sites structurally free ferrite
up to (Fig.2c) and after electric pulse treatment (Fig. 3c),
confirms the appearance of qualitative differences in the
structure of steel. One of them is the emergence of more
distinct additional boundaries in volumes of a
structurally free ferrite (designation "a", Fig. 3c).The
presence in volumes of structurally free ferrite distorted
lines, similar to the lines slipping of dislocations,
indicates a high degree of cold deformation with the
obligatory change in the direction of loading.
The action of pulses of electric current is
accompanied by the appearance of additional distortions,
which significantly distinguishes them from the lines of
slippage in the propagation of plastic flow in metallic
materials [11].Thus, we can assume that degree of
softening obtained from the action of pulses electric
current in reality is accompanied by significant changes
in the internal structure of the metal rim after cold plastic
deformation.
a
b
c
Fig.2. Microstructure of the steel near the rolling surface of the
wheel after decommissioning. Pearlite colonies (a,b) and site of
structurally free ferrite - (1) (c), magnification 250.
1
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a
b
с
Fig.3. The microstructure of the steel near the rolling surface of
the wheel after the treatment of the IES (a, b - perlite colonies,
c - a section of structurally free ferrite), magnification 250.
Considering the mechanism of development process of
softening in the cold deformed metal from the action of
pulses electric current, the cyclic change in the size of
the metal specimen undergoing processing becomes a
definite value. Indeed, in the conditions of a cyclic
loading, the micro volume of a metal is subject to the
action of a definite total deformation. However, under
the influence of the emerging internal stresses, the most
soft structural components will first be deformed. As a
result development of processes, the relaxation of
internal stresses in volumes of a structurally free ferrite,
the magnitude existing stresses may not be sufficient for
the start deformation of pearlite colonies.
Given the very small change in the geometric
dimensions of the specimen (Fig.1) with ferrite hardness
about 2 times below the pearlite colony, a large part of
the plastic deformation should be concentrated in the
regions of a structurally free ferrite of steel. According to
stoichiometry, in steel of railway wheels, the share of
structurally free ferrite can reach up to 20-25%.
Assuming that the main effect of action pulses
electric current is concentrated in ferrite, the additive
contribution to the general effect of softening at the
level of 10-15% is fully justified..
The results of the softening metal rim railway wheel
on the rolling surface after deformation were confirmed
by X-ray structural analysis (Table). The analysis of the
results on the measurement of the internal stresses (
σ
),
density of the dislocation by the interference (211) - (
211
ρ
), distortions of the second kind (
μ
) and the size of
the coherent scattering regions (D), regardless studied
areas of the wheel (I, II, III) development of very
complicated by the nature of structural transformations
in the metal under the influence of pulses electric
current.
Table. Changing of parameters fine crystalline structure
of the ferrite from pulses electric current.
Parameter Number areas of rim railway wheel
І ІІ ІІІ
σ
, GPа -0,064 -0,128 +0,210
11 2
211
10 cm
ρ
−
×
3,6/2,8 ∗ 3,6/2,2 ∗ 3,9/3,0 ∗
3
,10
μ
−
× 1,09/0,84 ∗ 1,22/0,87 ∗ 1,18/0.97 ∗
D, nm 28,9/32,7 ∗ 27,7/33,0 ∗ 27,7/31,6 ∗
Given the very complex conditions of loading the
railway wheel on the rolling surface (heterogeneous
hardened from cold plastic deformation, rapid heating
and cooling metal to different temperatures during
braking of rolling stock, etc.), an attempt was made to
assess the efficiency of pulses electric current treatment
compared to thermal technology.
The figure 4 shows effect of temperature heating on
the hardness steel of a railway wheel after cold plastic
deformation with a different dispersion of perlite. At the
conditions of equal degree of plastic deformation, the
hardness of metal increases in proportion to as cending
dispersion of the pearlite colony. At the same time, for
equal degree of plastic deformation, the softening effect
is to a large extent determined by the dispersion pearlite
colony of the steel. This is due ability of cementite plate-
а
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like form to be plastically deformed [11]. An increase of
the temperature heating will be accompanied by
appropriate metal softening. From the comparative
analysis it can be determined that the more finely the
plate cementite, the degree of softening will be lower
(Fig.4).
Fig.4. The influence temperature heating on hardness steel of
railway wheel after cold deformation (1,3-75, 2-25%) at high
(1,2) and low (3) disperse of pearlite colony.
The nature influence of the degree plastic
deformation and dispersion of pearlite colony on the
development of softening processes under heating is
similar to the known data. So, for the carbon content
close investigated steel after cold plastic deformation of
25%, softening at a level of 20% achieved by heating up
to 400° C, with duration of exposure up to 1 hour.
Reduction of hardness by 10% after cold plastic
deformation by 25% achieved already after the heating
of the metal to 300 - 350 C. Similar results are obtained
and for higher degrees of plastic deformation. Thus, for a
steel with a carbon content of 0.7%, deformed by 90%
dragging, the effect of softening by 20% is achieved
after heating to a temperature of 380°C, and a decrease
in hardness by 10% - by heating up to 300°C [11].
Analysis of the experimental data (table) shows that,
in general, there is a fairly good agreement both in
character and in absolute values with known results [11 -
13], which should be considered as confirmation of the
development of softening processes in carbon steel of
wheel fragment after treatment with electric pulses
current. Indeed, according results of the study, the
process of softening cold-deformed metal is
accompanied by decrease in the accumulated number of
dislocations, distortions of the second kind and an
increase in the size of the regions coherent dispersion
(table). At the same time, it is certain interest to evaluate
the rate of structural changes depending on the different
levels of metal hardening after cold deformation. Taking
only the magnitude of the decrease in hardness after
treatment pulses electric current ( v
H
Δ) as an indicator
of the degree metal softening, a formal construction of
the degree of change in the characteristics of the fine-
crystalline structure from (Fig. 5) was carried out. It
follows from the presented relationships that, in
proportion increase in the effect of metal softening after
being treated with electric pulses, decrease in size rate of
coherent scattering regions, dislocation density, and
second-type distortions is observed.
Fig.5. The dependence of change the size coherent scattering
regions ( DΔ, nm) - (1); dislocation density ( 211
ρ
Δ
11 2
10 сm−
×) – (2) and distortions of the second kind (
μ
Δ
3
10−
×) - (3) of the degree softening ( v
H
Δ, GPa) steel after
treatment with pulses of electric current.
Although, on the whole, the achievable softening
effect from impulse treatment with electric current
pulses is proportional to degree strain hardening of
metal, the observed change in the sign of residual
stresses indicates the development of quite complex
structural changes. At the same time, cumulative effect
of the non-uniformity of distribution plastic deformation
and its directivity on the rolling surface wheel indicates
the need to continue studying causes that led to a change
in the sign of residual stresses after treatment with pulses
electric current.
Thus, if we use the datas of the table and estimate the
degree of metal railway wheel softening after treatment
of pulses electric current according to the characteristics,
compared with the thermal technology, the softening
effect may correspond to the heating metal to
temperatures of 400-450°С [13]. At the same time, the
low temperatures of heating metal (up to 50°С) during
the processing of pulses electric current can’t be
attributed development processes of diffusion mass
transfer.
In order to determine mechanism of soften cold-
deformed metal after processing of pulses electric
current, an assessment was made of the stresses that
caused movement of dislocations from various external
influences. Low temperature heating of the metal at
processing of pulses electric current immediately
excludes the thermal nature of the influence. On other
hand, due to high values of dislocation density
3
4
5
150 250 350
GPa
ºC
1
2
3
0
1
2
3
4
5
0,4 0,6 0,8 1 ΔΗv
1
2
3
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MATEC Web of Conferences 294, 05004 (2019) https://doi.org/10.1051/matecconf/201929405004
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accumulated in crystallographic sliding systems (table),
one of the possible mechanisms explaining effect of
softening at ambient temperatures may be non-
conservative displacement of dislocations under the
action of pulses of electric current.
From the balance of forces that determine resistance
to displacement dislocation from one slip system to
another [14] and facilitate the displacement fragment of
the dislocation line into a parallel slip plane [15], the
relation obtained:
Sin bh
α
≈, (2)
where
α
is the angle of disorientation between the line
of dislocation and the Burgers vector (b), h is the
distance between the parallel sliding planes. Taking into
account that 2
h
ρ
−
=[16], where
ρ
- the density of
dislocations, the relation (2) takes the form:
Sin b
α
ρ
≈ (3)
After substituting (3) 7
2, 48 10b−
=⋅ mm [11], the
experimental values of the dislocation density in
accordance with areas rim of railway wheel under study
(I, II, III), the calculation shows that as a result of pulses
electric current treatment, the angle disorientation
between the dislocation line and Biurges vector is
reduced. So, for the regions I, II, and III cold-deformed
metal, the value
α
is from '
36 up to '
48 respectively. As
a result of the processing of pulses electric current
according to the estimated calculations, there was a
decrease of the angle of 48% for the III region, 28% for
II and 17% for I. The obtained results can be regarded as
evidence of a change in the ratio between the edge and
screw components of the dislocation structure in the
metal rim railway wheel in as a result of processing by
pulses of electric current. Thus, one of the explanations
softening effect of metal railway wheel as a result the
development processes of recombination dislocation
structure should be considered. Given that the edge
component of the dislocation structure [14] is
proportional to the angle, as a result of electrical impulse
processing, the part of the screw component should be
increased. As a result, the conditions for reducing the
metal resistance, of processes creeping dislocations,
which should contribute to the development of their
annihilation. After processing of pulses electric current,
the effect of reducing the hardness of cold-deformed
metal on the rolling surface railway wheel practically
adequate the heating in the middle temperature range.
4. Conclusions
1. Mechanism of softening of cold-deformed metal rim
railway wheel after processing by pulses of electric
current, based on recombination of dislocation structure.
2. Achieved the effect of softening cold-deformed metal
in comparison with thermal technologies, аdequate the
heating in the middle temperature range.
5. Acknowledgements
The authors are grateful to Dr. V.G. Anofriev for
providing material for research and Dr. V.A. Sokirko for
carrying out the processing of fragment railway wheel by
electrical pulses. The authors would like to express their
gratitude to the staff of the Department of Applied
Mechanics and Materials Science of the University for
their participation in the preparation of samples for
research.
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