Article citation info:
Vakulenko, I., Proidak, S., Perkov, O. Investigation of slide mechanism of tread during operation of railway wheel. Scientific Journal of Silesian University of Technology. Series Transport. 2016, 90, 187-193. ISSN: 0209-3324. DOI: 10.20858/sjsutst.2016.90.16.
Igor VAKULENKO[1], Svetlana PROIDAK[2], Oleg PERKOV[3]
INVESTIGATION OF SLIDE MECHANISM OF TREAD DURING OPERATION OF RAILWAY WHEEL
Summary. Causes of reasons and explanation of mechanism
forming damages of railway wheels tread were investigated. At slipping on
contact surfaces wheel-rail a between by simultaneous development of processes
of work-hardening and softening metal determines the terms of origin damages of
railway wheels tread were fixed.
Keywords: damage, railway wheel tread,
hardness, slipping, work-hardening, softening, energy of activation
1.
INTRODUCTION
During
operation of railway wheels different level of strength, the forming damages of
metal railway wheels tread are conditioned the simultaneous action forces of
friction and cyclic change of stresses [1]. Supposing that forming damages of
surface tread is mainly determined the internal structure in volumes of metall
near the tread of realway wheel [2], it is necessary to expect different of
character development of processes structural changes in a metal with the
different level of strength.
Taking
into account the tendency of produce railway wheels with the high level
strength as a result of microalloying, change of content carbon in steel and
use hardened heat treatments, the internal structure of metal near by the
surface of railway wheels tread in majority of cases corresponds quenching and
follow tempering in the middle interval of temperatures. It ensues from
experience of exploitation of railway wheels, that at the identical level of
strength and high-quality different structural state of metal, the processes of
wear can substantially
differ [2].
On the
basis of analysis reasons of premature withdrawal of railway wheels it is
discovered from exploitation, that except for the unrationed amount of
nonmetallic inclutions of different morphology and nature of origin at
arrangement near-by with railway wheels tread [3], substantial value, arising
up acquire additional moving of wheelpair in relation to the frame of light
cart [4]. The indicated displacements of wheelpair during exploitation are
reason of origin shear component of deformation to the constituent of metal on
a contact surface wheel-rail. As a result on-the-railway wheel tread areas are
formed with the different degree of wear and complicated internal structure of
metal from simultaneous influence of a few factors [5]. Forming of such areas
is a break by homogeneity of distributing of internal stresses in the metal of
wheel and results of bring nonhomogeneous strain hardening metal on the railway
wheels tread.
On the
basis of it the estimation terms of forming of the local slipping presents
certain interest in the process of exploitation of railway wheels with the
different level of strength, as one of the stages of complicated process of
formation damage on the railway wheels tread.
2. PURPOSE
OF WORK
The
purpose of this work was investigation of mechanism of slipping on the railway
wheels tread during operation.
3. MATERIAL
AND RESEARCH METHOD
For
research carbon steel of railway wheel served as material with contain of
carbon 0,6%. Heat treatment was reach at the different state of structure of
metal. As a result of quenching from the normal temperatures of heating the
structure of martensite was formed with hardness 65 HRC, and after subsequent
tempering at temperatures 450 − 470°C is a structure of the tempered
martensite with hardness 39 HRC. Heat higher than temperature 
, the required self-control for homogenization of аustenite and cooling
with a furnace allowed to get lamellar pearlitic structure in steel with
hardness 13 HRC.
The
analysis of behaviour speciment of the probed steel in conditions of the normal
contact rolling was carried out use the proof-of-concept machine of SMC-2 for
the normal load 18 kg on the speciment. At tests, the change of kinematics
chart of machine become possibility of receive of different sizes of slipping
on the contact surfaces of speciments, due to the change of their angulator
speed of rotation. Speed of turns spindle of proof-of-concept machine was 300
and 500 min
, at the temperatures of test 20 and 120°C (293 and 393°K).
As a result of test determined the beginning moment of forces in the area of
contact surfaces speciments.
For
explanation the character development processes changes of structure for tests
descriptions, analysis parameters the thin crystalline structure of metal, were
used. Estimation degree of tetragonality crystalline lattice of ferrite, size
of areas coherent dispersion (
), density of dislocations (
) and distortions of the second-type (
) carry out, through the methods of x-rays
analysis [6].
4. DISCUSSION
OF THE GOT RESULTS
The
analysis of known experimental of facts [1, 3, 7] bear witness that to the
formation of extrusions and intrusions (Fig. 1) on the railway wheels tread
which become the turn into of superficial damages in future, local changes are
preceded in the area of contact wheel-rail. Taking into account high-rate of
heating at the local slipping, the temperature of beginning of phase
transformations in steel of wheel can be arrived in thin of layer near railway
wheels tread. After completion of the stage of slipping a sharp cooling from
the heated metal in more remote volumes of rim of wheel is able to provide
development of phase transformations on intermediate or even to diffusionless
transformation [2, 5].
On that
was transformation railway wheels tread with the practically homogeneous strain
hardening of metal, transform into alternating areas with different hardness
and capacity for the deformation work-hardening. During subsequent exploitation
of wheel on the indicated areas with different hardness and internal structure
(cold work state and structures after quenching) it is necessary to expect
further growth of distinctions in character of the local slipping and proper
changes in the level of strength properties.
The make
use of disk brake systems far of the wheel light carts the estimation of
localization plastic flow of metal on a contact surface a wheel-rail is
presented by certain practical interest.
а
b
Fig. 1.
Chart of forming extrusion (a) and their real kind on the profile of railway
wheels tread (b) Magnification 100
4.1. Estimation of beginning slide on a contact surface wheel-rail
At
certain correlations of strain hardening of metal on the railway wheel tread
(degree and distributing of plastic deformation) and temperature of warming-up
during exploitation, there must be a change of balance between the indicated
effects. Above all things the process of slipping will be determined in size
tripping of metal of wheel with a rail. The indicated characteristic can be
estimated on the origin moment of forces in the area of contact surfaces. From
other side, the size of tripping of wheel with a rail must change with the
temperature of warming-up of metal near-by with the railway wheel tread. On the
basis of it, as description for the estimation of the looked after phenomenon
it is necessary to avail in size of energy, which must be expended for
achievement effect of slipping.
Taking into account that a process of slipping is
thermally activated, will take advantage of equalization of Arrhenius for
energy activation of process of plastic deformation. In a general view
equalization can be written [8]:
, (1)
where 
− speed of
deformation, 
−
value of constant, 
− energy
of activation process, 
− universal gas constant, 
−
temperature (°K), 
− moment
of forces at slipping, 
− index of
degree. After taking the logarithm of correlation (1) get:
(2)
For a condition of constant temperature, after
differentiation (2), determined
as
a tangent angle of slope from a graphic construction 
:
(3)
There is a size of relation at unchanging speed of
deformation 
determined
as an angular coefficient from dependence 
:
, (4)
and take
into account (3) get a numeral value [8].
Estimation of size energy activation of process
slipping, carried out from experimental data, on speciments of railway wheel
steel became able after quenching from the normal temperatures of heating (Fig.
2). The resulted structure is largely similar to the structure of metal railway
wheel after forming of slide-block onthe tread [2, 7].
a b
Fig. 2. Structure quenching railway wheel steel. Magnification (a)
− 1000, (b) – 13000
As speed of deformation the number turns of spindle
of proof-of-concept machine was accepted (
). As show in [7] a calculation of energy
activation carried out on tests at no less as two speeds of deformation and two
temperatures of loading. Size made values
300 and 500 mines, at temperatures +20 and +120°C and the
moment of forces in the area of contact surfaces was arrived at the size of
slipping 10%.
Beginning from the first cycles loadings at
rolling, from the diagrams change of size it was found
out high instability of values. In area of first hundred cycles of loading
decrease of size of moment of forces on contact surfaces arrived at a few
times. As far as the increase of number of cycles at rolling of unmonotony in
character change of the indicated description went down and only after set in
relation to the stable mode, determined the moment of forces. For the
estimation of size used values at reject 3 −
5% in relation to the average set level.
From a
correlation 
(Fig. 3)
values were certain , which were 0,83 and 0,2 for temperatures 293 and
393°K (20 and
120°C)
accordingly. Size estimated
rewritting correlation (4) with preliminary replacement on and by the
substitution of numeral values proper descriptions:
(5)
It was discovered from the analysis of the got
results, that in the interval of the use speeds (300, 500 мин) decrease of temperature from 120 to 20°C
accompanied decrease of the required energy for development of process slipping
approximately in 1,3 of time (from 20,6 to 15,3
).
Thus, at permanent level of loading and speed of
rotation, the indicated decrease of temperature on the tread metal is
accompanied decrease size of tripping with speciments approximately on 25%. In
the isothermal terms of loading the increase of speed of rotation in the use
range of values practically does not change the size energy of activating
process of slipping.
a
b
Fig. 3. Correlation
between and at the
temperatures of test (a) − 20, (b) − 120°C
At the change of the structural state of metal, depending on the terms
of rolling it is necessary to expect other character of dependence of tripping
in the area of contact surfaces high-quality. So, as compared to tempering on a
martensite, metal of railway wheel after the homogenizing annealing with the
level of hardness 13 HRC, at the temperature of test +20°C, with
the increase of speed of rotation in the use range (300 to 500 mines) a show growth in 1,5 of time moment of forces in
the area of contact surfaces. Moreover, confirmation was predictably got
high-quality other character of change of tripping of metal at slipping. As
compared to a quenching metal on a martensite, for speciments after annealing
there was absence of area with the unmonotonous change of size on the
initial stages of increase of cycles of loading.
It was found out the analogical character of
behaviour carbon steel and able after quenching with subsequent tempering in
the middle interval of temperatures, for example at the level of hardness 39
HRC. In both cases, since the first cycles of loading there was a monotonous
increase of hardness on-the tread of the tested speciments.
Thus, forming of structures on a diffusionless
transformation mechanism on the tread of railway wheels results in appearance
of neighbour areas with a high-quality different conduct at a loading.
4.2. Research mechanism of soften steel after
quenching at rolling
After
quenching (Fig. 2) the level hardness of carbon steel (65 HRC) is provided the beginning
certain degree of tetragonality crystalline lattice, which in same queue is
conditioned the concentration atoms of carbon, dissolve in austenite. With the
increase concentration of carbon in steel growth of degree of tetragonality of
crystalline lattice of ferrite (
) as a result of quenching estimated on correlation [9]:
, (6)
where 
− gravimetric %
carbon in steel, 
− a parameter of
crystalline lattice of ferrite, 
− size of rib of
lattice ferrite after forming crystal of martensite [9]. After a substitution
in (6) for the probed steel of size and the experimentally
got values and , there was a calculated degree of tetragonality of
crystalline lattice of ferrite after quenching, which made 
. Comparative analysis with the known experimental dates for
steel with analogical concentration of carbon after quenching on a martensite
[9] show a good enough coincidence. It is necessary to examine the got result
as a confirmation of absence of disintegration of super soturation solid
solution at cooling.
After
1200 cycles of rolling of specimens slip-free, to the discover decrease of
hardness from 5 to 7% diminish of degree of tetragonality of crystalline
lattice of ferrite of quenching steel corresponded from 1,027 to 1,0255 (on
0,15%). The decrease of the indicated parameter testifies to development of
processes of softened of quenching steel in the process of the contact
phenomena at rolling. Analogical results on influence of insignificant plastic
deformation on the strength middle carbon steel after quenching is got in [10].
For explanation of mechanism of looked after softened, will take advantage of
correlation (6). After a substitution in (6) values 
=1,0255 it was
discovered that size corresponds
concentration of carbon 0,56%.
It is
thus necessary to suppose that way out atoms of carbon from martensite of
crystals in the process of rolling, is principal reason of decrease of hardness
steel after quenching. However, if looked after soften at heating to the
temperatures higher 500 − 550°C the work-hardened carbon steel
accompanied the increase sizes of areas of coherent dispersion, reducing
the accumulated dislocations and decrease level of distortions of the
second-type [11], after rolling was found out an increase on 19% (from 
to 
), (from 
to 
) and
diminishing на 30% (from 618 to 445
). Resulted character of change parameters thinly crystalline structure are
show on the presence effect of the deformation work-hardening of
martensite crystals in the process of rolling. Moreover, the looked correlation
between by the processes of work-hardening and softened at rolling of metal
must continuously change depending on the total size of plastic deformation. If
on the initial stages of loading, when the effects increase amount of defects
crystalline structure are insignificant, a total result is a softened effect.
At growth of amount of cycles of loading at rolling, when the role of processes
of the deformation work-hardening will continuously increase, and soften from
disintegration of super saturated solid solution will go down, after the
certain number of cycles it is necessary to expect began exceedings effect of
work-hardening.
Research
of parameters of thinly crystalline structure of metal at rolling with a 10%
slipping did not result in the high-quality changes character of soften of
quenching metal. Already after 600 cycles of l0ading with slipping, hardness
was 62 HRC (before a test 65 HRC), here 504 made, and attained values 
and 
accordingly.
On the
basis of the known experimental results [1-3] and results of the conducted
researches there are the looked after cases of forming on the railway wheels
tread areas of «white layer», in actual fact by the appear result of
development structural transformations in a metal on a diffusionless mechanism
at slipping in the places of contact wheel-rail. Development of processes of
softening on these areas will be accompanied the inevitable deformation
work-hardening on near by areas without the local slipping. As a result there
will be growth of gradient of hardness on the boundary of section between them.
Even in the case of absence damage surface of rolling on the area of slipping,
a growth of strength properties rate on the indicated near by areas during
exploitation of wheel will not be identical.
Taking
into account the high enough degree of heterogeneity of distributing size of
plastic deformation on the railway wheels tread, an important practical value
has a between by development of the indicated processes, especially on the
stages of braking of railway carriage. In fact insignificant in size areas with
slipping after a roll-up in the process of exploitation of wheel will result in
the origin of high gradients of internal tensions in the volumes of metal
near by with the tread. The indicated areas during further exploitation of
railway wheels become the potential places of forming damages of railway wheels
tread.
5. CONCLUSIONS
1.
Forming damages of railway wheels tread is
conditioned correlation of the developed processes of work-hardening and
softening in the area of contact a wheel − rail.
2.
The found out the decrease hardness of quenching
steel at rolling is accompanied the increase of density dislocations, reduce of
areas coherent dispersion and growth of distortions of the second-type.
3.
Regardless of the structural state of carbon steel,
the decrease of temperature during exploitation of railway wheel is promote in
development of the local slipping on the railway wheels tread.
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Received 11.08.2015; accepted in revised form 21.09.2015
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