Article
citation information:
Labisz, K., Konieczny, J. Natural ageing effects
on microstructure and properties of rail fastening elements SKL-12. Scientific Journal of Silesian University of
Technology. Series Transport. 2020, 106,
85-96. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2020.106.7.
Krzysztof LABISZ[1],
Jarosław KONIECZNY[2]
NATURAL
AGEING EFFECTS ON MICROSTRUCTURE AND PROPERTIES OF RAIL FASTENING ELEMENTS
SKL-12
Summary. Rail transport is presently one of the most
supported means of transport in Europe; it existed from the end of the 18th
century. However, some issues especially concerning materials and its
exploitation are still actual and are a matter of scientific projects or
developments. In this paper, analyses concerning the characterisation of used
track infrastructure elements in form of sleepers of the popular rail fastening
system SKL 12 were performed. Specifically, the main objective of the work was
the characteristics of the material microstructure and properties after
long-term usage and natural ageing, reaching over a few decades. In this paper
was conducted investigations concerning the non-used and used fasteners by
reason of classic material research methods. The analysis was carried out based
on the results obtained through research using mainly light, scanning electron
microscopy (SEM) and transmission electron microscopy (TEM), as well as
electron diffraction for the lattice structure determination, EDS chemical
microanalysis and Rockwell hardness testing were also carried out in terms of
identification of the chemical analysis changes that occurred after long-term
application. The main reason was to characterise the long-term usage for the microstructure
changes on the surface layer of the used fasteners compared to newly produced
material.
Keywords: microstructure, rail
fastening system, sleepers, mechanical properties, rail steel, electron
diffraction
1. INTRODUCTION
This work is in the
mainstream of current activities to analyse the quality of materials leading to
improvements not only in the quality of the material itself but also to
streamline production, reduce susceptibility to damage, increasing the economic
factors of the product use and other aspects of the material, that is, its
processing and usage.
A rail fastening system
is a means of fixing rails to railroad ties (United States) or sleepers
(international). The terms rail anchors, tie plates, chairs and track fasteners
are used to refer to parts or all of a rail fastening system. Various types of
fastening have been used over the years. Their role is to keep the rails on the
sleepers and ensure that rails have an adequate tilt angle. Among the types of
rail track fastening can be distinguished such types as [1-3]:
- direct fastening (screw
spike, rail screw or lag bolt),
- screw fastening of the K
type,
- spring spikes or elastic
rail system SKL 12,
- elastic fastening SB
– 3, SB – 4, SB7.
The least frequently
used fastening is the direct fixing, which is only applicable for low-speed
trains, because at higher speeds they will loosen the screws and also if the
rails are badly fixed. Fastening of this type do not provide adequate clamping
rail to the sleeper, therefore, it is not suitable for standard rail. All these
disadvantages are eliminated in the elastic fastening, however, one thing that
should be investigated is the use in a long term perspective concerning the
stability of the obtained microstructure and properties.
While the fastening
screw - elastic system SKL 12 is a modern solution for rail elements fastening.
Thanks to the application of the elastic system, vibrations are effectively
absorbed. This is a universal system for fastening of wood, steel or concrete
blocks. The most modern solution is the application for fastening of
prestressed concrete to the sleepers. They are easy to install, decrease
vibrations and act as electrical insulation [4-7].
However, even in an
innovatory fastening solution, there are several issues, which should be
monitored and/or improved. One of them is the long-term durability of the
microstructure obtained after the production process, as well as the mechanical
properties stability.
For this reason in this
paper are presented basically, the investigation results of a structural
element used in rail transport, which is the elastic track fasteners as a part
of the SKL 12 rail fastening system. During the development of the rail
transport system, changes occurred not only in the drive methods by which the
railway machines were moved but also in improvement in the rail infrastructure
as well. In this work are presented particular investigations concerning the
research about the structure and properties, which have been subjected to the
material obtained from a used as well as non-used SKL fastener (obtained from
the Gliwice Railway Station during its renovation), after few decades of usage.
Samples of the material passed through a series of laboratory tests such as
ordinary light microscope examination, scanning electron microscope
microstructure evaluation (SEM), transmission electron microscope (TEM),
chemical microanalysis testing (EDS) and measurement of Rockwell hardness. This
work is placed in the mainstream of current activities to analyse the quality
of materials leading to improvements not only of the quality of the material
itself but also to streamline production, reduce susceptibility to damage,
increase the economy of use of the product and other aspects of the material
and its processing or application [7-8].
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Fig. 1. Screw fastening of the K type,
a-the rib plate, b-trap hook, c-fastening screw of the rib plate, d-bolt with
nut [3] |
Fig. 2. Spring spikes or elastic rail system SKL 12,
a-ribbed plate, b-screws that secure the backing plate rib, c-hex nut,
d-elastic clip SKL 12 [3] |
2. INVESTIGATION PROCEDURE
The analysis of changes
in the material of the elastic fastening element SKL 12 and comparing the
results to the state of a new element not used in railway transport was based
on microstructural analysis as well as mechanical properties investigation.
From each of them was cut off 6 samples-longitudinal and cross-section (12
samples, Fig. 3a), which are then mounted and prepared for further studies. The
samples were tested for the reason of comparing the corresponding pair in terms
of their microstructure and properties as well as changes that occurred during
long-time usage under real conditions. The following investigations were
particularly carried out for the purpose of investigating the microstructural
changes that occurred after the long-term use of the rail steel fasteners:
- light microscope -
comparison of the macrostructure of the material obtained from samples taken
from the used and non-used fastener, the samples have been cut in the same
adequate places in the horizontal and longitudinal direction, polished and then
etched in 10% nital in room temperature,
- scanning electron
microscope (SEM) – comparison of microstructure,
- transmission electron
microscope (TEM) – determination of the structure in nanoscale,
- chemical composition
investigation using the EDS method - an analysis of possible changes occurred
in the chemical composition of the used material,
- optical emission
spectroscopy using the Stationary Metal Analyzer SPECTROLAB type LAB 05
according to the PN-H/04045 for exact analysis of the chemical composition,
- Rockwell hardness
measurement – analysis of micro-hardness measurements,
- comparison and analysis
of changes occurring after usage of the SKL 12 fastener.
3.
MATERIAL FOR INVESTIGATION
A new elastic fastener
SKL 12 as well as a ca 40 years used fastener SKL 12 obtained during the repair
work of the railway station in Gliwice were as materials for these
investigations (Fig. 3b). According to the standard EN 10089:2002 the steel
50S2 is named as 46Si7 with the chemical composition presented in Table 3.
Steel 50S2 is the most commonly used steel to produce elastic rail fasteners.
It is characterised by a yield point equal or higher than 1080 MPa, tensile
strength equal or higher than 1280 MPa, an elongation equal or higher than 6%,
and a waist equal or higher than 30% (data presented for room temperature). SKL
12 fasteners should have a downforce of about 8 kN, while the deformation of
the central part should not be higher than 1 mm [4].
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Fig.
3. The investigated fasteners of the SKL 12 system (a), |
|
Tab. 1
Chemical composition of
the steel 46Si7 according to the PN-EN 10089 standard
|
Element |
Concentration, min.
vol. % |
Concentration, max.
vol. % |
|
C |
0.42 |
0.50 |
|
Mn |
0.50 |
0.80 |
|
P |
- |
0.025 |
|
S |
- |
0.025 |
|
Si |
1.50 |
2.00 |
The technological
production process of the SKL 12 fasteners consists of eight steps according to
data obtained from the producer:
- cutting - drawn rods
with a diameter of ø13 are cut by guillotine at parts wit a length of
approximately 514 mm,
- cold flat bending –
bending takes place in two steps on a hydraulic machine,
- cleaning – the
rods are treated in a shot-blasting machine for about 3 min,
- resistance heating –
the process of resistance heating takes place at 750-800°C in the resistive
heater,
- spatial bending –
bending of the rods is carried out on a high-speed hydraulic one rack press
marking using an eccentric press,
- heat treatment –
the heat treatment of rods consists of quenching for 35 min at 860°C, with
cooling in polymer, and annealing for 55 min at 450°C with cooling in the
water,
- deposition of a
protective coating – a protective coating in the form of a cataphoresis
with a minimum thickness of 20 µm, which is applied at a temperature of
350°C. The coating thickness should be about 50 microns.
Heat treatment of the
SKL 12 fastener. The elastic SKL 12 system in one of the final stages of its
production was subjected to hardening and tempering. In the case of hardening,
present in the microstructure of the fasteners was the formation of
fine-grained pearlite and martensite, with a content of at least 97.5%. A
sorbate structure was present after tempering.
4. INVESTIGATION RESULTS
At the beginning of the
investigations, the chemical composition was analysed in order to determine the
material property in terms of alloying additives (Table 1). The measured values
slightly differs from the PN standard additive concentrations of the
investigated fasteners, especially the carbon concentrations, which is higher
and exceed the standard value of more than 25%. Worth mentioning is the fact
that the phosphorus and sulphur content is below the permissible content
defined in the standard, hence, the quality of the original material was
ensured, and the higher carbon content could be caused by the long term usage
reaching even 30 years.
Tab. 2
Chemical composition
analysed using the emission spectrometer
according to the PN-H/04045 standard.
|
Chemical composition,
vol.% |
||||||||||||||||||||||||
|
element |
C |
Si |
Mn |
P |
S |
Cr |
Mo |
Ni |
Al |
|||||||||||||||
|
Concentr., min-max |
0.562-0.569 |
1.65-1.70 |
0.699-0.706 |
0.001 |
0.0133-0.0186 |
0.0705-0.0728 |
0.0783-0.0876 |
0.0942-0.0959 |
0.0020-0.0228 |
|||||||||||||||
|
Element |
Co |
Cu |
Nb |
Ti |
V |
W |
B |
Fe |
||||||||||||||||
|
Concentr., min-max |
0.00515-0.00774 |
0.149-0.153 |
0.00182-0.00273 |
0.00692-0.00725 |
0.00236-0.00349 |
0.006 |
0.001 |
96.59-96.70 |
||||||||||||||||
For microstructure,
investigations from each fastener were taken six samples from different places
presented in Fig. 3a, both in the longitudinal and cross-section direction.
After cut off of the sample material, the first study was related to a simple
analysis of the microstructure using light microscopy. Figure 4 shows the results
of the structure analysis of the elastic SKL 12 fastener carried out on a light
microscope. On the obtained images is shown a clearly visible
perlitic-martensitic microstructure, both in the used and non-used material.
Between diverse locations of the sampling collection, the samples revealed no
major differences. Only the sample take from point III had smaller grains,
whereas in the sample No. I are more microstructure discontinuities.
Additionally in the
microstructure of the long-term used material are found slightly dark coloured
places of some impurities and many discontinuity places were revealed, so this
material is not of the best possible quality. This indicates low-grade steel
used for the production of elastic tabs SKL 12 in the past decades (Figures 4b,
d, and f). The measured grain size of the material ranges from 8 to 10
µm.
The microstructures
presented on Figures 4d, e, and f of the used elastic SKL 12 fastener shows a
higher degree of complexity in terms of the size of the martensitic areas
compared with the non-used fastener. This is probably due to the slightly
different heat treatment as well as some more amount of impurities. The
detected amount of the microstructure discontinuity is also higher due to the
lower material quality. The measured grain size of the new fastener range from
10 to 12 µm. In addition, small precipitations can be seen especially in
Figure 6b. In the new fastener material was also detected a higher grain size
variations compared to the used fastener material.
Based on the light
microscopy analysis of the microstructure of the fastener surface located on
the protective coating was revealed the structure of the upper surface area in
the new SKL12 fastener, consisting of a ferrite mesh, which is characteristic
of this type of railway fasteners material (Figs. 5a and b). In the
microstructure are highly visible changes that occurred after carburizing and
application of cataphoresis of the material surface during the manufacturing
process of the fastener. The ferrite area is visibly very clear, the closer the
surface of the material, the more compacted and structured it is, and the
deeper into the material, the more visible the so-called ferrite grid. The grid
is typical of ferrite in the steel, which contains about 0.6 to 0.7%, and has
the form of a clear mesh that surrounds the perlite grains. The visible surface
of the material is smooth and consistent. Generally, it can be seen that the
microstructure of the sample in the place No II and III reveals a clearly
visible ferrite grid compared to other places in the fastener, especially on
the end part of the fastener (place No. I), where the grid occurs in a residual
form.
In Fig. 5b, there is
presented the material structure taken from the elastic SKL12 fastener, which
was used, can be observed a smaller area of the ferrite grid compared to the
new fastener material. In Fig. 5b there are only remnants of the ferrite grid.
More so, it can be noticed that the surface of the material has significant
corrosion signs, which proceeds to the core material. Additionally, in the
microstructure of both the new and used fastener material can be recognised
some precipitates in the steel microstructure of the size of ca. 5 µm.
Another investigation
method, which was used for the microstructure analysis is scanning electron
microscopy. In Fig. 6 are presented microstructures obtained during the
examination of samples from the non-used and used elastic tabs SKL 12 fastener.
In these structures is very visible the fine-grained martensite present in this
investigated railway steel, used for the production of the SKL12 elements. The
non-used material samples contain almost no precipitates in the micro-scale,
the entire microstructure in all the investigated places is relatively homogeny
with minor changes only resulting in a slightly different size of the structure
compounds. Within the sample structure are not
detected any impurities.
The grains of the used
material are slightly elongated compared to the non-used fastener material. In
Fig. 6c, some impurities are visible, as well as precipitate particles.
Compared with the new fastener, the grains are also more densely distributed
within the material.
According to the EDS
investigation results, which states the changes that have occurred within the
material compared between the sample material obtained from the non-used and
used SKL12 fastener. The most important issue is the change in the chemical
composition of the material phases occurred as impurities as well as
intermetallic phases or precipitates. Figures 6a and d show the elements
detected in the sample from the non-used 6a and used 6c. The used material
sample (Fig. 6c, point No. I) reveals numerous precipitates, consisting from
the following elements such as O, Mg, Al, S and Ca (Fig. 6d).
Based on Rockwell
hardness measurement, comparing the data from the producer, the hardness value
of the material should be in the range 42-46 HRC. Figure 9 presents the results
obtained for each sample. The measured average hardness of the non-used elastic
SKL12 fastener is equal 48 HRC and in used element 45 HRC. Both values are
within the established properties of the 50S2 steel, as the material for the
SKL 12 fastening system. The analysis of the results reveals that the mean
hardness value of the non-used fastener and the used one is very similar, and
does not exceed 3%, however, with different distribution along the entire
element.
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a) |
b) |
c) |
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d) |
e) |
f) |
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Fig. 4. Microstructure of the non-used: a),
b), c) and used d), e), f) fasteners of the SKL 12 system, depending on the
sampling place and direction, where: a) is from place no. I in Fig. 3a; b)
is from place II in Fig. 3a and c is from place III in Fig. 3a
|
a) |
b) |
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Fig. 5. Microstructure in the cross-section of
the surface layer of the non-used (a),
and used fasteners (b)
|
a) |
b) |
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c) |
d) |
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Fig. 6. Microstructure and the corresponding
EDS area microanalysis of the non-used: a), b) and used c), d) fastener
microstructure, carried out in point III (Fig. 3a), SEM
The final investigations
concern on examination of the SKL 12 fasteners using the TEM microscope. On
Figs. 7 and 8 are presented test results for the new as well as used element,
carried out in the bright and dark field technique for revealing the structure
in the nanoscale. The crystallite size estimated from the obtained images is
about 0.15 - 0.2 μm. In addition,
diffraction investigations were performed both for the used and non-used
fastener for determination of the d-spacing (Figs. 7c, d and 8c, d) presenting
the SAD diffraction patterns in the [220] and [100] zone axis as well as the
solution of this diffraction patterns. Based on the interplanar distance
calculations, it was found that there occur a small difference in the (00-1)
plane for the used fastener, reaching 0.51%.
In the material of the
used SKL 12 fastener was revealed a crystallite size of approximately 140 - 210
nm. It can be seen that in the material of the used SKL 12 fastener, there are
compounds of inclusions or contaminations, especially in the non-homogeneous
areas of the surface layer, which may come from the low quality of the steel
(Fig. 8a). For the sample of the used 50S2 fastener, were calculated the
d-spacing values for alfa ferrite Feα and diverse plains
occurred in the electron diffraction pattern and compared to the theoretical
values (Table 3).
|
a) |
b) |
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c) |
d) |
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Fig. 7. Structure of the
steel 50S2 from the non-used fastener: a) bright field, b) dark field, c) SAD
electron diffraction pattern, d) solution of the electron diffraction,
Feα phase, zone axis [220]
Tab. 3
D-spacing of the ferrite
Feα phase, zone axis [100] and [220]
|
plane |
hkl |
dexp[Å] |
dteor[Å] |
|
Used fastener |
|||
|
1 |
00-1 |
2.0371 |
2.0268 |
|
2 |
002 |
1.4267 |
1.4332 |
|
Non used fastener |
|||
|
3 |
011 |
2.0294 |
2.0268 |
|
4 |
002 |
1.4356 |
1.4332 |
|
a) |
b) |
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c) |
d) |
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Fig. 8.
Structure of the steel 50S2 from the used fastener: a) bright field, b) dark
field, c) SAD electron diffraction pattern, d) solution of the electron
diffraction,
Feα phase, zone axis [100]
5. CONCLUSIONS
Summarised, it can be
concluded that the material from which the elastic SKL12 fasteners are produced
reveals lower quality as it can be confirmed based on the occurrence of
structural compounds affecting microstructure and properties. The changes can
be observed in terms of the microstructure, where the minor amount of the
ferrite mesh at the surface of the material used for few decades due to the
wear process of the surface as well as the production process parameters. The
research hypothesis was confirmed, the material of the elastic fastener
revealed significant damaged and appearing in the microstructure, numerous
changes such as the presence of impurities, decrease of mechanical properties
and the occurrence of not desired intermetallic phases and/or precipitates. In
addition, it determined that the sample material, which was produced a few
decades ago, to be of low quality. Particularly, the following issues were
found:
-
the study of mechanical properties
suggests only a slight change in hardness between the material of the non-used
and the used SKL 12 fastener. For the non-used elastic SKL 12 fastener, the
determined hardness is 48 HRC and for the used element 45 HRC, which is
generally in the range of the allowed measurement error. Therefore, the
analysis of the presented hardness measurement results draws the conclusion
that the long-term usage of the SKL 12 fastening system does not affect the
material in any significant way, ensuring originally of the values obtained;
-
the occurrence and influence of additional
elements, mainly in form of impurities measured by reason of the chemical
composition on the hardness in only minimal. However, its impact of the
microstructure in form of discontinuities can be crucial for long-term
application, and special care devoted to the quality of pure material used for
the production of the fasteners is very important;
-
diffraction tests based on the Fα
phase parameter calculation allowed the determination of the interplanar
distances (00-1), (002), (011), (002) of the tested materials. Based on the
conducted analyses, differences of 0.51% in the d-spacing parameters were found
between the experimental d-spacing value and the theoretical interplanar
distance for the (00-1) plane;
-
the structure of the non-used elastic
SKL12 fastener occur a clear ferrite mesh in the top of the surface layer
reaching a depth of 100 μm.
However, in the used elastic SKL12 fastener, the ferrite mesh has a residual
character and reaches a depth of about 5 μm only;
-
the results of the tests carried out by
reason of optical microscopy show that the particle size of the non-used
elastic SKL12 fastener reaches from 8 to 10 μm, whereas in the used elastic SKL12 fastener, the
grain size ranges from 10 to 12 μm, depending on the measurement place in the fastener.
Fig. 9. Hardness measurement results for the
chosen places of the fasteners
References
1.
PKP Polish Railways. 2010. Conditions
for making and receiving elastic feet and springs that tie rails to sleepers
and dumpers Id – 109. Warsaw.
2.
Kaikai Wang, Zhunli Tan, Guhui Gao, Xiaolu Gui, R.DK. Misra,
Bingzhe Baia. 2016. “Ultrahigh strength-toughness combination in Bainitic
rail steel: The determining role of austenite stability during
tempering”. Materials Science and
Engineering A 662: 162-168. DOI:
https://doi.org/10.1016/j.msea.2016.03.043.
3.
Ryjáček Pavel, Miroslav Vokáč. 2014.
“Long-term monitoring of steel railway bridge interaction with continuous
welded rail”. Journal of
Constructional Steel Research 99: 176-186. DOI:
https://doi.org/10.1016/j.jcsr.2014.04.009.
4.
Urbańczyk Edward, Janusz Kasprowicz. 2004. “Railway
rails - requirements, technology and market position”. Seminar materials: Sixth seminar of
Diagnostics of Railroad Diagnostics: 293-306. Ironworks Katowice. Poland.
5.
Zygmunt Tomasz. 2008. The
effect of cyclic controlled cooling on the mechanical and operational
properties of the rails. Katowice.
6.
Pagliara Francesca, Luigi Biggiero,
Alessia Patrone, Francesca Peruggini. 2016. “An analysis of spatial
equity concerning investments in high-speed rail systems: the case study of
Italy”. Transport Problems
11(3): 55-68. DOI: 10.20858/tp.2016.11.3.6.
7.
Zaborowska Izabella. 2016.
“Evaluation of microstructure and properties of SKL-12 fastening after
long term usage”. MSc thesis. Gliwice. Silesian University of Technology.
8.
Gwoździk Monika, Zygmunt Nitkiewicz.
2009. “Wear resistance of steel designed for surgical instruments after
heat and surface treatments”. Archives
of Metallurgy and Materials 54(1): 241-246.
Received 05.11.2019; accepted in revised form 10.01.2020
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