Article
citation information:
Vakulenko, I., Vakulenko, L.,
Proydak, S. The
influence of nonmetallic inclusion on strain hardening carbon steel. Scientific Journal of Silesian University of
Technology. Series Transport. 2019, 103,
193-198. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2019.103.15.
Igor VAKULENKO[1],
Leonid VAKULENKO[2], Svetlana PROYDAK[3]
THE
INFLUENCE OF NONMETALLIC INCLUSION ON STRAIN HARDENING CARBON STEEL
Summary. On a
fragment of the rim of a railway wheel removed from service, the volume of the
metal with non-metallic inclusions located near the tread surface was
investigated. The use of the microhardness measurement technique made it
possible to establish the nature of strain hardening of carbon steel near
non-metallic inclusions. It showed that with a normal orientation of the
plastic flow relative to the inclusion surface, the metal volumes undergo
hardening. In proportion to the appearance of a fraction of the tangential
component of the deformation near the nonmetallic inclusion, a decrease in the
degree of hardening of the metal was observed.
Keywords: deformation, steel, railway wheel, microhardness,
non-metallic inclusion
1. INTRODUCTION
Improving the
operational safety of railway wheels is achieved through different qualitative
technological solutions [1]. Purposeful development of proposals for changing
the chemical and phase composition of steels for the manufacture of railway
wheels [1, 2], explaining the influence of dispersion and morphology of
structural components on the complex of properties [3], including after thermal
hardening [3, 4], is impossible without explaining the mechanism behind the
formation of damage to the metal on the rolling surface [5]. From the
interaction with the rail, the resulting non-uniform distribution of plastic
deformation of the metal over the rolling surface of the wheel [5, 6] receives
an additional gain from the presence of non-metallic inclusions [7, 8].
The assessment of
the level of internal stresses that are formed in the metal close to the
particles [9] indicates the total effect not only of size, shape and
distribution but also the nature of the origin of non-metallic inclusions.
Indeed, as shown in [7, 10], differences in the type of crystal lattice, the
ability to thermally expand, etc. metal matrix and particles of non-metallic
inclusions, leads not only to a change in the magnitude of residual stresses
but their sign as well [11, 12, 13].
2. MATERIALS AND METHODS
As an object of study, a fragment of a
whole-rolled railway wheel was taken out of service due to damage to the
rolling surface above regulatory restrictions. The chemical composition met the
requirements in the redistribution of steel grade used for the manufacture of
railway wheels: 0.62% C, 0.78% Mn, 0.30% Si, 0.031% S, 0.029% P, 0.09% Ni, 0.14
% Cr, 0.15% Cu. The selection of blanks for the manufacture of the object of
study for the microstructure was carried out from areas of the rim with
detected damage to the rolling surface.
The microstructure was studied under a
light microscope. Sample preparation for the study was carried out in
accordance with the methods of structural analysis. In order to prevent
distortion of the field of residual stresses in the metal, after the grinding
operation, the work-hardened layer was removed from the surface of the sample
using the electrolytic dissolution technique [14]. To identify the
microstructure of the metal, the samples were etched with nital. As a
characteristic, to assess the hardening of metal micro volumes, microhardness (Hm),
measured by a PMT-3 type instrument, was used with an indenter load of 0.49 N.
3. RESULTS AND
DISCUSSION
In addition to the influence of
carbon content in steel and the degree of thermal hardening on the
damageability of the rolling surface, there is a definite dependence of the
formation of the number of local metal absence on the rolling surface in the
presence of non-metallic inclusions [15]. Figure 1a shows the rolling surface
of a railway wheel with typical damage in the form of local metal absence,
which was removed by machining when the profile of the rim was restored. The
formation of local metal absence with the orientation of growing cracks inside
the rim (Fig. 1b, designation AB), is
one of the reasons for removing the railway wheel from operation as it is unfit
for future use.
As a
result of the layer-by-layer removal of metal from the surface of the rim
fragment, the effect of a nonmetallic inclusion particle in the change in the
direction of a growing crack was found (Fig. 1a, symbol B). This confirms the given growth
sequence, the magnitude of the divergence the crack faced, which is maximum in
point A and minimal in point C (Fig. 1b). Considering
the cyclical nature of the change in the effective stresses in the wheel-rail
contact area, the normal component of the deformation should ensure the metal
hardening from the development of the deformation hardening processes. With
increasing distance from the rolling surface, the damped effect of strain
hardening is accompanied by a decrease in residual stresses in the metal [16].
From the formal analysis, it follows that when the particle is located on the
path of propagation of plastic flow, distortion would arise in the monotonous
nature of the metal work hardening. Moreover, the higher the particle hardness
compared to the metal matrix, the more its effect in the change in the strength
characteristics of the metal. For the case under study, the immutability of the
particle shape (Fig. 1b) indicates that the nature of the effect on the metal
matrix should be classified as non-deforming inclusions [9, 17].
a
C B
b
Fig. 1.
Appearance of damage to the rolling surface of the railway wheel (a) and
fragment of the rim of study (b)
Considering
the differences between the metal lattices and the inclusion, the formation of
internal stresses from the phase hardening should contribute to the initiation
of a crack at the interface. However, the observed crack growth trajectory (point
B, Fig. 1 b) indicates that the
plastic flow in the bulk of the metal between the rolling surface and the
non-metallic inclusion is most likely due to the formation of compression
stresses. Indeed, if it is assumed that the strain hardening of the metal near
the particle surface is ensured by the action of the normal component of stress
from the wheel-rail interaction, it becomes clear that there is no effect of
reduced resistance to crack growth along the interface. The distribution of the
indenter prints across the thickness of the metal layer enclosed between the
crack and the inclusion surface (symbol B,
Fig. 1b) and the corresponding pattern of change Hm
depending on distance (L) with
distance from the particle are shown in Fig. 2.
The extreme nature of
the ratio Hμ ~ f (L) indicates the effect of hardening the metal from the
normal orientation of the plastic flow relative to the particle surface.
Despite the fact that the metal interlayer between the surfaces of the particle
and the crack (Fig. 1b, designation B)
is in the glued-up state to a hardness level of 3.6-3.75 GPa, the magnitude of
the increase still reaches 2 times. According to external signs, the presented
dependence is similar to that observed during thermal effects on the metal.
However, when the occurrence of internal stresses in the metal close to the
particles is the result of differences only in the coefficients of thermal
expansion, in the absence of texture, the stresses are oriented radial relative
to the inclusion surface [14]. Based on this, the cyclic change of the stages
of heating and cooling the railway wheel during operation is already
accompanied by an increase in internal stresses near the interfacial interfaces
[5, 6].
a
b
Fig. 2.
The location of the indenter hardness tester is perpendicular to the inclusion
surface (indicated by an arrow) (a) and the corresponding values Hm (b) as
they move away from the particle
In the case of propagation of
plastic deformation during the work hardening of the metal over the rolling
surface, the picture is somewhat different. Considering that the interfacial
interface is capable of performing the functions of both the nucleation and
annihilation sites of dislocations [14], the presence of the inclusion should
inevitably lead to a violation of the conditions of uniformity of plastic
flow propagation. Based on this, depending on the
degree of plastic deformation, the volumes of the metal near the particles
should be enriched to a different degree by defects in the crystal structure
(dislocations, vacancies, etc.).
With the frontal orientation of
the flow relative to the interfacial surface, after exhaustion of the resource
of accumulation and annihilation of dislocations, the number of inhibited
crystal structure defects should increase when approaching the particle, which
is confirmed experimentally (place N = 1 measurement Hm,
Fig.3b). On the other hand, in the neighbouring micro volumes of the metal,
where the influence of the particle is reduced or absent, the process of
accumulation of these defects should be different. As a result, we should
expect a change in the field of residual internal stresses and, as a
consequence, the nature of the distribution of the metal microhardness along
the contour near the particle of a nonmetallic inclusion (Fig. 3).
Experimental data Hm when
measuring along the interfacial surface (Fig. 2a) indicate a change in the
nature of the strain hardening of the metal when it deviates from the frontal
orientation of the deformation. Indeed, an increase in the proportion of
deformation with a tangential component is accompanied by a gradual decrease Hm (Fig.
3b), reaching minimum values (points 5.6). The result obtained indicates the existence, at
least, of a qualitative dependence of the level of residual stresses in the
metal layer (Fig. 2 b) on the contact phenomena in the interaction of the wheel
and the rail. Moreover, in the area of maximum tangential stresses (N = 5.6), a
decrease Hm was
found to 1.8 GPa, which is lower than the minimum values Hm (Fig. 2b).
Based
on this, it can be assumed that the closer the inclusion is to the incline, the
greater the distortion that must be made in the field of metal deformations in
volumes close to the rolling surface of the wheel. Given the current trend of
increasing the strength characteristics of railway wheels, we should expect a
decrease in the crack resistance of the metal from the non-uniform stress
distribution in the micro volumes of the metal close to the non-metallic
inclusions. One evidence of this is the result of the study of causes of
reduction in cyclic crack resistance of high-strength wheels during operation
[19]. The observed effect of non- metallic inclusion particles is in fact much
more complicated if one considers the effects of a thermal nature [2, 5, 7, 20].
In
addition to the work hardening on the rolling surface, changes are expected in
the metal structure from successive stages of intense heating during
deceleration of the rolling stock and accelerated cooling of the wheel rim from
the incoming airflow. The structural state of the steel will be determined by
the ratio in the development of qualitatively different mechanisms of
structural transformations from diffusion to shear [4, 19]. Therefore, the complex
processes of redistribution of defects in the crystal structure near the
particles can lead to unpredictable changes in internal stresses in the metal
of the railway wheel.
a
b
Fig. 3.
Layout of locations (N) of
measurement Hm close to non-metallic inclusion (a)
and the corresponding values Hm
depending on N (b);
the
direction of plastic deformation (OD)
is indicated by an arrow
4.
FINDINGS
1.
The character of the strain hardening of
carbon steel near non-metallic inclusions was investigated.
2.
In the case of normal orientation of the
plastic flow relative to the inclusion surface, the metal volumes were
subjected to hardening.
3.
The growth of the share tangential component
of the deformation near the non-metallic inclusion was accompanied by a
corresponding decrease in metal hardening.
Acknowledgements
The authors are
grateful to Dr. V.G. Anofriev for providing material for research. The authors
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 the research.
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Received 22.01.2019; accepted in revised form 29.04.2019
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