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614 MECHANICAL TESTING/MATERIALOGRAPHY
© Carl Hanser Verlag, München Materials Testing 60 (2018) 6
the heating rate and pattern [1,3,6]. 51CrV4
steel is a Cr-V alloyed heat-treatable steel
[11]. It can be used in many applications, for
example, commercially as spring steel [12].
The aim of the current study is to inves-
tigate quenching and tempering processes
at medium and low frequency induction of
51CrV4 steel. Another object of the study is
to investigate the possibility of minimizing
heat treatment duration in these processes.
Moreover, tempering temperatures and
their effects were investigated for both con-
ventional and induction heating.
Experimental studies
In this study 50 × 10 × 6 mm3 51CrV4 (SAE-
AISI 6150) steel samples were used. The
chemical composition of the steel is given in
and hardening have become increasingly
important thanks to advantages such as a
short process time, accurate temperature
control and cost effectiveness in comparison
with conventional heating methods [7, 8].
Because of its superior properties, it also
improves mechanical properties by provid-
ing for less decarburization [9, 10].
Induction heating relies on two mecha-
nisms: Joule heating and magnetic hystere-
sis. These two mechanisms generate heat-
ing. Non-uniform current distribution within
the conductor cross-section is called the skin
effect, since approximately 86 % of the power
will be concentrated in the surface layer of
the conductor. For this reason, induction is
generally used for surface heating. Moreo-
ver, coil design and selection of power-sup-
ply frequency and rating ensures control of
Electromagnetic induction is a heating
method employed for electrically conduc-
tive materials such as metals. It is used in
process heating prior to metalworking, heat
treating, welding and melting [1]. The most
important feature of induction heating is its
speed because heating occurs directly on
the metal. In general, induction is used for
surface heating. Morevover, heat transfer is
3,000 times better than that of other heat-
ing methods. In this way, the warming-up
process is completed more rapidly which
accounts for a reduction of time spent on
this phase [2–5]. Induction heating is con-
sidered one of the most powerful heating
methods for modern electromagnetic pro-
cessing of materials because it provides
energy-efficient heat in a minimal amount
of time [6]. Recently, induction tempering
Mechanical and microstructural properties of quenched steel are di-
rectly related to tempering time and temperature. In many applications,
conventionally quenched and tempered steel is widely used for acquir-
ing high strength and toughness. The present study was carried out
to investigate the variation in mechanical properties, observation of
diminished energy consumption and evaluation of the microstructural
properties in SAE-AISI 6150 steel components by induction heating,
compared with those of steel tempered by conventional method. Induc-
tion quenched and tempered steel provides a shorter process time, less
energy consumption and improved mechanical properties through the
inhibition of grain growth. In this study, quenching and tempering pro-
cesses were carried out on medium and low frequency induction units
and by using a conventional electrical resistance furnace for the sake
of comparison. It was observed that cementite particles began changing
their shape from spherical to fine-grained in the induction tempered
samples. The sample tempered by low frequency induction manifests
superior mechanical properties and offers a potential advantage for
significant cost savings.
Can Civi, Manisa, Metin Yurddaskal,
Izmir, Enver Atik, Manisa, and
Erdal Celik, Izmir, Turkey
Quenching and tempering of
51CrV4 (SAE-AISI 6150) steel
via medium and low frequency
induction
Article Information
Correspondence Address
Dr. Metin Yurddaskal
Department of Metallurgical and
Material Engineering
Dokuz Eylul University
35390 Izmir, Turkey
E-mail: metin.yurddaskal@deu.edu.tr
Keywords
Quenching, frequency, induction tempering,
microstructural investigation, mechanical
properties
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MECHANICAL TESTING/MATERIALOGRAPHY 615
60 (2018) 6
Table 1. The dimensions of the parts are
shown in Figure 1. Samples were first nor-
malized and quenched and then tempered
via induction and conventional mecha-
nisms, respectively. Normalizing was per-
formed through conventional heating. Two
different frequency models were used in the
induction mechanism (Figure 2). The induc-
tion mechanism used was a moderate fre-
quency unit (45 kHz-12 kW) for quenching
while a low frequency unit was used for
tempering (2.5 kHz-5 kW). Conventional
heating was carried out in an electrical re-
sistance furnace. The normalizing and aus-
tenitizing temperature was set for 870 °C.
Heating parameters are given in Table 2.
Heat treatment parameters were chosen ac-
cording to practical values currently applied
at a heat treatment company.
The quenching temperature of the oil was
40 °C. The properties of the quenching oil
are given in Table 3. All heat treatments
were performed in the air. Induction heating
was carried out within a copper coil (see Fig-
ure 3). A schematic illustration of the heat
treatments is given in Figure 4. The heating
rate was adjusted to 1 °C × s-1 and
100 °C × s-1 for conventional and induction
heating, respectively. During the induction
heating process, heating times were opti-
mized according to hardness values ob-
tained and the optimal duration determined
by the results of these values. Preliminarily,
half the traditional heat treatment periods
were targeted. The initial heating time was
reduced from 15 min to 1 min.
During the quenching process, medium
frequency was applied to the samples ow-
ing to rapid heating, During the tempering
process, low frequency was applied be-
cause of more uniform heating. A laser py-
rometer was used to keep the temperature
constant. Following the heat treatments, a
Rockwell-C test was performed on the sam-
ples according to ASTM E18-12, and the
Vickers hardness of the samples were
measured by loading 100 g of force for
10 seconds according to ASTM E384-11e1.
The Vickers hardness values of the sam-
ples were measured from the surface to the
bottom in order to maintain the skin effect
(see Figure 5). Moreover, the heat treated
samples were cut and microstructure im-
ages of the specimens were examined
through an optical microscope and via SEM
analysis. The images were taken on the
polished surface of the cross-section.
Results and discussion
Rockwell-C hardness measurements. In
this study, the conventional and induction
hardening processes of 51CrV4 heat treata-
ble steel were investigated and the mechani-
cal and microstructural properties of the
two conditions compared. For this compari-
son, tempering temperatures were signifi-
Figure 1: Dimensions of samples (mm) Figure 2: Induction equipment
Figure 3: Copper coil
Viscosity, cSt at 40 °C 26.6-28.0
Viscosity, cSt at 100 °C 4.8
Flash point, °C min COC 198
Table 3: Properties of quenching oil
Heating type
Heat treatment type
Normalizing
(870 °C)
Austenitizing
(870 °C)
Tempering
(420 °C)
Tempering
(240 °C)
Conventional heating 30 min 25 min 180 min 120 min
Induction –2 min 1 min 1 min
Table 2: Heating durations
CSi Mn P S Cr V
0.47-0.55 Max. 0.4 0.7-1.1 Max. 0.025 Max. 0.025 0.9-1.2 0.1-0.25
Table 1: Chemical composition of 51CrV4 steel(wt.-%)
Figure 4: Schematic
illustration of heat
treatments
Figure 5: Vickers hardness measurement points
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616 MECHANICAL TESTING/MATERIALOGRAPHY
60 (2018) 6
cantly affected hardness values. The Rock-
well-C hardness results of the specimens
are shown in Table 4. It can be seen that,
induction has shortened both the duration
of the austenitizing and tempering time.
Vickers hardness measurements. The
Vickers hardness values of the specimens
are given in Table 5. It was observed that
the hardness values did not deviate towards
the center. There was no skin effect in in-
duction sintering. This indicates that there
is no difference between the surface and
the center of the sample when considering
mechanical and microstructural properties.
The hardness values obtained for the same
temperature values by means of the induc-
tion hardening method were far superior to
those of the conventional method.
Microstructural investigation. The micro-
structures of the samples are shown in Fig-
ures 6 and 7.The microstructures of the nor-
malized samples consist of a mixture of ferrite
and pearlite with varied lamellar spacing. The
images of the samples obtained by the conven-
tional and induction methods appear to be
very similar. The Martensitic structure can
clearly be seen in the quenched samples. An
obvious difference between the microstruc-
tures of the tempered samples at varied tem-
peratures cannot be seen. However, the micro-
structure of the tempered samples varies from
both normalized and quenched samples when
considering the two methods for hardening.
SEM Investigation. SEM images of sam-
ples are given in Figures 8 and 9. The micro-
structure of both hardening methods reveals
fine carbides in the ferrite phase (see Fig-
ures 8a and 9a). Similar to the optical im-
ages, martensitic structures are clearly visi-
ble in the quenched samples. No size and
distribution differences of carbides can be
determined between specimens produced
by conventional and induction hardening.
Nevertheless, smaller grain size was ob-
tained via induction heating as compared to
conventional heating. Furthermore, the
presence of a martensitic structure in the
samples produced by induction is clearly in
Figure 6: Optical microscope images of conventional heated samples, a) normalized, b) quenched,
c) tempered at 240 °C for 120 min, d) tempered at 420 °C for 180 min
Figure 7: Optical microscope images of induction heated samples a) normalized, b) quenched,
c) tempered at 240 °C for 1 min, d) tempered at 420 °C for 180 min
Heating type 1 2 3 4 5 Average
Normalized 180.9 173.7 169.2 162.8 166.7 170.66
Quenched (conventional) 346.3 362.3 349.6 298.6 293.9 330.14
Tempered (conventional-240 °C) 304.3 290.1 245.2 242.3 256.4 267.66
Tempered (conventional-420 °C) 301.9 277.1 249.5 257.13 242.2 265.56
Quenched (induction) 473.3 499.6 449.6 448.5 433.1 460.82
Tempered (induction, 240 °C) 352.2 308.6 337.1 345.6 309.1 330.52
Tempered (induction, 420 °C) 286.3 305.7 289.6 338.4 345.9 313.18
Table 5: Vickers hardness values
Heating
type
Quenched
samples
Tempered
samples
(240 °C)
Tempered
samples
(420 °C)
Conven-
tional
heating
53 HRC 50 HRC 43 HRC
Induction 57 HRC 52 HRC 44 HRC
Table 4: Rockwell-C hardness values
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MECHANICAL TESTING/MATERIALOGRAPHY 617
60 (2018) 6
agreement with the literature [13–15].
These SEM results definitely prove that the
choice of hardening methods has a signifi-
cant effect on mechanical properties.
Conclusions
In this study, quenching and two varied
tempering processes were performed using
medium and low frequency induction
units. Conventional heat treatment param-
eters were obtained from a heat treatment
company. Induction heating was applied
for both quenching and tempering. Induc-
tion heating reduced the heating duration
for quenching (from 25 min to 2 min) and
for tempering (from 120-180 min to 1 min).
Moreover better hardness values were
achieved when low frequency induction
heating was used for tempering to attain
uniform heating. Better hardness values
and smaller grains sizes were obtained via
induction heating. The skin effect was not
observed. This study proves that induction
can be used not only for quenching but also
for tempering . This allows these processes
to be performed more quickly and with
lower energy consumption. Moreover, low-
frequency induction could prevent the skin
effect.Nevertheless, it is important to note
that medium frequency is more suitable for
quenching than low frequency because
rapid heating cannot be achieved at low
frequency. Furthermore, a lower frequency
in the tempering process can yield better
results since the process is performed at
lower temperatures.
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Materials Testing
60 (2018) 6, pages 614-618
© Carl Hanser Verlag GmbH & Co. KG
ISSN 0025-5300
The authors of this contribution
Dr. Can Civi, born in in 1987, graduated from
Celal Bayar University, Manisa, Turkey in 2009
with a degree in Mechanical Engineering. He com-
pleted his Master of Science thesis as well as his
PhD thesis at the Department of Mechanical Engi-
neering of Celal Bayar University in 2011 and
2016, respectively. He has been working as an
assistant professor in Department of Mechanical
Engineering at Celal Bayar University in Manisa.
Dr. Metin Yurddaskal, born in 1989, gradu-
ated from Dokuz Eylul University, Izmir, Turkey
in 2011 with a first degree in Metallurgical and
Materials Engineering. He completed his Master
of Science thesis and his PhD thesis at the De-
partment of Metallurgical and Materials Engi-
neering in 2014 and 2017, respectively. He has
been working as an assistant professor in the De-
partment of Metallurgical and Materials Enge-
neering at Dokuz Eylul University in Izmir.
He works on composite materials, nanoparticles,
thin films and materials characterization.
Prof. Dr. Enver Atik, born in 1963. He gradu-
ated from Istanbul Technical University in 1984
with a degree in Mechanical Engineering. He com-
pleted his Master of Science in Mechanical Engi-
neering at the Uludağ University, Bursa, Turkey
in 1987. In 1994, he received his PhD also from
that university. He has been working as a profes-
sor in the Mechanical Engineering Department at
the Celal Bayar University, Manisa, Turkey.
Prof. Dr. Erdal Celik, born in 1967, graduated
from Istanbul Technical University in 1993 with
a degree in Metallurgical and Materials Engineer-
ing. He is now director of the Center for Fabrica-
tion and Applications of Electronic Materials and
works as a professor at Dokuz Eylul University,
Izmir, Turkey. He also works on nanotechnology,
production techniques, electronic materials,
materials characterization and thin films.
Abstract
Induktives Abschrecken und Anlassen des Stahles 51CrV4 (SAE-AISI
6150) mit mittlerer und niedriger Frequenz. Die mechanischen und mik-
rostruturellen Eigenschaften abgeschreckter Stähle sind direkt von den An-
lasszeiten und -temperaturen abhängig. Konventionell abgeschreckte und
angelassene Stähle werden vielfach angewandt, um eine hohe Festigkeit
und Zähigkeit zu erhalten. Die diesem Beitrag zugrunde liegende Studie
wurde durchgeführt, um die Veränderung der mechanischen Eigenschaften
zu untersuchen, einen niedrigeren Energieverbrauch zu beobachten und
die mikrostrukturellen Eigenschaften in induktions-
wärmebehandelten
Komponenten aus dem Stahl SAE-AISI 6150 im Vergleich zu solchen mit
einer konventionellen Wärmebehandlung zu evaluieren. Induktiv abge-
schreckte und angelassene Stähle weisen niedrigere Herstellungszeiten,
einen geringeren Energieverbrauch bei der Herstellung und höhere me-
chanische Eigenschaften auf, da hierbei Kornwachstum verhindert wird.
In der Studie wurden Abschreck- und Anlassprozesse mittels Induktions-
einrichtungen bei mittlerer und niedriger Frequenz Im Vergleich mit ei-
nem konventionellen Widerstandsofen durchgeführt. Es stellte sich her-
aus, dass die Zementitpartikel in den induktions-wärmebehandelten Pro-
ben beginnen, ihre Form von kugelig zu einer feinkörnigen Struktur zu
verändern. Die Probe, die mit einer niedrigen Induktionsfrequenz wärme-
behandelt wurde, zeigte überragende mechanische Eigenschaften und ei-
nen potentiellen Vorteil bezüglich signifikanter Kosteneinsparungen.
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