Materials Sciences and Applicatio ns, 2011, 2, 572-577
doi:10.4236/msa.2011.26076 Published Online June 2011 (http://www.SciRP.org/journal/msa)
Copyright © 2011 SciRes. MSA
Quasi-Static and Dynanmic Deformation
Behaviors of Medium-Carbon Steels in a
Wide Temperature Range
Byoungchul Hwang
Ferrous Alloys Group, Korea Institute of Materials Science, Changwon, Korea.
Email: entropy0@kims.re.kr
Received December 10th, 2010; revised March 21st, 2011; accepted April 12th, 2011.
ABSTRACT
This paper presents a study of the quasi-static and dynamic deformation behaviors of conventional and microalloyed
medium-carbon steels in a wide temperature range. As strain rate increased, the flow stress increased at room tem-
perature, but occasionally did not at elevated temperatures. The flow stress of the microalloyed steel containing pre-
cipitates was less sensitive to stra in rate at room temperature than that of the con ventional steel due to a relatively lar-
ger activation leng th. Microstructural observation o f the steels deformed after compression test indicated that inhomo-
geneous deformation became more serious with increasing strain rate and temperature without fracturing in the highly
localized region.
Keywords: Qu asi - St at i c , Dy namic, Deformation Behavior, Medium-Car bon Steel, Precipitate
1. Introduction
Structural materials for automotive and machinery com-
ponents have frequently obtained the required mechani-
cal properties by heat-treating medium-carbon steels. In
order to reduce energy consumption by eliminating heat-
treatment steps, therefore, steady attention on non-
heat-treated steels has been paid for several decades.
Since the 49MnVS3 medium-carbon steel microalloyed
with V was first developed, non-heat-treated steels have
been extensively applied to machinery and automotive
components due to a good combination of strength and
toughness [1]. Presently, both the non-heat-treated and
heat-treated microalloyed medium-carbon steels where
stable carbon-nitride precipitates mainly form by adding
V, Ti, and Nb have been widely used as a substitute for
conventional heat-treated medium-carbon steels.
Although many researchers have reported the me-
chanical properties of medium-carbon steels under
quasi-static loading, the quasi-static and dynamic defor-
mation behaviors have not been systematically compared
over a wide temperature range, especially in conven-
tional and microalloyed medium-carbon steels. Since the
steels are highly likely to be subject to high-speed de-
formation during their manufacture and application, they
are required to acquire information on dynamic deforma-
tion behavior [2-4]. For example, plastic deformation can
become highly localized under the dynamic loading,
which is often called an adiabatic shear band. In this lo-
calized area, the load-carrying capability seriously dete-
riorates and cracks nucleate, thereby causing eventual
failures during hi g h-speed processing .
In the present study, quasi-static and dynamic com-
pressive tests were conducted on medium-carbon steels
using a universal testing machine and a compressive
Kolsky bar, and then the test results were analyzed in
terms of microstructure, strain rate, and temperature. The
aim of this study is to understand the effect of various
factors on quasi-static and dynamic deformation behav-
iors of the medium-carbon steels and the associated
mechanism.
2. Experimental Procedures
The materials used in this study were four kinds of me-
dium-carbon steels fabricated with different chemical
composition and heat-treatment conditions and were
supplied by Caterpillar Inc., IL, USA. Two kinds are
conventional SAE 1045 steels, and the others are
V-microalloyed SAE 15V45 steels. Table 1 compares
the composition range of the 15V45 steels with that of
Quasi-Static and Dynanmic Deformation Behaviors of Medium-Carbon Steels in a Wide Temperature Range573
the 1045 steels. The 15V45 steels are microalloyed with
V, and its Mn content is approximately doubled to that of
the conventional 1045 steel. Both steels were properly
heat-treated by quenching & tempering (Q&T) or
austempering in order to achieve a good combination of
strength and toughness. For convenience, they are re-
ferred to as ‘1045T’, ‘1045S’, ‘15V45T’, and ‘15V45S’
in this article, where the last letters ‘T’ and ‘S’ indicate
Q & T and austempering heat-treatments, respectively.
Compression tests were performed on them at low
strain rates of 0.001, 0.01, and 0.1 sec–1 using a universal
testing machine. Tests performed at these strain rates are
referred to as quasi-static in this article. The cylindrical
specimens with 5.8 mm diameter and 6.3 mm length
were machined from larger cylindrical disk using an
electrostatic discharge machine so as to minimize the
microstructure change. Specimen ends were lightly lu-
bricated with a silicon grease before testing to reduce
friction and ensure a flat surface.
A compressive Kolsky bar [2] was also used to test the
specimens at high strain rates above 1000 sec–1. Tests
performed at these strain rates are referred to as dynamic
in this article. The used specimens were cylindrical just
as the ones used for quasi-static compression tests. The
specimen situated between incident and transmission bar
was compressed by a steel projectile projected at a high
speed using argon gas pressure, and the strain rate could
be controlled by varying the length of a steel projectile
and the compressive gas pressure. During the dynamic
compression, the incident, reflected, and transmitted
pulses were respectively detected at strain gages, and
recorded at an oscilloscope. Among the recorded signals,
average compressive strain expressed as a function of
time was measured from the reflected signal, while com-
pressive stress expressed as a function of time was
measured from the transmitted signal. Dynamic com-
pressive stress-strain curves were obtained from these
two parameters by eliminating the time term. Both qua-
si-static and dynamic compression tests were conducted
over a temperature range of room temperature (25˚C) to
400˚C. Their microstructures were examined before and
after deformation by an optical microscope.
3. Results
3.1. Undeformed Microstructure
Figures 1(a) and (b) are optical micrographs of the
15V45T and 15V45S steels fabricated with different
heat-treatment conditions for the microalloyed 15V45
steels. The quenched and tempered steel (15V45T steel)
had typical fine-grained tempered martensite micro-
structure, whereas the austempered steel (15V45S steel)
showed conventional upper bainite structures. In all
Table 1. Composition of the conventional (SAE 1045) and
microalloyed0 (SAE 15V45) medium-crabon steels (wt%).
C Mn P S V
SAE
1045 0.43 ~ 0.500.60 ~ 0.90 <0.04 <0.05 -
SAE
15V45 0.43 ~ 0.501.35 ~ 1.65 <0.04 <0.05 0.10 ~ 0.25
(a)
(b)
Figure 1. Optical micrograph of the (a) 1045T and (b)
10V 4 5S steels. Arrows indicate MnS inclusions. Nita l e t c h ed.
steels, elongated MnS inclusions were present in the
form of globular inclusions and stringers, and were
mostly aligned along the rolling direction. The microal-
loyed 15V45 steels seemed to have finer microstructure
than the conventional 1045 steels.
3.2. Quasi-Static and Dynamic Stress-Strain
Curves
Figures 2(a) through (d) prent the quasi-static and es
Copyright © 2011 SciRes. MSA
Quasi-Static and Dynanmic Deformation Behaviors of Medium-Carbon Steels in a Wide Temperature Range
Copyright © 2011 SciRes. MSA
574
(a) (b)
(c) (d)
Figure 2. (a) and (b) Quasi-static and (c) and (d) dynamic compressive stress-strain curves of the steels tested at temperatures
from room temperature to 400˚C.
dynamic compressive stress-strain curves of the steels
tested at various temperatures. In dynamic tests, the total
plastic strain was relatively smaller than that obtained in
the quasi-static tests due to the constraint in the design of
Kolsky bar. The stress under quasi-static compression
was continuously increased with strain, whereas maxi-
mum stress appeared under dynamic compression be-
cause of the softening of material by local heating occur-
ring during dy namic loading [2].
In the steels tested at room temperature, the microal-
loyed 15V45 steels were relatively higher flow stress
than the convention al 1045 steels under both qu asi-static
and dynamic loadings, which resulted from the increased
amount of manganese and the precipitation of vanadium
carbides. Figures 2(c) and (d) are the quasi-static and
dynamic compressive stress-strain curves of the 1045S
and 15V45S steels tested at temperatures from room
temperature to 400˚C. The plastic deformation and flow
stress of materials are generally affected by test tem-
perature [2,5]. Since the flow stress expectedly decreased
with increasing temperature, the experimental results of
the 1045S and 15V45S steels showed typical behavior of
body-centere d cubi c ( bcc) m a t eri al s.
3.3. Deformed Microstructure
Figure 3(a) shows overall micrographs of the 1045S
steel dynamically deformed at an elevated temperature.
Examination of the cross-section of the deformed steels
showed a non-uniform deformation, which was more
visible in the middle region between corners and center
of the cross-section, whereas adiabatic shear bands were
not observed. The degree of heterogeneity became seri-
ous as strain rate or temperature increased. MnS inclu-
sions were mostly aligned along the deformation flow
direction and some of them were severely distorted in
heavily deformed region (Figure 3(b)).
In the specimens deformed at lower strain rates or
room temperature, however, MnS inclusions remained
Quasi-Static and Dynanmic Deformation Behaviors of Medium-Carbon Steels in a Wide Temperature Range575
(a)
(b)
Figure 3. Optical micrograph of the 1045S steel deformed
dynamically at an elevated temperature at a high strain
rate and (b) is a magnification of (a), showing a heavily
distorted MnS inclusion and the for m ation of voids.
slightly elongated and aligned along the rolling direction
as with those in the undeformed steels, and their shapes
did not change much. The distortion of MnS inclusions
was largely affected by strain rate than by test tempera-
ture.
4. Discussion
In general, it has been well recognized that the flow
stress slightly increases with logarithmic strain rate at
low strain rates, while it more rapidly increases with
logarithmic strain rate at high strain rates [2,4-6]. Figure
4(a) provides the effect of strain rate on the 5% flow
stress of the steels tested at room temperature. The flow
stress usually increased with the strain rate, and thus dy-
namic flow stress was higher than quasi-static flow stress.
Although the difference in flow stresses reflected scatter
for some specimens, the flow stress showed more in-
creased strain-rate dependency at high strain rates than at
low strain rates. Campbell and Ferguson [6] reported
from a study of the mechanical properties of mild-carbon
steel tested at strain rates from 10–3 to 4 × 104 sec–1 that
the shear flow stress was in linear proportion to the value
of strain rate on logarithmic scale at strain rates below
102 sec–1, and found that the strain-rate sensitivity of the
flow stress was a decreasing function of temperature. Lee
and Liu [4] obtained the dynamic flow stress of mild-
and medium-carbon steels at high strain rates between
103 and 104 sec–1, and confirmed that the magnitude of
the flow stress increased significantly with increasing
logarithmic strain rate at the high strain rates. In the
steels investigated, also, the strain-rate dependency of the
flow stress was relatively prominent at high strain rates
than at low strain rates, which indicates that the present
data results are in agreement with previous investigations
[2-6]. It should be noted that this behavior at low strain
rates is ascribed to the th ermal activation in which dislo-
cations require thermal energy to overcome obstacles
such as precipitates by cross slip, whereas there are still
many controversies over strain-rate controlling mecha-
nisms at high strain rates [2,5,7].
Figures 4(b) and (c) present the effect of strain rate on
the flow stress of the 1045S and 15V45S steels tested at
various temperatures of room temperature to 400˚C. The
flow stress of the 1045S steel was more largely affected
by strain rate at room temperature than that of the
15V45S steel having precipitates. The flow stress
con-
sists of two parts, a thermally activated part,
t, and an
athermal part,
a. Only
t is influenced by variation in
strain rate or temperature.
ta

(1)
The following equation is often used to describe a re-
lationshi p bet w een strain rate,
, and flow stress [8]:

0
0
exp
exp .
a
b
a
bbb
QV
kT
V
QV
kT kT kT






(2)
Here, 0
is a co nstant, Q is the activation energy, V is
the activation volume, kb is Boltzmann’s constant (1.38 ×
10–23 J·K–1), and T is an absolute temperature.
The activation volumes determined from the slopes in
Figures 3(b) and (c) were 6.45 × 10–28 m3 for the 1045S
steel and 7.82 × 10–28 m3 for 15V45S steel. The activation
Copyright © 2011 SciRes. MSA
Quasi-Static and Dynanmic Deformation Behaviors of Medium-Carbon Steels in a Wide Temperature Range
Copyright © 2011 SciRes. MSA
576
(a) (b) (c)
Figure 4. (a) 5% flow stress plotted as a function of strain rate for the steels tested at room temperature. (b) and (c) are 10%
flow stress plotted as function of strain rate for the 1045S and 15V45S steels tested at temperatures up to 400˚C.
volume for pure iron is 3.8 × 10–28 m3 [9]. Also, the acti-
vation volume is:
V = bdl (3)
where b is the Burgers vector, d is the activation distan ce
and l is the activation length. In this case, d is equal to b
the distance from on Peierls valley to the next.
As the Burgers vector is equal to 2.48 × 10–10 m for
bcc iron, the activation length of the 1045S and 15V45S
steels is about 1.05 × 10–8 m and 1.27 × 10–8 m, respec-
tively. The 15V45S steel having precipitates has a larger
activation length than the 1045S steel and thus lowers
locally the Peierls stress. This result is consistent with the
theory that precipitates interact with screw dislocations in
bcc iron locally [5,8]. As a result of the lowered Peierls
stress, the flow stress of the microalloyed 15V45S steel
having precipitates is less sensitive to strain rate than that
of the conventional 1045S steel that does not have pre-
cipitates (Figures 4(b) and (c)).
In the 1045S and 15V45S steels tested at temperatures
over 200˚C, the flow stress did not clearly increase with
increasing strain rate. As the strain rate increased, the
flow stress remained constant or rather decreased unlike
the results tested at room temperature. According to the
Equation (2), the flow stress had a relatively lower
strain-rate dependency at an elevated temperature than at
room temperature. It is possibly attributed to the fact that
a localized inhomogeneous deformation occurs by the
increased temperature.
On the other hand, deformation after compression tests
may be inhomogeneous and be concentrated into two
bands that formed an X-shaped pattern on a polished
section. In three dimensions, this pattern would be the
surfaces of two cones of deformation with their apices in
the center of the specimen [7,10]. Such deformation is
typical of material that has undergone barreling during
compression testing. At high-speed deformation of mate-
rials, adiabatic heating also may occur, where the heat
generated during the deformation in a particular region is
retained and causes a local rise in temperature [3,7].
Further deformation is concentrated preferentially in this
zone and the process may continue until fracture occurs.
One of the good examples of this phenomenon is the
formation of adiabatic shear bands in HY-100 and AISI
4340 steels having a tempered martensite structure [3,10],
which is the same microstructure to the 1045T and
15V45T steels. Observation results of the steels de-
formed under the current strain rates and temperatures,
however, indicated that all the steels were not fractured
after compression tests. Also, voids or cr acks was hardly
observed, but only a few isolated voids were occasionally
formed without being connected to form a crack even in
the steels dynamically deformed at elevated temperatures
(Figure 3(b)). Consequently, the medium-carbon steels
investigated in this study had the ability enough to with-
stand high-speed deformation at strain rates of up to 3000
sec–1 and temperatures from room temperature to 400˚C
without fracturing. This means that the medium-carbon
steels are suitable materials for high-speed deformation
processing.
5. Conclusions
The quasi-static and dynamic deformation behaviors of
conventional and microalloyed medium-carbon steels
were analyzed in terms of microstructure, strain rate, and
temperature. The following conclusions can be drawn. 1)
The flow stress increased with strain rate and was more
strain-rate sensitive at high strain rates than at low strain
rates, whereas it decreased with increasing temperature;
2) The flow stress of the 15V45S steel containing pre-
Quasi-Static and Dynanmic Deformation Behaviors of Medium-Carbon Steels in a Wide Temperature Range577
cipitates was relatively less sensitive to strain rate at
room temperature than that of the conventional 1045S
steel that did not have precipitates because of the in-
creased in activation length; 3) The quasi-static and dy-
namic deformation under the current strain rate and tem-
perature were non-uniform that was typically found in
compression tests, and the degree of heterogeneity in-
creased as strain rate or temperature increased. Therefore,
the present study provides a valuable reference for the
application of these medium-carbon steels.
6. Acknowledgements
This work was sponsored by the Korea Research Foun-
dation under Grant No. 2005-214-D00201. The author
thanks Professor C. L. Briant of Brown University and C.
Bissahoyo of Caterpillar Inc. for their help with the co m-
pressive dynamic Kolsky testing.
REFERENCES
[1] T. Gladman, “The Physical Metallurgy of Microalloyed
Steels,” 1st Edition, the Institute of Materials, London,
1997.
[2] M. A. Meyers, “Dynamic Behavior of Materials,” John
Wiley & Sons, New York, 1994.
doi:10.1002/9780470172278
[3] K. M. Cho, S. Lee, S. R. Nutt and J. Duffy, “Adiabatic
Shear Band Formation during Dynamic Torsional De-
formation of an HY-100 Steel,” Acta Materialia, Vol. 41,
No. 3, 1993, pp. 923-932.
doi:10.1016/0956-7151(93)90026-O
[4] W.-S. Lee and C.-Y. Liu, “Comparison of Dynamic
Compressive Flow Behavior of Mild and Medium Steels
over Wide Temperature Range,” Metallurgical and Mate-
rials Transactions A, Vol. 36A, No. 11, 2005, pp. 3175-
3186. doi:10.1007/s11661-005-0088-1
[5] M. A. Meyers, R. W. Armstron and H. O. K. Korchner,
“Mechanics and Materials—Fundamentals and Link-
ages,” John Wiley & Sons, New York, 1999.
[6] J. D. Campbell and W. G. Ferguson, “The Temperature
and Strain Rate Dependence of the Shear Strength of
Mild Steel,” Philosophical Magazine, Vol. 21, No. 169,
1970, pp. 63-82. doi:10.1080/14786437008238397
[7] Y. Bai and B. Dodd, “Adiabatic Shear Localization—
Occurrence, Theories and Applications,” Pergamon Press,
New York, 1994.
[8] S. Vaynman, M. E. Fine, S. Lee and H. D. Espinosa, “Ef-
fect of Strain Rate and Temperature on Mechanical Prop-
erties and Fracture Mode of High Strength Precipitation
Hardened Ferritic Steels,” Scripta Materialia, Vol. 55, No.
4, 2006, pp. 351-354.
doi:10.1016/j.scriptamat.2006.04.029
[9] A. Smith, H. Luo, D. N. Hanlon, J. Sietsma and S. van
der Zwaag, “Recovery Processes in the Ferrite Phase in
C-Mn Steel,” ISIJ International, Vol. 44, No. 7, 2004, pp.
1188- 1194. doi:10.2355/isijinternational.44.1188
[10] A. G. Odeshi, M. N. Bassim, S. Al-Ameeri and Q. Li,
“Dynamic Shear Band Propagation and Failure in AISI
4340 Steel,” Journal of Materials Processing Technology,
Vol. 169, No. 2, 2005, pp. 150-155.
doi:10.1016/j.jmatprotec.2005.03.016.
Copyright © 2011 SciRes. MSA