Scientia et Technica Año XXVIII, Vol. 29, No. 03, julio-septiembre de 2024. Universidad Tecnológica de Pereira. ISSN 0122-1701 y ISSN-e: 2344-7214 109
Toughness enhancement of low carbon steel
through bainitic transformation
Aumento de la tenacidad del acero bajo carbono mediante transformación bainítica
S. E. Bolaños-Bernal ; M. J. Monsalve-Arias ; R. Rodriguez-Baracaldo
DOI: https://doi.org/10.22517/23447214.25161
Scientific and technological research paper
Abstract— This study investigates the effect of continuous cooling
treatment on the impact toughness and ductile-brittle transition
temperature of a low carbon ferrite-pearlite dual-phase steel.
Impact testing was performed according to ASTM E23 at
temperatures ranging from -60ºC to 90ºC. The results indicate a
significant increase in toughness of approximately 64% and a
reduction in the ductile-brittle transition temperature from 50ºC
in the as-received condition to 0ºC after heat treatment. These
changes were analyzed through microstructural examination and
fractographic analysis. A bainitic transformation was observed,
leading to microstructural refinement and an associated toughness
improvement. Additionally, a change in fracture surface
morphology was noted in the heat-treated steel, as the bainitic
transformation resulted in an increased ductile fracture area
across the tested temperature range.
Index Terms— Toughness, Charpy impact test, dual-phase
steel, bainitic transformation.
Resumen— Este artículo estudia el efecto sobre la resistencia al
impacto y la temperatura de transición dúctil-frágil del
tratamiento de enfriamiento continuo para un acero de fase dual
de ferrita-perlita con bajo contenido de carbono. El ensayo de
impacto se ejecutó de acuerdo con ASTM E23 a una temperatura
entre -60ºC a 90ºC. Hubo un aumento en la tenacidad de
aproximadamente el 64%, y una disminución en la temperatura
de transición dúctil-frágil de 50ºC (acero estado entrega) a 0ºC
después del tratamiento térmico. Los cambios obtenidos se
analizaron a partir de la microestructura y las superficies de
fractura del material. Se evidenció una transformación bainítica
que permitió un refinamiento microestructural y, en consecuencia,
aumentó la tenacidad.
Palabras claves— Tenacidad, ensayo de impacto Charpy, acero de
fase dual, transformación bainítica.
I.
INTRODUCTION
The amount of energy required to fracture the material and the
ductile-brittle transition temperature (DBTT), the temperature
This manuscript was sent on June28, 2021 and accepted on September 22,
2024. This work was supported by Universidad Nacional de Colombia, Bogotá.
Sergio E. Bolaños-Bernal is with Grupo de investigación IPMIM,
Universidad Nacional De Colombia , Car 30 No 45-03, Bogotá D.C, Colombia,
(e-mail: sebolanosb@unal.edu.co).
at which the material changes its behavior from ductile to
brittle, are properties of great importance for purposes of
engineering design, which can be evaluated with impact tests.
The type of material behavior is evidenced on the fracture
surface: on the one hand, a granular, shiny, and relatively flat
surface appearance indicates a brittle material, and the on the
other hand, a dull, porous appearance, with some stretch marks,
shows a ductile material. [1-3]. Fracture avoidance in vessels
and pipelines is an important engineering challenge because
they are usually subjected to low temperatures and could enter
in the DBTT of the material [4]. If this condition is disregarded
in design, a brittle fracture may occur. Brittle fracture is defined
as the sudden rapid fracture under stress, where the material
exhibits little or no evidence of ductility [5]. This fracture is
unexpected and catastrophic since it can propagate at high
velocity [6]. Moreover, not only vessels and pipelines may
suffer low-temperature embrittlement [6], but also the
aerospace, nautical, nuclear and fossil-fuel power generation,
chemical and other industries suffer as well [7].
Heat treatments in steels allow obtaining a wide diversity of
mechanical properties in the material, without altering the
carbon content or alloy elements. The toughness in steels can
be enhanced by heat treatments of annealing (or tempering after
quenching), allowing internal stress relief, increase in grain
size, homogenization in the microstructure, nucleation of new
grains (in case of a previous material deformation), and other
mechanisms [1].
Due to the rapid cooling in the quenching treatment, there is
not enough time for the atoms in the steel arranged in an FCC
(austenite) crystalline structure to transform into a stable phase
BCC (ferrite). Annealing allows the atoms in the steel to be
located more stably from the metastable phase (generated by
continuous cooling), thus enabling stress relief, precipitation in
the matrix, formation of phases such as ferrite, and other
mechanisms, according to the treatment temperature and time.
Mónica Johanna Monsalve Arias is with Grupo de investigación AFIS,
Universidad Nacional De Colombia , Car 30 No 45-03, Bogotá D.C, Colombia,
(e-mail: momonsalvea@unal.edu.co).
Rodolfo Rodriguez-Baracaldo is with Grupo de investigación IPMIM,
Universidad Nacional De Colombia , Car 30 No 45-03, Bogotá D.C, Colombia,
(e-mail: rodriguezba@unal.edu.co).
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Since the presence of single-phase ferrite is greater in the
microstructure of the material, the energy absorption capacity
increases [8], and a lower amount of high hardness phases
allows an increase in material toughness [9,10]. The variation
in mechanical behavior obtained by heat treatments can be
evidenced and explained by a microstructural study.
Due to their high strength, toughness, and weldability
replacing the conventional quenched and tempered medium-
carbon steels, low-carbon bainitic steels have created enormous
interest among scientists across the world [11]. Bainitic steels
are regarded as relatively new steels because not long ago it was
impossible to produce them in the industry with the required
strength and toughness [12]. Toughness in bainitic steels is
directly related to the volume fraction of bainite and fine grain
size reached in the microstructure [11-13]. According to the
API grade, toughness is better in bainitic than ferrite-pearlitic
steels [14]. Specifically, within the bainite formation, the
mechanical stability of retained austenite is important to obtain
good toughness in bainitic steels [13,15]. Carbide free bainite
has achieved the highest strength and toughness combinations
to date for bainitic steels in as-rolled conditions [13].
Several works indicate the effect of annealed treatment on
the value of the impact toughness and DBTT in dual-phase
steel, increasing the amount of energy required to fracture the
material at low temperatures and also reducing the DBTT
[16,17]. Nevertheless, research to enhance toughness in steels
by bainitic transformation is limited. This study aims to
determine the relationship between the energy absorbing
capacity at a wide range of temperatures and microstructural
characteristics modified by continuous cooling heat treatment
in low carbon steel.
II.
MATERIALS AND METHODS
When A low-carbon steel (AISI 1020) with extensive
industrial and structural applications was selected. As-received
material was provided in a cold-rolled condition. Chemical
composition, determined by optical emission spectrometry, is
shown in Table I.
TABLE I
CHEMICAL COMPOSITION OF THE DUAL-PHASE STEEL.
Element
wt%
C
0.174
Si
Mn
P
S
Cr
Ni
Mo
Fe
0.156
0.788
0.022
0.012
0.010
0.036
0.012
98.67
Source: The authors
The heat treatment for steel samples included austenitizing at
840ºC for 30 minutes, continuous cooling in unshaken water,
and then annealing at 650ºC for 40 minutes followed by air-
cooling. In both cases, continuous cooling and annealing
treatment, the furnace was preheated to avoid excessive
decarburization.
The Charpy impact test was executed according to ASTM
E23-24 [18]. A Charpy impact machine, WPM Veb
Toffprufmachinen brand, with a capacity of 30 Kg-m (294.3 J)
was used. The dimensions of the samples were 10 mm x 10 mm
x 55 mm with a V-notch at 45º and 2 mm deep. The test was
carried out at a temperature between -60ºC to 90ºC to determine
the ductile-brittle transition temperature. Alcohol with liquid
nitrogen was used to cool the samples, and hot water to heat
them. Samples and centering tongs were stabilized at a set
temperature for 5 minutes. The test time per sample was lower
than 5 seconds.
Optical micrographs were analyzed using a Leco optical
microscope, and a Tescan Vega3 scanning electron microscope
(SEM). The fracture surface was analyzed using an Olympus
stereoscope and the SEM microscope previously used.
III.
RESULTS AND DISCUSSION
A.
Microstructure
Figure 1. Optical micrographs of as-received steel (a, b) and heat-treated steel
on the surface (c, d) and the center (e, f) of the sample. Source: The authors
Fig. 1 shows optical micrographs of as-received steel. Ferrite
grains for low carbon steel (Fig. 1a and 1b) are evidenced, with
a grain size ASTM 7-8 approximately [19]. The microstructure
obtained by heat treatment is evidenced on the surface (Fig. 1c
and 1d) and in the center of the material (Fig. 1e and 1f). The
presence of two phases is evident in both cases. The white phase
corresponds to the formation of ferrite; however, it is possible
to observe some very fine precipitates within this phase,
possibly due to carbides formation. Between the surface and the
center of the material, an increase in the white phase (ferritic
matrix) is observed, corresponding to a process of
decarburization of the steel during heat treatment. The dark
Scientia et Technica Año XXVIII, Vol. 29, No. 03, julio-septiembre de 2024. Universidad Tecnológica de Pereira.
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phase does not allow a clear identification with this
magnification; in some regions, short, acicular, and/or granular
precipitates are observed. Very thin and no uniform elongated
plates are also observed forming linear or islands clusters. The
morphology is not clear and very diffuse in other areas. The
dark phase regions are more diffuse and thinner in the center of
the material, thus making it difficult to identify the
microstructure.
Figure 2. SEM micrographs of heat-treated steel in (a) medium and (b) high
magnification. Source: The authors
Fig. 2 shows the microstructure of heat-treated steel obtained
by SEM in different scales. A low-relief ferritic matrix is
conserved, with some precipitates in greater relief, thus
presenting an acicular and/or granular morphology in most
regions, arranged in a linear distribution in some cases, and
more dispersed in others. A notable difference of sizes is
observed between these precipitates, with some of them being
still very fine to achieve a clear identification. Similar SEM
micrographs to Fig. 2 were reported by Vander in bainitic steel
[20] and Bhadeshia in 0,15C-2,25Cr-0,5Mo wt% steel [15].
According to the observations made above, and to comparisons
with some references, it is determined that the microstructure
corresponds mostly to granular bainite, due to the continuous
cooling treatment. References pointed out a granular bainite
microstructure with some regions of retained austenite and
tempered martensite [15,20,21]. Additional phases could be
part of the microstructure obtained in this study.
The formation of bainite by continuous cooling treatment can
be explained due to the overlapping of characteristic C-curves
for the formation of pearlite and bainite in TTT transformation
diagrams. For steels where the reaction rate is rapid, it becomes
experimentally difficult to distinguish the two C-curves as
separate entities [15,22]. For plain carbon and very low-alloy
steels, a significant overlap is observed between the
transformation temperature ranges of bainite and pearlite
[20,23], taking the form of just a single C-curve. Since the
different reactions overlap, it is difficult to distinguish the
curves using conventional experimental techniques [15].
However, Bhadeshia [23] has proposed mathematical models
based on thermodynamic analyses of isothermal
transformations for the prediction of TTT diagrams in an
extensive range of steels. In this C-curves perlitic and bainitic
transformations are distinguished and compared with
experimental results.
Fig. 3 shows the specific CCT (continuous cooling
transformation) diagram for the chemical composition in table
I. In addition to the initiation ferritic transformation line, the
Bainitic transformation line is highlighted between cooling
rates of 0,16 to 40 C/s. Vander Voort, G. [24] presents a small
region of bainitic transformation CCT diagram for an AISI
1010 steel. Granular bainite is only observed in low- or
medium-carbon steels and associated with continuous cooling
processes rather than isothermal treatments [15,20]. Because
the transformation occurs gradually during cooling, the bainitic
packets are coarse, giving the resultant microstructure a
granular appearance [20]. A characteristic feature (yet not
unique) of granular bainite is the lack of carbides in the
microstructure because the carbon partitioned from the bainitic
ferrite stabilizes the residual austenite; this typically results in
bainite, retained austenite, and some high-carbon martensite
being present in the microstructure [15,20]. The low carbon
concentration ensures that any film of austenite or regions of
carbide that might exist between sub-units of bainite is minimal,
making the identification of individual platelets within the
packets rather difficult using optical microscopy [15]. In
general, the peculiar morphology is a consequence of two
factors: continuous cooling transformation and low carbon
concentration [15].
Figure 3. Thermo-Calc continuous cooling transformation (CCT) diagram
analysis. Source: The authors
B.
Impact test (DBTT and energy absorption capacity)
Table II summarizes the impact toughness results for all the
temperatures analyzed and the applied sigmoid fitting function.
Fig. 4 shows the energy absorbed by impact test. Behavior is
observed for as-received and heat-treated steel, both curves
represent a sigmoidal function. For as-received steel, a brittle
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behavior is exhibited below 20ºC, and it is presented with a
completely ductile fracture above 80ºC approximately. This
range corresponds to the transition temperature of the ductile-
brittle behavior of the material. The temperature corresponding
to the average energy between the ductile and brittle regions
was selected as a criterion (within several applicable criteria) to
determine the DBTT [2]. Therefore, it is estimated in the middle
region, corresponding to 50ºC as the transition temperature of
the material.
TABLE II
CHARPY IMPACT ENERGY TEST FOR AS-RECEIVED AND HEAT-TREATED
STEEL.
Energy
absorbed (J)
Standard
Deviation (J)
Sigmoid Funtion
(J)
As-received
1,67
1,36
8,26
4,09
1,15
8,26
4,09
0,91
8,26
3,47
0,85
8,26
7,32
0,98
8,28
10,14
3,12
8,34
10,43
1,53
8,54
17,99
2,87
9,72
21,35
2,68
12,22
26,23
2,36
19,92
28,87
10,44
39,01
63,04
24,20
64,30
97,09
7,65
80,74
77,92
7,43
87,12
86,82
5,16
89,28
Heat-treated
28,63
1,39
8,26
71,42
26,35
8,26
91,72
0,00
8,26
159,58
29,70
8,26
139,30
6,94
8,28
154,95
25,44
8,34
141,02
20,02
8,54
139,06
2,71
9,72
150,88
5,55
12,22
Source: The authors
For heat-treated steel, a fully brittle behavior is exhibited
below -30ºC approximately, and the same way, it exhibits
ductile behavior above 20ºC. According to the selected
criterion, the transition temperature of the heat-treated steel is
determined between 0°C and 5ºC approximately. A notable
change in DBTT and energy absorption capacity by the steel
after heat treatment is observed.
Transition temperature range for both cases have a similar
size. However, there is a decrease of between 40ºC and 50ºC in
the value of the DBTT after heat treatment, indicating that the
steel has a higher ductile behavior, in a higher region of
temperatures than as-received steel. Moreover, a considerable
scattering in the DBTT region is observed in both cases,
showing more notable in the heat-treated steel. This behavior
was also described by Shi et al. in 0,09C-1,33Mn-0,13Mo-
0,34Ni wt% steel [4] and Cubides in 0,18C wt% steel [25].
Figure 4. Energy absorbed in Charpy impact test at different temperatures for
as-received (red) and heat-treated steel (blue). Source: The authors
The upper shelf energy by the as-received steel is 97 J at 70ºC
approximately. For heat-treated steel, upper shelf energy is 159
J at ambient temperature (22ºC) approximately. Therefore, after
the heat treatment, there is an increase in material toughness of
62 J approximately in energy absorbed by the impact,
corresponding to an additional 64% respect to the as-received
steel. Similar results have also been reported by Ibrahim in
0,18C-0,66Ni-0,58Mo wt% steel obtaining an increase in
energy absorption capacity of steel from 80 J to 200 J and
reduced DBTT from 25ºC to -25ºC, after continuous cooling
and annealed treatment [26].
This behavior obtained can be explained due to bainitic
formation, which causes a deviation in the crack propagation
path enhancing impact toughness and decreasing its transition
temperature [17]. The impact toughness and its scattering could
be determined by the bainitic packet’s size and its distribution.
[4].
C.
Fracture
Figure 5. Fracture surface after the Charpy impact test at different temperatures
of as-received and heat-treated steel.Source: The authors
Fig. 5 shows photographs of the fracture surface. For as-
received steel, a completely brittle fracture is observed below
0°C, with a percent shear fracture of 0% (value calculated
according to the fracture appearance procedure mentioned in
ASTM E23 [18]). At room temperature (22°C), a brittle
behavior is conserved; however, an increase in the percent shear
fracture of 10% is observed. At 40°C, the percent shear fracture
Scientia et Technica Año XXVIII, Vol. 29, No. 03, julio-septiembre de 2024. Universidad Tecnológica de Pereira.
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increases to 30%, showing a combined behavior. At 60°C, there
is a predominantly ductile fracture, with a percent shear fracture
of 85%. According to the criterion used, it is determined that
the temperatures of 40°C and 60°C belong to the ductile-brittle
transition region of the material. At 90°C, it presents a percent
shear fracture of 100%, therefore, at higher temperatures, the
material will have a ductile behavior.
For heat-treated steel, a brittle fracture is shown at -60°C and
-40°C, presenting a small variation in the percent shear fracture,
taking values of 0% and 10% respectively. However, at 0°C,
changes in its behavior are shown concerning as-received steel,
a significant increase in the percent shear fracture is observed,
with a value of approximately 50%, indicating that this
temperature corresponds (or it is very close) to the transition
temperature of the material. According to the impact test
results, the DBTT determined is very close to the value obtained
according to the fracture appearance. At room temperature
(22°C), a higher ductile behavior is evident, with a percent
shear fracture of 85%. Above 40°C, totally ductile behavior is
observed, with a percent shear fracture of 100%. The values
obtained indicate a reduction in the transition temperature of the
material after heat treatment in agreement with the results
obtained in impact testing, a correct tendency and reliability of
the observed behavior are ensured.
Figure 6. SEM micrographs of fracture surface for as-received (a, b) and heat-
treated (c, d) steel. Brittle behavior (a, c) and ductile behavior (b, d). Source:
The authors
Fig. 6 shows the fracture morphology obtained by SEM,
presenting an important difference between the surfaces of as-
received (Fig. 6a and 6b) and heat-treated steel (Fig. 6c and 6d).
The brittle behavior (Fig. 6a and 6c) has a smooth and flat
surface, with a granular appearance in both cases, although the
heat-treated steel shows a higher amount of "individual"
surfaces segments and smaller plastic deformation. For ductile
behavior (Figure 6b and 6d), a rough surface is evident in both
cases, with the presence of microvoids in the cross-section of
the material produced by the plastic deformation. However, as-
received steel remained with a smooth appearance in some
regions, while heat-treated steel presents a higher amount of
microvoids, accompanied by a refinement on the fracture
surface.
The fracture surface presents a notable change after heat
treatment, exhibiting an increased toughness caused by an
enhancement in the plastic strain capacity. The increase in
energy absorbed at low temperatures and decrease in the
cleavage and/or separation surfaces of the crystallographic
planes in the brittle fracture is caused principally by the
refinement of the microstructure by bainitic transformation and
slight plastic deformation evidenced. On the other hand, the
significant change in ductile fracture morphology is also due to
microstructure refinement (and other phases present in the fine
microstructure obtained), giving a considerable number of
stress concentrators, generating the formation of dimples by the
plastic deformation, and therefore, a significant increase in the
nucleation of plastic deformation.
The increase in toughness, concerning the microstructure and
the heat treatment used, is due to the microstructural refinement
obtained by the bainitic transformation. The fine size of the
bainitic plates acts as an obstacle to the propagation of cracks,
increasing its ability to absorb energy and, therefore, their
toughness, a phenomenon also analyzed in-depth by Pickering
[27]. A very higher microstructural refinement to nano bainite
levels has demonstrate an important increase in toughness
[28,29].
The high density of dislocations and the small precipitated in
martensite and ferrite are other factors that could increase
toughness by bainitic transformation [30]. Other studies report
the formation of granular bainite from a continuous cooling
treatment with the presence of austenite and martensite [21,31].
According to Qiao [31], the granular bainite formation can have
two mechanisms from the cooling rate: the growth of ferrite
equiaxial and the growth of plate-shaped ferrite that is evolved
from carbon-rich austenite. By increase in cooling rate, the
fraction of granular bainite increases, and consequently
toughness increases [8].
IV. CONCLUSIONS
Bainitic transformation in low carbon steel occurs due to
continuous cooling treatment and annealing. This
transformation produces a microstructural refinement regarding
ferrite-perlite, generating a deviation in the crack propagation
path enhancing impact toughness and decreasing its brittle-
ductile transition temperature.
An increase from 97 J to 159 J (64%) on impact toughness
steel was obtained by the heat treatment. The brittle-ductile
transition temperature had a reduction of 50ºC to -5ºC in the
DBTT region, also showing a scattering of results on the
transition region. In the same way, the fracture surface presents
a notable change after heat treatment, the refinement of the
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microstructure by bainitic transformation generates a fracture
surface with a major ductile area for temperatures tested.
ACKNOWLEDGMENT
The authors would like to thank to Dr.-Ing. L. Mujica from
INCITEMA (UPTC) for supporting Thermo-Calc analysis. We
recognize the financial support received from Universidad
Nacional de Colombia, Bogotá.
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Bolaños-Bernal, S., received the Bs. Eng
in Mechanical Engineering in 2019, from
the Universidad Nacional de Colombia,
Bogotá, Colombia, and now Master
candidate degree in Project Management
from Universidad de Nebrija, Madrid,
España. Currently, he is a manufacturing
processes and materials engineering group
investigation member. His research interests include:
Mechanical Metallurgy, Mechanical Properties of Advanced
Materials and heat treatments.
ORCID: https://orcid.org/0000-0001-9183-071X
Scientia et Technica Año XXVIII, Vol. 29, No. 03, julio-septiembre de 2024. Universidad Tecnológica de Pereira.
115
Mónica Johanna Monsalve Arias She
graduated from the University of
Antioquia where she obtained the
degrees of Materials Engineering in
2005, Master of Engineering in 2008
and Doctor of Engineering in 2014.
Doctor in Matériaux Céramiques et
Treatments de Surfaces from the
Université de Limoges ( France) in
2014. Since 2018 to date, she has been a full professor in
the Mechanical and Mechatronics Engineering program at
the National University of Colombia and is a member of
the IPMIM research group (Innovation in Manufacturing
Processes and Materials Engineering). He has carried out
research projects in the area of ceramic materials, ceramic
coatings, bioceramics and residual stresses. His fields of
interest in work and research include: Ceramic materials,
Coatings deposited by thermal spraying and PVD,
bioceramics and residual stresses.
ORCID: https://orcid.org/0000-0002-9902-8518
Rodolfo Rodríguez-Baracaldo,
received the Bs. Eng in Mechanical
Engineering (1997) from the
Universidad Nacional de Colombia,
the Ms degree in Mechanical
Engineering (1999), and the PhD
degree in Materials Engineering and
Metallurgy (2008) from Polytechnic University of
Catalonia. He worked for the Universidad Nacional de
Colombia since 2000. Currently, he is a Full Professor in
the Mechanical and Mechatronics Department, Faculty of
Engineering. Head of “Innovation in manufacturing
processes and materials engineering” research group. His
research interests include Mechanical Metallurgy,
Mechanical Properties of Advanced Materials, Metal
Forming, and Computational Materials: Modeling and
Simulation.
ORCID: https://orcid.org/0000-0003-3097-9312