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M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
Mathew J. Peeta , Hala S. Hasanb , Marie-Noëlle Avettand-Fènoëlc , Saud H. A. Raubyed ,
Harshad K. D. H. Bhadeshiaa
a
Materials Science and Metallurgy, University of Cambridge, U.K.
Mechanical Engineering Department, Al-Nahrain University, Bhagdad, Iraq
c
Unité Matériaux et Transformations, Université Lille 1, France
d
University of Technology, Bhagdad, Iraq
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For personal use only.
b
Low-temperature transformation to bainite
in a medium-carbon steel
The consequences of reducing the carbon concentration
from 0.8 to 0.6 wt.% in a bulk nanostructured bainitic-steel
have been investigated with the aim of enabling processing.
We observed that the average thickness of the product
phase plates increased from below 20 – 40 nm to about
100 nm after isothermal transformation at the same temperature. The increase in plate size led to decrease in hardness as expected. However the hardness reduction was not
as large as might be expected, a transition to lower bainite
at temperatures below 250 8C may have provided additional
strengthening. Transformation just below the martensite
start temperature resulted in a mixture of martensite and
bainite, with no observed increase in the rate of bainite
transformation.
Keywords: Hardness; Nanostructure; Bainite; Steel
1. Introduction
Bulk nanostructured steel is now a commercial reality,
some fifteen years following the initial work [1, 2]. A key
feature of the steel is that it has an attractive combination
of mechanical properties and can be manufactured in large
quantities without necessitating complex thermomechanical treatments. The major application is in the manufacture
of armour resistant to terrifying threats, and work is in progress in the context of shafts, bearings and wear-resistant
plates for the mining industries. There are, however, two
difficulties with the concept. The first is that some applications require the steel to be in a particularly soft condition
during forming operations [3], but the fine mixture of bainitic ferrite and carbon-enriched retained austenite proves to
be extremely resistant to tempering [4, 5]. Also, because of
its high carbon concentration (0.8 – 1 wt.%), the steel cannot be welded using conventional technologies [6], and this
greatly limits its wider application.
Achieving nanostructure is reliant on transformation at
low-homologous temperatures, primarily by the use of carbon which stabilises austenite relative to ferrite. This also
maintains a substantial separation between the bainite and
martensite-start temperatures (BS and MS). An alternative
way in which transformation can be suppressed is to reduce
the carbon concentration and increase that of substitutional
solutes that stabilise the austenite. Unfortunately, there is a
theoretical limitation which arises from the fact that carbon
partitions during the nucleation of bainite but not during
that of martensite [7]. It is this difference that results in the
distinction between the two transformations, so it is unlikely that a truly low-carbon nanostructured bainite can be
created [8]; that this limitation exists has been verified experimentally [9].
The original nanostructured steel [10] contained bainitic
ferrite plates with a true thickness of 20 – 40 nm, with a
yield strength of about 2 GPa; detailed properties have been
summarised in [11]. Previous work has reported on relatively low-carbon nanostructured [12 – 15] transformed at
temperatures 220 8C–350 8C. Kundu et al. [13] produced a
steel containing only 0.61 wt.% carbon, Fe-0.61C-1.5Mn1.66Si-0.34Mo-1.39Cr wt.% transformed at 300 8C and the
plate widths were reported to be in the range 70 – 110 nm,
resulting in a reduced yield strength of 1.4 GPa. The bainitic ferrite plate thickness, which determines the total interfacial area per unit volume, is a major component of
the strength of these steels [11, 16]. Others have referred
to \nanobainite" [17] for plate thickness in the range 200 –
400 nm so that the strength achieved is less than 960 MPa.
Table 1. Chemical composition, wt.%, and transformation temperatures determined.
Medium carbon
High carbon
C
0.58
Si
1.76
Mn
2.21
Ni
0.76
Mo
0.33
Cr
0.54
Co
0.45
BS (8C)
&370
MS (8C)
173
S
0.02
P
0.01
Al
0.03
Cu
0.08
V
0.01
Ti
0.01
W
0.01
Ac1 (8C)
755
Ac3 (8C)
788
C
0.80
Si
1.41
Mn
2.03
Bi
1.05
Mo
0.38
Cr
0.22
Co
–
Cu
0.03
Al
0.06
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
89
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M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
The purpose of the present work was to modify an existing alloy design [10], aiming to achieve the same structure
and properties but with carbon concentration reduced to
about 0.6 wt.%. This was expected to provide a steel that
could be transformed at temperatures below 220 8C to form
a nanostructured steel, but one that might soften to a greater
degree on tempering, thereby allowing processing. We expect that such a reduction in the carbon concentration may
not compromise strength severely since the amount of retained austenite would be reduced, and this in itself may
be an advantage in the context of bearing steels. Transformation just above, and just below the martensite start temperature was applied to investigate the resulting structure
and properties.
Transmission electron microscopy (TEM) was also performed on thin foils by using a JEOL 200CX microscope
at 200 kV. Thin foils were ground to 50 lm and finally
electrochemically thinned using a twin jet polishing system
and an electrolyte consisting of 5 % perchloric acid and
15 % glycerol, in methanol. The optimum polishing conditions for the samples varied from 22 – 30 V at temperatures
in the range –37 to –31 8C, this resulted in current densities
from 4.3 – 6.5 A mm–2.
2. Experimental procedure
A 640 g cylindrical sample (35 mm in diameter and 90 mm
in length) was made by arc melting, and homogenised in
vacuum at 1200 8C for 2 days and slowly cooled in a furnace. The chemical composition of the medium-carbon
steel was determined using inductively coupled plasma
spectroscopy, and is given in Table 1. A high-carbon variant of the alloy with similar composition was also produced, which is listed in Table 1.
Many of the heat treatment experiments were conducted
using a Thermecmastor–Z thermomechanical simulator with
cylindrical, homogenised specimens 8 mm in diameter and
12 mm in length. One such sample was heated at 1 K s–1 to
1050 8C and then cooled at 10 K s–1. The resulting dilatation
curves were used to determine the temperatures, Ac1 where
austenite first begins to form on heating at the specified rate,
Ac3 where austenite formation is completed; and the martensite-start temperature MS during cooling. The data were interpreted using the offset method [18], determining an MS
of 173 8C, Ac1 of 755 8C and Ac3 of 788 8C (as shown in
Fig. 1).
Isothermal transformation experiments to monitor the
formation of bainite involved heating cylindrical samples
at 10 K s–1 to 850 8C and then holding for 30 min to achieve
the fully austenitic state; they were then cooled at the same
rate to a variety of isothermal transformation temperatures
below the bainite-start temperature BS and held there for
time periods ranging from 4 to 22 h depending on the time
needed for the reaction to complete. Due to the longer time
needed for complete transformation at 220 8C and below,
experiments were carried out in which austenitisation was
conducted with argon atmosphere before transfer to an oven
for holding at the bainite transformation temperatures for
10 days.
Experiments to examine the case of bainite formation below MS involved isothermal transformation 10 8C below MS
at 163 8C. Using the Thermecmastor–Z, with isothermal
transformation for 10 h, and under vacuum in sealed quartz
tubes for times of 1 to 10 days. For comparison, samples
with a fully martensitic microstructure resulting from direct
quenching to water were tempered at 163 8C for times from
10 h to 10 days in order to directly compare the effect of
tempering time on the microstructure and the hardness.
Samples for optical and scanning electron microscopy were
etched in 4 % nital solution. Vickers hardness tests were
performed using a load of 30 kg, and the hardness values
determined by an average value of at least five indentations.
90
(a)
(b)
(c)
Fig. 1. (a) Heating curve, (b) at higher magnification to show the interpretation of the transformation temperatures, and (c) martensite-start
temperature determination.
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
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X-ray diffraction (XRD) was performed on polished
samples in a Bragg–Brentano configuration using a Philips
PW1730 diffractometer with unfiltered Cu-Ka radiation
(wavelength 1.54182 Å). The resulting data were interpreted using Rietveld analysis of the whole diffraction pattern using Philips Highscore-plus software. This enabled
the phase volume fractions and lattice parameters to be determined, with the latter useful in estimating the carbon
concentration of the austenite and ferrite according to analysis provided by Dyson and Holmes [19] and Bhadeshia
et al. [20].
3. Results and discussion
3.1. Determination of transformation temperatures
The equilibrium phase diagram and the equilibrium carbon
concentration of the austenite, xc, as a function of the temperature are shown in Fig. 2. These results were calculated
using MTDATA [21], permitting austenite (c), ferrite (a),
cementite (h), M23C6 and M7C3 to exist, and including all
the components listed in Table 1. It should be noted that
none of the carbides included in the analysis are expected
to form during cooling or transformation to bainite. Silicon
retards the precipitation of cementite [22] and M7C3 is unlikely given the lack of substitutional atom mobility at the
temperatures where bainite forms. Holding for sufficient
time at temperature would be necessary to form the equilibrium carbides, and as described below, the austenitisation
conditions were designed such that all elements would be
in solution.
The transformation temperatures were measured as illustrated in Fig. 1. The difference between the Ac1 = 755 8C
and Ac3 = 788 8C temperatures (Table 1) is much smaller
than expected from equilibrium calculations (Fig. 2) whereby Ae1 = 635 8C and Ae3 = 740 8C, but as expected from kinetic effects, the measured temperatures are in both cases
greater than the corresponding equilibrium temperatures.
The MS determined to be 173 8C is in agreement with the
range of values predicted by the various models shown in
Table 2, an experimental error of up to 15 8C can be expected based on previous assessment [18, 23 – 25].
A calculated time–temperature–transformation diagram
using methods described elsewhere [26], showing the time
for a detectable volume of ferrite (a) and bainite (ab) is
shown in Fig. 3a. Our calculation predicted lower martensite-start temperature and bainite-start temperature than experimentally observed. The microstructure obtained after
furnace cooling from the homogenisation heat treatment
(1200 8C, 2 days, furnace cooled) is shown in Fig. 3b, contained a substantial amount of bainite, and exhibited a hardness of 469 ± 5 HV30. In contrast, the higher carbon alloy
(Table 1) containing 0.8 wt.% carbon given an identical
heat treatment resulted in the formation of martensite, with
a hardness of 694 ± 4 HV30, as shown in Fig. 3c.
3.2. Transformation to bainite above MS
Fig. 2. Equilibrium phase data estimated using MTDATA [21]. The
terms a, c, h and xc refer to ferrite, austenite, cementite and the carbon
concentration of austenite, respectively.
Isothermal transformation to bainite between 300 – 370 8C
revealed as expected that the maximum quantity of bainite
transformed decreases as the transformation temperature is
increased towards BS, although at lower temperatures this
was only approximately the case. The results indicate that
the bainite-start temperature is likely to be close to 370 8C,
which is somewhat greater than indicated by calculations
(343 8C), Fig. 3.
Our primary interest is in the bainitic transformations
that occur at the lowest temperatures since it is these that
produce the finest structures [27 – 29]. In the absence of carbide precipitation, isothermal transformation should stop
when the carbon concentration of the residual austenite
reaches the T0’ curve (Fig. 4). It is evident from Fig. 4c that
the austenite carbon concentration xc, as measured via the
lattice parameter, is consistent with the T0’ curve, i. e., diffusionless transformation to bainite followed by the partitioning of carbon from the bainitic ferrite into the residual austenite. In contrast, the concentrations are far less than
would be expected if carbon was partitioned during transformation, so that the bainitic ferrite is never supersaturated
Table 2. Calculated MS temperatures. Mucg models from the map website [23, 24], committee model of linear models and vanilla neural network [25].
MS ( 8C)
Medium carbon
High carbon
Mucg46
159
Mucg73
171
Mucg83
176
Mucgn
169
49
66
84
53
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
Committee
214 ± 21
Neural Net
194 ± 24
Observed
173 ± 15
116 ± 31
144 ± 25
130 ± 15
91
M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
at any stage of its existence; xc << xAe30 , where Ae30 represents the curve giving the concentration of carbon in austenite that is in paraequilibrium with ferrite. This observation
is consistent with many previous studies in silicon alloyed
carbide-free bainitic steels. The carbon concentrations of
ferrite obtained by isothermal transformation for 10 days
at 220, 200 and 180 8C, as determined from the lattice parameters, were estimated to be 0.05, 0.02, 0.05 wt.%, noting
that the variation observed is similar to the expected error in
the measurement.
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3.3. Microstructure and mechanical properties
(a)
Optical and scanning electron microscopy was conducted to
characterise general features and archived at [30]. The
structures illustrated in Fig. 5 are as expected, consisting
of bainitic ferrite and retained austenite. Additionally, at
the two lowest transformation temperatures carbides were
observed within the bainitic ferrite. This transition to lower
bainite during transformation at 200 and 180 8C is shown in
Fig. 6, electron diffraction confirmed that the carbide particles are cementite, and the retained austenite established by
electron diffraction is typical of all the samples studied and
further evidence is archived on [30].
Many more transmission electron micrographs were taken in order to obtain the bainite plate thickness t, stereologically corrected from measurements of the apparent plate
a
thickness LT [24]:
a
LT ¼ pt=2;
(b)
(c)
Fig. 3. (a) Calculated time–temperature–transformation diagram [26].
(b) Microstructure of the 0.58 wt.% carbon alloy following homogenisation heat-treatment and furnace cooling. (c) Microstructure of the
0.80 wt.% carbon alloy following homogenisation heat-treatment and
furnace cooling.
92
with
r2e ¼
r2La
NðLT Þ2
a
ð1Þ
where re is the dimensionless statistical-error in the detera
mination of LT , dependent on the number of measurements
N and the standard deviation rL in the distribution of the
measurements. The true thickness values measured in this
way are listed in Table 3. As expected, the thickness decreases as the transformation temperature is reduced. The
plate widths obtained are significantly larger than those observed in the high carbon nanostructured bainite [31],
Fig. 7. This may be attributed to the fact that the plate thickness depends on additional factors other than transformation temperature, for example strength of the austenite [29,
32].
X-ray determined retained austenite contents are also
listed in Table 3 [33], and are generally smaller than those
observed in the higher carbon concentration variants. The
X-ray diffraction patterns (Fig. 8) reveal that the ferrite
peaks broaden as the transformation temperature is reduced, consistent with refinement of the plate size.
It is interesting to compare the data obtained here with
that previously reviewed [11]. The hardness should vary approximately with t1 [34], one approximation being that we
deal here with a mixture of phases (bainitic ferrite and austenite) rather than just the boundaries between ferrite plates.
Figure 7 shows that the hardness obtained is somewhat
greater than the general trend of the higher carbon alloys
as a function of the transformation temperature, possibly
because of the presence of fine cementite particles within
the lower-bainitic platelets in this work, compared to previous results for carbide-free bainite [35].
Estimation of the various contributions to the hardness is
shown in Table 4, using a previously developed model for
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
Isothermal transformation at 163 8C (10 8C below MS ) produced a microstructure of primary martensite formed during cooling below MS , bainitic ferrite formed during holding, retained austenite, and secondary martensite formed
during final cooling. Figure 9a shows the dilatation curve
for sample austenitised at 850 8C for 30 min then held at
163 8C for 10 h before cooling to room temperature, compared to the dilatation curve for the sample directly
quenched to room temperature. More analysis for the dilatation data using the offset method shows that the fraction
of primary martensite is around 0.2 (Fig. 9b). For comparison, a fraction of 0.10 martensite should be expected from
the Koistinen and Marburger equation [38], although the
equation is not universally applicable. Also, it should be
noted that the difference could easily be explained by the
expected error in determining the MS [18]. Figure 10a–d
shows the microstructures of samples transformed at
163 8C with larger fractions of bainite as the transformation
time increases, which leads to decrease in hardness
(Fig. 11). The hardness after 10 days transformation
(472 HV) is lower than that of the fully bainitic microstructure transformed at 180 8C for 10 days (613 HV). This
decrease is not attributable to tempering of the primary
martensite during the isothermal transformation, as demonstrated by the lack of any significant change in hardness as a
result of repeating the heat treatment on the fully martensitic microstructure (Fig. 10e, also shown in Fig. 11).
X-ray analysis results in Table 3 and Fig. 8b demonstrated clearly that the reduction in hardness is due to the
large fraction of retained austenite (0.32) in the microstructure after 10 days transformation at 163 8C.
The bainitic transformation below MS is much slower
than that above although it is just 17 8C different, a slower
transformation rate may be expected due to the influence
of temperature on the transformation rate of bainite – for
example this difference increases the calculated start time
by a factor of 6. As shown in Fig. 3 there is a logarithmic
(a)
(b)
(c)
(d)
yield strength [36, 37] and dividing by 3 to convert to a
hardness value. Dislocation densities were calculated as a
function of transformation temperature based on measurements reported in the literature as in [37]. Due to these approximations, the model overestimates the absolute value
of the hardness by around 10 – 30 %. Application of the
model demonstrates the relatively large contribution of the
microstructure, for example transformation at 163 8C leads
to lower contribution from the plate thickness due to the larger fraction of austenite in the final structure.
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3.4. Transformation below MS
Fig. 4. (a,b) Isothermal transformation to bainitic ferrite above MS . (c) Comparison of xc obtained after 10 days isothermal transformation (open
diamonds) at 220, 200 and 180 8C against calculated phase boundaries. (d) Dilatometer curves representing cooling from the indicated isothermal
transformation temperature. Martensite is only observed to form for cooling from 350 and 370 8C transformation temperatures.
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
93
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M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
(a)
(b)
(c)
(d)
Fig. 5. Transmission electron micrographs illustrating the general structure obtained by transformation at: (a) 250 8C, 12 h; (b) 220 8C, 22 h; (c)
200 8C, 10 days; (d) 180 8C, 10 days.
94
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
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M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 6. Sample transformed at 180 8C for 10 days. (a–c) Bright and dark field images of cementite, and diffraction evidence. (d–f) Bright and dark
field images of retained austenite, and diffraction evidence.
Table 3. Stereologically corrected values of the true thickness of
bainitic ferrite plates as a function of the transformation temperature. The volume fraction of retained austenite (Vc), determined
using X-ray diffraction, is also listed. The uncertaintly in measurement of austenite volume fraction is approximately 1 vol.%
[33].
Heat treatment
True thickness t
(nm)
Vc
220 8C, 10 days
200 8C, 10 days
180 8C, 10 days
163 8C, 10 days
163 8C, 10 h
Water quenched
109 ± 13
107 ± 16
99 ± 14
0.11
0.16
0.10
0.32
0.06
0.10
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
dependence of transformation start time on temperature below around 200 8C. The fact that a quantity of bainite had
formed after holding for 10 days may even be considered
as evidence for a slight acceleration in the bainite transformation kinetics due to the formation of a quantity of martensite, although the large amount of austenite retained indicates transformation was far from complete. Compared
to transformation just above the MS the time needed for
complete transformation is undoubtedly longer.
4. Conclusions
1. A reduction in the carbon concentration permits a bainitic structure to be generated by continuous, slow cooling
from the austenitic state. This helps to avoid a hard
95
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M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
martensitic microstructure forming during production
which has a propensity to microcracking [39].
2. The hardness of the microstructures produced is less
than that obtained in higher carbon nanostructured bainite generated at the same transformation temperature,
due to the larger plate size of the bainitic ferrite. The decrease in strength is less than expected from the plate
size, possibly due to carbide formation in lower bainite
forming below 220 8C.
3. In the fully bainitic state, the retained austenite content
is never found to exceed a fraction of about 0.16, which
may be an advantage in circumstances where the dimensional stability of the final component is important.
(a)
(b)
(a)
Fig. 8. X-ray diffraction patterns (a) above MS and (b) below MS .
Table 4. Contributions to hardness calculated using yield strength
model [36] after characterisation of structures, and comparison to
measured hardness.
(b)
Fig. 7. A comparison of published data on high-carbon (0.78 –
0.98 wt.%) nanostructured bainite shown as open circles [35] and the
present work (0.58 wt.% carbon) shown as filled circles. (a) Hardness
versus the reciprocal of the true bainitic ferrite plate thickness. (b) Volume fraction of retained austenite versus transformation temperature.
Note that the transformation times are not constant, due to differing
times needed for completion.
96
Temperature
( 8C)
163
180
200
220
Substitutional
elements (HV)
53
69
65
69
Interstitial
elements (HV)
91
129
89
128
Dislocations
in ferrite (HV)
144
191
178
189
Plate thickness
(HV)
263
348
301
313
Prediction
(HV)
619
760
667
722
Measurement
(HV30)
472
613
600
580
Prediction/
Measurement
1.30
1.24
1.11
1.24
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
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M. J. Peet et al.: Low-temperature transformation to bainite in a medium-carbon steel
(a)
Fig. 11. Hardness measurements for samples transformed at 163 8C for
different times (open circles) and also hardness via tempering time for
martensitic microstructure (filled circles).
4. It is possible to form a mixture of martensite and bainite
at temperatures just below MS but the transformation
rate observed was much slower than that just above MS .
The authors express their gratitude to the Scholars Rescue Fund of International Institute of Education in Washington DC for supporting
Hala Salman Hasan during her stay in Cambridge.
References
(b)
Fig. 9. (a) Dilatation curve for sample transformed at 163 8C for 10 h
before cooling to room temperature, compared to the dilatation curve
for a sample directly quenched to room temperature. (b) Volume fraction of primary martensite in the microstructure.
(a)
(b)
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(Received September 26, 2016; accepted November 22,
2016; online since January 11, 2017)
Correspondence address
Dr. Mathew J. Peet
University of Cambridge
Materials Science and Metallurgy
27 Charles Babbage Road
Cambridge, CB3 0FS
United Kingdom
Tel.: +44 1223334336
E-Mail: mathew@mathewpeet.org
Bibliography
DOI 10.3139/146.111461
Int. J. Mater. Res. (formerly Z. Metallkd.)
108 (2017) 2; page 89 – 98
# Carl Hanser Verlag GmbH & Co. KG
ISSN 1862-5282
Int. J. Mater. Res. (formerly Z. Metallkd.) 108 (2017) 2
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