Mossbauer study of the retained ...

Mossbauer study of the retained austenitic phase in, Polibuda, Magisterka, Stale typu TRIP, Obróbka cieplna

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Materials Science and Engineering A283 (2000) 65 – 69
www.elsevier.com:locate:msea
M ¨ ssbauer study of the retained austenitic phase in
multiphase steels
A. Mijovilovich
a,
*, A. Gon¸alves Vieira
b
, R. Paniago
a
, H.D. Pfannes
a
,
B. Mendon¸a Gonzalez
b
a
Departamento de Fsica
,
Uni
6
ersidade Federal de Minas Gerais
,
C
.
P
. 702, 30123-970
Belo Horizonte
,
Brazil
b
Escola de Engenharia
,
Uni
6
ersidade Federal de Minas Gerais
,
Rua Esprito Santo
35, 30160-030
Belo Horizonte
,
Brazil
Received 31 May 1999; received in revised form 21 December 1999
Abstract
Samples of steels with composition 0.30%C-1.5%Mn-1.5%Si-0.5%Al-0.5%Mo (wt.%) were subjected to different thermomechan-
ical treatments to produce ferrite:pearlite:bainite (FPB), spheroidized (ESF) and martensite (MAR) microstructures. Subsequently
they underwent a two stage annealing to obtain a nal structure comprising of ferrite, bainite, martensite and austenite. The
samples were studied by means of M ¨ ssbauer spectroscopy (transmission and conversion electron M ¨ ssbauer spectrocopy
(CEMS)), X-ray diffraction (XRD), and metallographic analysis. Austenite contents were found to be the same for all samples
except for the spheroidized sample annealed at 750°C that showed an increase of the austenite with increasing temperature of the
treatment. M ¨ ssbauer spectroscopy and quantitative XRD analysis exhibited signicant discrepancies ascribed to texture effects.
It is shown that the thermal treatment was successful in retaining signicant quantities of the austenite phase for steels of this
composition. © 2000 Elsevier Science S.A. All rights reserved.
Keywords
: Multiphase steel; M ¨ ssbauer; Austenite; Martensite; Bainite
1. Introduction
spite that the result may be strongly inuenced by
texture effects. M ¨ ssbauer spectroscopy is a well-known
technique used in the study of Fe-containing alloys [4].
The different phases can be distinguished from their
different signals, and different magnetic behaviors re-
gardless of the state of aggregation of the phases.
Martensite and austenite are easily distinguished from
their different hyperne patterns in the M ¨ ssbauer
spectra with better accuracy than by other techniques.
Due to the low solubility of carbon in
In order to enhance the ductility in high-strength
steels it was shown that it is necessary to increase the
content of their retained austenite. Alloys with high
ductility and excellent levels of mechanical strength can
be obtained by the transformation of austenite to
martensite during plastic deformation (i.e. trip: trans-
formation induced plasticity effect) [1]. Matsumura et
al. [2] increased the content of retained austenite in an
alloy of C – Mn – Si by a two stage thermal treatment:
an annealing followed by a quick quenching to the
range of temperatures for the bainitic transformation.
The amount of retained austenite increased with in-
creasing content of Mn and Si in the alloy [3].
It is usual to determine the phases present by metal-
lographic analysis as well as X-ray diffraction. The last
method is sometimes used for quantitative analysis in
-Fe in equi-
librium, the interstitial solute C can not be detected by
M ¨ ssbauer spectroscopy. In the transmission made sig-
nals from all the
57
Fe atoms in the sample are obtained
regardless of the state of the aggregation or crystallinity
in the material. In the case of conversion electron
M ¨ ssbauer spectrocopy (CEMS) the spectrum stems
from a region of
a
10 – 100 nm below the surface of the
sample, and thus becomes an efcient tool to analyse
the surface. With M ¨ ssbauer spectroscopy the texture
effect does not affect the total area of the subspectra
corresponding to the different phases.
* Corresponding author. Present address: EMBL c:o DESY,
Notkestrasse 85, Geb. 25A, Notkestrasse 85, 2603, Hamburg, Ger-
many. Tel.: 49-40-89902120; fax: 49-40-89902149.
0921-5093:00:$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0921-5093(00)00620-1
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We used transmission M¨ ssbauer spectroscopy to
determine the amount of austenitic retained phase after
the thermal treatments. By CEMS we were able to
study the mechanical stability and transformation to
martensite during the laminating process. Quantitative
X-ray diffraction analyses were employed to determine
the relative amounts of retained austenite, and ferritic
phases present.
2. Experimental
Fig. 1. Schematic diagram of two stage annealing.
T
1
intercritical
temperature;
t
1
annealing time;
T
2
bainitic temperature (425°C),
t
2
bainitic transformation time.
By a specic thermal treatment of an alloy of compo-
sition 0.30%C-1.5%Mn-1.5%Si-0.5%Al-0.5%Mo three
initial structures were obtained: ferrite:pearlite:bainite
(FPB), spheroidized (ESF) and martensite (MAR).
Subsequently they underwent a two stage annealing
(Fig. 1) to obtain a nal structure of ferrite, bainite,
martensite and austenite. We will keep the acronyms of
the initial phases when we refer to the samples after the
two-stage annealing. For the metallographic analysis a
selective etching with Nital 2%, Picral 5% and Na-thio-
sulfate [5] was used.
Fig. 2. Photographs of the microstructures: (a), FPB780; (b), MAR780; (c), ESF750 and (d), ESF840. Ferrite is gray color, bainite in dark gray
and austenite-martensite in light gray.
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Table 1
Hyperne parameters and relative areas (%) for the identied phases
from transmission M ¨ ssbauer spectra
a
were able to take into account the thickness of the
samples. The sample FPB780 was investigated also by
CEMS using the same source as above. In this case, as
the effective thickness is small, we used a least
squared t with simple Lorentzian lines [11]. The values
of
Sample
H
(
T
)
d
(mm s
1
)
D
(mm s
1
) Relative area
(%)
2
to measure the quality of the t ranged from 1.7
to 3.4.
FPB
780
Ferrite 1
33.0
0.047
–
45.42
Ferrite 2
30.6
0.067
–
28.02
Ferrite 3
27.9
0.072
–
6.84
Austenite 1
–
0.023
–
13.67
Austenite 2
–
0.032
0.6
6.08
MAR
780
Ferrite 1
33.0
0.045
–
45.70
Ferrite 2
30.7
0.064
–
26.07
Ferrite 3
28.4
0.073
–
8.34
Austenite 1
–
0.027
–
13.51
Austenite 2
–
0.033
0.6
6.38
Mar
810
Ferrite 1
33.0
0.008
–
43.80
Ferrite 2
31.0
0.03
–
22.83
Ferrite 3
29.4
0.033
–
12.46
Austenite 1
–
0.059
–
13.72
Austenite 2
–
0.005
0.6
7.20
Esf
750
Ferrite 1
33.0
0.008
–
48.2
Ferrite 2
31.0
0.031
–
28.4
Ferrite 3
29.2
0.025
–
13.0
Austenite 1
–
0.053
–
7.1
Austenite 2
–
0.051
0.6
3.2
Esf
840
Ferrite 1
33.0
0.005
–
40.2
Ferrite 2
31.3
0.028
–
22.8
Ferrite 3
29.5
0.041
–
17.4
Austenite 1
–
0.062
–
13.6
Austenite 2
–
0.004
0.6
6.0
a
H
(
T
) is the hyperne magnetic eld in Tesla,
d
(mm s
1
)isthe
isomer shift refered to
a
-Fe in mm s
1
, and
D
(mm s
1
)isthe
quadrupole splitting.
Integrated intensities of X-ray diffraction peaks were
used to determine the content of retained austenite [6].
The samples were grounded, embedded in epoxy, pol-
ished with 1
m
). The reections used for the quantitative analysis
were: (200), (220) and (311) for the
Fig. 3. M ¨ ssbauer spectra for: (a), MAR780 and (b), FP780 samples.
g
-phase; (200) and
Table 2
Hyperne parameters and percentages of phases from CEMS spectra
(surfaces)
a
-phase.
We measured transmission M ¨ ssbauer spectra of all
samples. The measuring temperature was room temper-
ature and the source was
57
Co in Rh matrix. Two
samples (FPB780 and MAR780) were also measured at
77 K. Since the steels were 50 nm thick foils the lines
were broadened because of thickness effect. It is com-
mon to use ts with hyperne eld distributions or
Voigtian line proles [7] to take this effect into account.
In tting these spectra we used the program WOTAN
[8] which is based on the integral form of the absorp-
tion line calculated by Margulies [9,10]. In this way we
a
H
(
T
)
d
(mm s
1
)
D
(mm s
1
)
Relative area
(%)
FPB780
Ferrite 1
33.0
0.00
–
48.12
Ferrite 2
31.0
0.04
–
39.36
Ferrite 3
27.0
0.08
–
3.37
Austenite 1
–
0.18
–
5.38
Austenite 2
–
0.00
0.6
3.79
a
Symbols as in Table 1.
x
m diamond paste and measured in a
Philips diffractometer (
K
a
-Cu radiation, 0.01°-steps of
2
u
(211) for the
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Materials Science and Engineering A
283 (2000) 65–69
Fig. 4. CEMS spectra for sample FP780.
bainite shows dark gray, and both, the martensite and
the austenite become light gray. The microstructure of
the FPB sample is heterogeneous with ne and coarser
regions (Fig. 2a). The nal structure for the sample
obtained from an initial martensite is ner (Fig. 2b).
For the samples obtained from the spheroidized initial
structure an increase of the amount of bainite and
martensite-austenite with increasing intercritical tem-
perature is found (Fig. 2c, d).
The main contributions to the M¨ ssbauer spectra
arise from the ferrite and martensite phases of the steel,
which lead to magnetically split spectra. We used the
designations Ferrite 1, 2 and 3 to denote subspectra
corresponding to Fe atoms in the ferritic:bainitic:
martensitic matrix with three different environments,
following the nomenclature of Uwakweh et al. [12]. But
due to the different nomenclatures used in the literature
it is not possible to give unique assignments for the
different Fe – C congurations [13]. The austenitic para-
magnetic phase is clearly distinguishable with two con-
tributions: austenite 1 and 2 for the singlet and the
doublet, respectively. It amounts
Table 3
XRD results for retained austenite
g
phase (volume%) for different
two-stage thermal treatments
21% in all samples.
Since there is insignicant contribution of cementite or
other carbide precipitates seen in the spectra, they have
been neglected in the tting. The hyperne parameters
and the percentages of each phase as determined from
the M¨ ssbauer spectra are listed in Table 1. Typical
M¨ ssbauer spectra are shown in Fig. 3.
The M¨ ssbauer results indicate similar austenite con-
tents for the MAR and FPB samples. For the ESF
samples it is clearly observed that by the treatment at
higher temperature more austenite is retained. By com-
paring the room temperature and liquid nitrogen spec-
tra of FPB780 (ferrite-perlite system) and MAR780
(martensite rich sample), we conclude that both exhibit
similar austenite contents.
We measured the FPB780 sample also with CEMS
and deduced a decrease of the austenitic phase, indicat-
ing a transformation from austenite to martensite in the
surface during the polishing process (see Table 2 and
the corresponding spectrum in Fig. 4). This was simi-
larly observed by [14].
The quantitative determination of phases by XRD is
given in Table 3 and a characteristic pattern is shown in
Fig. 5. The M¨ ssbauer results for the austenite content
differs signicantly from the XRD results, except for
the ESF samples where both techniques indicate the
same trend. This is ascribed to a cristallographic texture
effect that strongly inuences the XRD measurements.
M¨ ssbauer results concerning the spin texture in these
steels will be published elsewhere [15].
B
Sample
Thermal treatment
Volume (%) of
g
phase
FPB780
780°C 20 min425°C 10 min
13.2
9
1.5
ESF750
750°C 20 min425°C 10 min
7.1
9
1.5
ESF840
840°C 20 min425°C 10 min
16.1
9
1.5
MAR780 780°C 20 min425°C 10 min 17.8
9
1.5
MAR810 810°C 20 min425°C 10 min 17.2
9
1.5
Fig. 5. X-ray diffraction pattern of a multiphased structure obtained
from martensitic initial structure-MAR-780 (intercritically annealed
at 780°C for 20 min and at 425°C for 10 min).
3. Results and discussion
Photographs of optical microscopy are shown in Fig.
1. Three phases can be distinguished in the photo-
graphs, namely martensite plus austenite, bainite and
ferrite. With the used etching, ferrite becomes gray,
4. Conclusions
Metallographic analysis showed the presence of
bainite, ferrite and martensite-austenite in all samples.
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Materials Science and Engineering A
283 (2000) 65–69
69
The ner microstructure is present in the samples ob-
tained from the martensitic phase due to its acicular
morphology.
From the micrographs it is seen that the amount of
carbide precipitates is negligible which is in concordance
with the M¨ ssbauer results, which do not indicate any
signicant contribution from carbides. For the evalua-
tion of the retained austenite the M¨ ssbauer spectra
indicate that the annealing at higher temperature is
effective in stabilizing the austenite phase in the
spheroidized sample. Results for other samples under
different thermal treatments are similar, in disagreement
with XRD measurements. The difference was attributed
to a texture effect.
From CEMS results a signicant decrease of the
austenite content in the surface due to the mechanical
polishing is deduced.
References
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Vieira, B. Mendon¸a Gonzalez, (in press).
Acknowledgements
The support of the Brazilian research agencies
Fapemig, CAPES and CNPq is greatfully acknowl-
edged.
.
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