Thermoelectric Assessment of Aging in Duplex Stainless Steel

NDT&E International 44 (2011) 463–468
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NDT&E International
journal homepage: www.elsevier.com/locate/ndteint
Nondestructive assessing of the aging effects in 2205 duplex stainless steel
using thermoelectric power
Noemi Ortiz Lara a, Alberto Ruiz a,n, Carlos Rubio b, Ricardo R. Ambriz c, Ariosto Medina a
a
NDE Group, Institute of Metallurgical Researches, UMSNH, Mexico
CIDESI, Qro, Mexico
c
Instituto Politécnico Nacional CIITEC-IPN, Mexico
b
a r t i c l e i n f o
abstract
Article history:
Received 3 November 2010
Received in revised form
9 April 2011
Accepted 13 April 2011
Available online 27 April 2011
The microstructural transformation of ferrite into secondary austenite and sigma phase during long
term exposure to high-temperatures (650–900 1C) in a 2205 duplex stainless steel has been
investigated using the thermoelectric power (TEP) technique, scanning electron microscopy (SEM),
Charpy-impact test (CIT), equivalent ferrite and sigma phases content measurements. The possibility of
using the TEP coefficient as a nondestructive assessment technique to characterize the aging kinetics of
2205 duplex stainless steel is discussed. Experimental results indicate that TEP coefficient is sensitive to
the gradual microstructural transformation of ferrite phase experienced by the 2205 duplex stainless
steel during the aging treatments.
& 2011 Elsevier Ltd. All rights reserved.
Keywords:
Thermoelectric power
Ferrite transformation
Impact test
Sigma phase
1. Introduction
It is well-known that the thermoelectric power technique
coefficient, TEP of metals is sensitive to diverse material properties
that can affect the measurement. Chemical composition exerts the
strongest effect on the thermoelectric properties, and accordingly,
the primary application of thermoelectric materials characterization is metal sorting [1]. However, it is also known that as a result of
fabrication processes of certain components, materials of identical
chemical composition can also create an efficient thermocouple.
This characteristic can be further exploited for nondestructive
testing of materials [2–7]. The TEP coefficient, S, is a temperaturedependent electronic property of the material that can be described
as the entropy of the free electrons in the alloy [8]. Several factors
control the TEP coefficient, which includes: electron concentration,
effective mass of the electrons and electronic scattering behavior in
an alloy [9]. All these factors are influenced by the solute content,
lattice strain, microstructural changes, material processing and
time-dependent phase changes. The thermoelectric effect leads to
a number of interesting phenomena, which can be exploited for
nondestructive testing (NDT) and materials characterization. Essentially, all existing thermoelectric NDT methods are based on the
well-known Seebeck effect that is commonly used in thermocouples to measure temperature at the junction between two different
n
Corresponding author.
E-mail address: [email protected] (A. Ruiz).
0963-8695/$ - see front matter & 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ndteint.2011.04.007
conductors [10]. Fig. 1 shows a schematic diagram of the thermoelectric measurements commonly used in nondestructive materials
characterization. The system includes two reference electrodes. One
of the electrodes is heated by electrical means to a preset temperature Th, while the other electrode is left cold at room temperature
Tc. The measurement is done rapidly in a few seconds to assure that
the hot reference electrode is not cooled down by the specimen
while the rest of the specimen beyond the close vicinity of the
contact point is not warmed up perceivably. The measured thermoelectric voltage is determined as follows:
Z Th
Z Th
V¼
½SS ðTÞSR ðTÞdT ¼
SSR ðTÞdT
ð1Þ
Tc
Tc
where T is the temperature, and SS and SR denote the absolute TEP of
the specimen and the reference electrode, respectively. Any variation
in material properties can affect the measured thermoelectric voltage
via SSR ¼SS SR, which is the relative TEP coefficient of the specimen
to be tested with respect to the reference electrode. In most cases,
the temperature-dependence of the TEP can be neglected over the
range of operation and the thermoelectric voltage can be approximated as VE(Th Tc) SSR. Ideally, regardless of how high the
temperature difference between the junctions is, only thermocouples
made of different materials, or more precisely, materials of different
TEP, will generate a thermoelectric signal. This unique feature makes
the simple thermoelectric tester one of the most sensitive material
discriminators used in nondestructive inspection.
Duplex stainless steels have wide applications in different
industrial fields like the naval, petrochemical and chemical
464
N. Ortiz Lara et al. / NDT&E International 44 (2011) 463–468
V
+
−
reference
electrodes
cold tip
hot tip
specimen
Fig. 1. Schematic diagram of the dual-tipped probe thermoelectric measurements
most often used in nondestructive materials characterization.
industries. A wide range of applications of these steels are related
to their high strength and excellent toughness properties as well
as their high corrosion resistance [11]. The excellent mechanical
properties exhibited by these steels are due to a duplex microstructure that consists of approximately the same amounts of
austenite (g) and ferrite (d). To some degree, austenite ensures the
toughness of the material and d-ferrite is responsible for the
strength [12]. However, during some fabrication processes such
as welding, the steel is subjected to elevated temperatures raising
the susceptibility of this steel to the precipitation of second
phases. Of main concern is the precipitation of s-phase during
exposure to elevated temperatures in the range 600–900 1C [13].
Sigma phase is a hard and brittle intermetallic phase. Studies
of the microstructural evolution in 2205 duplex stainless steel
aged at high temperatures show that s-phase precipitates in the
interface g/d at short aging times through a heterogeneous
nucleation and growth process [14–18]. It has been observed that
the fast formation kinetics of s-phase dramatically decreases
impact properties and corrosion resistance [19,20]. Several
researchers have investigated the phenomenon of ferrite decomposition and sigma phase precipitation [21–24] therefore the
ferrite transformation phenomenon is well understood.
Few investigations have been conducted to nondestructively
characterize the thermal effect on duplex stainless steels. Kawaguchi and Yamanaka [7] investigated the thermal effect in a cast
duplex stainless steel at a temperature of 400 1C. They found that
the TEP signals of cast duplex stainless steel increases with the
distribution of the Cr concentration. Ruiz et al. [25] investigated the
thermal effect on duplex stainless steel using ultrasonic attenuation
and shear wave velocity measurements. Their results indicate that
the attenuation measurements can predict an early thermal degradation. Elmer et al. [26] studied the microstructural transformation
of ferrite, austenite and sigma phase of 2205 DSS using synchrotron
radiation. Based on the amounts of sigma, ferrite and austenite
measured at 850 1C, they concluded that the dissolution temperature is under predicted by the thermodynamic calculations.
In an effort to find a rapid and reliable alternative NDE
technique, this investigation undertakes an interdisciplinary
research to study TEP measurement capabilities to assess material
microstructure, processing and mechanical characteristics of
2205 duplex stainless steels.
2. Experimental
2.1. Material preparation and methods
Type 2205 duplex stainless steel plate with 12.7 mm thickness
was used in this study. The experimental work consisted of TEP,
CIT, SEM, ferrite and sigma phase content measurements. The TEP
coefficient was used as the electronic measurement to assess the
microstructural changes endured by the 2205 duplex during
thermal aging. TEP measurements were performed on four sets
of specimens, each specimen measuring 25 20 12.7 mm3. A
series of aging treatments were performed at 900, 800, 700 and
650 1C for different holding times ranging from 1 min to 240 h
depending on the holding temperature. These treatments were
interrupted by water quenching for each sample.
The thermoelectric voltage was measured with a Walker
Scientific alloy thermo-sorter model ATS-6044T using a probe
with cold and hot tips of copper and gold, respectively. The TEP of
each sample was calculated using a lineal model obtained from
best-fitting the measured thermoelectric voltages in materials
with known TEP namely copper, alumel and chromel. In order to
determine the effect of temperature and aging time on impact
properties a series of Charpy-impact bar specimens of square
cross section (10 mm 10 mm) with 451 V-notch were fabricated
and heat-treated at 900, 700 and 650 1C for different aging times.
For microstructural analysis, samples were prepared by metallographic electro-polishing and etching in a KOH electrolyte
(10 g KOH, 100 ml water) with a voltage of 3 V for approximately
10 s. Microstructural examination of the samples was done using
a JEOL JSM-5910LV scanning electron microscope. The equivalent
ferrite content was measured in each sample after each treatment
using a MP30E FERITSCOPEs. Finally, sigma phase content was
measured using commercial software and compared to an existing sigma phase predicting model.
3. Results and discussion
3.1. Effect of the aging treatment on the microstructural evolution
In the temperature range between 650 and 900 1C, duplex
stainless steel experiences a phase transformation through the
reaction d-g2 þ s, which reduces the ferrite content and significantly affects the impact properties of the 2205 duplex stainless
steel. As it can be observed in the scanning electron microscopy
micrographs of Fig. 2, the sample aged at 650 1C the microstructure does not show noticeable changes for times lower than
30 min and the duplex phase characteristic of this steel is
maintained with large and elongated island-like austenitic grains
(bright) in a more or less continuous matrix of ferrite (dark) as
shown in Fig. 2(a). For the sample aged for 1 min at 900 1C
Fig. 2(b), the ferrite phase experiences noticeable changes, due
to the formation of small ferrite grains with cellular shape, which
breaks down the elongated morphology of the ferrite grain.
As an example of the effect of aging time in the ferrite
transformation, Fig. 3(a–f) shows a sequence of SEM micrographs
of the microstructural changes that take place in the 2205 duplex
stainless steel aged at 700 1C for times up to 240 h. As depicted in
Fig. 3(a), the microstructure shows no appreciable change in
percentage of ferrite. However, as in the case of the sample
treated at 900 1C, there is a gradual formation of new grain
boundaries in the initially elongated ferrite matrix to form again
small ferrite grains with cellular shape. Also, as in the case of
Fig. 2(b), the g/d interface is more defined showing bright areas
that indicate the beginning of the precipitation of second phases.
This condition leads to a loss of chromium in the neighboring
ferrite region, and therefore a migration of the initial g/d grain
boundary into the ferrite grain. It has been reported that in an
early stage of the ferrite transformation, the higher diffusivity of
carbon atoms produces the precipitation of chromium carbides at
the g/d interfaces where Cr-rich ferrite intersects with C-rich
austenite [20,21]. In Fig. 3(c), it is observed that at 1 h holding
N. Ortiz Lara et al. / NDT&E International 44 (2011) 463–468
465
Fig. 2. Microstructure of the 2205 duplex stainless steel: (a) aged for 30 min at 650 1C and (b) aged for 1 min at 900 1C.
Fig. 3. Scanning electron microscopy sequence of the microstructural changes of 2205 duplex stainless steel aged at 700 1C. (a–b) formation of new grain borders in the
ferrite, (c) sigma phase precipitation at the grain boundaries and (d–f) diminishing of equivalent ferrite and growth of sigma phase and austenite.
time, the formation of new grains of ferrite with cell-like form is
completed and a new defined border in the ferrite phase gives rise
to the formation of secondary austenite (g2), which has different
chemical composition than the original austenite phase [16].
Sigma phase begins to precipitate slightly after the precipitation of chromium carbides and as it is observed in Fig. 3(c) sigma
phase preferentially nucleates at the g/d interface where depending on the aging temperature and time it will continuously grow.
This is clearly seen in Fig. 3(d–f), where a significant and
progressive diminishing of ferrite phase occurs followed by a
continuous growth of secondary austenite and a coarsening of
particles of sigma phase. The microstructural analysis of Figs. 2
and 3 indicates that it is clear that temperature plays an
important role in the rate of ferrite transformation that affect
the impact properties of this steel. From the literature, it is not
possible to establish a universal master model that can predict
N. Ortiz Lara et al. / NDT&E International 44 (2011) 463–468
the kinetics of ferrite decomposition since numerous physical and
metallurgical parameters could affect the transformation kinetics
and the modes of ferrite transformation. These parameters
include variations in the chemical composition, ferrite grain size
and volume fraction as well as temperature of solution treatment
and conformation treatment, the effect of aging time and holding
temperature. Among the models to predict the fraction of sigma
phase, one of the most used theories is the Johnson–Mehl–Avrami
(JMA) model.
3.2. TEP measurements
Fig. 4 shows the experimentally determined TEP coefficient of
the 2205 duplex stainless specimens aged at the different holding
times. For the case of samples aged at 900 1C, the TEP presents a
lower value in comparison to the other holding temperatures. Also,
there is a maximum value of the TEP coefficient after 5 min holding
and thereafter the TEP shows a monotonic decrease as aging time
increases. The TEP coefficient measured in samples aged at 800 1C
temperature presents a maximum value at 5 min and then slowly
changes for the next 25 min and starts to decrease rapidly after
that. A quite different behavior was exhibited by the samples aged
at 700 and 650 1C. In both cases and at low holding times, the
measured TEP shows a small but continuous decrement up to
30 min and 2 h for the 700 and 650 1C cases, respectively. After
that, the TEP begins to increase and reach maximum value of
1.16 mV/1C after 1 and 6 h for the 700 and 650 1C treatments,
respectively. As shown in Fig. 3(c) the maximum values in the TEP
have been related to the formation of a more defined border in the
ferrite grains and has been related to carbide precipitation [27].
Following these maximum values, the TEP begins to decrease
monotonically and presents a sharp drop. Finally, for longer holding
times at 900 and 800 1C, the TEP stabilizes after the 10 h treatment to
reach values of approximately 1.5 and 1.68 mV/1C, respectively.
3.3. Equivalent ferrite and sigma phase content measurements
Fig. 5 shows the measured equivalent ferrite as a function of
aging time in four sets of samples. The results indicate that the
loss of equivalent ferrite content is highly influenced by the
amount of thermal exposure. The initial equivalent ferrite content
of the samples aged at 900 1C is approximately 25 pct lower than
that exhibited at lower holding temperatures. At this temperature
and time up to 5 min, equivalent ferrite content remains constant
and decreases rapidly after 10 min of exposure to reach a
minimum value of approximately 3.8 pct at 30 h.
At a temperature of 800 1C, equivalent ferrite content remains
constant for low holding times and shows a more rapid reduction
after 10 min to reach a minimum value of 1.9 pct at 20 h. At
700 1C, content of equivalent ferrite remains constant up to 1 h.
After that, its content continuously decreases up to 12 h holding
time and then sharply decreases for higher holding times. After
48 h, the rate of equivalent ferrite reduction slows down reaching
about 6.9 pct at 240 h. Finally at 650 1C, the equivalent ferrite
content stays constant for up to 2 h to sharply decrease after 2 h
holding time to reach a value of 13.1 at 240 h.
From Figs. 4 and 5, it is clear that for some temperatures, TEP
coefficient and equivalent ferrite content exhibit similar trends.
Aiming to establish a relationship between equivalent ferrite
content and TEP coefficient, a graph correlating the TEP coefficient
and the normalized fraction of equivalent ferrite is shown in
Fig. 6. It is evident that a somewhat linear correlation exists
between the two quantities in the equivalent ferrite fraction
range of 0.2–0.72. The equivalent ferrite fraction here is obtained
by normalizing the measured equivalent ferrite content values to
50
Equivalent ferrite content [%]
466
40
800 °C
900 °C
35
30
25
20
15
10
0
0.01
1
10
Aging time [h]
0.1
100
1000
Fig. 5. Change on the equivalent ferrite content as function of aging time in aged
2205 duplex stainless steel.
2
900 °C
800 °C
700 °C
900 °C
650 °C
800 °C
700 °C
650 °C
1.5
Thermoelectric power [µV /°C]
1.5
Thermoelectric power [µV /°C]
650 °C
700 °C
5
2
1
0.5
0
-0.5
-1
1
0.5
0
-0.5
-1
-1.5
-1.5
-2
0.01
45
-2
0.1
1
10
Aging time [h]
100
1000
Fig. 4. Measured TEP coefficient as function of aging time in 2205 duplex stainless
steel aged at different temperatures.
0
0.2
0.4
0.6
0.8
Normalized equivalent ferrite content
1
Fig. 6. Measured equivalent ferrite content as function of measured TEP in 2205
duplex stainless steel aged at different temperatures.
N. Ortiz Lara et al. / NDT&E International 44 (2011) 463–468
467
the maximum equivalent ferrite value measured at 1 min holding
temperature. Also, Fig. 6 shows that the TEP does not change after
an approximate value of 0.72 equivalent ferrite fraction is reached.
At these holding temperatures austenite and sigma phase are
the main products of the ferrite transformation; however, it is
well-known that sigma phase plays the most important role in
the reduction of impact properties in this type of steel. Therefore,
the amount of sigma phase was measured in SEM micrographs
using commercial software and the results were compared to JMA
theory that relates the transformed fraction of the initial phase to
the transformation time [23,24].
ð2Þ
where X is the fraction of the initial phase transformed at time t,
the Avrami exponent n varies from 0.2 to 0.8, b is a constant that
can be expressed by the Arrhenius equation: b ¼ b0 expðQ =RTÞ
that depends on the temperature T and activation energy for
transformation Q and R is the universal gas constant. To apply this
model to predict the phase transformation kinetics, the following
assumptions are made: transformation occurs under isothermal
conditions and nucleation frequency is either constant or maximum at the beginning of transformation and decreases towards
the end of transformation and the nucleation is random. Eq. (2)
can be transformed into
1
ln ln
¼ n lnðtÞ þ lnðbÞ
ð3Þ
1X
Under certain conditions, the phase transformation kinetics
obeys the classical JMA model, i.e., when the evolution of ln[ln[1/
(1 X)]] as a function of ln(t) gives a straight line. The kinetic
parameters n and b are related to the transformation mechanism
and transformation rate, respectively, and they can be found
using simple linear regression of the experimental data. Fig. 7
shows the plot of ln[ln[1/(1 X)]] as a function of ln(t) results of
sigma phase precipitation for two aging temperatures. The values
of the Avrami exponents n800 1C, n700 1C are consistent with
reported values [23]. To compare the predicted and measured
values of sigma phase precipitated, the set of values of n800 1C,
b800 1C and n700 1C, b700 1C were used in Eq. (2). The experimental
and predicted values are in good agreement as shown in Fig. 8.
3.4. Impact energy determination
Fig. 9 shows the impact characteristics of the 2205 duplex
stainless steel as a function of aging time. It is evident that the
excellent impact characteristics of this steel deteriorate very fast
depending on the holding temperature of the aging treatment. For
example, at 900 1C and 5 min of aging time the capability of
Fig. 7. Johnson–Mehl–Avrami plots for two differently aging treatments.
Fig. 8. Changes in phase fraction of sigma phase for two aging temperatures.
400
900 °C
350
Absorved energy [J]
X ¼ 1exp½bt n 700 °C
650 °C
300
250
200
150
100
50
0
0.01
0.1
1
10
Aging time [h]
100
1000
Fig. 9. Changes in absorbed energy in Charpy-impact test of the aged 2205 duplex
stainless steel.
absorbing impact energy diminishes considerably. Comparing the
behavior at 5 min for 900 and 700 1C, impact properties decrease by
77 pct. Similar behavior is observed at the 650 1C holding
temperature, i.e., the steel withstands the thermal degradation of
impact properties for up to 30 min. After this time, impact characteristics decay rapidly to very low values for longer holding times.
4. Conclusions
The microstructural transformation in the 2205 stainless steel
was investigated for different temperatures in the interval
650–900 1C. The transformation effect was analyzed based on
TEP, SEM, ferrite phase measurements, sigma phase measurements and CIT determination.
The observations performed allowed the following conclusions:
The TEP measurements in 2205 duplex stainless steel are
sensitive to the gradual transformation of d-ferrite into secondary
austenite and sigma phase. Changes in the TEP coefficient can be
related to the reduction of ferrite content caused by the aging
treatment.
The TEP measurements in 2205 duplex stainless steel exhibited two characteristic maxima at 700 and 650 1C that could be
related to the time of formation of new grain boundaries during
the aging process.
There is a somewhat linear relationship between TEP and
d-ferrite content that can be used to predict the transformation
kinetics of this steel and to indirectly infer the presence of
sigma phase.
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N. Ortiz Lara et al. / NDT&E International 44 (2011) 463–468
Acknowledgment
This work was performed at the Universidad Michoacana de
San Nicolas de Hidalgo with funding from Universidad Michoacana de San Nicolás de Hidalgo under project: 1.30 (2010). The
authors also wish to thank CONACYT-MEXICO for its support of
Noemi Ortiz Lara during her doctoral studies.
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