metals

Article
Effect of Solution Treatment Temperature on
Microstructural Evolution, Precipitation Behavior,
and Comprehensive Properties in UNS S32750 Super
Duplex Stainless Steel
Junhe Li 1 , Wei Shen 1 , Ping Lin 2 , Fuming Wang 1 and Zhanbing Yang 1, *
1 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing,
Beijing 100083, China; lijunhe_ustb@163.com (J.L.); shenweiustb@163.com (W.S.);
wangfuming@metall.ustb.edu.cn (F.W.)
2 Zhejiang Tsingshan Steel Co Ltd., Zhejiang 323000, China; lp@ruipt.com
* Correspondence: yangzhanbing@ustb.edu.cn; Tel.: +86-10-6233-2872




Received: 18 October 2020; Accepted: 4 November 2020; Published: 6 November 2020


Abstract: The changes of microstructures, element distribution, and comprehensive properties were
studied to explore their interactions with each other, induced by solution treatment of UNS S32750
super duplex stainless steel. The results showed that the ferrite content improved, while the austenite
content declined as the temperature increased. From 900 to 1000 ◦ C, the σ phase existing at α/γ grain
boundaries and in ferrite grains gradually dissolved. At 1050 ◦ C, the microstructures consisted of
only ferrite and austenite. From 1050 to 1300 ◦ C, the Cr2 N precipitated in ferrite and gradually grew
and coarsened. The impact energy and pitting potential of UNS S32750 first improved and then
weakened, while the hardness is the opposite, owing to the combined effects of element distribution,
microstructures, and precipitates. In the presence of the σ phase, the corrosion resistance and
mechanical properties of UNS S32750 correspond directly to the σ phase fraction. Subsequently,
the rise in temperature promoted γ → α phase transformation, and the elements partitioning ratios of
Cr and Mo declined, resulting in reduced toughness and corrosion resistance and a rise in hardness.
Consequently, when the solution treatment temperature is 1050 ◦ C, the α/γ ratio of UNS S32750
approached 1:1, with excellent overall properties.

Keywords: UNS S32750 super duplex stainless steel; solution treatment temperature; microstructure;
precipitates; mechanical property; corrosion resistance

1. Introduction
Super duplex stainless steel (SDSS) has a double-phase ferritic-austenitic microstructure in
approximately equal proportions, which provides it with excellent properties that cannot be achieved
with austenitic or ferritic stainless steels [1–3]. In addition, its high resistance to pitting corrosion,
with a pitting resistance equivalent number (PREN = wt% Cr + 3.3 wt% Mo + 20 wt% N) greater than
40 [4,5], renders it a high corrosion-resistant stainless steel. Compared with ordinary duplex stainless
steel, super duplex stainless steel has higher strength and superior corrosion resistance, and is widely
used in harsh environments such as petrochemicals and seawater desalination [6–8].
The high alloying of super duplex stainless steels increases the risk of secondary phase precipitation,
and the corrosion resistance and mechanical properties of super duplex stainless steels are limited
during the thermal processing in the range of 400–1000 ◦ C, owing to the precipitation of sigma (σ),
chi (χ), chromium nitrides (Cr2 N), G phases, and so on [9–11]. Among them, the σ phase is commonly
considered to be the most hazardous component and has been extensively studied by scholars [12–14].

Metals 2020, 10, 1481; doi:10.3390/met10111481 www.mdpi.com/journal/metals

Metals 2020, 10, 1481 2 of 15

It is a brittle intermetallic phase rich in Cr and Mo with a tetragonal crystallographic structure, which
mainly precipitates at 600–1000 ◦ C [15–19]. Its precipitation seriously affects the plasticity, toughness,
and corrosion resistance of materials [20]. The study indicates that the precipitation of a 5% σ phase
leads to an 80% drop in impact energy and a 25 ◦ C reduction in critical pitting temperature [21–24].
The σ phase is able to re-dissolve after the solution treatment temperature rises to a certain value,
but during the rapid cooling process after solution treatment above 1200 ◦ C, chromium nitride
precipitates within the ferrite grains, forming a high-density Cr2 N zone and reducing the material
properties [25–27]. Therefore, proper control of the solution treatment temperature seems to be the
simplest solution to obtain the desired performance.
However, the solution treatment temperature also affects the ratio of ferritic (α) and austenitic (γ)
phases, the distribution of alloying elements between the two phases, and the corrosion resistance
of each phase. It has been noted that alloying elements have different partition coefficients in the
two phases [28]; chromium and molybdenum are enriched in ferrite, while nickel and nitrogen are
concentrated in austenite. The reduction of Cr and Mo content in ferrite leads to a drop in the
precipitation dynamics of the σ phase, which effectively inhibits σ-phase precipitation and enhances the
toughness and corrosion resistance of steel [9,12]. In the process of rapid cooling at a high temperature,
the supersaturated N in ferrite grains easily combines with Cr to form Cr2 N [29,30]. The corrosion
resistance of duplex stainless steels is mainly attributed to the action of the elements Cr, Mo, and N.
The different content of alloying elements in the austenitic and ferritic phases leads to variations in the
corrosion resistance of the two phases [31–33]. In this regard, it is of great significance to study the
correlation between the solution treatment parameters and the microstructural characteristics.
In the current studies, however, few works have provided a complete and detailed description of
the precipitation phase, biphasic ratio, alloying element content, mechanical properties, and pitting
resistance of UNS S32750 as a function of variable thermal treatment temperature. At present,
few scholars have investigated elemental partitioning ratios as a function of σ phase content and
biphasic ratios, revealing mechanical properties as a function of σ phase content, biphasic ratios,
and elemental partitioning ratios. In the present work, various microstructural states were obtained
by varying the treatment temperature between 900 ◦ C and 1300 ◦ C to investigate the variation of
microstructure characteristics and overall properties, such as α/γ ratio, precipitates behavior, element
partitioning ratios, hardness, impact toughness, and corrosion resistance of UNS S32750 duplex
stainless steel. The aim of this paper is to provide a sufficient theoretical basis for establishing the
reasonable heat treatment process for UNS S32750 duplex stainless steel and analyze the interaction
among the microstructure transition, precipitation phase, and alloying element partitioning ratios, as
well as their influence on the mechanical properties and corrosion resistance.

2. Materials and Methods

The material used for this experiment was a 75 mm diameter UNS S32750 super duplex stainless
steel bar, the main chemical composition of which is shown in Table 1. The size of the solid solution
treated specimen was 10 mm × 10 mm × 12 mm, the size of the electrochemical test specimen was
10 mm × 10 mm × 5 mm, and the impact specimen was 10 mm × 10 mm × 55 mm in the standard form
of a Charpy V-impact specimen.
The thermodynamic equilibrium phase diagram of UNS S32750 super duplex stainless steel was
calculated by Thermo-Calc thermodynamic software, as shown in Figure 1. The solution treatment
temperature range of experimental steel was determined according to the equilibrium phase diagram.
The specimens were held at solution treatment temperatures of 900, 950, 1000, 1050, 1100, 1150, 1200,
1250, and 1300 ◦ C for 40 min using a muffle furnace, the holding time including the time required to
reach the target temperature, and then water quenched.
After mechanical grinding and polishing, the specimens were electrolytically etched with a 20%
NaOH solution at a voltage of 10 V and an electrolysis time of 10–20 s. The microstructure was observed
by the MX6R optical microscope (OM, SDPTOP, Shanghai, China), and the area fractions of ferrite and
Metals 2020, 10, 1481 3 of 15
Metals 2020, 10, x FOR PEER REVIEW 3 of 15

austenite were
of ferrite andcounted using
austenite ImageJ
were imageusing
counted analysis software.
ImageJ imageEach samplesoftware.
analysis was measured at 20 fields
Each sample was
ofmeasured
view and at the20average value was taken. The structure morphology and chemical
fields of view and the average value was taken. The structure morphology andcomposition of
ferrite and austenite
chemical composition wereofstudied
ferritebyand
JSM-6701F scanning
austenite electron by
were studied microscopy
JSM-6701F (SEM, JEOL, Tokyo,
scanning electron
Japan) and NS7(SEM,
microscopy energyJEOL,
dispersive
Tokyo,spectroscopy
Japan) and(EDS,
NS7 Thermo, Waltham, MA,
energy dispersive USA), and(EDS,
spectroscopy nitrides were
Thermo,
observed and analyzed by JEM-2200FS transmission electron microscopy (TEM, JEOL,
Waltham, MA, USA), and nitrides were observed and analyzed by JEM-2200FS transmission electron Tokyo, Japan).
Tomicroscopy
analyze the(TEM,
microstructural changes
JEOL, Tokyo, of the
Japan). To precipitates,
analyze the the specimens were
microstructural electrolytically
changes eroded
of the precipitates,
with
the aspecimens
10% oxalicwereacid solution. The specimens
electrolytically were aetched
eroded with 10% in the reagent
oxalic of 1 g KThe
acid solution. S O
2 2 5 + 15 mL
specimens HCl
were
+ etched
85 mL H in2 O forreagent
the 10–20 sofbefore
1 g Kcounting
2S2O5 + 15 the
mLarea
HClratio
+ 85ofmL
theHσ2phase.
O for 10–20 s before counting the area
ratio of the σ phase.
Table 1. Composition analysis of UNS S32750 super duplex stainless steel (mass fraction, wt%).
Table 1. Composition analysis of UNS S32750 super duplex stainless steel (mass fraction, wt%).
C Si Mn P S Cr Ni Mo N Fe
C Si Mn P S Cr Ni
0.024 0.39 0.67 0.022 0.002 25.49 6.18 3.47 0.288 Bal. Mo N Fe
0.024 0.39 0.67 0.022 0.002 25.49 6.18 3.47 0.288 Bal.

Figure1.1. Equilibrium
Figure Equilibriumphase
phasediagram of of
diagram thethe
UNS S32750
UNS duplex
S32750 stainless
duplex steel calculated
stainless by Thermo-
steel calculated by
calc software.
Thermo-calc software.

TheThe Rockwell
Rockwell hardness
hardness (HRC)(HRC) of each heat-treated
of each heat-treated sample was was
sample measured with with
measured 150 kg load
150 kgfor 10 for
load s
by10TH300 Rockwell
s by TH300 hardness
Rockwell testertester
hardness (Beijing TimeTime
(Beijing HighHigh
Technology Co Ltd.,
Technology Beijing,
Co Ltd., China),
Beijing, withwith
China), at
least 10 indentations per specimen was determined and the average hardness
at least 10 indentations per specimen was determined and the average hardness value was calculated. value was calculated.
The
TheCharpy
Charpyimpact impacttest
testwas
wascarried
carriedoutoutatatroom
room temperature
temperaturewith withJB-30B
JB-30Bimpact
impacttester
tester(Wuzhong
(Wuzhong
material
material testing
testingmachine
machine Co.,Co.,
Ltd.,Ltd.,
Ningxia, China),
Ningxia, and data
China), andwere
datathewere
average
the values
average of three
values standard
of three
impact samples.
standard impact samples.
Electrochemical
Electrochemical teststests
werewere
carriedcarried
out withoutAUTOLAB PGSTAT302N
with AUTOLAB electrochemical
PGSTAT302N workstation
electrochemical
(Metrohm,
workstation Beijing, China)
(Metrohm, in a typical
Beijing, China)three-electrode system, withsystem,
in a typical three-electrode the specimen
with theasspecimen
the workingas the
electrode, the saturated calomel electrode (SCE) as the reference electrode, and
working electrode, the saturated calomel electrode (SCE) as the reference electrode, and the platinum the platinum electrode
aselectrode
the auxiliaryas theelectrode.
auxiliaryThe 3.5% NaCl
electrode. solution
The 3.5% NaCl simulating seawater was
solution simulating used aswas
seawater theused
test medium
as the test
atmedium
room temperature. The working electrode was first poled at −1 V for 10 min
at room temperature. The working electrode was first poled at −1 V for 10 min to remove to remove the oxide
film
the oxide film formed on the surface of the sample in air, and then the open circuit potential for
formed on the surface of the sample in air, and then the open circuit potential was measured was
50measured
min. After forthe
50open
min. circuit
After thepotential was stabilized,
open circuit potential the waselectrochemical impedance spectroscopy
stabilized, the electrochemical impedance
and potentiodynamic
spectroscopy polarization curves
and potentiodynamic were tested.
polarization Electrochemical
curves were tested. impedance spectroscopy
Electrochemical (EIS)
impedance
was carried out(EIS)
spectroscopy at open-circuit
was carriedpotential, the test frequency
out at open-circuit potential,range was
the test 100 mHz–100
frequency range kHz,
was 100andmHz–
the
amplitude
100 kHz, of andthethe
ACamplitude
excitation of signal
the was 10 mV. Thesignal
AC excitation scanning
was range
10 mV. of the
Thepotential
scanningpolarization
range of the
curve is −0.3–1.4
potential V (relative
polarization curveto the open circuit
is −0.3–1.4 potential),
V (relative to theandopen
the scanning frequencyand
circuit potential), is 1the
mV/s.scanning
frequency is 1 mV/s.
3. Results and Discussion
3. Results and Discussion
3.1. Effect of Solution Treatment Temperature on Microstructural Evolution
3.1.The proportion
Effect of Solutionof ferrite (α)
Treatment to austenite
Temperature (γ) has an important
on Microstructural Evolutioninfluence on the mechanical
properties and corrosion resistance of duplex stainless steel. The solution treatment temperature plays
The proportion of ferrite (α) to austenite (γ) has an important influence on the mechanical
a key role in the ratio of α/γ at a given chemical composition [24]. Figure 2 illustrates the optical
properties and corrosion resistance of duplex stainless steel. The solution treatment temperature
plays a key role in the ratio of α/γ at a given chemical composition [24]. Figure 2 illustrates the optical
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Metals 2020, 10, x FOR PEER REVIEW 4 of 15

microstructures for UNS S32750 specimens as solution treated at 900–1300 ◦ C. The ferrite gradually
microstructures
increases for UNS S32750
and the austenite specimens
gradually as solution
decreases withtreated
the rise at 900–1300
in solution°C. treatment
The ferrite gradually
temperature;
increases and the austenite gradually decreases with the rise in solution treatment
the austenite and ferrite structures recover and recrystallize and grow, and the austenite structure temperature; the
austenite and ferrite structures recover and recrystallize and grow, and the austenite structure
gradually changes from long strips to islands distributed on the ferrite matrix. The σ phase distributes at
gradually changes from long strips to islands distributed on the ferrite matrix. The σ phase distributes
the grain boundary of austenite and ferrite and within ferrite grain at 900–1000 ◦ C. When the specimen
at the grain boundary of austenite and ferrite and within ferrite grain at 900–1000 °C. When the
is treated at 1050 ◦ C, the microstructure of the specimen consists of ferrite and austenite phases,
specimen is treated at 1050 °C, the microstructure of the specimen consists of ferrite and austenite
withphases,
no σ phase.
with no It can be appreciated
σ phase. that the twothat
It can be appreciated main thephases
two main are found
phases to arebefound
in approximately
to be in
equal proportions.equal proportions.
approximately
Figure 3 presents
Figure 3 presentsthe area fractions
the area of ferrite
fractions and austenite
of ferrite and phase
and austenite ratio after
and phase ratiosolution treatment
after solution
at different temperatures. As can be observed, the ratio of α/γ improves rapidly,
treatment at different temperatures. As can be observed, the ratio of α/γ improves rapidly, indicating indicating the sharp
increase in the
the sharp content
increase in of
theferrite
content during solution
of ferrite duringtreatment at 900–1000
solution treatment ◦ C. At solution
at 900–1000 treatment
°C. At solution
treatment temperatures
temperatures ranging fromranging 1000 ◦ C from 1000 ◦°C
to 1100 C,tothe
1100 °C, the austenite
austenite transformstransforms into slowly,
into ferrite ferrite slowly,
the phase
ratiothe
is phase ratio and
balanced, is balanced,
the σ/γand theisσ/γ
ratio ratio is 1.07–1.28.
1.07–1.28. When the When the solution
solution treatmenttreatment temperature
temperature exceeds
exceeds
◦ 1100 °C, the transformation rate from austenite to ferrite is accelerated,
1100 C, the transformation rate from austenite to ferrite is accelerated, and the ratio of two phases and the ratio of two(α/γ)
phases (α/γ) rises rapidly. The reason is that the eutectoid reaction, α → γ2 + σ, is reversible, and the
rises rapidly. The reason is that the eutectoid reaction, α → γ2 + σ, is reversible, and the σ phase and
σ phase and austenite phase rapidly transform into the ferrite phase, leading to a significant rise in
austenite phase rapidly transform into the ferrite phase, leading to a significant rise in ferrite content.
ferrite content. The nitrogen is a strong austenitic forming element, which can expand and stabilize
The nitrogen is a strong austenitic forming element, which can expand and stabilize the austenitic zone.
the austenitic zone. In the temperature range of 1000–1100 °C, the N element inhibits the
◦ C, the N element inhibits the transformation from austenite to
In the temperature range of 1000–1100
transformation from austenite to ferrite. However, the stabilizing effect of the N element on austenite
ferrite. However,
begins to weaken thewith
stabilizing
the rise ineffect of the N resulting
temperature, element on austenite
in the begins
accelerated to weaken with
transformation the rise in
of austenite
temperature,
to ferrite. resulting in the accelerated transformation of austenite to ferrite.

Figure 2. Microstructures of the UNS S32750 super duplex stainless steel treated at different solution
treatment temperatures. (a) 900 ◦ C; (b) 1000 ◦ C; (c) 1050 ◦ C; (d) 1100 ◦ C; (e) 1200 ◦ C; and (f) 1300 ◦ C.
Metals 2020, 10, x FOR PEER REVIEW 5 of 15
Metals 2020, 10, x FOR PEER REVIEW 5 of 15

Figure 2. Microstructures of the UNS S32750 super duplex stainless steel treated at different solution
Figure 2. Microstructures of the UNS S32750 super duplex stainless steel treated at different solution
Metals treatment
2020, 10, 1481
temperatures. (a) 900 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C; (e) 1200 °C; and (f) 1300 °C. 5 of 15
treatment temperatures. (a) 900 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C; (e) 1200 °C; and (f) 1300 °C.

Figure 3. Relationship
Relationship between solution
solution treatment temperature
temperature and (a)
(a) area fraction
fraction of αα and
and γ phase
Figure 3. Relationship between
Figure 3. between solution treatment
treatment temperature and
and (a) area
area fraction of
of α and γγ phase
phase
and
and (b) phase ratio (α/γ) (error bars represent standard deviation).
and (b)
(b) phase
phase ratio
ratio (α/γ)
(α/γ) (error
(error bars
barsrepresent
representstandard
standarddeviation).
deviation).

3.2.
3.2. Effect of Solution
Solution Treatment
TreatmentTemperature
Temperatureon onAlloying
AlloyingElements
ElementsDistribution
DistributionininTwo-Phase
Two-PhaseStructures
Structures
3.2. Effect of Solution Treatment Temperature on Alloying Elements Distribution in Two-Phase Structures
The
The energy
energyspectroscopic
spectroscopic analysis of the
analysis of ferrite and austenite
the ferrite phases phases
and austenite is performed under scanning
is performed under
The energy spectroscopic analysis of the ferrite and austenite phases is performed under
electron microscopy and the results are shown in Figure 4, where the N element content
scanning electron microscopy and the results are shown in Figure 4, where the N element content is is calculated
scanning electron microscopy and the results are shown in Figure 4, where the N element content is
by Thermo-Calc
calculated thermodynamic
by Thermo-Calc software. The
thermodynamic Cr and Mo
software. Theelements
Cr and Mo areelements
mainly enriched in the
are mainly ferrite
enriched
calculated by Thermo-Calc thermodynamic software. The Cr and Mo elements are mainly enriched
phase, and the
in the ferrite content
phase, andofthe
N content
and Ni of
in Ntheand
austenitic
Ni in thephase is significantly
austenitic greater thangreater
phase is significantly that inthan
the
in the ferrite phase, and the ◦ content of N and Ni in the austenitic phase is significantly greater than
ferritic
that in phase. At 900–1050
the ferritic phase. At C, the content
900–1050 of Cr
°C, the and Mo
content in ferrite
of Cr and Mo rises significantly,
in ferrite while the Crwhile
rises significantly, and
that in the ferritic phase. At 900–1050 °C, ◦the content of Cr and Mo in ferrite rises significantly, while
Mo
the content
Cr and Modrops gradually
content dropsatgradually
1050–1300atC. With the °C.
1050–1300 riseWith
in temperature, the content ofthe
the rise in temperature, Mocontent
and N in of
the Cr and Mo content drops gradually at 1050–1300 °C. With the rise in temperature, the content of
austenite
Mo and Nincreases significantly,
in austenite increases while the content
significantly, whileofthe
Ni content
reducesof slightly;
Ni reducesthe content
slightly;ofthe
Nicontent
and N in of
Mo and N in austenite increases significantly, while the content of Ni reduces slightly; the content of
ferrite
Ni andcontinuously improves. improves.
N in ferrite continuously
Ni and N in ferrite continuously improves.

Figure 4. Chemical
Figure 4. Chemical composition
composition of
of austenite
austenite and
and ferrite
ferrite phase
phase in
in the
the UNS
UNS S32750
S32750 steel
steel varies
varies with
with
Figure 4. Chemical composition of austenite and ferrite phase in the UNS S32750 steel varies with
solution
solutiontreatment
treatmenttemperature.
temperature.(a)
(a)Cr
Cr content;
content;(b)
(b) Mo
Mo content;
content; (c)
(c) Ni
Ni content;
content; and
and (d)
(d) NN content
content (error
(error
solution treatment temperature. (a) Cr content; (b) Mo content; (c) Ni content; and (d) N content (error
bars
bars represent
represent standard
standarddeviation).
deviation).
bars represent standard deviation).
The
The σσphase
phaseprecipitates
precipitatesatatthe
theferrite area
ferrite areaalong thethe
along phase boundaries
phase or grain
boundaries boundaries
or grain of γ/α
boundaries of
and Theinσ the
α/α phase precipitates
900–1000 at the
◦ C range. ferrite area
Analysis of along
the the phase
relationship boundaries
between the σ or grainproportion
phase boundariesandof
γ/α and α/α in the 900–1000 °C range. Analysis of the relationship between the σ phase proportion
γ/αcontent
the and α/αofinCrthe 900–1000
and °C range.
Mo in ferrite, Analysis
as shown of the
in the relationship
Figure 5, shows between the in
that the rise σ phase proportion
the precipitation
of the σ-phase leads to the reduction of the content of Cr and Mo in ferrite. This is because, during
Metals 2020,
Metals 2020, 10,
10, xx FOR
FOR PEER
PEER REVIEW
REVIEW 66 of
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and the
Metals
and the
2020,content
10, 1481 of Cr and Mo in ferrite, as shown in the Figure 5, shows that the rise in
content of Cr and Mo in ferrite, as shown in the Figure 5, shows that the rise in6 ofthe the
15
precipitation of
precipitation of the
the σ-phase
σ-phase leads
leads toto the
the reduction
reduction of of the
the content
content of of Cr
Cr and
and Mo Mo in in ferrite.
ferrite. This
This isis
because,
because, during
during the the precipitation
precipitation of the
of the σ phase,
σ phase, the Cr and
the Cr and Mo elements
Mo ferrite
elements in the
in the ferrite diffuse to the
the precipitation of the σ phase, the Cr and Mo elements in the diffuse to ferrite diffuseand
the σ phase to the
are
σ
σ phase
phase andand are enriched,
are enriched, resulting
resulting in a decline
in content in the
a declineofinCrthe content
content of Cr and Mo in the ferrite. With the
enriched, resulting in a decline in the and Mo inofthe Crferrite.
and MoWith in thetheferrite.
increaseWithof the
the
increase of
increase of the
the two-phase ratio,
ratio, the
the Cr
Cr and
and Mo
Mo partitioning ratio ratio gradually drops, drops, whilewhile the
the NiNi
two-phase ratio,two-phase
the Cr and Mo partitioning ratiopartitioning
gradually drops, graduallywhile the Ni partitioning ratio
partitioning
partitioning ratio continuously rises, as shown in Figure 6. The increase of solution treatment
continuouslyratio rises, continuously rises, 6.asThe
as shown in Figure shown in Figure
increase 6. The
of solution increase
treatment of solution
temperature treatment
promotes the
temperature
temperature promotes
promotes the transformation
the transformation of austenite phase to ferrite phase, the ferrite content rises,
transformation of austenite phase to ferriteofphase,
austenite
the phase
ferriteto ferriterises,
content phase, thethe
and ferrite content
austenite rises,
content
and the
and the austenite
austenite content
content decreases,
decreases, soso that
that the
the concentration
concentration of of CrCr and
and MoMo inin ferrite
ferrite improves,
improves, while
while
decreases, so that the concentration of Cr and Mo in ferrite improves, while the concentration of Cr
the concentration of
the of Cr and
and Mo
Mo inin austenite declines.
declines. The Ni Ni in
in the
the austenite diffuses
diffuses into into ferrite,
ferrite, and
andconcentration
Mo in austeniteCr declines. Theaustenite
Ni in the austeniteThe diffuses intoaustenite
ferrite, and the Ni content inand the
the Ni
the Ni content
content in in the
the austenite
austenite reduces
reduces slightly,
slightly, while
while the
the Ni
Ni content
content in in the
the ferrite
ferrite increases.
increases.
austenite reduces slightly, while the Ni content in the ferrite increases.

Figure 5.
Figure 5. The
The relationship between
The relationship
relationship between the
between the area
the area ratio
area ratio of
ratio of the
of the σ
the σ phase and
σ phase
phase and the
and the mass
mass fraction
mass fraction of
fraction of Cr
of Cr and
and Mo
Mo
in ferrite.
in ferrite.

Figure 6.
Figure The elements
6. The elements partitioning
partitioning ratio
ratio varies
varies with
with the
the α/γ ratio.
α/γ ratio.

The
The NN content
N content
content in in
in austenite
austenite is is
is far
far higher
higher than
than that
that inin ferrite. The
ferrite. The capacity
The capacity
capacity of of the
of the face-centered
the face-centered
face-centered
The austenite far higher than that in ferrite.
cubic
cubic structure
structure ofof austenite
austenite toto accommodate
accommodate interstitial
interstitial atoms
atoms is is
much much stronger
stronger than than
that ofthat
the of the
body-
cubic structure of austenite to accommodate interstitial atoms is much stronger than that of the body-
body-centered
cubiccubic
centered cubic structure
structure of ferrite,
of ferrite,
ferrite, andandthe the maximum
maximum solubility
solubility of of
NN ininthe
theferrite
the ferritephase
phase is is about
about
centered structure of and the maximum solubility of N in ferrite phase is about
0.05%, which
0.05%, which makes
which makes
makes N N almost
N almost completely
almost completely soluble
completely soluble in
soluble in the
in the austenitic
the austenitic phase.
austenitic phase.
phase. As As the
As the solution
the solution treatment
solution treatment
treatment
0.05%,
temperature
temperature increases,the
increases, theNNcontent
contentinin austenite
austenite increases
increases rapidly,
rapidly, while
while the the
N N content
content in in ferrite
ferrite rises
temperature increases, the N content in austenite increases rapidly, while the N content in ferrite rises
rises slightly
slightly [9,12].[9,12].
[9,12]. In In addition,
In addition,
addition, the the
the bindingbinding
binding abilityability between
ability between
between N N
N and and
and Cr Cr
Cr is is strong,
is strong,
strong, whichwhich hinders
which hinders
hinders the the
the
slightly
dissolution
dissolution of
of Cr
Cr in
in ferrite
ferrite phase
phase during
during phase
phase transformation
transformation [4].
[4]. Therefore,
Therefore, Cr
Cr is
is more
more easily
easily soluble
soluble
dissolution of Cr in ferrite phase during phase transformation [4]. Therefore, Cr is more easily soluble
in austenite
in austenite phase
austenite phase during
phase during
during thethe phase
the phase transformation
transformation of
phase transformation of austenite
austenite andand ferrite.
ferrite.
in of austenite and ferrite.
3.3. Effect of Solution Treatment Temperature on the Precipitation Phase
3.3. Effect
3.3. Effect of
of Solution
Solution Treatment
Treatment Temperature
Temperature on on the
the Precipitation
Precipitation Phase
Phase
Figure 7 shows the microstructure of the precipitated phase after solution at different temperatures.
Figure 77 shows
Figure shows the the microstructure
microstructure of of the
the precipitated
precipitated phase
phase after
after solution
solution atat different
different
It is noticed that a large amount of σ phase precipitates in the matrix, after solution treatment at
temperatures.
temperatures. It is noticed that a large amount of σ phase precipitates in the matrix,
It is noticed that a large amount of σ phase precipitates in the matrix, after solution after solution
900 ◦ C. With the increase of solution treatment temperature, the precipitation of the σ phase gradually
treatment at 900 °C. With the increase of solution treatment
treatment at 900 °C. With the increase of solution treatment temperature, temperature, the precipitation
the precipitation of of the
the σ
σ
decreases, and as the solution treatment temperature rises to 1050 ◦ C, the σ phase is completely
phase gradually
phase gradually decreases,
decreases, and
and as as the
the solution
solution treatment
treatment temperature
temperature rises
rises toto 1050
1050 °C,
°C, the
the σ
σ phase
phase isis
dissolved. When the solution treatment temperature is between 1100 ◦ C and 1300 ◦ C, the black nitrides
completely dissolved. When the solution treatment temperature is between 1100
completely dissolved. When the solution treatment temperature is between 1100 °C and 1300 °C, the °C and 1300 °C, the
precipitates at the ferrite grain boundaries as well as inside the grains. With the increase of solution
black nitrides
black nitrides precipitates
precipitates atat the
the ferrite
ferrite grain
grain boundaries
boundaries as as well
well as
as inside
inside the
the grains.
grains. With
With the
the increase
increase
treatment temperature, these fine and dispersed precipitates begin to grow into the ferrite grains,
and gradually grow coarse to form high-density regions.
Metals 2020, 10, x FOR PEER REVIEW 7 of 15

of solution
Metals treatment
2020, 10, 1481 temperature, these fine and dispersed precipitates begin to grow into the ferrite
7 of 15
grains, and gradually grow coarse to form high-density regions.

Metallographic picture
Figure 7. Metallographic picture of precipitated
precipitated phase morphology
morphology at different solution treatment
◦
temperatures. (a) 900 °C; ◦
C; (b) 950 °C;
C; (c)
(c) 1000 ◦
1000 °C;
C; (d)
(d) 1050 ◦ C; (e) 1100 ◦ C; (f) 1200 ◦ C; and (g) 1300 ◦ C.
1050 °C; (e) 1100 °C; (f) 1200 °C; and (g) 1300 °C.

The
The σ phase is
σ phase formed during
is formed during the
the eutectoid
eutectoid reaction, → γγ22 ++σ,
reaction,αα → andititpreferentially
σ,and preferentially precipitates
precipitates
at
at the
the interface between the
interface between the austenite
austenite and
and ferrite
ferrite phase
phase and
and the
the grain
grain boundary
boundary of of the
the ferrite
ferrite phase.
phase.
The precipitation temperature range of the σ phase is 600–1000 ◦ C [34,35], as indicated in Figure 1.
The precipitation temperature range of the σ phase is 600–1000 °C [34,35], as indicated in Figure 1.
Figure
Figure 88 shows
shows the
the SEM
SEM image
image ofof the
the σσ phase,
phase, andand Figure
Figure 99 indicates
indicates the the variation
variation ofof the
the σσ phase
phase
precipitation at different solution treatment temperatures. It is clear that the σ
precipitation at different solution treatment temperatures. It is clear that the σ phase is widely phase is widely
distributed ◦ C.
distributed within
withinthe
theferrite
ferritegrains
grainsand
andatatthe α/γphase
theα/γ phaseboundaries,
boundaries, after solution
after treatment
solution treatmentat 900
at 900
With the increase of solution treatment temperature, the precipitation of the σ phase decreases, and the
°C. With the increase of solution treatment temperature, the precipitation of the σ phase decreases,
andMetals
the σ2020,
phase10, x changes
FOR PEER REVIEW
from concentrated distribution to dispersed existence in ferrite. At 8 of 15 °C,
1000
the σ phase distributed in the ferrite is completely dissolved; only a small amount of the σ phase
°C. With the increase of solution treatment temperature, the precipitation of the σ phase decreases,
exists at the grain boundaries of the two phases, which is only 0.8%. After solid-solution treatment at
and the σ2020,
Metals phase changes from concentrated distribution to dispersed existence in ferrite. At8 1000
10, 1481 of 15 °C,
1050the°C,σ no σ phase is present in the matrix. There are two reasons
phase distributed in the ferrite is completely dissolved; only a small amount of the σ for the dissolution of the σ phase.
phase
First, withatthe
exists the increase of solution
grain boundaries of the treatment
two phases, temperature,
which is only the0.8%.
N element content in ferrite
After solid-solution treatment increases,
at
σ phase changes from concentrated distribution to dispersed existence in ferrite. At 1000 ◦ C, the σ phase
which
1050reduces
°C, no σ the
phaseactivity
is presentof Cr in andmatrix.
the inhibits There theareprecipitation
two reasons of
for the
the σ phase [14,28].
dissolution of the Second,
σ phase. the
distributed in the ferrite is completely dissolved; only a small amount of the σ phase exists at the grain
austenite structure
First,boundaries
with decreases
the increase
of the twoofphases,gradually;
solutionwhich treatment subsequently,
is only temperature,
0.8%. the
the interface
N element
After solid-solution between
treatment at 1050inferrite
content no σ and
ferrite
◦ C, austenite
increases,
phase
decreases
which and the
reduces
is present nucleation
the
in the activity
matrix. site
of
There Cr
areofand
twothereasons
σ phase
inhibits forthedecreases
the precipitation
dissolution [16,18]. σ Therefore,
of the
of the σ phase
phase. the solution
First, [14,28].
with Second,
the increase treatment
the
austenite structure
solution decreases
treatment gradually;
temperature, the N subsequently,
element content inthe interface
temperature should be at least higher than 1000 °C in order to eliminate the σ phase in UNS S32750
of ferrite between
increases, which ferrite
reduces and
the austenite
activity
of Cr and
decreases
duplex stainlessandinhibits the precipitation
the nucleation
steel. site of the of the σ phase
σ phase [14,28]. Second,
decreases [16,18].the austenite the
Therefore, structure decreases
solution treatment
gradually;
temperature subsequently,
should be the
at the interface
leastNhigher between
thancontentferrite
1000 °C in and austenite
in order decreases
to eliminate and the nucleation
the σasphase site
in UNS S32750of
Figure 4d shows that element the ferrite increases, the solution treatment
duplexthe stainless
σ phase decreases
steel. [16,18]. Therefore, the solution treatment temperature should be at least higher
temperature
than increases.
1000 ◦ in order However, thethe solubility inofUNS
theS32750
N element instainless
the ferrite phase is relatively low;
Figure 4dCshows to eliminate
that the N element σ phase
content in the ferrite duplex
increases, steel.
as the solution treatment
it easily reachesFigure the saturation
4d shows that thestate.
N element Therefore,
content in thetheNferrite
element and asCrtheelement
increases, are combined to
solution treatment
temperature increases. However, the solubility of the N element in the ferrite phase is relatively low;
precipitate Cr2N, during
temperature increases. theHowever,
rapid cooling process
the solubility after
of the dissolution
N element in the [29,30].
ferrite phaseFigure 10 exhibits the
is relatively
it easily reaches the saturation state. Therefore, the N element and Cr element are combined to
morphologylow; it easily
ofCrCr reaches the saturation state. Therefore, the N element and
2N analyzed by SEM and TEM. The Cr2N appears short and rod-like, and the Cr2N
Cr element are combined to
precipitate
precipitate 2N, during the rapid cooling process after dissolution [29,30]. Figure 10 exhibits the
Cr2 N, during the rapid cooling process after dissolution [29,30]. Figure 10 exhibits the
precipitates
morphology in ferrite
of Cr Ngrains
analyzedare fine
by SEM or coarse.
and TEM. TheThe different size ofshort
Cr2N appears Cr2Nand is probably
rod-like, andduethe to sub
Cr2Ngrain
morphology of2Cr 2 N analyzed by SEM and TEM. The Cr2 N appears short and rod-like, and the Cr2 N
boundaries
precipitates andininhomogeneous
precipitates ferrite grains
in ferrite grains are nucleation
arefine
fineor orcoarse.at The
coarse. defects
The [27].size
different
different sizeofof
CrCr
2 N2N is probably
is probably duedue to sub
to sub graingrain
boundaries andand
boundaries inhomogeneous
inhomogeneousnucleation
nucleation at
at defects [27].
defects [27].

◦ C; (b) 950 ◦ C.
Figure
Figure 8. Scanning
8. Scanning
Figure
8. Scanning electron
electron
electron microscopy (SEM)
microscopy
microscopy (SEM) micrograph
(SEM) micrograph
micrograph ofof phase.
σof (a) (a)
σ phase.
σ phase. 900(a)
900 900 °C; 950
°C; (b) 950 °C.
(b) °C.

Figure 9. Area fraction of σ phase at different solution treatment temperatures (error bars represent
Figure Figure
9. Area
standard Area fraction
9.fraction
deviation). of σofphase
σ phase
atatdifferent
different solution
solutiontreatment temperatures
treatment (error bars
temperatures represent
(error bars represent
standard deviation).
standard deviation).
MetalsFigure
2020, 10,9.1481
Area fraction of σ phase at different solution treatment temperatures (error bars represent9 of 15
standard deviation).

Metals 2020, 10, x FOR PEER REVIEW 9 of 15

Metals 2020, 10, x FOR PEER REVIEW 9 of 15

Figure 10. SEM and transmission electron microscopy (TEM) micrograph of Cr2N. (a) SEM image; (b)
TEM image; (c) energy dispersive spectroscopy (EDS) of Cr2N.
Figure SEMand
10. SEM
Figure 10. andtransmission
transmissionelectron
electronmicroscopy
microscopy (TEM)
(TEM) micrograph
micrograph of of
Cr2Cr
N.2 N.
(a) (a)
SEM SEM image;
image; (b)
(b)
TEM TEM image;
image; (c) energy
(c) energy dispersive
dispersive spectroscopy
spectroscopy (EDS)
(EDS) of 2Cr
of Cr N.2 N.
3.4. Effect of Solution Treatment Temperature on Mechanical Properties
3.4. Effect of Solution Treatment Temperature on Mechanical Properties
Theofmechanical
3.4. Effect properties
Solution Treatment of UNS S32750
Temperature changeProperties
on Mechanical curvilinearly with the increase of solution
treatment temperature,
The mechanical as shown
properties ofin Figure
UNS 11. The
S32750 hardness
change of UNS S32750
curvilinearly with thedecreases
increase from 900 °C to
of solution
The mechanical properties of UNS S32750 change curvilinearly with the increase of solution
1050 °C and
treatment increases in
temperature, as the 1050–1300
shown in Figure °C 11.
range.
TheThe hardness
hardness of steel
of UNS is the
S32750 lowest atfrom
decreases °C,◦while
1050900 C to
treatment temperature, as shown in Figure 11. The hardness of UNS S32750 decreases from 900 °C to
the ◦impact
1050 energy isin
C and increases the theopposite,
1050–1300 ◦ C range.
reaching theThe
highest value.
hardness ofThe
steelchange at 1050 ◦ C,
of mechanical
is the lowest properties
while theis
1050 °C and increases in the 1050–1300 °C range. The hardness of steel is the lowest at 1050 °C, while
consistent
impact energywith is that of microstructure.
the opposite, reaching At the1050 °C, no
highest σ-phase
value. The precipitation is present,properties
change of mechanical and the ratio
is
the impact energy is the opposite, reaching the◦highest value. The change of mechanical properties is
of ferrite with
consistent to austenite is well balanced,Atapproaching
that of microstructure. 1:1, resulting
1050 C, no σ-phase in the best
precipitation performance.
is present, and the ratio of
consistent with that of microstructure. At 1050 °C, no σ-phase precipitation is present, and the ratio
ferriteWhen the solution
to austenite is welltreatment
balanced,temperature
approachingis1:1, in the range of
resulting in 900–1000 °C, the σ phase precipitates
the best performance.
of ferrite to austenite is well balanced, approaching 1:1, resulting in the best ◦ performance.
at the boundaries
When the solutionof ferrite and austenite
treatment temperature and inside
is in thethe ferrite
range of grains.
900–1000 TheC, σ the
phase is hard
σ phase and brittle,
precipitates
When the solution treatment temperature is in the range of 900–1000 °C, the σ phase precipitates
atwhose precipitation
the boundaries has aand
of ferrite great effect onand
austenite theinside
impact thetoughness and hardness
ferrite grains. The σ phase of UNS S32750
is hard duplex
and brittle,
at the boundaries of ferrite and austenite and inside the ferrite grains. The σ phase is hard and brittle,
stainless
whose steel. Whenhas
precipitation theaspecimen is exposed
great effect on the impactto an impact
toughness load,and
the hardness
σ phase takes the lead
of UNS in cracking,
S32750 duplex
whose precipitation has a great effect on the impact toughness and hardness of UNS S32750 duplex
causingsteel.
stainless the grain
Whenboundary
the specimen to become
is exposed brittle
to anand fracture
impact load,along the grain,
the σ phase takesthereby
the leadreducing the
in cracking,
stainless steel. When the specimen is exposed to an impact load, the σ phase takes the lead in cracking,
impactthe
causing toughness of the material.
grain boundary to becomeFigure
brittle 12andshows
fracturethat the the
along hardness of UNSreducing
grain, thereby S32750 corresponds
the impact
causing the grain boundary to become brittle and fracture along the grain, thereby reducing the
directly toofthe
toughness theproportion of σ phase.
material. Figure 12 showsIt is that
noteworthy that aofsmall
the hardness UNSamount of the σ phase
S32750 corresponds significantly
directly to the
impact toughness of the material. Figure 12 shows that the hardness of UNS S32750 corresponds
reduces impact
proportion toughness,
of σ phase. with only about
It is noteworthy that a 0.8%
smallof σ phase
amount ofdecreasing
the σ phasethe impact energy
significantly from
reduces 315.87
impact
directly to the proportion of σ phase. It is noteworthy that a small amount of the σ phase significantly
J to 100.93 J.
toughness, with only about 0.8% of σ phase decreasing the impact energy from 315.87 J to 100.93 J.
reduces impact toughness, with only about 0.8% of σ phase decreasing the impact energy from 315.87
J to 100.93 J.

Figure 11.Mechanical
Figure11. Mechanicalproperties
propertiesofofthe
theUNS
UNSS32750
S32750steel
steelvary
varywith
withsolution
solutiontreatment
treatmenttemperature.
temperature.
(a) Hardness; (b) impact energy (error bars represent standard deviation).
(a) Hardness; (b) impact energy (error bars represent standard deviation).
Figure 11. Mechanical properties of the UNS S32750 steel vary with solution treatment temperature.
(a) Hardness; (b) impact energy (error bars represent standard deviation).
Metals 2020, 10, x FOR PEER REVIEW 10 of 15

Metals 2020, 10, 1481 10 of 15

Metals 2020, 10, x FOR PEER REVIEW 10 of 15

Figure 12. Effect of σ phase on impact energy and hardness.

As the solution treatmentFigure 12.

Figure 12. Effect of
temperature
Effect of σ phase
σ phase on
rises,on impact
the energy
σ phase
impact and hardness.
dissolves
energy and completely and the structural
hardness.
transformation proceeds slowly, the complete degree of recrystallization of the structure of the steel
As the solution treatment temperature rises, the σ phase dissolves completely and the structural
gradually
As theincreases, its constituent
solution treatment phasesrises,
temperature gradually growdissolves
the σ phase larger, and the strength
completely and theof structural
the steel
transformation proceeds slowly, the complete degree of recrystallization of the structure of the steel
gradually
transformation proceeds slowly, the complete degree of recrystallization of the structure of the UNS
weakens. Therefore, when the solution treatment temperature reaches 1050 °C, steel
gradually increases, its constituent phases gradually grow larger, and the strength of the steel gradually
S32750
graduallyhas increases,
the highestitsimpact toughness
constituent and gradually
phases the lowest growhardness.
larger,Figure
and13a
the shows the
strength effect
of the of α/γ
steel
weakens. Therefore, when the solution treatment temperature reaches 1050 ◦ C, UNS S32750 has the
phase ratioweakens.
gradually on impactTherefore,
energy and hardness,
when and it istreatment
the solution clear that temperature
the impact toughness
reaches 1050 and hardness
°C, UNS
highest impact toughness and the lowest hardness. Figure 13a shows the effect of α/γ phase ratio on
directly
S32750 hascorrespond
the highest to the biphasic
impact ratio. At
toughness androomthe temperature,
lowest hardness. the strength
Figure 13aof the
showsferrite
thewith
effecta of
body-
α/γ
impact energy and hardness, and it is clear that the impact toughness and hardness directly correspond
centered
phase ratiocubic
on structure
impact energy is higher
andthan that ofand
hardness, the it
austenite
is clear with a face-centered
that the impact toughnesscubic structure
and hardness [36].
to the biphasic ratio. At room temperature, the strength of the ferrite with a body-centered cubic
The toughness
directly correspondof UNSto theS32750
biphasic is ratio.
mainly derived
At room from the austenitic
temperature, the strength tissue
of theand thewith
ferrite strength
a body- is
structure is higher than that of the austenite with a face-centered cubic structure [36]. The toughness
primarily reflected
centered cubic in ferrite
structure structure.
is higher thanAs a result,
that of the the impactwith
austenite toughness weakens,cubic
a face-centered whilestructure
the hardness[36].
of UNS S32750 is mainly derived from the austenitic tissue and the strength is primarily reflected
increases with the
The toughness raise of
of UNS α/γ ratio.
S32750 In addition,
is mainly derivedthe influence
from of alloying
the austenitic elements,
tissue and theespecially
strength N is
in ferrite structure. As a result, the impact toughness weakens, while the hardness increases with
elements, needs to be considered. With the rise in solution treatment temperature,
primarily reflected in ferrite structure. As a result, the impact toughness weakens, while the hardness the N partitioning
the raise of α/γ ratio. In addition, the influence of alloying elements, especially N elements, needs
ratio increases,
increases with theandraise
the hardness of theInmaterial
of α/γ ratio. addition, improves, whileof
the influence thealloying
toughness weakens,
elements, as shown
especially N
to be considered. With the rise in solution treatment temperature, the N partitioning ratio increases,
in Figure 13b.
elements, needsNitrogen exists as interstitial
to be considered. atoms
With the rise in the octahedral
in solution treatmentinterstices
temperature,of the
thematrix, forming
N partitioning
and the hardness of the material improves, while the toughness weakens, as shown in Figure 13b.
an interstitial
ratio increases,solid
and solution
the hardness reinforcement that is
of the material about two
improves, orders
while of magnitude
the toughness largeras
weakens, than
shownthe
Nitrogen exists as interstitial atoms in the octahedral interstices of the matrix, forming an interstitial
substitution
in Figure 13b.solid solution
Nitrogen existsreinforcement formedin by
as interstitial atoms the chromium, nickel, and
octahedral interstices molybdenum
of the matrix, forming [29].
solid solution reinforcement that is about two orders of magnitude larger than the substitution solid
Furthermore,
an interstitial itsolid
should be noted
solution that the solubility
reinforcement that isofabout
N in ferrite is low,of
two orders and during thelarger
magnitude water-cooling
than the
solution reinforcement formed by chromium, nickel, and molybdenum [29]. Furthermore, it should be
process, supersaturated
substitution solid solution N combines with Crformed
reinforcement to form by Cr2N, which enhances
chromium, nickel, the
andhardness
molybdenumof steel [29].
and
noted that the solubility of N in ferrite is low, and during the water-cooling process, supersaturated N
reduces the impact
Furthermore, toughness.
it should be noted that the solubility of N in ferrite is low, and during the water-cooling
combines with Cr to form Cr2 N, which enhances the hardness of steel and reduces the impact toughness.
process, supersaturated N combines with Cr to form Cr2N, which enhances the hardness of steel and
reduces the impact toughness.

13. (a)
Figure 13.
Figure (a) Effect
Effectof α/γphase
ofα/γ phaseratio
ratioonon
impact energy
impact andand
energy hardness; (b) effect
hardness; of Nof
(b) effect partitioning ratio
N partitioning
on impact energy and hardness.
ratio on impact energy and hardness.
Figureof13.
3.5. Effect (a) Effect
Solution of α/γ phase
Treatment ratio ononimpact
Temperature energy and
the Corrosion hardness; (b) effect of N partitioning
Resistance
3.5. Effect of Solution
ratio on Treatment
impact energy Temperature on the Corrosion Resistance
and hardness.
It is evident from Figure 14a that the Nyquist diagrams at different solution treatment temperatures
are in
It is form
the of
evident from Figurearc,
of theTreatment
14aindicating
that the that
capacitiveTemperature
Nyquist diagrams at
a more complete
different solution treatment
3.5. Effect Solution on the Corrosion Resistancepassivation film is formed on the
temperatures are in the form of the capacitive arc, indicating that a more complete passivation film
surface of the material and the corrosion mechanism of UNS S32750 steel does not change. The radius
is formed on the surface of the material and the corrosion mechanism
of the capacitive loop increases first and then decreases, and reaches the maximum solution
It is evident from Figure 14a that the Nyquist diagrams at of UNS
differentS32750 steeltreatment
at 1050 ◦ C.
does not
A larger
change.
temperaturesThe radius of form
are in the the capacitive loop increases
of the capacitive arc, first and
indicating then decreases, and reaches filmthe
radius of the capacitive arc indicates a stronger resistance to that a more
charge complete
transfer at thepassivation
metal–solution
maximum
is formed at the
on 1050surface
°C. A larger
of the radius ofand
material the the
capacitive
corrosionarcmechanism
indicates a of stronger
UNS resistance
S32750 steeltodoes
charge
not
interface, which means better corrosion resistance of the metal [37]. Therefore, from the changing trend
transfer
change. at
Thetheradius
metal–solution
of the it interface, loop
capacitive which means better
increases corrosion resistance of thereaches
metal [37].
of the capacitive arc radius, can be observed that the first and
increase then
of decreases,
solution and
treatment the
temperature
maximum at 1050 °C. A larger radius of the capacitive arc indicates a stronger resistance to charge
transfer at the metal–solution interface, which means better corrosion resistance of the metal [37].
Metals 2020, 10, 1481 11 of 15

makes the corrosion resistance of UNS S32750 steel first stronger and then weaker, and the corrosion
resistance performance is optimal at 1050 ◦ C.
Figure 15 shows the dynamic potential polarization curves of UNS S32750 at different solid
solution treatment temperatures, and it can be seen that all the anodic polarization curves show a
passivation zone, which is related to the formation of the passive film on the metal surfaces. Based on
the analysis of the polarization curves, Table 2 presents the main electrochemical parameters. As the
solution treatment temperature rises, the pitting potential (Ep ) of UNS S32750 duplex stainless steel first
improves and then weakens, and the pitting potential reaches the highest at 1050 ◦ C, which indicates
that the tendency of pitting corrosion at this solution treatment temperature is inferior. Because of the
precipitation of the σ phase between 900 and 1000 ◦ C, chromium and molybdenum depleted zones
are easily formed in the vicinity, which leads to pitting corrosion. The corrosion resistance of duplex
stainless steel directly corresponds to the proportion of the σ precipitation phase. The two-phase
structure (ferritic and austenitic phase) in UNS S32750 at 1050 ◦ C is more uniformly distributed, which is
conducive to the stability of the passivation film on the steel surface and has a better inhibitory effect
on pitting corrosion, manifested by a higher pitting potential (Ep ). When the temperature of solution
treatment rises above 1100 ◦ C, the distribution of elements in the two phases is uneven. With the
rise in solution treatment temperature, the ferrite phase ratio improves, and the concentration of Cr
and Mo, the key elements for pitting corrosion resistance, decreases in ferrite, causing the corrosion
rate to rise. The element N, an austenite-forming element, has a high solubility in the austenitic
phase [26], leading to preferential corrosion in the ferrite phase due to the lack of N, which makes
an outstanding contribution to pitting resistance. At the same time, with the increase of the solution
treatment temperature, the Cr2 N precipitates in the ferrite during the water-cooling process, resulting
in the reduction of pitting resistance of the ferrite phase. Therefore, the pitting potential (Ep ) of UNS
S32750 shows a downward trend at 1050–1300 ◦ C.

Table 2. The electrochemical parameters for UNS S32750 super duplex stainless steel (SDSS) after
solution treatment at different temperature. Icoor : corrosion current density, Ecorr : corrosion potential,
Ep : pitting potential. SCE, saturated calomel electrode.

Temperature/◦ C Icoor /A·cm−2 Ecoor vs. SCE/V Ep vs. SCE/V (Ep -Ecoor )/V
900 9.8877 × 10−8 −0.2686 0.8460 1.1146
950 7.4826 × 10−8 −0.2831 0.8772 1.1603
1000 6.1035 × 10−8 −0.2956 0.9634 1.2590
1050 4.9744 × 10−8 −0.2812 1.0019 1.2831
1100 5.7373 × 10−8 −0.2928 0.9451 1.2380
1150 5.1575 × 10−8 −0.2898 0.9694 1.2592
1200 5.8594 × 10−8 −0.2884 0.9616 1.2500
1250 5.8948 × 10−8 −0.2719 0.9425 1.2144
1300 6.1156 × 10−8 −0.2634 0.9317 1.1951

In addition, the (Ep -Ecoor ) values listed in Table 2 show the same trend as the (Ep) values, with a
tendency to rise and then fall. Because the (Ep -Ecoor ) value represents the resistance of the nucleus [15],
the resistance of the pitting nucleus also increases and then reduces, i.e., the pitting resistance of the
steel improves and then weakens as the solid solution temperature increases. Furthermore, as the
solution temperature increases, the self-corrosion current density (Icoor ) first becomes smaller and then
larger. At 1050 ◦ C, the lowest self-corrosion current density is 4.9744 × 10−8 A·cm−2 , indicating that
UNS S32750 has quite good corrosion resistance.
Metals 2020, 10, 1481 12 of 15
Metals 2020, 10, x FOR PEER REVIEW 12 of 15

Metals 2020, 10, x FOR PEER REVIEW 12 of 15

Figure
Figure 14.Electrochemical
14.
Figure14. Electrochemicalimpedance
Electrochemical impedancespectroscopy
impedance spectroscopy(EIS)
(EIS)diagrams
diagramsof
diagrams ofUNS
of UNSS32750
UNS S32750duplex
S32750 duplexstainless
duplex stainless
stainless
steel
steel treated
treated at
at different
different solution
solutiontreatment
treatmenttemperatures.
temperatures.(a)
(a)Nyquist
Nyquistplot;
plot;(b)
(b)Bode
Bodeplot.
plot.
steel treated at different solution treatment temperatures. (a) Nyquist plot; (b) Bode plot.

Figure
Figure15.
15.Polarization curvesof
Polarizationcurves ofUNS
UNSS32750
S32750solution
solutiontreated
treatedatatdifferent
differenttemperatures.
different temperatures.
temperatures.

The
Thesolution
The solutiontreatment
solution treatmenttemperature
treatment temperatureaffects
temperature affectsthe
affects thealloying
the alloyingelement
alloying elementcontent
element contentof
content ofaustenite
of austeniteand
austenite andferrite
and ferrite
ferrite
in
in the
in the steel,
the steel, resulting
steel, resulting
resulting in in a variation
in aa variation
variation in in the
in the corrosion
the corrosion resistance
corrosion resistance
resistance of of both
of both phases.
both phases.
phases. The The main
The main alloying
main alloying
alloying
elements
elementsthat
elements that
that affect
affect
affectpitting
pittingcorrosion
pitting corrosion
corrosion in chloride
in
in chloride environment
chloride environment
environment are Cr,are Mo,
are Cr, andMo,
Cr, N. The
Mo, and
and pitting
N.
N. The resistance
The pitting
pitting
equivalent
resistance number of the austenitic and ferritic phases can be
resistance equivalent number of the austenitic and ferritic phases can be calculated using the
equivalent number of the austenitic and ferritic calculated
phases using
can be the elemental
calculated contents
using the
of the austenitic
elemental
elemental contents
contentsandof ferritic
of the phases atand
theaustenitic
austenitic different
and ferritic
ferritic solution
phasestreatment
phases at
atdifferent
different temperatures
solution
solutiontreatment in Figure
treatment 4. PREN is
temperatures
temperatures
an
in empirical
inFigure
Figure4.4.PREN valueisfor
PREN isan
anpredicting
empiricalthe
empirical value
valuecorrosion
for resistance
forpredicting
predicting the of an alloyresistance
thecorrosion
corrosion or phase;of
resistance a an
of larger
analloy PREN
alloy or valueaa
orphase;
phase;
represents
larger
largerPREN a better
PRENvalue the corrosion
valuerepresents
representsaabetter resistance
betterthe of the
thecorrosion alloy or
corrosionresistance phase
resistanceof [38,39],
ofthethealloyPPREN
alloyororphase
γ = w(Cr)
phase[38,39], + 3.3
[38,39],PPRENw(Mo)
PPRENγγ
+==20 w(N),
w(Cr)
w(Cr) PREN
++3.3
3.3w(Mo) α = w(Cr)
w(Mo) ++20 + 3.3PREN
20w(N),
w(N), w(Mo)
PRENαα=[1]. =w(Cr)From
w(Cr) Figure
++3.3
3.3w(Mo)
w(Mo)16, it[1].
is evident
[1].From
FromFigure that the
Figure 16,PREN
16, α of thethat
ititisisevident
evident ferrite
that the
the
phase
PREN first
of increases
the ferrite and
phase then decreases
first increases with
and the
thenrise in the
decreases solution
with
PRENα of the ferrite phase first increases and then decreases with the rise in the solution treatment
α treatment
the rise in temperature,
the solution and
treatment the
PREN γ of theand
temperature,
temperature, austenite
and the
the PRENphase
PREN gradually
γγ of
of the increases,
the austenite
austenite phase
phasewhich is caused
gradually
gradually by the which
increases,
increases, change
which isis ofcaused
the chemical
caused byby the
the
composition
change
changeof ofthe inchemical
the each
chemical phase, and this change
composition
composition in
ineach
each is phase,
consistent
phase,and andwith thisthe
this changevariations
change of the composition
isisconsistent
consistent with
withthe of each
thevariations
variations
phase
of thein
ofthe Figure 4. When
composition
composition of
ofeachthephase
each solution
phasein treatment
inFigure
Figure4.4.When temperature
When the exceeds
thesolution
solution 1000 ◦ C,temperature
treatment
treatment no σ phase precipitates
temperature exceeds
exceeds1000 in
1000
UNS
°C,
°C,no S32750,
noσσphase and the
phaseprecipitates PREN
precipitatesin value
inUNS of both
UNSS32750, ferrite
S32750,and andthe and
thePRENaustenite
PRENvalue valueofis greater
ofboth than
bothferrite 40,
ferriteand which
andaustenite complies
austeniteisisgreater with
greater
the
than standard of super duplex stainless steel. It should be pointed
than 40, which complies with the standard of super duplex stainless steel. It should be pointedout
40, which complies with the standard of super duplex out
stainless that the
steel. Itformula
should only
be considers
pointed out
the
that
thatrole
theof
the alloyingonly
formula
formula elements,
only considers
considersand the
does
the role notof
role take
of into account
alloying
alloying elements,
elements, the and
effects
and does of tissue
does not
not take inhomogeneity
take into accountand
into account the
the
precipitation
effects
effectsof oftissue phases.
tissue It is not appropriate
inhomogeneity
inhomogeneity and
andprecipitationto use only
precipitation the PREN
phases.
phases. ItItisisnotvalue
not to assessto
appropriate
appropriate the
to use
usepitting
only corrosion
onlythethePRENPREN
resistance
value
valueto of duplex
toassess
assess the stainless
thepitting
pitting steel, because
corrosion
corrosion resistance
resistance the of distribution
ofduplex
duplexstainless of the steel,
stainless decisive
steel,becauseCr, Mo,
because theand
the N alloying
distribution
distribution of
of
elements
the
thedecisive between
decisiveCr, Cr,Mo, the
Mo,and two
andN phases
Nalloying is
alloyingelements not balanced.
elementsbetween betweentheThe poor
thetwo zones
twophases of
phasesisisnot these elements
notbalanced.
balanced.The are
Thepoor prone
poorzones to
zones
pitting
of
ofthese
thesecorrosion,
elementsand
elements are the
areprone
proneactual
to pittingcorrosion,
topitting
pitting resistanceand
corrosion, of the
and the
the steel
actualis lower
actual pitting
pitting than what isof
resistance
resistance represented
ofthe steelisisby
thesteel the
lower
lower
PREN
than value.
what is represented by
than what is represented by the PREN value. the PREN value.
Metals 2020, 10, 1481 13 of 15
Metals 2020, 10, x FOR PEER REVIEW 13 of 15

Figure 16.
Figure 16. Pitting resistance
Pitting equivalent
resistance number
equivalent (PREN)
number of UNS
(PREN) of S32750 solutionsolution
UNS S32750 treated at different
treated at
temperatures.
different temperatures.

4.4. Conclusions
Conclusions
(1)
(1)AsAsthe
thesolution
solutiontreatment
treatmenttemperature
temperatureincreases,
increases,the theferrite
ferritecontent
contentrises,
rises,while
whilethetheaustenite
austenite
content ◦ ◦
content declines. At 1050 °C, the dual-phase ratio approaches 1:1. From 900 to 1000 °C, theσσphase
declines. At 1050 C, the dual-phase ratio approaches 1:1. From 900 to 1000 C, the phase
fraction ◦ C, Cr N precipitates
fractiondrops
drops sharply fromfrom
sharply 19.33% to 0.80%.
19.33% When the
to 0.80%. Whentemperature exceeds 1050
the temperature exceeds 21050 °C, Cr2N
in the ferrite grain,
precipitates and continuously
in the ferrite grows and coarsens
grain, and continuously grows and to form a dense
coarsens Cr2 Naregion.
to form dense Cr2N region.
(2)
(2) The Cr and Mo are enriched in the ferrite, while Ni and N are concentrated in
The Cr and Mo are enriched in the ferrite, while Ni and N are concentrated in the
the austenite.
austenite.
From 900 to 1050 ◦ C, the Cr and Mo content in the ferrite phase rises with the drop of σ phase
From 900 to 1050 °C, the Cr and Mo content in the ferrite phase rises with the drop of σ phase
precipitation. ◦ C, the elements partitioning ratios of Cr and Mo decline with the
precipitation. FromFrom 1050
1050 to
to 1300
1300 °C, the elements partitioning ratios of Cr and Mo decline with the
increase
increaseof ofthe α/γratio,
theα/γ ratio,while
whilethose
thoseofofNi Niand
andNNcontinue
continuetotorise.rise.
(3)
(3) With the rising solution treatment temperature,the
With the rising solution treatment temperature, themechanical
mechanicalproperties
properties of ofUNS
UNSS32750
S32750
vary in a curve. From 900 to 1050 ◦ C, the hardness drops with σ phase dissolution, and the impact
vary in a curve. From 900 to 1050 °C, the hardness drops with σ phase dissolution, and the impact
toughness ◦ C, as the α/γ ratio rises, the strength and hardness of the
toughness improves.
improves. From
From 1050
1050 to to 1300
1300 °C, as the α/γ ratio rises, the strength and hardness of the
material ◦ C, the two-phase structure equilibrates,
material rise, while the impact toughness weakens. At
rise, while the impact toughness weakens. At 1050
1050 °C, the two-phase structure equilibrates,
and
andUNS
UNSS32750
S32750hashasaaminimum
minimumhardnesshardnessofof25.7425.74HRC HRCand andaamaximum
maximumimpact impactenergy
energyofof315.87
315.87J.J.
(4) As the solution treatment temperature rises, the
(4) As the solution treatment temperature rises, thepEp and p(Ep-E E and (E -E coor ) values of UNS S32750
coor) values of UNS S32750 show
show a
trend
a trendof increasing
of increasingandand
thenthen
decreasing,
decreasing, while Icoor Ishows
while the opposite trend. UNS S32750 has the best
coor shows the opposite trend. UNS S32750 has the
resistance to pitting corrosion treated at 1050 ◦ C. The AC impedance test indicates that the capacitive
best resistance to pitting corrosion treated at 1050 °C. The AC impedance test indicates that the
arc radius and
capacitive arcmaximum
radius and phase angle vary
maximum in a curve,
phase angle which
vary in is consistent
a curve, whichwith theisconclusions
consistent obtained
with the
from the potentiodynamic polarization curve.
conclusions obtained from the potentiodynamic polarization curve.
Contributions:Conceived
AuthorContributions:
Author Conceivedandand
designed the experiments,
designed J.L., W.S.J.L.,
the experiments, andW.S.,
Z.Y.; performed
and Z.Y.;the experiments,
performed the
J.L. and W.S.; contributed
experiments, J.L. and W.S.; reagents/materials/analysis tools, P.L. andtools,
contributed reagents/materials/analysis F.W.; P.L.
analyzed the data
and F.W.; and prepared
analyzed the data the
and
paper, J.L.; the
prepared review and J.L.;
paper, revised the manuscript,
review and revisedZ.Y.
theAll authors have
manuscript, Z.Y.read
All and agreed
authors to the
have published
read version
and agreed of
to the
the manuscript.
published version of the manuscript.
Funding: This work was supported by the Open Funds of State Key Laboratory of Advanced Metallurgy (KF18-01).
Funding: This work was supported by the Open Funds of State Key Laboratory of Advanced Metallurgy (KF18-
Acknowledgments:
01). The authors wish to thank Lianqi Li, Xi Chen, Kai Lu, and Yubin Lu for their advice and help
with the experiments, and Lianqi Li and Qiang Cheng for their comments and suggestions on revising the article.
Acknowledgments:
Conflicts of Interest: The authors declare
The authors wish tono
thank Lianqi
conflict Li, Xi Chen, Kai Lu, and Yubin Lu for their advice and
of interest.
help with the experiments, and Lianqi Li and Qiang Cheng for their comments and suggestions on revising the
article.
References
Conflicts of Interest: The authors declare no conflict of interest.
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