九州大学大学院総合理工学府報告第 40 巻 第 1 号 7-14 頁 平成 30 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 40, No. 1, pp. 7-14, SEP, 2018

Precipitation Behavior of Super Duplex StainlessSteel in Friction-Stir-Welding Processes
Itto SUGIMOTO*1, † Hikaru SAITO*2 and Satoshi HATA*2†

E-mail of corresponding author: itto.sugimoto.nu@hitachi.com(Received July 19, 2018, accepted July 20, 2018)

To investigate the precipitation behavior of super duplex stainless steel in its weld by friction
stir welding (FSW) at a low welding speed, we carried out microstructural observation andanalysis. An intermetallic compound phase, σ, was observed in the heat affected zone (HAZ). The
σ phase precipitated at the interfaces between δ-Fe (ferrite) and γ-Fe (austenite) grains 1 - 2 mm
away from the stir zone (SZ) / HAZ boundary. On the other hand, the Cr2N phase was observed
together with the belt-like γ-Fe grain aggregates in the vicinity of the advancing side (AS) of the
SZ. The other intermetallic phase, χ, was also observed at a triple point of γ-Fe grains. One caninterpret that the precipitation of the Cr2N and χ phases correlates with the transformation of
δ-Fe to γ’-Fe (secondary austenite).
Key words: Super duplex stainless steel, Friction stir welding (FSW), Sigma (σ) phase, Chi (χ)
phase, Chromium nitride, Electron back-scatter diffraction (EBSD), Transmission
electron microscopy (TEM)

1.

Introduction

Super duplex stainless steels are the next
generation high durability structural material
having both high strength and high corrosion
resistance, and are expected to use for plants,
water treatment equipment, pipelines, etc.
The microstructure of duplex stainless steels
consists of ferrite (δ-Fe) and austenite (γ-Fe),and their volume ratio is approximately 1: 1.
The duplex stainless steels are mainly
Fe-Ni-Cr ternary systems containing otherelements to improve strength, corrosion
resistance, workability and weldability. For
example, pitting corrosion resistance ofstainless steel is enhanced by adding asignificant amount of Cr, Mo, W and nitrogen.
The pitting resistance equivalent number(PREN) expressed by the following formula isan index showing the pitting corrosion
resistance of stainless steel. Super duplex
stainless steels are those of duplex stainlesssteels with PREN of 40 or more 1):
PREN = 1%Cr + 3.3(%Mo + 0.5%W) + 16%N.
*1

Department of Applied Science for Electronics andMaterials. Hitachi Ltd.
*2 Department of Advances Materials Science

Although super duplex stainless steels haveexcellent material properties, their corrosion
resistance is remarkably impaired by
precipitation of an intermetallic phase, σ, inwelding processes 1), 2). Fig. 1 shows that
optical microscopic images of super duplex
stainless steel used in this study. In Fig. 1 (a),a base material without heat treatment showsa two-phase structure, ferrite (δ-Fe) and
austenite (δ-Fe), with a lamellar morphology.
On the other hand, Fig. 1 (b) shows a test pieceafter heating at 800 °C for 30 min. The σ phaseforms in the lamellar structure, as recognizedin the darkest region. One can interpret thatthe σ phase precipitated during the heat
treatment. The interfacial energy of the duplex
stainless steel is highest at the interface
between δ-Fe and γ-Fe, and the σ phase
precipitates preferentially at the δ / γ
interfaces 3), 4). The precipitation of the σ phaseis related to the formation of the secondary
austenite, γ'-Fe 5), 6). That is, Ni is partitioned
to the γ'-Fe grains during the transformationfrom δ-Fe to γ'-Fe due to the differences insolubility of each element. On the other hand,
Cr and Mo are discharged from γ'-Fe and the σphase containing a significant amount of Cr8Precipitation Behavior of Super Duplex Stainless Steel in Friction-Stir-Welding Processes

and Mo precipitated. Fig. 1 (b) also shows thatthe σ phase precipitates to erode δ grains.

show a symmetrical structure in many cases.
The left and right sides are distinguished as
the followings respectively: an advancing side(AS) where the directions of the tool rotationand the material joining coincide with eachother and a retreating side (RS) where thesetwo directions are opposite to each other.

Fig. 1 Cross-sectional optical microscopy
images of a super duplex stainless steel. (a) A
base material without heating and (b) aheat-treated material (800oC for 30 min).
The present study focuses on friction stir
welding (FSW) of super duplex stainless steel.
The FSW is a metal bonding method invented
in the Welding Institute (TWI) in 1991, and its
process is a kind of solid phase bonding not
through the process of melt solidification7).
Joining by the FSW was initially applied to
low melting point metals such as aluminumalloys. Its joining part has little welding
trouble such as grain coarsening, residual
stress, deformation, porosity and solidification
crack after construction. Therefore, the FSW ofhigh melting point materials such as various
steels and titanium alloys has been vigorously
studied. Fig. 2 shows (a) a bird's-eye view of
the FSW joining and (b) a schematiccross-sectional image of the weld. The FSW
device consists of rotation tools with a probe
and a shoulder at the tip. The rotation tool isinserted into the weld material and is moved
along a joining line under a loaded state. The
softened parts by frictional heat are stirred
and plastically flowed, and then joined byrecombining at the back of the rotation tool.
The cross-sectional structure after the FSW
consists of a stir zone (SZ), a heat affected zone
(HAZ) and a base material. The HAZ formedby the FSW can be divided into athermo-mechanically affected zone (TMAZ),
which is affected, by plastic deformation and a
HAZ with no influence of the plastic flow. One
should notice that the left and right sides of
the cross-sectional image of the welds do not

Fig. 2 (a) A bird’s eye view of joining by the
FSW and (b) a schematic cross-section of theweld formed by the FSW.
The FSW of super duplex stainless steels ischallenging to join them, but there have been
several reports after developments of various
high durability tools. According to thesereports 8) ~11), the welds of super duplex
stainless steel consist of fine grains of δ-Fe andγ-Fe. However, as far as the authors'knowledge, there has been no report on
precipitates in the FSW welds of super duplex
stainless steels. In this study, the FSW isconducted on a super duplex stainless steel ata lower welding speed than usual, and its weld
part was observed in detail by electron
microscopy. Based on the electron microscopy
experiment, we will clarify the precipitation
behavior at the weld of the super duplex
stainless steel, which is an indispensable
knowledge for using the welded material
safely.

2.

Experimental procedure

The super duplex stainless steel prescribedin UNS-S32750 was used for the present study.
A rolled material, which has a length of 300
mm, a width of 100 mm and a thickness of 12mm, was prepared, and a bead-on-plate test of平成 30 年度

九州大学大学院総合理工学報告

the FSW with a welding length of 200 mm wascarried out at the center of the adjacentmaterials to join.
Table 1 The chemical composition of thesuper duplex stainless steel (UNS-S32750)
used in the present study.

Table 1 shows the chemical composition of
the super duplex stainless steel used in thepresent study. The nitrogen concentration,
0.24 wt% N, is higher than those of otherstainless steels and is adjusted so that the
PREN described as formula (1) becomes 42.5
or more.
In the welding test of the FSW, the rotation
speed was 400 rpm, the rotation tool tilt angle
was 3°, and the plunging depth was 5.1 mm.
Fig. 3 shows a rotation tool made of acommercially available ceramic material,
SIALON (Si-Al-O-N), with a probe length of
4.5 mm and a shoulder diameter of 18.0 mm.
The welding speed was 0.42 mm / s which is
1/4 of the proper speed of 1.7 mm / s. The
welding speed is an important parameter toobtain good weld joints by the FSW. For thepresent experimental settings and materials,
no welding defect was formed by setting thewelding speed to 1.7 mm/s or less.
Cross-sectional specimens of the weld from thecentral part of the weld bead were prepared for
microstructural observations. Firstly, thesample was electrolytically etched with 10
kmol/m3 KOH solution for macroscopic
characterization using optical microscopy.
Secondary, micrometer-scale microstructuralcharacterizations have been carried out using
scanning electron microscopy (SEM) combinedwith an electron back-scatter diffraction(EBSD) technique. Thirdly, nanometer-scale
microstructural characterizations have been
performed
using
transmission
electronmicroscopy (TEM) and scanning transmission
electron microscopy (STEM). A focused ion
beam (FIB) sampling technique was utilized
for the TEM/STEM specimen preparation.
Phase stability calculations were carried out
using Thermo-calc. software to evaluate

第 40 巻第1号

9

possible precipitates formed during the FSW
process. Kinetic behaviors of the precipitates
were also calculated using JmatPro software.
For both the calculations, the same chemicalcomposition as the material used in theexperiment (Table 1) was assumed.

Fig. 3 A rotation tool for the FSW made of
SIALON.

3.

Results and discussion

3.1

Calculated phase stability
precipitation kinetics

andFig. 4 shows calculated mole fractions of thethermal equilibrium phases as a function oftemperature and Fig. 5 shows calculated
time-temperature-transformation(TTT)
diagrams for the possible precipitates in thepresent alloy system. The plots in Fig. 5
express the conditions under which the volume
fractions of the phases are more than 0.5%.
Let us explain the phases shown in thesefigures. δ-Fe and γ-Fe are the matrix phases ofthe duplex steel. σ is the tetragonalintermetallic phase described as Fe-Cr(-Mo).

Fig. 4 Calculated mole fractions of thethermal equilibrium phases of the super
duplex stainless steel as a function oftemperature.

10

Precipitation Behavior of Super Duplex Stainless Steel in Friction-Stir-Welding Processes

high-temperature range. On the other hand,
the intermetallic σ phase and the nitride Cr2N
phase are also main thermal equilibriumphases at 1000 °C or lower. The volume
fraction of the σ phase increases up to 0.4 at
700 °C.
As for Fig. 5, the σ, Cr2N, χ and LAVES
phases precipitate after relatively shortdurations than M23C6, M6C and CrN phases ina temperature range of 700-1000 °C. The Cr2N
phase precipitates in much shorter durations
than those for the CrN phase.
Fig.
5
Calculated
time-temperature-transformation (TTT) diagrams for the
possible phases in the super duplex stainlesssteel.
χ is a cubic intermetallic phase having acomposition of Fe36Cr12Mo10. Because the χphase is a metastable phase, it does not appear
in Fig. 4. Cr2N is a hexagonal nitride phase
commonly observed in super duplex stainlesssteels containing significant amounts ofnitrogen. CrN is a cubic nitride phase. The
LAVES phase is a cubic or hexagonalintermetallic phase described as AB2.
As for Fig. 4, the mole ratio between δ-Feand γ-Fe changes at higher than 1000 °C while
no precipitation is expected in this

3.2 M icrostructural observations
Fig. 6 shows cross-sectional views of theweld. By setting the low welding speed, 0.42mm/s, no welding defect occurred. However,Fig. 6(a) shows unknown image contrast in the
SZ near the AS. This unknown image contrast
showing the belt-like morphology is recognizedin both the optical microscopy (a) and SEM (b)images and notably different from the uniform
image contrast of the (δ-Fe + γ-Fe) two-phasematrix. In the following EBSD analysis, area A
and area B denoted in Fig. 6 (b) were selected
for the fields of view. Area A is located 2.0 mm
away from the SZ/TMAZ interface in the HAZ
outside the AS, and area B is in the belt-likestructure of the SZ.

Fig. 6 Cross-sectional optical microscopy (a) and SEM (b) images of the FSW sample under thewelding speed of 0.42 mm/s.
Fig. 7 demonstrates the SEM-EBSD
analysis in area A including the TMAZ/HAZ.
No precipitate was recognized near the
SZ/TMAZ interface. However, some phases
other than the δ-Fe and γ-Fe matrix phases
were observed in the HAZ being 1.5 mm awayfrom the SZ/TMAZ interface. The observed
precipitates showed brighter contrast than thematrix grains in the SEM images andconfirmed to be of the σ phase, as displayed inyellow on the EBSD phase map. The σ phase
precipitates from the δ-Fe (red) / γ-Fe (green)interface. This fact coincides with theprecipitation behavior of σ phase during heat
treatments reported previously 3), 4). The
precipitation of the σ phase in the HAZ was平成 30 年度

九州大学大学院総合理工学報告

also recognized at a location about 1 mm awayfrom the SZ/TMAZ interface at the RS. The
calculated results in Fig. 5 and Fig. 6 indicatedthat the σ phase is most likely to precipitate
around 950 °C and the alloy should be kept at

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11

the temperature for more than 1 min topromote the σ precipitation. In the weld, theregion 1-2 mm away from SZ was kept just inthat temperature region, and the σ phase
precipitated.

Fig. 7 SEM-EBSD analysis of area A denoted in Fig. 6(b) including the TMAZ/HAZ.
(a) Low-magnification SEM images, (b) magnified views of areas 1, 2 and 3 depicted withrectangles in (a), (c) an image quality map and (d) a phase map acquired from a square area
denoted in area 2 of (b).

12Precipitation Behavior of Super Duplex Stainless Steel in Friction-Stir-Welding Processes

Fig. 8 demonstrates the SEM-EBSD
analysis in area B of the SZ. The SZ shows amicrostructure in which the crystalline grains
are small: a few micrometers as a whole. The
belt-shaped structure showing the darker
contrast than the matrix is notably coarser
than the grain sizes at the SZ. This coarse
belt-shaped structure is interpreted as a traceof the stirred fluid. The EBSD analysis foundthat the belt-shaped structure is an aggregateof γ-Fe in which δ-Fe does not exist eventhough the surrounding matrix consists of the
(γ-Fe
+
δ-Fe)
two-phase
structure.
Furthermore, fine Cr2N precipitates indicatedby the blue color in Fig. 8 (c) are formed in thegrain boundaries of γ-Fe. It should be notedthat there is a microstructure composed of
Cr2N and γ-Fe phases which has a higher solid
solubility of nitrogen than that of δ-Fe.

Fig. 8 SEM-EBSD analysis of area B denoted in Fig. 6(b) in the belt-shaped structure near the
SZ/TMAZ interface at the AS. (a) An SEM image, (b) an image quality map and (c) a phase map
acquired from a square region depicted in (a).
Fig. 9 demonstrates STEM/TEM analysis ofan area showing the fine Cr2N precipitates atwhich the specimen was prepared by FIB. The
STEM high-angle annular dark-field (HAADF)
image in Fig. 9(a) indicates that Cr2N showing
the darker image intensity precipitates at thegrain boundaries of the γ-Fe matrix. Fig. 9(b)shows a magnified view of the square area
depicted in Fig. 9(a). At the triple junction ofthe γ-Fe grains, a precipitate showing thebrighter image intensity is formed with thetwo adjacent Cr2N particles. Fig. 9(c) shows a
TEM bright-field (BF) image of the same fieldof view as in Fig. 9(b). A selected area
diffraction pattern in Fig. 9(e) acquired fromthe precipitate at the triple junction denoted
as area 2 in Fig. 9(c) is different from adiffraction pattern in Fig. 9(d) acquired fromarea 1 in the γ-Fe matrix. Indexing thediffraction pattern in Fig. 9(e) revealed thatthe precipitate at the triple junction is of the χphase. The STEM/TEM analysis in Fig. 9
indicates the existence of aggregates of theγ-Fe matrix containing the grain boundary
precipitates of Cr2N and χ phases in the SZ. It
is interpreted that the Cr2N phase in the form

平成 30 年度

九州大学大学院総合理工学報告of sheets precipitated at grain boundaries ofthe γ-Fe matrix and the χ phase precipitated

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13

at triple junctions of the γ-Fe matrix.

Fig. 9 STEM/TEM analysis of an area showing Cr2N precipitates. (a) A STEM-HAADF image, (b)a magnified view of a square area depicted in (a), (c) a TEM-BF image, and (d) and (e) selectedarea diffraction patterns acquired from areas 1 and 2 depicted respectively in (c).
The precipitation of Cr2N is commonly
observed in super duplex stainless steels
containing a large amount of nitrogen.
Ramirez et al. 12) investigated the precipitation
behavior of Cr2N and reported the following
relationship between Cr2N
and γ'-Fe(secondary austenite) phases: Cr2N nucleatesfrom δ-Fe phase side at δ-Fe / γ-Fe interfaces.
Because the Cr2N precipitation consumes
nitrogen in the γ-Fe matrix, the grainboundaries of γ-Fe containing the Cr2N

precipitates reduce nitrogen concentrations.
As the nitrogen contained in the γ-Fe matrix
diffuses toward the δ-Fe / γ-Fe interfaces, theδ-Fe matrix partially transforms to austenite
which is called secondary austenite, γ'-Fe. The
phase transition processes described above issummarized as follows:δ + γ → (δ + Cr2N) + γ → (δ + Cr2N + γ') + γ.
In the present super duplex stainless steel,

14

Precipitation Behavior of Super Duplex Stainless Steel in Friction-Stir-Welding Processes

Cr2N was observed in the aggregates of theaustenite not containing δ-Fe phase. If thephase evolution proceeds along the reactions
described above, the interpretation is possible:
Cr2N nucleated at the grain boundary of the δphase. At the SZ, the grain sizes of the (δ-Fe +γ-Fe) matrix became smaller to micrometersizes during the friction stirring process. The
δ-Fe phase around the Cr2N phasetransformed to γ'-Fe because nitrogen
diffusion from the γ-Fe phase stabilizes γ'-Fethan δ-Fe. As a result, aggregates consisting ofthe γ-Fe and γ'-Fe grains were formed aroundthe Cr2N precipitates. Mo was also released
when the δ-Fe phase transformed to γ'-Fe andthe χ phase was then formed at the triple
junctions of the γ'-Fe and δ-Fe grains bybounding Mo with the surrounding Fe and Cr.
Fig. 10 illustrates model precipitation sites
revealed in this study. The σ phase
precipitated in the HAZ about 1-2 mm awayfrom the SZ which was not influenced byplastic flow during the FSW process. The Cr2N
and the χ phase precipitated in the vicinity of
the AS in the SZ, and aggregates of γ-Fe and
γ'-Fe formed with the Cr2N and χ precipitates.

precipitated in the SZ, and the γ'-Fe phase andthe χ phase formed secondarily. According toEscriba et al. 13), it is possible that the χ phaseformed as a precursor stage of the σ phase.
Since these precipitates influence material
properties, it is necessary to optimize the FSW
conditions for suppressing the precipitation as
future work.

4.

Conclusion

The precipitation behavior of super duplex
stainless steel in the weld formed by the FSW
at a low welding speed was investigated using
optical and electron microscopy techniques.
The results are shown below.
1) The σ phase precipitated in the HAZ
which was 1-2 mm away from theinterface with the SZ. The interface
between the δ-Fe and γ-Fe phases is theprecipitation site for the σ phase.
2) The Cr2N precipitated in the austenite
aggregates with the belt-like morphology
in the SZ near the AS. Also, the χ phase
precipitated at a triple junction of theaustenite grains.
References

Fig. 10 Model precipitation sites in the FSW
weld.
The vicinity of the AS is a place where theplastic fluid circulating with the rotating tool
accumulates, as shown in Fig. 10. It seems
that the belt-like fluid as shown in Fig. 6
circled with the tool multiple times, wasexposed in a heating environment for a long
duration, and precipitated Cr2N grains. The
calculated TTT diagram of Fig. 5 indicatesthat Cr2N precipitates at a high temperature
in a short duration compared with the σ phase.
Based on the calculation, Cr2N preferentially

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