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SIGMA PHASE PRECIPITATION IN DUPLEX PHASE STAINLESS STEELS
Zbigniwe Stradomski1, Dariusz Dyja 21) Proffesor, Director
Institute of Materials Engineering, 2) MSc, PhD students,1,2)
Czestochowa University of Technology, Institute of Materials
Engineering
[email protected] financial support by the Polish S tate
Committee for Scientific Research No 3 T08 B 001 26 is gratefully
acknowledged
Introduction. Precipitation processes occurring in duplex steels
are determined by the possibility of formation of
various intermetallic phases (, , , R) and carbides. The sigma
phase has particular importance among them, as
already its small amount causes a considerable reduction in
plasticity and impairment in corrosion resistance. The
Fe-Cr-Ni system indicates that all duplex steels are subject to
the precipitation of the sigma phase, with ferrite
stabilizing elements, chiefly chromium and molybdenum,
increasing the rate of its precipitation. From a practical
viewpoint, the occurrence of this phase as early as in the
production process with associated brittleness constitute
a problem of paramount importance, more especially as the duplex
cast steel belongs to a family of steels, whoseequilibrium
structure is composed of ferrite, austenite and intermetallic
phases. The desirable structure of those
materials, composed of approximately equal parts of ferrite and
austenite, without intermetallic phases, is
a non-equilibrium structure that can be achieved solely by means
of properly performed heat treatment.
Materials and Methods.
The GX2CrNiMoCu25-6-3-3 stainless steels used
in this study were in as-cast condition. The chemical
composition of the two test materials are listed in Tab.1
Table 1
C Cr Ni Cu S Si Mn P Mo
0.06 24.2 7.5 2.6 0.01 0.9 0.13 0.02 2.41
0.10 24.0 7.6 2.6 0.01 0.9 0.25 0.02 2.35
The optical microscope (OM) was used for
microstructure observation of duplex steel with
different C contents. The ImagePro Plus analyzer was
used to estimate the stainless steel phase contents.
The redistribution of the substitutional alloying
elements Cr, Mo, Ni, Mn and Si between the ferrite,the austenite
and the intermetallic phase was followed
by means of scanning electron microscopy (SEM) and
energy dispersive X-ray spectroscopy (EDX).
Theoretical thermodynamic analysis sigma phase
participation with Thermo-Calc software package was
done.
Results.In cast steel of duplex type the main ferrite-
stabilizing elements are chromium, molybdenum and
silicon. To provide high corrosion resistance for these
steels, a large amount of either chromium or
molybdenum is necessary. In the presence of large
quantities of these elements the possibility of occurring
the brittle tetragonal sigma phase in this steel increases.
The effect of chemical composition and cooling rate on
the range of occurrence of various precipitates isillustrated in
Fig. 1. The carbon present at a level of
0.06-0.1% results in a possibility of carbides
precipitation in the cast steel examined. As the
locations of nucleation of M23C6 carbides are primarily
ferrite/austenite interfaces, hence these precipitates
may constitute preferable spots for phase nucleation.
Figure 1
An analysis carried out using the Thermo-Calc
program has shown that the precipitation of the
intermetallic sigma phase in investigated steel takes
place during cooling a cast after its solidification in a
temperature range from 8500C to approx. 550
0C. The
chemical composition of this phase includes, inaddition to Fe,
approx. 30-60% Cr and 4-10% Mo.
The sigma phase appears much more quickly in
ferrite than in austenite, not only due to the fact
that the ferrite is closer to the sigma phase in chemical
Table 2 '
Cr 27.829.2 20.922.3 20.322.4 32.166.9
Ni 4.545.60 7.929.79 8.179.64 1.583.31
Mo 2.653.95 1.372.26 2.154.06 6.3510.6
Si 0.791.23 0.831.19 0.711.18 1.411.54
Cu 1.352.25 3.063.91 2.744.4 1.41
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composition, but primarily because of a higherdiffusion rate.
The formation of phases enriched in
chromium and molybdenum causes the ferrite to be
depleted in these alloying constituents, as a
consequence of which it may transform into austenite
depleted in the above-mentioned elements (Fig. 2). Asthe
diffusion in the austenite surrounding the depleted
ferrite, as a result of which the equalization of
chromium and molybdenum contents should occur, is
slow, therefore the corrosion resistance of the metal
bordering the sigma phase, and thus that of the whole
alloy, decreases.
Figure 2
The sigma phase nucleates at the ferrite/austenite
interface and grows towards the ferrite grains as a
result of the following eutectoid reaction: + (1).
A schematic diagram of sigma phase nucleation at the
ferrite/austenite interface and its increase in the ferrite
is shown in Fig. 2. The structure of the alloyexamined, as shown
in Fig. 4, confirm that the sigma
phase forms by way of decomposition according to
reaction (1). Sigma phase precipitation may also occur
directly in the ferrite, with preferable nucleation
locations being other precipitates, particularly carbides.
In order to establish of how the rate of cooling after
the solutioning process influences sigma phase
precipitation in massive casts, samples taken from the
cast were soaked in laboratory conditions at 10900C for
3 hours, and then cooled in: 1-water, 2-oil, 3-
compressed air, 4-air, and 5-with the furnace.
The results show that phase precipitation can be
avoided by using high cooling rates after the solution
heat treatment (Tab. 3). With decreasing cooling rate
Figure 4
Table 3Cooling rate
1 2 3 4 5
% 48.9 48.4 51.2 50.5 48.2 % 48.8 45.2 44.7 42.5 37.7
% 2.30 3.40 4.30 6.50 13.1
the amount of the undesirable sigma phase increases,
and this increase is accompanied by a reduction in delta
ferrite content of the microstructure.
Fine, rounded phase precipitates are distributed not
only at the ferrite/austenite interfaces, but also inside
the austenite and ferrite grains, and it can also be
noticed that sigma phase precipitates tend to surround
the austenite grains, which substantially influences theservice
properties of the cast steel.
Conclusions and discussion.
During solidification and subsequent cooling
process, duplex steel casts, especially those of large
cross-sections, will tend to develop sigma phases.
Therefore, in respect of casts, the necessity of carrying
out heat treatment after removing the cast from the
mould is obvious. Every cast must be heat treated at a
high temperature that assure the dissolution of
undesirable phases, and then cooled quickly so as to
prevent the appearance of the sigma phase. In the light
of the investigation carried out it should be noted,
however, that no complete elimination of the sigmaphase is
possible in massive casts, and properly
performed heat treatment operations will only allow its
adverse effect to be minimized.
The above-mentioned factors favouring sigma
phase precipitation in cast steel of duplex type show
that the higher cooling rate after the cast solidification
process the lower tendency of sigma phase
precipitation in the material. Therefore, in order to
increase cooling rate, casts of this material should be
removed from the mould immediately after being
solidified. A lot of attention should also be given to the
design of the cast, chills and the supply of metal to the
mould cavity (thermal centres in the cast should be
avoided, and in addition, the machining allowanceshould be as
small as possible, not only for saving
reasons, but also with a view to increasing cast wall
thickness and thus lengthening the solidification time).
References
[1] Dong L., Effect of phase formation on mechanical
properties
of stainless steel SUS309L isothermal aging, Steel Research,
2004,
[2] Premachandra K.,Effect of stabilising elements on formation
ofphase in experimental ferritic stainless steel containing 39%
Cr,
Materials Science and Technology, 1992, [3] Erneman
J.,Quantitative metallography of sigma phase precipitates in AISI
347
stainless steel, Materials Science and Technology, 2004
