Influence of phase transformations
on mechanical properties and corrosion
properties in duplex stainless steels
Jan-Olof Nilsson and Pasi Kangas, Sandvik Materials Technology, Sweden
Introduction
The increased use of duplex stainless steels (DSS) derives from their
attractive combination of mechanical strength, corrosion resistance in
various types of environments and
weldability (1). The attractive properties of DSS may be destroyed by
the formation of precipitates. In DSS
the most common precipitates are
Cr2N and intermetallic precipitates
(IP) such as (-phase, (-phase and Rphase. These can be formed in the
temperature interval 600-1000 °C as
a result of improper cooling, whereby both corrosion resistance and
toughness are adversely affected.
Hence, a thorough knowledge of the
kinetics of precipitate formation is
important in determining the heat
treatment and cooling rates required
during production to ensure a precipitate-free material.
There is a continual development of
DSS towards more corrosion resistant grades. The resistance to pitting
corrosion can be increased by
adding nitrogen, chromium and
molybdenum but the concentrations accepted in practice are limit-
The content of secondary phases was quantified in an SEM using Image analysis. The SEM images
were taken at 400 and 600 times magnification on unetched samples. Images were taken with
the SEM using back scattered electrons (BSE) to enhance the contrast between particles with
large variations in atomic number.
ed by the solubility of nitrogen in
combination with the fact that the
kinetics of intermetallic phase formation is enhanced by chromium
and molybdenum. There is also evidence that tungsten, a group 6B
Critical pitting temperature for alloys with various PREnw values
80
75
CPT ASTN G48C (°C)
70
65
60
55
50
38
40
41
42
43
44
PRENW (% Cr+3,3 (%Mo+0,5%N) +16% N)
Figure.1 Critial pitting temperature for alloys with various PRENW values (ref 1)
56
S T A I N L E S S
S T E E L
W O R L D
M A Y
2 0 0 7
45
46
transition element like chromium
and molybdenum and sometimes
used as an alloying element in SDSS,
enhances the kinetics of intermetallic phase formation (2). Figure 1
shows an example where the critical
pitting temperature has been plotted against the PRENW number for
various experimental DSS grades (3).
It is clear that there is an optimum
PRENW number where the CPT
reaches a maximum. The alloys
with higher PRENW numbers in the
diagram probably have an unfavourable phase balance and/or a
presence of precipitates which deteriorate the corrosion resistance. A
balanced alloy where care has been
taken to design the composition to
suppress formation of sigma phase
is hence essential for optimum
properties.
Given the fact that it is difficult to
warrant a material that is entirely
www.stainless-steel-world.net
500
1000
300
1%
950
200
300
200
Hardness
Impact toughness
100
900
Impact toughness (J)
Hardness (HV1)
400
DUPLEX
Duplex
850
800
5%
750
700
100
650
0
0
2
0
50
10
20
30
40
Volume pct of intermetallic phase
1100
SAF 2507
1000
900
29Cr-6Ni-2Mo-0.38N
800
27J impact toughness
600
1
2
3
4
5
6
Log aging time in sec
Figure 3. Time Temperature Transformation curve for 5% intermetallic
phase compared with SAF 2507. In the diagram the 27J curve for SAF
2906 has been plotted
free of intermetallic phase it is interesting to study its effects on properties. It is quite relevant to ask the
question how much IP (IP is used as
a generic name for intermetallic
phase) we can tolerate in a particular application without sacrificing
the performance. In earlier work (4)
the correlation between intermetallic phase content, impact toughness, hardness and corrosion properties was investigated. It was noted
that 0,2% IP had no effect on hardness, impact toughness or critical
pitting temperature. At 0,6% IP a
decrease in the impact toughness
was observed but several percent IP
was necessary to seriously deteriorate the materials properties.
5
6
Figure 2. Time Temperature Transformation curve for 1% and 5% intermetallic phases in SAF 2906
Klockars et al (5)
demonstrated
that a material
with 2 and 6% IP
will pass a crevice
corrosion test in
natural seawater
at 35°C but will
fail at 50°C with
8% IP and at
60°C with 5% IP.
At 95°C in deaerated seawater a
material with 3%
IP will pass the
test at free corroding potential.
Experiment
The experiments were carried out
on mainly two duplex stainless
steels, SAF 2507 and SAF 2906. The
tests involved heat treatments so as
to produce secondary phases in the
structure followed by testing of
properties on material with various
amounts of precipitates.
The first experimental material was
SAF 2906 extruded bar of 25 mm diameter of chemical composition, expressed in wt % , given in Table I.
The second material was SAF 2906
round bar of 14 mm diameter. The
third test material was SAF 2507
seamless tubes size 15,7x1,5 mm.
Table I. Nominal chemical composition of the test alloys in weight percent (wt%)1
C
max
Si
max
Mn
max
Cr
Ni
Mo
N
SAF 2906
0.025
0.8
1.2
29
7
2.2
0.4
SAF 2507
0.03
0.8
1.2
25
7
4
0.3
1 In the following it is implicitly assumed that all concentrations of alloying elements are given in wt%
www.stainless-steel-world.net
4
Log aging time, s
Figure 1. Influence of intermetallic phase content on impact toughness
and hardness of SAF 2906
700
3
S T A I N L E S S
Isothermal heat treatment of SAF
2906
Impact testing was performed on
SAF 2906 specimens using standard
10mm(10mm(55mm Charpy V
specimens. Light optical microscopy
(LOM) was performed in a Nikon inverted microscope. A 95% confidence interval of the volume fractions of ferrite, austenite and intermetallic phase was estimated using
manual point counting according to
the standardized ISO 9042 procedure. Ferrite and austenite were
quantified using Beraha’s etchant
(NH4)HF2, 0.2g K2S2O5, 100 ml
distilled H2O and 18 ml concentrated HCl(and intermetallic phase using Murakami’s etchant (15g
K3Fe(CN)6, 30g KOH and 60 ml distilled H2O(.
The content of secondary phases
was quantified in an SEM using
Image analysis. The SEM images
were taken at 400 and 600 times
magnification on unetched samples.
Images were taken with the SEM using back scattered electrons (BSE) to
enhance the contrast between particles with large variations in atomic
number.
Results
Isothermally heat treated SAF
2906
Mechanical testing
Results from impact testing and
hardness testing of different aging
conditions are shown in Fig 1 in
which both hardness and impact
toughness are plotted against volume fraction of intermetallic phase
S T E E L
W O R L D
M A Y
2 0 0 7
57
CPT(°C)
Duplex
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0,001
0
0,01
0,1
1
10
0,01
0,1
1
IP%
Figure 4. Critical pitting temperature in 3% NaCl potentiostatically at
600 mV SCE for various Intermetallic phase (IP) contents for SAF 2906
(see section B below). First of all,
this diagram shows that there is a
close relation between these mechanical properties and the volume
percentage of intermetallic phase.
Furthermore, it is quite apparent
that very small amounts of intermetallic phase are required to cause
embrittlement. For instance, only
about 5 vol pct reduces the impact
toughness to 27J, which is the lower
tolerance limit according to the
specification of DSS. However, the
same amount is insufficient to influence hardness significantly. In fact,
from Fig 1 it can be concluded that
at least 10 vol pct of intermetallic
phase is needed to give a substantial
hardness increase.
10
100
IP %
Figure 5. Critical pitting temperature ASTM G48C for SAF 2507 tubes
with various intermetallic phase (IP) contents
There is a continual development of DSS towards more corrosion resistant grades.
seems that the situation is analogous
to SAF 2906, i.e. also SAF 2507 can
tolerate about 1% IP before pitting
corrosion is affected significantly.
References
1. J.-O. Nilsson: Mater. Sci. Techn., 1992,
vol. 8, pp. 685-700.
2. US Patent number 6,312,532
3. H. Okamoto: Proc. Conf. Applications
Microstructure
Figure 2 shows a TTT diagram for
1% intermetallic phase for SAF
2906. In figure 3 the TTT diagram
for SAF 2906 and SAF 2507 have
been plotted. In the same diagram
the 27J curve for SAF 2906 has been
plotted. There is a good correlation
between the curves corresponding
to a toughness of 27J and a volume
percentage of 5% intermetallic
phase in the structure.
Continuously cooled SAF 2906
The results are presented graphically
in figure 4. It can be seen that about
1% of IP can be tolerated before a
substantial decrease in pitting corrosion resistance occurs.
Isothermally heat treated SAF
2507 tubes
The results have been summarised in
figure 5 where the amount of IP has
been plotted against the critical pitting temperature. Although there is
a lack of information below 1% IP it
www.stainless-steel-world.net
Concluding remarks
of Stainless Steel ‘92, H. Nordberg and J.
Two superduplex stainless steels, SAF
2507 and SAF 2906, were tested.
Various amounts of intermetallic
precipitates were introduced into the
structure. The volume fraction of intermetallic phase was correlated to
the mechanical properties and corrosion properties.
It was demonstrated that DSS can
tolerate relatively large amounts of
intermetallic phases (IP) before failing in tensile and fatigue tests. For
instance, SAF 2507 with 6% IP will
pass a corrosion test in seawater at
35°C. Impact toughness will not
reach the critical level of 27J until
the IP content exceeds approximately 4%.
Impact toughness and pitting corrosion resistance are considered to be
the most sensitive tools for revealing
the presence of IP in the structure.
The critical level of IP for a particular application has to be determined
by the method that simulates service
conditions most appropriately.
Björklund eds., ASM International,
S T A I N L E S S
Stockholm, Sweden, 1992, pp. 360-369.
4. J-O Nilsson, A. Wilson, B. Josefsson, T
Thorvaldsson. Applications of Stainless
Steel 92. vol 1. pp 280-289
5. M Klockars et al, Conf Proc Stainless
Steel World Conference Expo America,
12-14 February 2002, Houston USA, paper 0212
About Jan-Olof
Nilsson
Jan-Olof Nilsson gained
his PhD in Physics from
Chalmers University of
Technology where he later became
Adjunct Professor of Physics - a position which he still retains today. He is
also Senior Advisor at the R&D
Centre of Sandvik Materials
Technology . Mr. Nilsson has published more than 130 scientific papers while being employed at Sandvik.
S T E E L
W O R L D
M A Y
2 0 0 7
59