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The high
corrosion resistance and the excellent mechanical properties
combination of duplex stainless steels can be explained by
their chemical composition and balanced ('duplex') microstructure
of approximately equivalent volume fractions of ferrite and
austenite. Firstly, the chemical composition based on high
contents of Cr and Mo, improves intergranular and pitting
corrosion resistance, respectively. Moreover, additions of
nitrogen can promote structural hardening by interstitial
solid solution mechanism, which raises the yield strength
and ultimate strength values without impairing toughness.
Secondly, the two-phase microstructure guarantees higher resistance
to pitting and stress corrosion cracking in comparison with
conventional stainless steels.
The duplex stainless steels comprise a family of grades with
a range in corrosion performance depending on their alloy
content. Modern duplex stainless steels are often addressed
in four groups:
- Lean
duplex, such as 2304 (S32304), which contain little or no
deliberate Mo addition;
- 2205
(S32205), the workhorse grade accounting for more than 80%
of duplex use;
- 25
Cr duplex, such as 255 (S32550) and S31260;
- Superduplex,
with 25-26 Cr and increased Mo and N, such as 2507 (S32750).
The table
lists the duplex stainless steels covered in ASTM specifications
for plate, sheet, and bar products. Although few of the duplex
grades are patented, many of these grades are closely associated
with particular steel producers as proprietary developments.
This situation is often reflected in the naming of the grades.
Only one duplex stainless steel has an AISI designation, Type
329, a first generation duplex stainless steel without intentional
nitrogen addition. Many of the grades have become commonly
known by a number that reflects their typical chromium and
nickel contents, e.g., 2205 with 22% Cr and 5% Ni. These composition-based
names, used by many producers, have recently been added to
A 240 and are being added to the ASTM specifications for other
product forms. Trademarks of individual producers are not
permitted in the ASTM specifications. All grades in the ASTM
specifications are all listed by their designations in the
Unified Numbering System (UNS). It is important to reference
the UNS number when ordering to the standard specifications
in order to get the desired grade.
Composition
of Duplex Stainless Steelsa
UNS
Number
Duplex Grades
|
Typeb |
C |
Mn |
P |
S |
Si |
Cr |
Ni |
Mo |
N |
Cu |
Other |
| S31200 |
... |
0.030 |
2.00 |
0.045 |
0.030 |
1.00 |
24.0-26.0 |
5.5-6.5 |
1.20-2.00 |
0.14-0.20 |
... |
... |
| S31260 |
... |
0.03 |
1.00 |
0.030 |
0.030 |
0.75 |
24.0-26.0 |
5.5-7.5 |
2.5-3.5 |
0.10-0.20 |
0.20-0.80 |
W0.10-0.20 |
| S31803 |
... |
0.030 |
2.00 |
0.030 |
0.020 |
1.00 |
21.0-23.0 |
4.5-6.5 |
2.5-3.5 |
0.08-0.20 |
... |
|
| S32001 |
... |
0.030 |
4.0-6.0 |
0.040 |
0.030 |
1.00 |
19.5-21.5 |
1.00-3.00 |
0.60 |
0.05-0.17 |
1.00 |
|
| S32205 |
2205 |
0.030 |
2.00 |
0.030 |
0.020 |
1.00 |
22.0-23.0 |
4.5-6.5 |
3.0-3.5 |
0.14-0.20 |
... |
|
| S32304 |
2304 |
0.030 |
2.50 |
0.040 |
0.030 |
1.00 |
21.5-24.5 |
3.0-5.5 |
0.05-0.60 |
0.05-0.20 |
0.05-0.60 |
|
| S32520 |
... |
0.030 |
1.50 |
0.035 |
0.020 |
0.80 |
24.0-26.0 |
5.5-8.0 |
3.0-4.0 |
0.20-0.35 |
0.50-2.00 |
|
| S32550 |
255 |
0.04 |
1.50 |
0.040 |
0.030 |
1.00 |
24.0-27.0 |
4.5-6.5 |
2.9-3.9 |
0.10-0.25 |
1.5-2.5 |
|
| S32750 |
2507 |
0.030 |
1.20 |
0.035 |
0.020 |
0.80 |
24.0-26.0 |
6.0-8.0 |
3.0-5.0 |
0.24-0.32 |
0.50 |
|
| S32760 |
... |
0.030 |
1.00 |
0.030 |
0.010 |
1.00 |
24.0-26.0 |
6.0-8.0 |
3.0-4.0 |
0.20-0.30 |
0.50-1.00 |
c |
| S32900 |
329d |
0.06 |
1.00 |
0.040 |
0.030 |
0.75 |
23.0-28.0 |
2.5-5.0 |
1.0-2.0 |
... |
... |
|
| S32950 |
... |
0.03 |
2.00 |
0.035 |
|
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a
Weight percent, maximum unless otherwise noted.
b Unless otherwise indicated, a common name, not
a trademark, widely used, not associated with any one producer,
as listed in ASTM A 240.
c W 0.50-1.00; Cr+3.3Mo+16N=40 min.
d AISI designation
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