©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
V 5.9 DUPLEX
STAINLESS
STEELS
Duplex means "two". Duplex stainless
steels consist of the two "building stones" (microstruc-
ture phases) ferrite and austenite
and are often termed ferritic-austenitic stainless steels.
Typically, duplex stainless steels
have a microstructure consisting of approximately 50% ferrite
and 50% austenite.
5.9.0.1 In
simple terms, the ferrite could be said to give high strength and some resistance
to stress corrosion cracking, the austenite
provides good toughness, and the two phases in combination
give the duplex steels their attractive corrosion resistance.
5.9.0.2 The
most important alloying elements of duplex stainless steels are Cr, Ni, Mo and
N. These elements largely govern
the properties of the steels. Some grades also contain
additions of copper (Cu) or tungsten
(W). 5.9.0.3
A wide range of different versions
of duplex stainless steel is
currently available on
the market. At present, the 22% chromium (Cr), 5% nickel (Ni), 3% molybdenum
(Mo), 0.15% nitrogen
(N) grade (commonly called 2205) is the most common type of duplex stain-
less steel and is used in a wide range
of applications. Higher alloyed duplex steels, the so-
called super duplex
stainless steels, have also been introduced into the market. The
25% chromium (Cr),
7% nickel (Ni), 4% molybdenum (Mo), 0.25% nitrogen (N) grade (commonly
called 2507) is one example of a modern
high alloy super duplex stainless steel. These steels
are designed for use in demanding applications
where even greater corrosion resistance or higher
strength is required. 5.10
ELECTRODE SELECTION
There are a great many AISI grades
of stainless steel, and in many cases there is a matching
electrode for the AISI type. For
instance, if both members of a weldment are AISI type 316, the
electrode to be used would be 316 also.
It is not necessary to have a matching electrode for
every type of stainless steel, however,
because some electrodes produce satisfactory welds even
though the chemical analysis of the steel may be slightly different.
5.10.0.1 Type
308 stainless steel electrodes may be used for welding AISI 201 and 202 that
have a lower nickel content and a high
manganese content. Type 308 electrodes may also be
used to weld types 301, 302, 304, 305
and of course, 308 itself. Even though their chromium-
nickel contents vary slightly, all
of these steel types may be considered as one family of alloys.
The chart in Figure 13 shows the proper
Arcaloy electrode to be used for the various types of AISI
steels.
