254SMO(UNS S31254,1.4547 ) Super A.
S.S(DUPLEX STAINLESS STEEL)
254 SMO(UNS S31254) is an austenitic stainless
steel which due to its high molybdenum content possesses very high resistance to
pitting and crevice corrosion. The metal was developed for use in
halide-containing environments such as seawater. 254 SMO also shows good
resistance to uniform corrosion and is superior to conventional stainless steels
especially in acids containing halides.
The high levels of molybdenum in particular but
also of chromium and nitrogen endow 254 SMO with extremely good resistance to
pitting and crevice corrosion. The addition of copper provides improved
resistance in certain acids. Futhermore, due to its relatively high nickel
content in combination with the high levels of chromium and molybdenum 254 SMO
possesses good resistance to stress corrosion cracking. duplex stainless steel
Even highly alloyed stainless steels have a vary
limited applicability in environments which contain halide ions, e.g. chloride,
bromide and fluoride solutions. For this reason it has often been necessary to
use expensive materials such as nickel based alloys or titanium for these
environments. 254 SMO was developed with a view of offering a less expensive
Due to its high nitrogen content 254 SMO has
higher mechanical strength than most other austenitic stainless steels. Like
these 254 SMO is characterized by high ductility and impact strength as well as
good weldability. The high molybdenum content of 254 SMO can lead to an
increased oxidation rate at annealing and as a consequence a rougher surface
finish after pickling than standard stainless steels. This has, however, no
negative influence on the corrosion resistance of the material.
Chemical Composition (%)
Nickel Ni 18
Chromium Cr 20
Nitrogen N 0.20
Carbon C 0.01
Density 8030 kg/m
Ultimate Tensile Strength 650 N/mm(min.)
(0.2% offset) 300 N/mm(min.)
Hot working should be
carried out in the temperature range 1000-1200C. Higher temperatures will reduce
the workability. Fairly heavy scaling occurs at tempertaures exceeding
To ensure dissolution of possible precipitations of secondary phases
from the hot forming, the subsequent heat treatment should take place at minimum
254 SMO possesses very good cold formability. Bending, pressing and other
forming operations which are used in fabrication can be carried out without
difficulty. Practical experience obtained from the pressing of heat exchanger
plates has been very favourable. The steel work hardens rapidly.
254 SMO possesses good weldability. When delivered, sheets and plates
have a homogeneous composition. Remelting of the parent metal, such as during
welding without filler metal, may cause microscale variations in composition for
elements such as chromium, nickel and particularly molybdenum. This phenomenon
occurs in all highly alloyed stainless steels. These variations may reduce the
pitting resistance of the weld. GTA and plasma arc welding without filler metal
should therefore be avoided.
may not be subjected to abrasive contact with copper/brass items. Penetration of
copper/brass into the grain boundaries can give rise to crack
Avesta P12 or P16 welding
consumables should be used for all welding methods.
Ignite the electrode in the joint since ignition burns
beside the weld can give rise to corrosion attacks.
Heat treatment is normally not necessary after welding.
However, welding without filler metal should be followed by solution annealing
at 1150-1200绡� and subsequent rapid cooling in order to achieve the best
possible corrosion resistance.
optimum corrosion resistance the welds should be cleaned, preferably by