Duplex stainless steel
Duplex stainless steels are called “duplex” because they have a two-phase
microstructure consisting of grains of ferritic and austenitic stainless steel.
The picture shows the yellow austenitic phase as “islands” surrounded by the
blue ferritic phase. When duplex stainless steel is melted it solidifies from
the liquid phase to a completely ferritic structure. As the material cools to
room temperature, about half of the ferritic grains transform to austenitic
grains (“islands”). The result is a microstructure of roughly 50% austenite and
Duplex stainless steels have a two-phase microstructure of austenite and
The duplex structure gives this family of stainless steels a combination of
Strength: Duplex stainless steels are
about twice as strong as regular austenitic or ferritic stainless steels.
Toughness and ductility: Duplex
stainless steels have significantly better toughness and ductility than ferritic
grades; however, they do not reach the excellent values of austenitic grades.
Corrosion resistance: As with all
stainless steels, corrosion resistance depends mostly on the composition of the
stainless steel. For chloride pitting and crevice corrosion resistance, their
chromium, molybdenum and nitrogen content are most important. Duplex stainless
steel grades have a range of corrosion resistance, similar to the range for
austenitic stainless steels, i.e from Type 304 or 316 (e.g. LDX 2101©) to 6%
molybdenum (e.g. SAF 2507©) stainless steels.
Stress corrosion cracking resistance: Duplex stainless steels show very good stress corrosion cracking (SCC)
resistance, a property they have “inherited” from the ferritic side. SCC can be
a problem under certain circumstances (chlorides, humidity, elevated
temperature) for standard austenitics such as Types 304 and 316.
Cost: Duplex stainless steels have
lower nickel and molybdenum contents than their austenitic counterparts of
similar corrosion resistance. Due to the lower alloying content, duplex
stainless steels can be lower in cost, especially in times of high alloy
surcharges. Additionally, it may often be possible to reduce the section
thickness of duplex stainless steel, due to its increased yield strength
compared to austenitic stainless steel. The combination can lead to significant
cost and weight savings compared to a solution in austenitic stainless
Please download the updated brochure "Practical Guidelines for the
Fabrication of Duplex Stainless Steels" for more information.