lean duplex stainless steel __ key to metals article
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7/27/2019 Lean Duplex Stainless Steel __ KEY to METALS Article
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Lean Duplex Stainless Steel
Abstract:
The early grades were alloys of chromium , nickel and molybdenum.
Thermodynamically, because the austenite is forming from the ferrite, it is impossiblefor the alloy to go past the equilibrium level of austenite.
Lean Duplex Stainless Steels have many end uses in oil and gas due to their high
strength and good corrosion properties. They have been used for subsea applications
including flexible flowlines, umbilical tubing, subsea manifolds, water injection lines
and downhole chemical injection tubing.
Historical Evolution
Duplex stainless steels (DSSs), meaning those with a mixed microstructure of
approximately equal proportions of austenite and ferrite, have existed for more than 80 years
[1]. The early grades were alloys of chromium, nickel and molybdenum.
Thermodynamically, because the austenite is formed from the ferrite, it is imposs ible for the
alloy to go pas t the equilibrium level of austenite. The main problem with Duplex is that it very
easi ly forms bri ttle intermetalic phases, such as Sigma, Chi, R and Alpha Prime. Prolonged
heating in the range 350°C to 550°C can cause 475°C temper embrittlement.
The first wrought duplex stainless steels were produced in Sweden in 1930 and were used
in the sulfite paper indus try. These grades were developed to reduce the intergranular
corrosion problems in the early high-carbon austenitic stainless steels. Duplex castings
were produced in Finland in 1930, and a patent was granted in France in 1936 for the
forerunner of what would eventually be known as Uranus 50. It became obvious that a
balance of ferrite and austenite had better resistance to chloride stress -corrosion cracking
than a fully austenitic microstructure.
Engineers have exploited this advantages of duplex over aus tenitic steels ever s ince. In
France, the UR 50 grade wi th 20-35% ferrite (UNS S32404) was marketed in various product
forms, including forging, for such industries as oil refinement, food processing, pulp and
paper, and pharmaceutical.
These steels were produced in high frequency induction furnaces using precisely weighed
alloying additions. Partial vacuum ensured carbon removals, rudimentary de-oxidation and
restricted nitrogen ingress. Nevertheless , plate products remained sensi tive to edge cracks.
During the late 1960s and early 1970s, there were two main factors which advanced the
development and use of duplex alloys.
First, there was a nickel shortage that pushed up the price of austenitic steels , in
combination wi th increased activity in the offshore oil indus try which demanded a s tainless
steel material to handle aggress ive environments. Second, steel production techniques
improved dramatically with the introduction of the vacuum and argon oxygen decarburization
(VOD and AOD) practices. These techniques made it poss ible to produce much cleaner
steels with a very low carbon level and well controlled nitrogen content.
In the 70’s, the introduction of continuous casting in s tainless steel production has
contributed to lower production costs and higher quali ty. From 1970 onwards, the addition of
nitrogen and the lowering of carbon content improved corrosion resis tance and high
temperature stability of the duplex structure, e.g. the HAZ, by stabilizing the austenite.
Development of new steels inevitably brings new problems in manufacturing and joining.This is particularly true for welding where the desired m aterial properties, carefully produced
by the steel maker, can be radically changed by a process that locally melts and recasts part
of the micros tructure.
Because the cooling rate determines the amount of ferrite that can transform to aus tenite,
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cooling rates following high temperature exposures influence the phase balance. Since fast
cooling rates favor retention of ferrite, it is possible to have more than the equilibrium
amount of ferrite. Another beneficial effect of nitrogen is that it raises the temperature at
which the aus tenite begins to form from the ferrite. Therefore, even at relatively rapid cooling
rates, the equilibrium level of austenite can almos t be reached.
In the new generation of duplex stainles s s teels (higher nitrogen content), this effect reduces
the problem of excess ferrite in the HAZ. There is definitely a continuing demand for
increased productivity in welding, while maintaining the parent material properties. The last
two decades have seen the introduction of the "super" stainless steels . Super- ferritic
grades with very low interstitial levels and high chromium and molybdenum contents have
superior corrosion resistance compared to standard ferritic grades. However, although
these steels have found certain applications, their success has been limited.
The highly alloyed super-austenitic and superduplex stainles s s teels, with excellent
corrosion resis tance and better fabricability and weldability than the ferritic steels , have
found a more widespread use and are today important engineering alloys. The hot
workability and rolling experience was greatly improved, making pos sible the production of
wide sheets and coils.
Figure 1: Schaffer diagram: Stainless families
Table 1: Typical chemistry of several s tainless steels
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Applications
Lean Duplex Stainless Steels have many end uses in oi l and gas due to their high strength
and good corrosion properties . They have been used for subsea applications including
flexible flowlines, umbilical tubing, subsea manifolds, water injection lines and downhole
chemical injection tubing. They are also us ed for topside applications such heat
exchangers, separation units, blast and fire walls , cable trays, cable ladders , pipe supports,
pipe racks and pipe insulation wraps.
Duplex stainless steels are used for many applications in the chemical process industryincluding reactors, heat exchangers, distillation columns and storage tanks. Desalination
plants and power generation units use s ignificant amounts of duplex stainless steels.
Duplex grades are also used for structural and architectural applications. Photographs
showing some of these applications are shown in Figures 2 and 3.
Figure 2: The roof of the new airport in Doha, Qatar, which will be the larges t stainless s teel
roof in the world, is being constructed from ATI 2003™ lean duplex stainless s teel
Figure 3: 2205 continuous sulphate pulp digester and impregnation tower, Sodra Cell
Mönsteras Sweden
Date Published: Feb-2012