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,





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





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

Lean Duplex Stainless Steel __ KEY to METALS Article

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