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An Introduction to Duplex Stainless Steels

Duplex Stainless Steels and their characteristics

Duplex Stainless steels were developed more than 70 years ago in Sweden for use in the sulfite paper industry. Duplex alloys were originally created to combat corrosion problems caused by chloride-bearing cooling waters and other aggressive chemical process fluids.

Called Duplex because of its mixed microstructure with about equal proportions of ferrite and austenite, duplex stainless steels are a family of grades, which range in corrosion performance depending on their alloy content. The chemical composition based on high contents of Cr and Mo, improves intergranular and pitting corrosion resistance, respectively. Additions of nitrogen can promote structural hardening by interstitial solid solution mechanism, which raises the yield strength and ultimate strength values without impairing toughness. Moreover, the two-phase microstructure guarantees higher resistance to pitting and stress corrosion cracking in comparison with conventional stainless steels.

The first-generation grades were alloys of chromium, nickel and molybdenum and while they provided good performance characteristics, welds had low toughness due to excessive ferrite, which limited the use of Duplex steel to a few specific applications.

Coinciding with the 1970's development of offshore gas and oilfields in the North Sea, a second-generation Duplex was introduced and quickly became the Duplex steel of choice. With deliberate addition of nitrogen as an alloying agent, both improved toughness in the heat-affected zones of welds and excellent chloride corrosion resistance was achieved thus quickly becoming the workhorse of stainless steels, accounting for more than 80% of duplex use. Its popularity is also cost-based as the replacement of the nickel content with the less-expensive addition of nitrogen.

Duplex alloys are essentially a compromise possessing some of the ferritic stress corrosion cracking resistance and much of the superior formability of the common austenitic stainless alloys, at a cost saving over the high nickel alloys.

The various Alloys

The development of the Duplex stainless steels has continued, graded for their corrosion performance depending on their alloy content. Today, modern duplex stainless steel can be divided into four groups:

Lean Duplex such as 2304, which contains no deliberate Mo addition;
2205, the work-horse grade accounting for more than 80% of duplex usage;
25 Cr Duplex such as Alloy 255 and DP-3;
Super Duplex; with 25-26 Cr and increased Mo and N compared with 25 Cr grades, including grades such as 2507, Zeron 100, UR 52N+, and DP-3W

Composition of Duplex Stainless Steels^a

The table lists the duplex stainless steels covered in ASTM specifications for plate, sheet, and bar products.

UNS Number Duplex Grades	Type^b	C	Mn	P	S	Si	Cr	Ni	Mo	N	Cu	Other
S31200	...	0.030	2.00	0.045	0.030	1.00	24.0-26.0	5.5-6.5	1.20-2.00	0.14-0.20	...	...
S31260	...	0.03	1.00	0.030	0.030	0.75	24.0-26.0	5.5-7.5	2.5-3.5	0.10-0.20	0.20-0.80	W0.10-0.20
S31803	...	0.030	2.00	0.030	0.020	1.00	21.0-23.0	4.5-6.5	2.5-3.5	0.08-0.20	...
S32001	...	0.030	4.0-6.0	0.040	0.030	1.00	22.0-23.0	1.00-3.00	0.60	0.05-0.17	1.00
S32205	2205	0.030	2.00	0.030	0.020	1.00	19.5-21.5	4.5-6.5	3.0-3.5	0.14-0.20	...
S32304	2304	0.030	2.50	0.040	0.030	1.00	21.5-24.5	3.0-5.5	0.05-0.60	0.05-0.20	0.05-0.60
S32520	...	0.030	1.50	0.035	0.020	0.80	24.0-26.0	5.5-8.0	3.0-4.0	0.20-0.35	0.50-2.00
S32550	255	0.04	1.50	0.040	0.030	1.00	24.0-27.0	4.5-6.5	2.9-3.9	0.10-0.25	1.5-2.5
S32750	2507	0.030	1.20	0.035	0.020	0.80	24.0-26.0	6.0-8.0	3.0-5.0	0.24-0.32	0.50
S32760	...	0.030	1.00	0.030	0.010	1.00	24.0-26.0	6.0-8.0	3.0-4.0	0.20-0.30	0.50-1.00	^c
S32900	329^d	0.06	1.00	0.040	0.030	0.75	23.0-28.0	2.5-5.0	1.0-2.0	...	...
S32950	...	0.03	2.00	0.035

a Weight percent, maximum unless otherwise noted.
b Unless otherwise indicated, a common name, not a trademark, widely used, not associated with any one producer, as listed in ASTM A 240.
c W 0.50-1.00; Cr+3.3Mo+16N=40 min.
d AISI designation

BENEFITS

High strength,
High resistance to pitting, crevice corrosion resistance.
High resistance to stress corrosion cracking, corrosion fatigue and erosion,
Good sulfide stress corrosion resistance,
Low thermal expansion and higher heat conductivity than austenitic steels,
Good workability and weldability,
High energy absorption.

APPLICATIONS

Heat exchangers, tubes and pipes for production and handling of gas and oil,
Heat exchangers and pipes in desalination plants,
Pressure vessels, pipes, tanks and heat exchangers for processing and transport of various chemicals,
Pressure vessels, tanks and pipes in process industries handling solutions containing chlorides,
Rotors, fans, shafts and press rolls where the high corrosion fatigue strength can be utilized,
Cargo tanks, piping and welding consumables for chemical tankers.

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