Duplex Stainless Steels for Sea Water Service

Introduction

1. Duplex stainless steels were developed in the early thirties in Sweden and in France. The driving force to the development was the sensitivity to intergranular corrosion of existing austenitic steels which often contained 0.08-0.10% carbon. The duplex steels had the same carbon content but proved much less sensitive to this type of corrosion. Over the last several decades, there has been an increased interest in the use of duplex stainless steel for pumps used in marine environments. The synergistic effect of increased levels of chromium, molybdenum and nitrogen have been shown to provide outstanding benefits for localized corrosion resistance of the duplex alloys comparable to those of the highly alloyed austenitic alloys. Today both the super-austenitic and super-duplex stainless steels are widelybeing utilized in seawater pumps.

2. Duplex Stainless Steels have a structure that contains both ferrite and austenite. Duplex alloys have higher strength and better stress corrosion cracking resistance than most austenitic alloys and greater toughness than ferritic alloys, especially at low temperatures. It has been noted for many years that it is mainly the lack of localized corrosion (i.e. pitting and crevice corrosion) resistance that has limited the suitability of stainless steels for seawater services. This understanding has provided the motivation for developing more highly alloyed duplex stainless steels that offer superior corrosion resistance in seawater and other chloride media.

3. None of the earlier first or second-generation duplex stainless steels were resistant enough for seawater use until nitrogen and higher molybdenum alloying was introduced, and the so-called super-duplex stainless steels were made available.

4. The corrosion resistance of duplex alloys depends primarily on their composition, especially the amount of chromium, molybdenum, and nitrogen they contain. 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. Modern seawater resistant duplexes usually contain at least 25% chromium and have increased levels of molybdenum and nitrogen alloying over the standard duplex offerings such as wrought UNS S32900 (AISI Type 329), and cast UNS J93370 (ACI Type CD4Mcu).

5. Duplex alloys are often divided into three sub-classes: Lean Duplex (AL 2003™ alloy), Standard Duplex (AL 2205™ alloy), and Superduplex (AL 255™ Alloy and UNS S32760), as described in Table 1.

Table 1.Duplex steel types.

Stainless Steel - Duplex

Alloy(UNS Designation) End Use

Compositionnominal wt% Specifications

Densitylb/in3 (g/cm3)

TensileStrengthksi. (MPa)

0.2% YieldStrengthksi. (MPa)

Elong-ation % Hardness

AL 2003™ S32003

Piping, tubing in general corrosion and choride environments, architectural structures, roofing, topside applications on oil platforms

C 0.03 max, Mn 2.0 max, Si 1.0 max, Cr 19.5-22.5, Ni 3.0-4.0, Mo 1.5-2.0, N 0.14-0.2, Fe Balance

ASTM A-240 ASME Code Case 2503

0.279(7.72)

100 min (sheet) / 95 min (plate)(690min (sheet) / 655 min (plate)

70 min (sheet) / 65 min (plate)(485 min (sheet) / 450 min (plate)

25 min

31 Rockwell C max

AL 2205™ S31803/S32205

Pipe, Tubing in general corrosion and chloride stress corrosion environments

C 0.03 max, Mn 2.0 max,Si 1.0 max, Ni 4.5-6.5, S31803: Cr 21.0-23.0, Mo 2.5-3.5, N 0.08-0.20, S32205: Cr 22.0-23.0, Mo 3.0-3.5, N 0.14-0.20, Fe Balance

ASTM A-240 ASME SA-240

0.280(7.75)

90 min (31803) / 95 min (32205)(620 min (31803) / 655 min (32205))

65 min(450 min)

25 min

31 Rockwell C max

AL 255™ S32550

Pipe, containers for CPI, Oil & Gas

C 0.04 max, Mn 1.5 max, P 0.04 max, S 0.03 max, Si 1.0 max, Cr 24.0-27.0, Ni 4.5-6.5, Mo 2.9-3.9, N 0.1-0.25, Cu 1.5-2.5, Fe Balance

ASTM A240 0.279(7.73)

110 min(760 min)

80 min(550 min)

15 min

32 Rockwell C max

Benefits

6. The benefits of these steels over Austenitic or Martensitic steels can be summarized as follows:

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

Factors Affecting Seawater Resistance

7. From a corrosion viewpoint, seawater may be considered a neutral chloride salt solution that promotes localized corrosion in stainless steels. In saline waters or seawater environments at or near neutral pH levels the protective passive film on

stainless steels renders them virtually immune to general corrosion. Unfortunately, this passive film can break down locally in certain environments containing chlorides. At the intermediate temperatures existing in many seawater pumps, crevice and pitting corrosion are the major forms of corrosion damage most often observed.

8. Over the past 25 years, a number of highly alloyed duplex stainless steels offering superior corrosion resistance have been introduced for seawater service. Many researchers and much of the materials/corrosion literature indicates that excellent resistance to localized corrosion can be achieved in neutral and acid chloride containing media by increasing chromium, molybdenum and nitrogen content in these materials. In addition more noble values are exhibited in seawater with increases of these key elements.

9. Increased nitrogen content has led to improved corrosion resistance due to a better balance of alloying elements between the dual austenite and ferrite phases, resulting in a decreased susceptibility towards alloy partitioning between these two phases.

10. Regarding other alloying elements, it should be noted that some duplex stainless steels contain tungsten additions in the range of 0.5 % to 2.0 %. Tungsten additions have been shown to improve crevice corrosion resistance, with benefits similar to that of molybdenum. Copper additions in certain duplex alloys along the order of 0.5 % to 3.25 % are used primarily to extend the application to include sulfuric acid service, and offer better resistance in polluted seawater that contains hydrogen sulfide or other reducing reactants.

Pitting Resistance Equivalent

11. A familiar aspect of these newer seawater worthy duplex stainless steels is a high content of the alloying elements chromium, molybdenum, and nitrogen. Attempts have been made to establish a measure of the localized corrosion resistance by assessing the relative effect of these important alloying elements in a weighted form. This calculated sum is often referred to as the PRE (pitting resistance equivalent) or PREN factor (when nitrogen is included), with a common expression for the austenitic and duplex steels as follows:

PREN = %Cr + (3.3 x %Mo) + (16 x %N)

12. The passive film on stainless steels is significantly improved in seawater services when the alloy composition contains higher levels of these key elements and provides a PREN value of at least 38 or more. Long-term exposure in natural stagnant seawater has determined that only the higher molybdenum and nitrogen bearing duplex stainless steels with a PREN greater than 40 are highly resistant to localized (pitting & crevice) corrosion. Those super-duplex alloys with PREN greater than 40 successfully compete with the most resistant super-austenitic 6% molybdenum alloys such as UNS S31254 (Alloy 254SMO) and UNS N08367 (Alloy AL6XN), and are considered to be most suitable for seawater service. It should be noted that it is only those alloys offering a PREN value of 40 and greater that are usually referred to as "super" stainless steels. Several of these are listed as cast alloy grades in the ASTM Duplex Standard

Specification (i.e. ASTM A890 (2) as grades 1C, 5A and 6A). Table 2 summarizes some of the various cast duplex stainless steels that are suitable for seawater service.

Importance of Nitrogen and Phase Balancing

13. As mentioned earlier, the two phases in duplex stainless steel have different compositions, resulting in a different resistance to localized corrosion in seawater. The austenite phase generally is less resistant to localized corrosion than the ferrite phase. This is because the chromium and molybdenum are more concentrated in the ferrite phase; while the nickel and nitrogen are more concentrated in the austenite phase. This is known as alloy partitioning, and one of the drawbacks of dual phase or duplex alloy systems.

15. Nitrogen enhancement is beneficial to increase localized corrosion resistance of the austenite where it is mainly concentrated, and it also reduces the partitioning of chromium and molybdenum between the two phases. The only way to make the austenite phase as resistant as the ferrite phase in a duplex stainless is to alloy the material with high amounts of nitrogen. Since nitrogen helps to maintain higher levels of chromium and molybdenum in the austenite, it improves its corrosion resistance. Therefore, duplex stainless steel enhancement due to nitrogen is mainly due to the improved corrosion resistance of the austenite.

16. In low-nickel duplex stainless steels, the diffusion of nitrogen into austenite is possible due to its large diffusion coefficient, but molybdenum and chromium (small diffusion coefficients) cannot be significantly redistributed, so they are unevenly distributed (higher in ferrite) between the ferrite and austenite phases. The main role of nickel is to control the ferrite/austenite phase ratio and the partitioning of the alloying elements between the two phases.

17. An optimum range of nickel content is about 4 to 8% in a 25% Chromium duplex stainless steel. Increasing nickel content above this optimum increases the austenite ratio in such a way that the dilution of nitrogen in larger volumes of austenite would be detrimental and lowers its resistance to pitting and crevice corrosion.

18. Increasing the chromium content raises the pitting potential, and is beneficial in both phases. The trend in materials development seems to be towards moving from the 25% to 27%

Chromium duplex alloys for increased corrosion resistance. Molybdenum content well in excess of 3% is needed within a 25% Cr nitrogen enhanced duplex to be fully resistant to pitting and crevice corrosion in seawater. Molybdenum content cannot be increased indefinitely because of severe problems with intermetallic phase precipitation, and it rarely exceeds 4.5 to 5.0% in even the highest alloyed duplex stainless steels.

Applications

19. Duplex steels find application typically in:

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.

Summary and Recommendations

20. To summarise, for seawater services the duplex alloy selected should have a balanced nitrogen enhanced composition to provide for a PREN greater than 38 or higher to ensure freedom from localized corrosion. Those duplex alloys that offer PREN values of 40 or more are ideal for pumps and valves and are highly recommended for use in seawater. An excellent way to specify any duplex alloy intended for seawater service would be to request that the chemical composition be balanced to provide a PREN greater than or equal to 40 as a minimum using the pitting resistance equivalent expression given above. This will ensure that the alloy selected has the optimum chemistry control to provide adequate localized corrosion resistance in the seawater environment.

21. Highly alloyed 25% Chromium duplex stainless alloys such as Goulds Code 1338 (Modified Cast Alloy CD4MCuN with 3% molybdenum), Goulds Code 1384 (ASTM A890 Grade 1C - CD3MCuN), or Goulds Code 1361 (ASTM A890 Grade 5A - CE3MN) are logical choices for sea water services.

References:

(1) Bengt Walle'n, “Corrosion of Duplex Stainless Steels In Seawater,” Avest Sheffield AB, Research and Development, acom 1-1998, SE-77480 Avesta, Sweden

(2) ASTM A890, “Standard Specification for Castings, Iron-Chromium-Nickel-Molybdenum Corrosion - Resistant, Duplex (Austenitic-Ferritic) for General Application,” Annual Book of ASTM Standards 2002, Volume 01.02.

(3) Stephen Morrow, “Duplex Stainless Steels – Several Generations In The Making,” PUMPLINES – Spring 2000 issue, ITT Industries- Industrial Products Group, Seneca Falls, NY

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Duplex UNS S31803 Technical Information

Overview

Duplex is an austenitic ferritic Iron Chromium-Nickel alloy with Molybdenim addition. It has good resistance to pitting, a high tensile strength and higher resistance to stress corrosion cracking at moderate temperatures to that of conventional austenitic stainless steels.

Duplex is a material having an approximate equal amount of austenite and ferrite. These combine excellent corrosion resistance with high strength. Mechanical properties are approximately double those of singular austenitic steel and resistance to stress corrosion cracking is superior to type 316 stainless steel in chloride solutions. Duplex material has a ductile / brittle transition at approximately -50°C. High temperature use is usually restricted to a maximum temperature of 300°C for indefinite use due to embrittlement.

Benefits

There are several benefits of Duplex including:

High Strength

High resistance to pitting, crevice corrosion resistance High resistance to stress corrosion cracking, fatigue and erosion Excellent resistance to Chloride stress corrosion cracking Low thermal expansion and higher heat conductivity than Austenitic steels High energy absorption Good workability and weldability

Applications

Pipe – ASTM A790

The method of manufacture can be either seamless or automatic welding, with no addition of filler metal. Pipe may be hot or cold finished but must always be furnished in the heat treated condition.

BUTT WELD – ASTM A815

This class covers the class of WP, made up of 4 categories and meet the requirements of ANSI B16.9. Pressure ratings are the same compatability of matching pipe.

Categories :-

WP-S : Seamless Construction

WP-W : Welded Construction where construction welds are radiographed WP-WX : Welded Construction where all welds are radiographed WP-WU : Welded Construction where al welds are Ultrasonically Tested.

Flanges ASTM A182

ASTM specifications regulate approved raw materials from which flanges can be made. Forged or rolled alloy steel pipe flanges, forged fittings and valves for high temperature applications.

Technical Details

CHEMICAL COMPOSITION (All values are maximum unless stated otherwise)

%C %Cr %Ni %Mo %Mn %S %P %Si %N

0.03 21.0-23.0 4.5-6.5 2.5-3.5 2.00 0.020 0.030 1.00 0.08-0.20

MECHANICAL PROPERTIES

Yield Strength

Tensile Strength

Elongation (Minimum)

Reduction of Area (Minimum)

Hardness (Maximum)*

(Ksi) (Mpa) (Ksi) (Mpa) (Bhn)

65 450 90 620 20 - 290

* (N.A.C.E. MR-01-75 latest revision may limit hardness in certain applications)

PREn (PITTING RESISTANCE EQUIVALENT) - (%Cr) + (3.3 x %Mo) + (16 x %N)

HEAT TREATMENT: SOLUTION ANNEALED AT 1020 DEG C - 1100 DEG C WATER QUENCH

EQUIVALENT GRADES

UNS BS EN SWEDEN SS GERMANY DINFRANCE AFNOR

SANDVIK +

31803 1.4462 2377X2 CrNiMoN 22.5.3

Z2 CND 22.05.03

SAF 2205

19. Are there temperature limits, low and high, on the use of duplex stainless steels?The toughness of the duplex stainless steel mill plate does not undergo an abrupt ductile-brittle transition. Rather it decreases gradually from its high shelf energy to a very low impact energy as temperature decreases from about ambient to temperatures in the range of -45 to -75° C (-50 to -100° F). So the minimum application temperature is determined in accordance with the tough of the duplex stainless steel. To date, there have been few applications with minimum design metal temperature below -40° C (-40° F).

The maximum temperature for ASME Code applications is 315° C (600° F). The temperature was chosen because it represents the lowest temperature for the transformation curve for 475° C (885° F) embrittlement. Below that temperature, the steel will not be embrittled by this reaction in many years of exposure. In non-Code applications, it would be possible to consider use of 2205 in applications where there are limited excursions in the range just slightly above the limiting temperature. However, the embrittling reaction is real and exceptions to the 315° C (600° F) limit should not be undertaken without full knowledge and evaluation.

20. How do the properties of duplex stainless steels affect wall thickness, thermal expansion, and heat transfer in comparison to austenitic stainless steels?Although it is generally correct to say that the yield strengths of the duplex stainless steels are twice that of the common austenitic stainless steels, that relationship does not imply that the thickness of the duplex stainless steel will be simply half that of the austenitic stainless steel in the same design. The higher strength of the duplex grades is reflected in higher allowable design stresses in the ASME Code. Depending on the shape of the construction, it is possible to reduce significantly the thickness of the material required when using duplex stainless steel, an opportunity for cost savings.

The thermal expansion of a duplex stainless steel is intermediate to that of carbon steel and austenitic stainless steels. This difference can be an advantage in structure with cyclic heating because there is less necessity to accommodate the large expansions associated with the austenitic materials. On the other hand, using duplex stainless steel within a construction of austenitic stainless steel may create problems when the duplex steel does not expand to the same extent. The combination of high strength and lower expansion may mean that the duplex stainless steel will impose high stresses at the point where it is joined to the austenitic structure.

Because the duplex stainless steel has a ferritic matrix, it is more efficient in heat transfer than the austenitic stainless steels. This property, combined with the thinner material that results from economical

use of the higher strength of the duplex grades, can be used to significant advantage in heat transfer applications.

Duplex Stainless SteelThese grades combine high strength with excellentcorrosion resistance, especially to chloride stresscorrosion cracking, however a tendency to brittlenesslimits their use to approx 300 to 315oC maximum, sub-zerouse is also restricted to approx 50oC because of brittlenessdue to the ferrite content. Main uses include offshore piping, chemical tanks, flue gas scrubbers and chimneys.

Super Duplex Stainless SteelSimilar qualities and limitations to duplex grades above.These grades are widely used to handle seawater andother brackish waters, marine pumps, oil and gasproduction and desalination plants are typical applications.