w w w. s t a i n l e s s - s t e e l - w o r l d . n e t s t a i n l e s s s t e e l w o r l d d e c e m b e r 2 0 0 8 71

du

pl

ex

The use of duplex stainless steel grades in tubular products*

The increasing use of duplex in the stainless steel tubular business has been one of the major breakthroughs in the past decades. Thanks to their excellent combination of corrosion resistance and high strength, duplex steel grades provide a very interesting alternative to conventional stain-less and structural steels. In recent years, price increases for important alloying elements such as nickel and molybdenum have resulted in a gradual change to more price-stable duplex grades. However, from a technical point of view, this price stability may be just considered as an extra bonus. It is thought that the new low-nickel duplex grade LDX 2101® (trademark of Outokumpu, EN 1.4162, UNS S32101) will achieve a bright future among the various duplex grades. The corro-sion-resistance, welding, and strength properties of LDX 2101® make it interesting from a tech-nical point of view, while its lean composition, with a very low-nickel content, make it interesting from an economical perspective. This paper compares the duplex family to traditional austenitic grades. Its main focus is to show duplex’s weight-saving possibilities in not only high-pressure tub-ing systems, but also in construction applications where both round and square hollow tubes are used. Some new experiences gained with the new LDX 2101® grade will be related.

Presented by Mikael Paijkull, Asko Kähönen, Sören Nytomt, Outokumpu Stainless Tubular Products AB

1. IntroductionThe fi rst duplex stainless steels were developed during the late 1920’s. The oldest test results for them archived at Outokumpu, Avesta Research Centre are dated December 23, 1930 [1,2] (see Fig.1). The earliest duplex grades developed by Avesta Ironworks in the 1930’s were typically of type 25Cr-5Ni with or without the addition of about 1.5% Mo. One of them 453S (25%Cr-5%Ni-1.5%Mo) is a forerunner to AISI 329. The early grades had a relatively high ferrite content,

about 60–70% in the solution-annealed state. As in all stainless steels of thisperiod, the early duplex grades had a high-carbon content of typically 0.1%. The main advantages of these steels, compared to the austenitic grades available, were their superior strength and a greater resistance to inter-granular corrosion. It was later found that duplex stainless

The use of duplex stainless steel

*This paper was presented at the Stainless Steel World 2008 Conference and Exhibition in Houston, TX, USA September 9–10, 2008.

outokumpu.indd 71 27-11-2008 15:45:54

w w w. s t a i n l e s s - s t e e l - w o r l d . n e ts t a i n l e s s s t e e l w o r l d d e c e m b e r 2 0 0 872

du

pl

ex

steels could be used in environments where standard aus-tenitic stainless steels suffered from stress corrosion crack-ing (SCC). After the AOD (Argon Oxygen Decarburisation) process was introduced during the 1970’s, it became possible to efficiently reduce the carbon content of stainless steels and to add nitrogen with high precision. This has resulted in modern duplex stainless steels that are low in carbon and rich in nitrogen. These steels have improved pitting resis-tance and weldability compared to their earlier counterparts. The first modern duplex grade was 2205. This grade has a much higher mechanical strength and corrosion resistance than types 316L and 317L. 2205 has been found to be a cost-efficient alternative for many applications and has been the duplex “workhorse” for a long time. The so-called super duplex stainless steel 2507 was developed in the 1980s for use in environments that were too aggressive for 2205. The leaner alloyed grade 2304 was developed at the same time, but has only been used in a few applications. Last year’s rapid increase in the nickel price led to an increased interest in grade 2304, which in most environ-ments can be considered as a high-strength alternative to type 316L. The chemical com-position of the modern duplex stainless steels appears in Table 1.

The members of the duplex stainless steel family have established themselves as materi-als for use in such applications as: pressure vessels and equipment exposed to erosive/abrasive conditions in the pulp and paper industry, cargo tanks in chemical tankers, storage tanks in various industries, and in civil engineering constructions such as bridges. The world annual consumption of duplex stainless steels, which in 1997 was around 70,000 metric tonnes, has probably doubled today. This paper describes the general properties of modern duplex stainless steels and some case studies where these properties can be utilised in appli-cations for welded tubes, pipes and fittings.

2. basic characteristics of modern duplex stainless steels

2.1. Mechanical propertiesA common characteristic for all duplex grades is their high mechanical strength. Their yield

Fig. 1. Test records (left) from 1930 describing a test with stainless steels, including duplex grade 453S. A reactor vessel (right) of grade 453S used for the production of gun powder, supplied to a Belgian chemical industry in August 1933.

Table 1. Typical chemical composition, wt%, and PRE numbers* for some duplex stainless steels. Austenitic and ferritic grades included for comparison

Steel grade UNS Cr Ni Mo N Other PRELDX 2101® S32101 21.5 1.5 0.3 0.22 5Mn 262304 S32304 23 4.8 0.3 0.10 - 262205 S32205 22 5.7 3.1 0.17 - 352507 S32750 25 7 4 0.27 - 42304L S30403 18.1 8.1 - - - 18316L S31603 17.2 10.1 2.1 - - 24317L S31703 18.2 13.7 3.1 - - 28904L N08904 20 25 4.3 - 1.5Cu 34254 SMO® S31254 20 18 6.1 0.2 Cu 43430 S43000 16.5 - - - - 16.5444 S44400 18 - 2 - Ti, Nb 25

*PRE (Pitting Resistance Equivalent), calculated according to: PRE = %Cr+3.3%Mo+16%NLDX 2101® and 254 SMO® are registered trade marks of Outokumpu

strength is about twice that of the most common austenitic grades, and is much higher than that for ferritic stainless steels. This higher strength can often be utilised in terms of reduced gauge and thereby reduced weight for construction. The mechanical properties for modern duplex stainless steel are presented in Table 2.

In the solution-annealed state, duplex grades are harder than austenitic and ferritic grades. This is an advantage in most applications involving erosion and abrasion. With respect to formability, involving a significant element of stretch forming, duplex grades can be ranked in-between the highly formable austenitic and ferritic grades. Due to their lower ductility, they can, in general, not be formed in as compli-cated geometries as are the common austenitic grades and due to their higher yield strength, greater forces have to be applied in forming operations.

2.2. Corrosion propertiesThe entire group of duplex stainless steels available on the market today cover the same range of resistance against chloride induced pitting and crevice corrosion as the group of austenitic stainless steels, ranging from 304 to 6Mo grades. This is illustrated by the PRE values presented in Table 1. PRE values act only as a rough measure of the rela-tive resistance to these forms of localised corrosion, but simi-lar rankings are obtained when doing laboratory testing, e.g. determination of CPT (Critical Pitting Temperature) accord-ing to ASTM G150. The resistance to chloride-induced stress corrosion cracking is much better for all duplex grades than

Table 2. Mechanical properties, cold rolled coil, minimum values at room temperature. Austeni-tic and ferritic grades included for comparison

ASTM A240, min values

Steel grade UNS Proof strength, Tensile strength, Elongation Hardness, Rp0.2, RM, in 2” or 50mm Outokumpu [MPa] [MPa] [%] typical values [HB]LDX 2101® S32101 530 700 30 2302304 S32304 400 600 25 2252205 S32205 450 655 25 2502507 S32750 550 795 15 255304L S30403 170 485 40 165316L S31603 170 485 40 165317L S31703 205 515 40 195904L N08904 220 490 35 190254 SMO® S31254 310 690 35 220430 S43000 205 450 22 170444 S44400 275 415 20 -

outokumpu.indd 72 27-11-2008 15:45:56

w w w. s t a i n l e s s - s t e e l - w o r l d . n e ts t a i n l e s s s t e e l w o r l d d e c e m b e r 2 0 0 874

du

pl

ex

for the common austenitic-type 304 and 316 grades [3,4]. Fig. 2 illustrates the relative ranking of some duplex and aus-tenitic grades tested according to the Drop Evaporation Test (DET) [5,6], a method developed to simulate conditions where a high risk of stress corrosion cracking occurs due to evapo-ration of aqueous solutions on hot stainless steel surfaces (often referred to as “external SCC”).

2.2.1. Laboratory erosion corrosion testsErosion corrosion tests were performed in three different test solutions with or without an immersion period followed by a wearing cycle [7]. Duplicate test samples were used in all tests. In the 24 h erosion corrosion test cycle (Test1) without any immersion pre-treatment. All the tested materials remained passive and the weight losses obtained were due to pure wearing. However, when the test materials were pre-treated by immersion for one week in the high-chloride test solution (1000 mg/l) followed by 5 h wearing (Test2Env2), the passive film broke down locally, resulting in both some pitting and wear. In the 1 N H2SO4 solution all the test materials corroded actively and wear took place very rapidly during the 5 h wear-ing cycle (Test3Env3), corresponding to a erosion corrosion rate of about 1 mm/year. The results are presented in Fig. 3.The erosion corrosion rate of 316L was 24–30% higher than the erosion/corrosion rate of LDX 2101® in the mildest environment containing 200 mg/l chloride (Env1). In 1000 mg/l chloride-(Env2)-containing environment, the erosion/corrosion rate difference was 16– 8%. In the most aggressive environment (Env3) the erosion corrosion rate of 316L was only 6% higher than the rate of LDX 2101®. The laboratory

Fig. 2. Minimum relative stress for failure during a 500h test duration in the DET test. 304* and 316 *suffered SCC at 10% of Rp0.2, which was the lowest relative test load.** Undefined threshold stress, test specimen failed due to general corrosion thinning.

erosion corrosion tests show that the mechanical strength of the material has a considerable impact on the overall perfor-mance as grade LDX 2101® experienced the lowest weight losses in all test environments.Duplex stainless steels are increasingly taking over the role of austenitic stainless steels like 304 and 316L in the paper-making equipment industry. Nitrogen as an alloy ad-dition makes the duplex steels stronger and more resistant to pitting. They also have the added advantage that they can resist stress corrosion cracking that can occur to higher temperature components such as steam boxes.

2.3. Physical propertiesMany physical properties are similar for duplex and auste-nitic stainless steels. One important difference, however, is the thermal expansion (linear expansion), which is lower for the duplex grades. Data for duplex and type 304 grades are presented in Table 3. The lower linear expansion is usually an advantage, especially in constructions where stainless steel is combined with carbon steel. It also reduces the risk of thermal fatigue.

Table 3. Linear expansion at (RT→T)°C, x10-6/°C

Steel grade 100°C 200°C 300°CType 304 16.0 16.5 17.0All duplex 13.0 13.5 14.0

3. case studies3.1. Process pipe design In process plants the transportation of chemicals and liquids with tubular products is sometimes a demanding task. This is due to the fact that corrosive substances have to be trans-ported from one point to another. Pipeline and structural designs are based on the use of standardized tube, pipe and component sizes as much as possible. The size and inspec-tion standards are most often issued by European system (EN-standards, e.g. 13480-3) [8], or the American system (ASME/ANSI-standards) [9].The design of the process factory/plant is based on the fol-lowing main phases and documentation:• Processchartforthewholeplant.• Flow-linediagram,whichincludesallthechemical,me-

chanical and physical processes.• Pipelineassemblydiagram.One criterion in choosing the tube or tubular component is often the total purchase price, which is mainly decided upon by the weight and length of the tube needed. This is the reason why design engineers and purchase managers are keen to develop new material grades that may lead to more economical pipelines. There are also other aspects to be considered when choosing lighter weights, i.e. reduced wall thicknesses. The transportation and installation costs will be less and the amount of welding on site is reduced remark-ably. Lighter supporting structures also provide additional cost savings.The thickness reduction possibilities of the wall in pres-surized tubes are dependant upon several factors. Designs according to ASME specification gives somewhat more conservative wall thickness than those attributed to EN standard. But both design standards give about 40% thick-ness reduction from 304L to lean duplex stainless steel LDX 2101®. But then the actual design thickness is dependant on

Fig. 3. Erosion corrosion test results presented as a function of weight loss [mg/m2*h].

outokumpu.indd 74 27-11-2008 15:46:14

w w w. s t a i n l e s s - s t e e l - w o r l d . n e t s t a i n l e s s s t e e l w o r l d d e c e m b e r 2 0 0 8 75

du

pl

ex

the specification, e.g. ASME schedules. Also the design pres-sure has to be taken into account. With lower pressures the thickness reduction potential is less and then if the original design has already very thin wall thickness, no additional thickness reduction is practically possible.

Fig. 4. (a) Comparison between EN and ASME standards results for a 40 bar, 457 mm pipe with 304L or LDX 2101 material grades. (b) Com-parison curves for a whole DN-range with realistic thicknesses.

There are also other aspects to be taken into account when designing a pipeline. Internal pressure is not the only load case for the whole pipeline design. Support spacing has to be appropriate to the case in question. Support spacing depends on:• Thetypeofsupport,• Tubediameterandwallthickness,• Contentinthetube,• Isolationifneeded,• Potentialvibrationcontrol,• Or,moreoften,acombinationofaboveaspects.Ambient or content temperatures variations can also cause extra loading that has to be taken into account either in material selection or pipeline design.If we look at a more realistic case where an existing pipe line is described and designed according to European design standard EN 13480-3 (8). Two material grades compared in this case study, are 304L and LDX 2101®. Main parameters for this design case are listed in Fig. 5.

- Application = Liquid (1300 kg/m3) transport

- Line length total 22 m- DO 457,2 mm- Inside Pressure 25 bar- Design Temperature 120°C / 248 F- Material choices 304L / LDX 2101

(S32101)- Design standard EN 13480-3- Weld factor 1.0

Fig. 5. Isometric drawing of a pipeline and main design information.

The design comparison shows that in this type of pipeline we can go down in material thickness from 5.6 to 2.42 mm theoretically when changing from 304L to LDX 2101. In practice we have to account for the dimension standards. We then end up having a realistic thickness of 6.0–3.2 mm. This means about 47% weight reduction in this case. With sizes like DN 450, the higher material strength gives substantial benefit. The support of the pipeline does not provide any problems even with higher strength material – the support intervals can be even lengthened. Standard sup-ports and fixtures can be used. Less weight means also easier

installation, larger sub-components can be lifted (e.g. pipe bridge together with pipes). Welding work in the field will also be reduced, which means substantial cost savings.

3.2. Structural hollow section designStainless steel tubes are traditionally used in architecture as decorative elements. Structural hollow sections used in architecture, building, construction and transportation have often been carbon steels. The trend in the carbon steel structural design is towards higher strength steels. This is of course due to the possibility to do more cost effective design when the material strength can be fully utilized. However, the one aspect that makes the carbon steels vulnerable in many environments is corrosion. They need a well-planned corrosion inspection and protection program including the planned – and unplanned – production or utility stops because of the renovation, re-protection, painting, etc. It is also expected that the relative price of maintenance for painted carbon steel increase in the future.Duplex stainless steel is already gaining more interest in structural applications in architecture. The material is more cost-effective than many other materials used in a corrosive atmosphere. When compared to 300 series stainless steel, the cost reduction is realized in a different manner. Duplex’s yield strength is double that of the 300 series stainless steel. This factor allows the use of a thinner material to support a similar load. The cost of fabricating duplex usually is lower than for other materials because of its comparative ease of machining and welding. The ferrite content makes weld-ing less intimidating than when welding high-nickel alloys. This factor alone saves significant weld time. Machining of especially lean duplex grade LDX 2101® is similar or easier than that of 316 stainless steel with high-speed steel tool-ing. Duplex also requires fewer machining labor hours than high-nickel alloys.

a) Bus-shelter b) Stair constructionsFig. 6. Examples of use of stainless steel structural tubes in architec-ture, building and construction.

Structural design can be based on European standard EN 1993-1-4: 2006 [10]. The design of structures in EN 1993-1-4 is based on a limited-state analysis. The limits used are ultimate limit and serviceability limit states. Ultimate limit states are those, which, if exceeded, can lead to collapse of part or the whole of the structure. The serviceability limit state corresponds to a situation beyond which specified ser-vice criteria are no longer met. The design rules presented in standard EN 1993-1-4 are suitable for annealed materials up to yield strength fy = 480 N/mm2 and cold worked materials up to strength classes C700 and CP350.According to the Euro-Inox Design Manual [11], the en-hanced yield strength and tensile strength values of cold-worked austenitic stainless steels can be used as a basis for design up to strength class CP 500. According to the Euro-Inox Design Manual, the design strength for mem-

a) b)

outokumpu.indd 75 27-11-2008 15:46:16

w w w. s t a i n l e s s - s t e e l - w o r l d . n e t s t a i n l e s s s t e e l w o r l d d e c e m b e r 2 0 0 8 77

du

pl

ex

bers loaded in axial compression and members subject to combined axial compression and bending is the 0.2% proof strength, depending on the temperature of the material. The behavior of stainless steel differs from that of other metals at fire temperatures in that its mechanical properties (mainly modulus of elasticity and yield strength) maintain their val-ues comparatively well up to temperatures corresponding to a 30-minute standard fire. The stability of the yield strength of stainless steel depends on the alloy of the material i.e. the chosen stainless steel grade. The basis of the fire design of stainless steel structures is presented in standard EN 1993-1-2 and in Euro-Inox “Design Manual for Structural Stainless Steel” (2006). Stainless Steel Design Manual by Euro-Inox [11] helps in the design and includes also a lot of other interesting information about the use of stainless steel in structural applications. A first quite comprehensive information package about tubular stainless steels in structural application “Stainless Hollow Sections Handbook” [12] has been published by FCSA (Finnish Con-structional Steelwork Association). The structural design itself is not more complicated than with carbon structural steel. Duplex stainless steel has practically the same thermal expansion coefficient as carbon structural steel, i.e. much less than with standard austenitic stainless steel. Welded joints have at least as good a fatigue strength as carbon steel joints. Their higher structural strength than standard austenitics gives better capacity for loaded members.Cold-working the material can enhance the strength values of stainless steel hollow sections. This can either be done at the steel works by means of temper rolling the strip intend-ed for hollow section manufacture or in the tube production line. In these cases, the mechanical values for the hollow section batch are given in an inspection document indicat-ing values tested from the hollow section. The strength clas-sifications of stainless steel hollow sections with enhanced strength values supplied by Outokumpu are presented in Table 4.

Table 4. Strength classification of stainless austenitic steel hollow sec-tions supplied by Outokumpu when hardening of the material during cold working is taken into account.

Strength class Strength values fy [N/mm2] fu [N/mm2]CP3502) 350 6001)

CP5002) 500 6501)

1 Minimum tensile strength indicated by the hollow section supplier.2 Dimension program and available steel grades must be confirmed by the

hollow section supplier.

In using higher strength structural hollow section in a vertically loaded support, like a column in a building or the supporting component in a bus-shelter construction – like in Fig. 6 gives us possibilities of having an architecturally ap-pealing and a structurally cost-effective solution.In designing a column structure, the stability of the struc-ture/component, i.e. the buckling strength is often a limit-ing criterion. If we assume that the size of the column tube is fixed and we calculate the load supported by the column according to European instructions [10] we get a results like in Fig. 7. Another case could be e.g. a roof truss construction with support length (construction span) of 15 m, maximum height 3 m and minimum of 0.5 m (Fig. 8). Loading of the

truss consists of: Snow load, own weight, glass roof weight, and secondary supports = 4 kN/m2. Minimum wall thickness according to European regulations is 2.5 mm.

Fig. 8. Roof-supporting truss, consisting of tubular components (FCSA).

When designing the truss with 3 different strength class stainless steels, we get the results presented in Table 5. The total weight will reduce from 480 kg with standard stainless steel to 300 kg with high strength LDX 2101®, i.e. about 37% less weight. This gives big savings in material costs and also additional savings in installation of the roofing.

Table 5. Total weight of the structure designed with different material strength levels

Material/Grade Stainless Stainless high Stainless high 304L/316L, strength strength, LDX 2101 f 0.2 = 220 MPa f 0.2 = 350 MPa f 0.2 = 450 MPaTotal weight (kg) 480 370 300

3.3. Water heatingThis construction contains a heat exchanger in aluminum that is sealed with a stainless steel tube on the outside. The principal is to let heated gas flow trough the aluminum heat exchanger (Fig. 9) and let the gas heat the surrounding wa-ter. In order to protect the aluminum heat exchanger from corrosion, a stainless steel tube is cold drawn and sealed to the aluminum core.

Fig. 9. Principal drawing of water heating.

Fig. 7. Structural tube as a load-carrying column, calculation results for compression and shear loading.

outokumpu.indd 77 27-11-2008 15:46:50

w w w. s t a i n l e s s - s t e e l - w o r l d . n e ts t a i n l e s s s t e e l w o r l d d e c e m b e r 2 0 0 878

du

pl

ex

Fig. 10. Pressure testing of the unit.

The customer has been using the stainless steel grade 316Ti earlier, but with the LDX 2101® a weight reduction of 30% has been obtained. The customer also gains the benefits of a considerable lower Ni and Mo content that provides him with a much more stable purchase price (Fig. 10). All the duplex grades are much more resistant than the austenitic type 316 grades to stress corrosion cracking. Due to differences in properties between the different groups of stainless steels, the change from austenitic or ferritic stainless steels to duplex typically calls for some adjustment of manufacturing equipment and procedures. Due to the higher yield strength of duplex grades, higher forces need to be applied in forming operations. In cases when the higher strength is utilized in terms of reduced wall thickness this effect is however less pronounced. The duplex grades show a significantly higher spring-back at forming.At automatic TIG-welding, the travel speed of the welding torch might have to be reduced when changing from auste-nitic to duplex stainless steels, while the welding of duplex is generally considered easier compared to welding ferritics.2205 and other duplex grades are frequently used in heat exchangers of tube and shell type. A general advantage of duplex grades compared to austenitic when used in heat exchangers is the lower thermal expansion.

3.4. MiningThe case study describes a pipeline when pumping water from the mine 450m below ground level at the Boliden mine in northern part of Sweden (Fig. 11). The pipe dimen-sion is DN 600 with a pressure of 45 bar from 450 m down up to 200 m and thereafter 32 bar up to ground level. The length of the pipeline is 1200 m. However, the Duplex grades service temperature range stretches from temperature range stretches from –40ºF (40ºC) to 572ºF (300ºC), i.e. can-not always replace austenitic grades.If we compare between 304L, that is normally specified for this application, and LDX 2101®, using the EN standard calculation with a weld factor of 0.7 the calculation shows

that 304L tubes need a wall thickness of 13.4 mm and LDX 2101® only 7.2 mm for the 45 bar section. For 32 bar sec-tion the thickness are 9.6 respectively 5.1 mm. The closest standard dimensions are 14mm (Sch 30) and 10 mm (Sch 20) when you are designing for the 45 bar pressure and 8mm (Sch 20) and 6mm (Sch 10) for the 32 bar design. The weight will in this case be 196 metric tons if designed in grade 304L and 113 tons for LDX 2101®. It is possible to save 83 tons, or 42.5%. With today’s pricing this gives a potential saving of around half a million USD. Looking at potentials beyond the direct weight saving for the pipe system, you will also receive a lot of other benefits; i.e. the weld time at the site can be reduced. With 230 girth welds the total weld length is 437 m. To weld 304L the total weld time is about 2100 hours and for LDX 2101®you will only need 1325 hours due to the thinner wall thickness. You save 775 hours and can shorten the setup time on site. Depending on the lower weight of the pipe system you will save energy in all handling steps, which reduce the amount of CO2. Even the transportation cost will be cheaper from tube manufacturer to the site. In this case the transportation distance is 1500 km and us-ing a power driven train the CO2 reduction is 125 kg. If the transport is made by truck the CO2 missions are 75 times higher. As a result of the smaller amount of primary material for LDX 2101® in the melting process the reduction of CO2 is also here an advantage compared to 304L.

Savings• Weightsavings on pipes: • Reduced labor costs for welding: • Welding consumables • Energy savings for all handling of material: • Energy savings for transportation: • Reduction of CO2 emission (melting, handling, etc.) Total savings between $3 – 500,000, depending on raw mate-rial cost and currency rate are possible.

Fig. 11. Mine overview.

outokumpu.indd 78 27-11-2008 15:46:51

w w w. s t a i n l e s s - s t e e l - w o r l d . n e t s t a i n l e s s s t e e l w o r l d d e c e m b e r 2 0 0 8 79

du

pl

ex

4. conclusions4 case studies have been presented, where duplex stainless steel in welded tubular product form is successfully used in applications where alternative materials could be austenitic and ferritic stainless steels, but also other metals, such as corrosion protected carbon steel and aluminum, or poly-mers. The applications presented are related to such diverse processes as constructions, extracting water in the mining industry, and production of hot water in private homes. Since the growing global economy is creating an increas-ing demand for energy and transports and allowing a larger number of people to install modern conveniences such as running hot water at home, it seems very likely that the demand for these applications will increase in the future.After being more or less synonymous with grade 2205, the general awareness is increasing of duplex stainless steel as a family of grades with different levels of corrosion resistance. From being something to consider in situations where 316L can be expected to fail from corrosion, duplex stainless steel is today considered by a continuously increasing number of people for a much wider spectrum of corrosive conditions. The list of references where duplex stainless steels is chosen because it is a cost efficient way of ensuring light, strong and corrosion resistant constructions is increasing, more or less day by day. It cannot be neglected that the high nickel price during the period 2006–2007 has contributed to an increased interest in duplex grades, and that the increased sales volume of duplex grades also include examples where duplex grades have substituted austenitic stainless steels

just because of price and without really taking benefit of the technical merits. However, as shown by the case studies presented here, duplex grades posses a combination of prop-erties, which make them well suited to many old and new applications for stainless steel. The duplex family is more complete today than ten years ago in terms of covering a larger range of corrosion resistance. It is thus more than likely that the positive development of demand for duplex stainless steel seen over the past decade will also continue in the future. ■

references[1] J. Olsson and M. Liljas, Proc. Corrosion´94 , paper NoXXX

(1994)[2] R. Vishnu and H. Loucif, Proc. International Symposium on

Stainless Steels, Chennai, India, (2007)[3] E .Johansson,T. Prosec, Corrosion 2007, NACE, Paper No 07475

(20 07)[4] H. Anderssen, W. Wasielewska, L. Wegrelius, C. Wolfe, Proc.

Corrosion´98, NACE, paper No. 251 (1998)[5] S. Henrikson, M. Åsberg, Corrosion Vol. 35 No. 9, 1979, pp

429-431[6] P-E Arnvig, W Wasielewska, Proc. The Danish Metallurgical

Society´s Winter Meeting, (1993) pp 37-49[7 ] T. Laitinen, L.Wegrelius, A. Bergquist, 6th European Stainless

Steel Conference - Science and Market, Helsinki, Finland (2008)[8] European Standard EN 13480-3: Metallic industrial piping - Part

3: Design and calculation.[9] ASME B31.3-2006. ASME Code for Pressure Piping, B31[10] European standard EN 1993-1-4. Eurocode 3. Design of steel

structures. Part 1-4: General rules. Supplementary rules for stainless steels.

[11] Design Manual for Structural Stainless Steel. Euro Inox and The Steel Construction Institute (2006). (http://www.euro-inox.org )

[12] Stainless Hollow Sections Handbook, FCSA (Finnish Constructio-nal Steelwork Association).

outokumpu.indd 79 27-11-2008 15:46:51