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PAST, PRESENT AND FUTURE OF THE DUPLEX STAINLESS STEELS Dr. J.CHARLES

ARCELOR MITTAL STAINLESS / UGINE&ALZ /Dir. I-R&D 5-7, rue Luigi Cherubini, 93210 LA PLAINE SAINT-DENIS Cdx, FRANCE.

E-mail address: jacques.charles@arcelor.com -Tel: 0033171920650 ABSTRACT Stainless Steels world wide crude steel production is close to 30 millions of tons. The average annual growth since 30 years is 6%, much more than others materials including wood, aluminum, copper, steel….This results from there very attractive properties resulting from “multi-grades” offer making it possible to cover a wide range of corrosion resistance and mechanical properties. Furthermore when properly selected, they are almost maintenance free and secure materials. The main stainless steels families are the austenitic and the ferritic ones. They cover more than 95% of the global stainless steel production. Recently among the austenitic, due to a sharp increase of the Ni cost, austenitic grades with partial substitution of Nickel by manganese additions have been developed (the 200 series). Duplex grades production, worldwide, represent less than 200KT i.e. less than 1% of the total stainless steel production although the production has growth of more than 100% in the latest decade. Most of the production concerns the so-called quarto plate’s i.e. wide –more than 2M- and thick – more than 6mm- plates. For those productions weight savings are often obtained taking advantage of their high mechanical properties. Most of the applications concerns highly corrosion resistance properties encountered in process industry. (Chemical, petrochemical, off-shore, chemical tankers, pulp and paper industry, pollution control equipments-FGD-, desalination, sea-water applications...). The paper presents present also the new trends in alloying duplex grades including the new low alloying grades so-called lean duplex- as well as new grades with specific properties. The extended duplex family with new grades makes it possible to extend the applications areas including in huge markets requiring less corrosion resistance properties than process industry. One of the main challenges is the industrialization of thinner gauges. The possible new offer of duplex grades is also optimized in alloying cost with some new grades with partial replacement of nickel by manganese additions. The target to be more cost competitive than 304 austenitic is now achieved at least for the quarto plates. The next target is the thinner gauges. Duplex developments are nowadays extremely dynamic with high potential of successes in many new markets. They offer the right answer with a unique combination of high mechanical properties, appropriate corrosion resistance properties and optimum alloying costs. For all this reasons a yearly growth with two digits may be expected. KEYWORDS Duplex, Markets and applications, Chemistries, Mechanical Properties, Corrosion resistance. INTRODUCTION The first international duplex conferences ( St LOUIS/USA/82 and Den HAGUE/NL/86 ) were mainly focused on “scientific” aspects since they appeared very attractive for the metallurgical aspects including phase precipitations (structures, kinetics’,…), corrosion resistance, mechanical properties. The world wide industrial production in that time was almost marginal and standardization has still to come.

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BEAUNE ’91/France conference was the first duplex conference with a mix between scientific and market applications. The new grades with increased nitrogen additions were presented. Duplex grades gained in structure stability, weld ability and corrosion resistance properties! New standards were proposed. The duplex family included the popular 2205 grades with increased nitrogen additions (0.16/0.18%N instead of 0.12/0.15%N) and optimized Mo contents. PREN values were proposed between 33 and 36 with a most common value 34/35. Sigma free grades were recommended as well as a minimum Mo level of 3%. Grade 2304 was already developed but for marginal applications. Several super duplex grades were also promoted – SAF 2507, UR 52N+, Zeron 100… - for the most severe applications including off-shore. Those super duplex grades were also redesigned before the BEAUNE’91 conference in order to have a PREN value minimum of 40 and a nitrogen level minimum of 0.25 %. The grades were mainly produced with EF + VOD or AOD + continuous casting devices i.e. the most efficient stainless steels tools. First extensive applications were reported as well as new areas of developments. Confidence in duplex grades gained the end-users and the cost savings aspects partially obtained through the high mechanical properties were expected. New codes for duplex grades had to come. The potential growth of the grades was clearly emphasized. All this partially has explained the extremely wide audience of the BEAUNE’91 conference. York’94, Maestricht’97 and Venice’2000 conferences were also successful conferences with increasing return of information on practical experiences. Most of the applications still concerned the quarto plates and tubing. The first lean duplexes appeared. More concerning the technical content of those conferences will be discussed in this article. STAINLESS FAMILIES and ALLOYING COSTS.

Figure 1 : Schaeffler diagram.

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Although Schaeffler diagram is mainly used for welded structures, it is very useful to illustrate the different areas of stability for the stainless steel microstructures. The classical austenitic grades – the so called 300 series- contains a minimum of 9/10%Ni while the more alloyed grades in Cr and/or Mo needs even more Ni to stabilise the austenitic phase. The most popular stainless steel – the 304 – is the lowest alloying grade of the austenitic area. Ferritic stainless may be produced when Ni content is almost 0% and Cr minimum content 12% Most of the ferritic grades have a Cr content included in the 12/18% range. More recently >20%Cr grades with Ti+Nb or Cu+Nb are developed to extend the Mo-free ferritic family. Between the austenitic and the ferritic areas, we obtain a mixed ferrite + austenite microstructure. Most of the duplex industrial grades have a typical 50%ferrite/50%austenite microstructure. The three main families of duplex grades are plotted in the Schaeffler diagram. As observed it is almost impossible to develop duplex grades having less than 19%Cr without formation of martensite. With the extreme volatility of alloying element costs ( Figure 2 ) – Ni for example has increased of more than 500% this latest 3 years - new grades have been introduced recently in the market. This concerns austenitic grades with partial replacements of Ni by combined Mn and N additions. Such grades were already developed on a marginal production since more than 50 years. The newly developed grades have Cu additions and often lower Cr contents in order to reduce the interstitial elements like C and N. Those grades are softer than the former 200 grades and make it possible to utilise the same tools to manufacture equipments - drawings for ex… - than those used for 304 grades (Table I and Figure 3). Typical mechanical properties of the different stainless steel families are presented Figure 4. Mechanical properties of the different stainless steels are directly linked to the nitrogen + carbon contents and microstructure. Ferritics have slightly higher yield strength than austenitics at room temperature but can not be strengthen by interstitial additions. (lack of solubility) The duplex grades have the highest mechanical properties due to a grain refining effect obtained by the two phase microstructure. This will be very useful for the design of pressure vessels where weight reductions can be expected when properly designed. Figure 2 : Cr and Ni volatility this latest years.. TABLE I : Typical chemistry of several stainless steels.

LME Ni Evolutions

4 000 $/T

9 000 $/T

14 000 $/T

19 000 $/T

24 000 $/T

29 000 $/T

34 000 $/T Chromium

0.2 $/lb 0.3 $/lb 0.4 $/lb 0.5 $/lb 0.6 $/lb 0.7 $/lb 0.8 $/lb 0.9 $/lb

02 03 04 05 06

02 03 04 05 06

52 000 $/T 04/2007

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FAMILY AISI(USA) EURONORM Cr Mo Ni Mn N Cu Others PRE PREN

304L 1.4307 18 0 9 1 0 18 18316L 1.4401 17 2 11 1 0 24 24

904LN 1.4339 20 4 25 1 0,1 1,5 Cu 32 362101 1.4162 21 0 2 5 0,2 21 242304 1.4362 23 0 4 1 0,13 23 252205 1.4462 22 3 6 1 0,17 32 352507 1.4410 25 3,5 7 1 0,25 37 41430 1.4016 17 0 0 0 0 17 17439 1.4510 17 0 0 0 0 Ti;Nb 17 17445 21 0 0 0 0 Ti;Nb 20 20434 1.4113 18 1 0 0 0 Ti;Nb 22 22444 19 2 0 0 0 Ti;Nb 26 26201 1.4372 16,5 0 5 6 0,15 16 18204 15,5 0 2 9 0,2 15 18

A NO 17 0 4,8 3,7 2 0,12 17 18B NO 15 0 4 7 1,6 0,05 15 16C NO 15 0 1 9 1,7 0,1 C=0,1 16 17

200New

PRE = %Cr + 3.3%Mo ; PREN = %Cr + 3.3%Mo + 16%N

200

300

400

DUPLEX

Figures 3 and 4 : Stainless Steel families and mechanical properties STAINLESS CRUDE WORLDWIDE PRODUCTION AND CONSUMPTION. Figure 5 presents the worldwide yearly crude steel production for flat stainless steels. An average growth of about 6% is observed since many decades. Figure 6 shows that if the nowadays biggest geographic area of consumption is located in Europe, in the future most of the growth is expected to take place in Asia. Production capacities are booming mainly in China and are expected to come in India. Asia witch has since many years imported part of their needs will be soon able to export stainless steel due to possible overcapacities. The booming demand in Asia particularly in China and a certain lack of regulations made it possible to develop new grades with less alloying elements. China and India are the countries where the new 200 series were developed. More recently China has used more than 1 million of tons of those new grades which do not have specific norms. (Figures 7 and 8) Austenitics remains the most popular grades thanks’ to their unique combination of high ductility, high potential of strengthening, weld ability, toughness even at extra-low temperatures and of course corrosion resistance.(Figures 9 and 10) Ferritics represent about 25% of the total production and

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Figure 6 : Flat Stainless Steel worldwide 2002 consumption and 2010 expectations.

Figures 7 and 8 : Stainless Steel crude steel production (2001-2006) by grades. (ISSF documents)

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Figure 5 :Flat Stainless Steel Crude worldwide production.

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58%

8%

1%1%

11%

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11%2%1%

304316>Mo AUST200OLD200NEW

12CR FERR17CR FERRMo FERRDUPLEX

Figures 9 and 10 : Stainless worldwide crude production in 2004 by grades. due to welding aspects and toughness properties, they are restricted to thinner gauges even if they are often the cost saving grades. Duplex grades cover about 1% of the total production. The recent evolutions of raw material costs particularly Ni have a drastic impact on 304 and 316 alloy surcharges. They are nowadays higher than the base material price!! As a result differences of several thousands of euros per ton are observed for the 300 series prices in a few years. Of course ferritic steels are almost not affected by those effects. Replacement of 304 grades by ferritic grades containing some molybdenum is nowadays cost savings solutions! This concerns particularly thin gauges applications. Duplex grades start to be very cost competitive answers for thicker gauges and particularly for very corrosive applications. Replacements of 316 grades by 2205 duplex grades are now to be considered: cheaper grade for much more corrosion resistance properties and high mechanical properties! Same results are observed for 2304 duplex versus 304 and 904L versus superduplex grades!!! This provides new powerful driving forces for a further increase duplex uses in new applications!

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304L316L210123042205250743043944543444420120417 5 Cu15 1 Cu

Figure 11 : Evolutions of raw material costs ( 2004/2007).

Ni Mo Cr 1 6 7 0,8 2 10 17 1 3 14 35 1,5 4 30 40 1,5 5 52 42 1,5

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DUPLEX STAINLESS STEELS IN THE FUTURE. Some new duplex grades have been introduced in the market or are concerned by R&D activities. Nowadays the main target is the development of so-called lean duplex having much less alloying elements than the standard 2205 duplex grade. There targets is the replacement of 316 and even 304 grades. Some of those chemistries are presented Table II. Chromium contents are in the range 20/22 while Ni additions are reduced by further increase of Mn contents. For some more corrosion resistance applications Mo contents are considered. Table II presents also Table II : Chemical analysis of duplex grades.

FAMILY USA EURONORM Cr Mo Ni Mn Cu N Others PRE PREN

304L 1.4307 18 0 9 1 0 18 18316L 1.4401 17 2 11 1 0 24 24

904LN 1.4339 20 4 25 1 1,5 0,1 33 350 0

S 32304 1.4362 23 0 4 1 0,13 23 25S 32205 1.4462 22 3 6 1 0,17 32 35S 32750 1.4410 25 3,5 7 1 0,27 37 41S 32760 1.4501 25 3,8 7 1 0,7 0,27 0,7W 38 42

S 32520 1.4507 25 3,5 7 1 1,5 0,25 37 41

S 31500 18,5 2,7 5 1 0,1 27 29S 32101 1.4162 21 0 1,5 5 0,2 21 24S 32001 20 0,3 1,7 5 0,3 0,15 21 23S 32003 20 1,7 3,5 2 0,15 26 28S 31260 27 3 7 1 0,5 0,16 0,3W 37 39S 39274 25 3 7 1 0,6 0,27 2W 35 39S 32906 1.4362 29 2 6 1 0,4 36 42

PRE = %Cr + 3.3%Mo ; PREN = %Cr + 3.3%Mo + 16%N

300

Standard DUPLEX

(1996)

New DUPLEX

(EX)

Figure 12 : Pitting resistance corrosion of stainless steels.

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some R&D or ongoing work on new grades. Alloying with copper and tungsten are also considered. Those developments may boost the developments of duplex grades in the future. The real huge boost in duplex production is still pending upon the successful industrialisation of thinner gauges! Duplex grades, particularly the lower alloying grades are know to have poor hot workability properties. Production of wide hot coils free of defects remains a technical challenge! In case of success, taking in to account their corrosion resistance properties and alloying costs they have promising future even we can not expect for thin gauges weight savings due to their improved mechanical properties. Typical corrosion resistance properties and PREN ranking are presented figures 12 and 13. Figure 13 : PREN ranking and stainless steels families. DUPLEX STAINLESS STEELS AND MARKET APPLICATIONS.

Figure 14 and 15 : Main duplex production and market applications.(ISSF documents)

PREN = %Cr + 3,3 x %Mo+ 16 x %N

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25

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35

304

316

317LN317LNM

904

6Mo

15 430434

446

409

444

FERRITIC 300 AUST. DUPLEX

2205

2304

2XX

201-202

200 AUST.

Most severeconditions( acids, temperature.)

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2003

2205+

2101

2507

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35

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317LN317LNM

904

6Mo

15 430434

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444

FERRITIC 300 AUST. DUPLEX

2205

2304

2XX

201-202

200 AUST.

Most severeconditions( acids, temperature.)

Standard grades

2003

2205+

2101

2507

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Most of the duplex grades in services are 2205 hot rolled products. In the recent past strong demand and developments concerns cold finished flat products and hot finished long products ( rebars). Typical applications are presented Figures 16 to 19.

Figure 16 York millennium bridge Figure 17 : La grande Arche –La défense - Paris Figure 19 : Chemical tanker. Figure 18 : Arco project Figure 20 : Pollution controll

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CONCLUSIONS The paper has presented the impressive growth of stainless steels grades in the latest decades. An average yearly growth of 5/6 % is observed. Among the stainless steel grades the austenitics and particularly the 304 grades are the most popular ones. They cover more than 80% of the total stainless steel market. In the recent past – since 2000- a huge increase of new austenitic grades has been observed particularly in Asia. Those grades replace partially Ni by combined Mn, N and Cu additions. The real cost savings grades are the ferritic grades since they do not contain Nickel additions but heir uses are restricted to the thinner gages due to welding and toughness aspects. There formability when considering complex shapes is also reduced when compared to austenitic Finaly, duplex grades which still covers nowadays less than 1% of the worldwide stainless markets may presents a very attractive answer taking in to account their lower alloying cost than austenitic and their exceptional combination of corrosion resistance and mechanical properties. Already for the most severe corrosive conditions superduplex grades are the cost saving grades; they are economical substitutions to some Ni grades, 6 Mo grades and even 904 grade. They targeted applications are still marginal when considering the stainless steel markets. Duplex 2205 grade is now a standard which is widely used for the severe corrosion applications like pulp and paper, chemical tankers, desalination plants, petrochemical industry, phosphoric acid production plants, pollution control equipments… energy including sour gas applications. The grades are cost savings when considering the alternative solutions like 904LN grades and even 316L even when weight savings are obtained ( pressure vessels..).Duplex 2205 grades represent nowadays about 85% of the total duplex production. The new coming duplex grades are designed to replace 304L grades. Their Ni and Mo contents are decreased. Further cost savings with partial replacement of Ni by combined Mn and N additions are are even considered! The grades nevertheless are more difficult to manufacture. The targeted markets are huge! This is particularly through when thinner gauges applications are also considered. Technical improvements are still to come. Duplex family is now an industrial success and represents about one percent of the total stainless steel market. The next steps are duplex grades to replace 304 grades and the developments of thinner gauges for new markets. An annual growth of more than 10% is expected. REFERENCES : 1) J.CHARLES and S.BERNHARDSON Duplex Stainless Steels ’91 Conference Beaune, France 2) R.ROBERI and A.TAMBA Duplex conference 2000 Venezia, Italy AIM 3) P.LOVLAND Duplex Stainless Steels 97 The Hague, NL Stainless Steel World 4) J.A.ODRIOZOLA and A.PAUL Stainless Steel ’05 Sevila, Spain 5) D.LEANER Stainless Steels 2003 Maastricht, NL pp115-121 6) Y.YAMADA at all Stainless Steels 2003 Maastricht, NL pp 164-170 7) J.J.DUNN at all all Stainless Steels 2003 Maastricht, NL pp 415-422 8) W.SCUMACKER at all. Stainless Steel USA 2002 Houston USA pp 130-136. 9) C.ERICKSSON at all. Stainless Steel USA 2002 Houston USA pp 423-431. 10) J.W.ENERWAY. Stainless Steel USA 2002 Houston USA pp 92-122. 11) J.W.ENERWAY. Stainless Steels 2001 The Hague, NL pp 245-264. 12) P.JOHANSSON 4th Stainless Steel Steels science and Markets Paris France 2002,pp153-163. 13) L.COUDREUSE 4th Stainless Steel Steels science and Markets Paris France 2002,pp120-125 14) C.ERICSON Stainless Steels America 2004 pp 230-240 15) S.AMcCOY Stainless Steels America 2004 pp 374-379. 16) J.CHARLES Duplex conference 2000 Venezia, Ilaly pp 1-12.