OA12657A - Duplex steel alloy. - Google Patents

Abstract

The present invention relates to a duplex stainless steel alloy, with high resistance to corrosion in combination with good structural stability and hotworkability. The duplex stainless steel has the following composition in percent by weight: C max 0,03% Si max 0,5% Mn 0 - 3,0% Cr 24,0 - 30,0% Ni 4,9 - 10,0% Mo 3,0 - 5,0% N 0,28 - 0,5% B 0 - 0,0030% S max 0,010% W 0 - 3,0% Cu 0 - 2,0% Ru 0 - 0,3% Al 0 - 0,03% Ca 0 - 0,010% Ti 0 - 0,35 % V 0 - 0,55 % balance Fe and normal occurring impurities

Description

012657 1

DUPLEX STEEL ALLOY

TECHNICAL FIELD OF THE INVENTION

The présent invention relates to a stainless Steel alloy, doser determined a duplex stainless Steel alloy with ferritic-austenitic matrix and withhigh résistance to corrosion in combination with good structural stability andhotworkability, particularly a duplex stainless Steel with a content of ferrite of 40-65 volume-% and a well balanced composition, which imparts the materialcorrosion properties, which make it more suitable for use in chloride-containingenvironments than earlier been considered being possible.

BACKGROUND OF THE INVENTION

Over the recent years, the environments in which corrosion résistant metaliic materials were used, became more aggressive, therequirements on the corrosion properties as well as on their mechanicaiproperties increased. Duplex Steel alloys, which were established as analternative to until that used steel grades, as for example high alloyed austeniticsteels, nickel-base alloys or other high alloyed steels, are not exempted fromthis development.

An established measure for the corrosion résistance in chloride-containingenvironments is the so-called Pitting Résistance Equivalent (abbreviated PRE),which is deftned as

PRE=%Cr+3,3%Mo+16%N where the percentages for each element allude to weight-percent. A higher numerical value indicates a better corrosion résistance in particularagainst pitting corrosion. The essential alloying éléments, which affect thisproperty, are according to the formula Cr, Mo, N. An example for such a Steelgrade is évident from EP0220141, which hereby through this reference isincluded in this description. This Steel grade with the denotation SAF2507 (UNSS32750) was mainly alloyed with high contents of Cr, Mo and N. it isconsequentîy developed against this property with above ail good résistance tocorrosion in chloride environments. 012657 2

In recent times also the éléments Cu and W hâve shown to beefficient alloying additions for further optimization of the steel's corrosionproperties in chloride environments. The element W has by then been used assubstitute for a portion of Mo, as for example in the commercial aiioy DP3W 5 (UNS S39274) orZeronlOO, which contain 2,0% respectively 0,7% W. The latercontains even 0,7% Cu with the purpose to increase the corrosion résistance ofthe alloy in acid environments.

The alloying addition of tungsten led to a further development of themeasure for the corrosion résistance and thereby the PRE-formula to the 10 PREW-formula, which also makes the relationship between the influence of Moand W on the alloys corrosion résistance clearer: PREW=%Cr+3,3(%Mo+0,5%W)+16%N,such as described for example in EP 0 545 753. This publication refers to aduplex stainless alloy with generaliy improved corrosion properties. 15 The above-descrïbed described Steel grades hâve a PRE-number, irrespectivemethod of calculation, which lies above 40.

From the alloys with good corrosion résistance in chlorideenvironments also SAF 2906 shall be mentioned, which composition appearsfrom EP 0 708 845. This alloy, which is characterized by higher contents of Cr 20 and N compared to for example SAF25O7, has shown being especially suitablefor use in environments, where résistance to intergranular corrosion andcorrosion in ammonium carbamate is of importance, but it has also a highcorrosion résistance in chloride-containing environments. 25 US-A-4 985 091 describes an alloy intended for use in hydrochloric and sulfuric acid environments, where mainîy intergranular corrosion arises. It isprimarily intended as alternative to recently used austenitic steels. US-A-6 048 413 describes a duplex stainless alloy as alternative toaustenitic stainless steels, intended for use in chloride-containing environments. 30

The disadvantage with the above-described alloys, ail with highPRE-numbers, is the appearance of hard and brittle intermetaliic précipitations 012657 3 in the Steel, as for example sigma phase, especially after heat-treating, such asfor example by welding under later processing. That results in a harder materialwith poorer workability and finafly a deteriorated corrosion résistance.

In order to further improve among others the pitting corrosionrésistance of duplex stainless steels, an increase of the PRE-number in boththe ferrite phase and the austenite phase is required, without for that sakejeopardizing the structural stability or workability of the material. If thecomposition in the two phases is not équivalent with regard to the activealloying components, one phase becomes more sensitive for pitting and crevicecorrosion. Consequently, the phase which is more sensitive to corrosiondecreases the résistance, while the structural stability is decreased by thehighest alloyed phase.

SUMMARY OF THE INVENTION

It is therefore an object of the présent invention to provide a duplexstainless steel alloy, which shows high corrosion résistance in combination withimproved mechanical properties and which is most appropriate for use inenvironments where a high résistance to general corrosion and localizedcorrosion is required, such as for example in chloride-containing environments.

It is another object of the présent invention to provide a duplexstainless steel alloy with a content of ferrite in the range of 40 to 65 volume-%and a PRE-number of at least between 46 and 50 in both the austenite andferrite phase and with an optimum relationship between PRE austenite andPRE ferrite in the range of 0,90 to 1,15; preferablÿ between 0,9 and 1,05.

It is a further object of the présent invention to provide a duplexstainless steel alloy with a Critical Pitting Corrosion Température (henceforthabbreviated CPT) value higherthan 90°C, preferablÿ higher than 95°C and aCritical Crevice-corrosion Température (henceforth abbreviated CCT) value oflowest 50°C in 6% FeCÎ3, preferablÿ at least 60°C in 6% FeCl3.

It is a further object of the présent invention to provide an alloy with impact strength of at least 100 J at room température and an élongation after tensile test of at least 25% at room température. 012657 4

For ils high alloying content, the material according to the présent inventionshows remarkably good workabiiity, in particular hotworkability and shallthereby be very suitable to be used for example the production of bars, tubes,such as welded and seamiess tubes, plate, strip, wire, welding wire,constructive parts, such as for example pumps, valves, flanges and couplings.

These objects are fulfilled according to the présent invention withduplex stainless Steel alloys, which contain (in weight-%) up to 0,03% C, up to0,5% Si, 24,0-30,0% Cr, 4,9-10,0% Ni, 3,0-5,0% Mo, 0,28-0,5% N, 0-3,0% Mn,0-0,0030% B, up to 0,010% S, 0-0,03% Al, 0-0,010% Ca, 0-3,0% W, 0-2,0%

Cu, 0-3,5% Co, 0-0,3% Ru, balance Fe and inévitable impurities.

SHORT DESCRIPTION OF THE DRAWINGS

Figure 1 shows CPT-values from tests of the test heats in the mod'rfied ASTMG48C test in "Green Death'-solution compared with the duplex steels SAF2507,SAF 2906 as weil as the high alloyed austenitic Steel 654SMO.

Figure 2 shows CPT-values attained with the help of the modified ASTM G48Ctest in " Green Death'-solution for the test heats compared with the duplex SteelSAF2507 as well as the austenitic steel 654SM0.

Figure 3 shows the average amount of érosion in mm/year in 2%HCI at atempérature of 75°C.

Figure 4 shows the results from hot ductility testing for most of the heats.

DETAILED DESCRIPTION OF THE INVENTION A systematic development work has surprisingly shown that one bymeans of a well-balanced combination of the éléments Cr, Mo, Ni, N, Mn andCo can obtain optimal dispensation of the éléments in the ferrite and austenite,which enables a very corrosion résistant material with only an insignificantamount of sigma phase in the material. The material obtains also goodworkabiiity, which enables extruding to seamiess tubes. It shows that with theintention to obtain a combination of high corrosion résistance in connection withgood structural stability a much narrow combination of the alloying éléments in 012657 5 the material is required. The alloy according to the invention contains (in weight%): C Max 0,03% Si Max 0,5% Mn 0 - 3,0% Cr 24,0 - 30,0% Ni 4,9-10,0% Mo 3,0 - 5,0% N 0,28 - 0,5% B 0-0,0030% S max 0,010% Co 0-3,5% W 0-3,0% Cu 0-2,0% Ru 0-0,3% Al 0-0,03% Ca 0-0,010% balance Fe and normal occurring impurities and additions, whereby the contentof ferrite is 40-65 volume-%.

Carbon (C) has limited solubility in both ferrite and austenite. The limitedsolubility implies a risk of précipitation of chromium carbides and the contentshould therefore not exceed 0,03 weight-%, preferably not exceed 0,02 weight-%.

Silicon (Si) is utilized as desoxidation agent in the steel production as well as itincreases the flowability during production and welding. However, too highcontents of Si lead to précipitation of unwanted intermetallic phase, whereforethe content is limited to max 0,5 weight-%, preferabiy max 0,3 weight-%. 012657 6

Manganèse (Μη) is added in order to increase the N-solubility in the material.However, it has shown that Mn oniy has a limited influence on the N-solubility inthe type of alloy in question, instead there are found other éléments with higherinfluence on the solubility. Besides, Mn in combination with high contents of 5 sulfur can give rise to formation of manganèse sulfides, which act as initiation-points for pitting corrosion. The content of Mn should therefore be limited tobetween 0-3,0 weight-%, preferably 0,5-1,2 weight-%.

Chromium (Cr) is a much active element in order to improve the résistance to a10 majority of corrosion types. Furthermore, a high content of chromium implies that one gets a very good N-solubility in the material. Thus, it is désirable tokeep the Cr-content as high as possible in order to improve the corrosionrésistance. For very good amounts of corrosion résistance the content ofchromium should be at least 24,0 weight-%, preferably 27,0 -29,0 weight-%. 15 However, high contents of Cr increase the risk for intermetallic précipitations, for what reason the content of chromium must be limited up to max 30,0 weight-%.

Nickel (Ni) is used as austenite stabilizing element and is added in suitablecontents in order to obtain the desired content of ferrite. In order to obtain the 20 desired relationship between the austenitic and the ferritic phase with between40-65 volume-% ferrite, an addition of between 4,9-10,0 weight-% nickel,preferably 4,9-8,0 weight-%, is required.

Molybdenum (Mo) is an active element which improves the résistance to 25 corrosion in chloride environments as well as preferably in reducing acids. A toohigh Mo-content in combination with that the Cr-contents are high, implies thatthe risk for intermetallic précipitations increases. The Mo-content in the présentinvention should lie in the range of 3,0-5,0 weight-%, preferably 3,6-4,7 weight-%, in particular 4,0-4,3 weight-%. 30 '

Nitrogen (N) is a very active element, which increases the corrosion résistance,the structura! stability as well as the strength of the material. Further, a high N- 012657 7 content improves the recovering of the austenite after welding, which givesgood properties within the welded joint. In order to obtain a good effect of N, atleast 0,28 weight-% N should be added. At high contents of N the risk forprécipitation of chromium nitrides increases, especially when simultaneousiy thechromium content is high. Further, a high N-content implies that the risk forporosity increases because of the exceeded solubility of N in the smelt Forthese reasons the N-content should be limlted to max 0,5 weight-%, preferably>0,35 - 0,45 weight-% N is added.

Boron (B) is added in order to increase the hotworkabiiity of the material. At atoo high content of Boron the weldability as well as the corrosion résistancecould deteriorate. Therefore, the content of boron should be limited to 0,0030weight-%.

Sulfur (S) influences the corrosion résistance negatively by forming solublesulfides. Further, the hot workability detorlates, for what reason the content ofsulfur is limited to max 0,010 weight-%.

Cobalt (Co) is added in order to improve foremost the structural stability as wellas the corrosion résistance. Co is an austenite-stabilizing element. In order toobtain effect should at least 0,5 weight-%, preferably at least 1,5 weight-% beadded. Because cobalt is a relatively expensive element, the addition of cobaltis therefor limited to max 3,5 weight-%.

Tunqsten increases the résistance to pitting- and crevice corrosion. But theaddition of too high contents of tungsten in combination with that the Cr-contents as well as Mo-contents are high, means that the risk for intermetallicprécipitations increases. The W-content in the présent invention should lie in therange of 0-3,0 weight-%, preferably between 0,5 and 1,8 weight-%.

Copper is added in order to improve the general corrosion résistance in acidenvironments such as sulfuric acid. At the same time Cu influences the 012657 8 structurai stability. However, high contents of Cu imply that the soiid solubilitywill be exceeded. Thereforthe Cu-content should be iimited to max 2,0 weight-%, preferabîy between 0,5 and 1,5 weight-%.

Ruthénium (Ru) is added in order to increase the corrosion résistance. Becauseruthénium is a very expensive element, the content should be iimited to max 0,3weight-%, preferabîy more than 0 and up to 0,1 weight-%.

Aluminum (Al) and Calcium (Ca) are used as desoxidation agents at the Steelproduction. The content of Ai should be iimited to max 0,03 weight-% in order toOmit the forming of nitrides. Ca has a favorable effect on the hotductility.However, the Ca-content should be Iimited to 0,010 weight-% in order to avoidan unwanted amount of slag.

The content of ferrite is important in order to obtain good mechanical propertiesand corrosion properties as well as good weidability. From a corrosion point ofview and a point of view of weidability a content of ferrite between 40-65% isdésirable in order to obtain good properties. Further, high contents of ferriteimply that the impact strength at low températures as weil as the résistance tohydrogen-induced brittieness risks deteriorating. The content of ferrite istherefore 40-65 volume-%, preferabîy 42-60 volume-%, in particular 45-55volume-%.

DESCRIPTION OF PREFERED EMBODIEMENTS

In the examples below the composition of a number of test heats is presented, which iliustrate the effect of different aiioying éléments on theproperties. Heat 605182 represents a référencé composition and isconsequently not a part of the field of this invention. Neither shall the remainingheats be considered limiting the invention, without only specifying examples ofheats, which iliustrate the invention accordîng to the daims.

The specified PRE-numbers or -values consider always amounts calculatedaccordîng to the PREW-formula, even though this is not explicitly mentioned. 012657 EXAMPLE 1

The test heats according to this example were produced by castingof 170kg ingots in the laboratory, which were hotforged to round bars. Those 5 were hotextruded to bars (round bars as well as fiat bars), where test materialwas taken out from the round bars. Further on the fiat bars were annealedbefore cold rolling took place, whereafter further test material was taken out.From a material engineering point of view, the process can be considered to bereprésentative for the préparation in bigger scale, for example for the production 10 of seamless tubes by the extrusion method, foilowed by cold rolling. Table 1shows the composition of the first batch of test heats.

Table 1. Composition for test heats, weîght-%.

Heat Mn Cr Ni Mo W Co V La Ti N 605193 1,03 27,90 8,80 4,00 0,01 0,02 0,04 0,01 0,01 0,36 605195 0,97 27,90 9,80 4,00 0,01 0,97 0,55 0,01 0,35 0,48 605197 1,07 28,40 8,00 4,00 1,00 1,01 0,04 0,01 0,01 0,44 605178 0,91 27,94 7,26 4,01 0,99 0,10 0,07 0,01 0,03 0,44 605183 1,02 28,71 6,49 4,03 0,01 1,00 0,04 0,01 0,04 0,28 605184 0,99 28,09 7,83 4,01 0,01 0,03 0,54 0,01 0,01 0,44 605187 2,94 27,74 4,93 3,98 0,01 0,98 0,06 0,01 0,01 0,44 605153 2,78 27,85 6,93 4,03 1,01 0,02 0,06 0,02 0,01 0,34 605182 0,17 23,48 7,88 5,75 0,01 0,05 0,04 0,01 0,10 0,26 15 in purpose to investigate the structural stability samples from everyheat were annealed at 900-1150°C with steps of 50°C as well as they werequenched in air, respective water. At the lowest températures intermetaliicphase was formed. The lowest température, where the amount of intermetaliic 20 phase became insignificant, was determined with the help of studies in lightoptical microscope. New samples from respective heat were annealedafterwards at said température under five minutes, whereafter the samples were'»cooled down with the constant cooling rate of -140°c/min to room température.Subsequently, the area fraction of sigma phase in the matériels was determined 25 with digital scanning of the pictures with back-scattering électrons in a scanningélectron microscope. The results appearfrom Table 2. 012657 10

Tmax sigma was calculated with Thermo-Cale (TC version Nthermodynamic database for steel TCFE99) based on characteristic amountsfor ail specified éléments in the different variations. Tmax sigma is the dissolvingtempérature for the sigma phase, where high dissolving températures indicatelower structural stability.

Table 2.

Heat Heat treatment Amount σ [vol-%] Tmaxa 605193 1100°C, 5min 7,5% 1016 605195 1150’C, 5min 32% 1047 605197 1100°C, 5min 18% 1061 605178 1100’C, 5min 14% 1038 605183 1050’C, 5min 0,4% 997 605184 1100’C, 5min 0,4% 999 605187 1050’C, 5min 0,3% 962 605153 1100’C, 5min 3,5% 1032 605182 1100’C, 5min 2,0% 1028

The purpose of this investigation is to be able to rank the materialwith regard to the structural stability, i.e. this is not the real content of sigmaphase in the samples, which were heat treated and quenched before forexample the corrosion testing. One can see thatTmax sigma, which wascalculated with Thermo-Cale does not directly coïncide with the measuredamounts of sigma phase, however it is distinct that the test heats with thelowest calculated Tmax sigma contain the lowest amount sigma phase duringthis investigation.

The pitting corrosion properties of ail heats were tested for rankingin the so-cailed "Green Death"-solution, which consiste of 1%FeCI3, 1%CuCt2,11% H2SO4, 1,2% HCl. The test procedure is équivalent to the pitting corrosiontesting according to ASTM G48C, however, it will be carried out in the moreaggressive "Green Death'-solution. Further, some of the heats were testedaccording to ASTMG48C (2 tests per heat). Also the electrochemical testing in3%NaCI (6 tests per heat) was carried out. The results in form of the CriticalPitting Température (CPT) from ail tests appear from Table 3, such as thePREW-number (Cr+3,3(Mo+0,5W)+16N) for the total composition of the alloy as 012657 11 well as for austenite and ferrite. The indexing alpha refers to the ferrite andgamma refers to the austenite.

Table 3.

Heat PRE a PRE y PRE y/PRE a PRE CPT°CModifiedASTM G48CGreen death CPT °C ASTM G48 C6% FeCI3 CPT °C 3% NaCI 605193 51,3 49,0 0,9552 46,9 90/90 64 605195 51,5 48,9 0,9495 48,7 90/90 95 605197 53,3 53,7 1,0075 50,3 90/90 >95 >95 605178 50,7 52,5 1,0355 49,8 75/80 94 605183 48,9 48,9 1,0000 46,5 85/85 90 93 605184 48,9 51,7 1,0573 48,3 80/80 72 605187 48,0 54,4 1,1333 48,0 70/75 77 605182 54,4 46,2 0,8493 46,6 75/70 85 62 654SMO 90/85 SAF2507 70/70 SÀF2609 60/50

It is established that there exists a linear ratio between the lowestPRE-number in the austenite or ferrite and the CPT-value in the duplex steel,but the results in Table 3 show that the PRE-number not solely explains the 10 CPT-values. In Figure 1 the CPT-vaiues from test in the modified ASTM G48Ctest are shown diagrammatically. The duplex steels SAF2507, SAF2906 as wellas the high alloyed austenitic steel 654SMO are included as reference. It isdistinct from these results that ail test materials show better CPT in the modifiedASTM G48C than SAF2507 as well as SAF2906. Furthermore some of thè test 15 materials show CPT results in the modified ASTM G48C at the same level as orin excess of 654SMO. The test heat 605183, alloyed with cobalt shows goodstructural stability at a controlled cooling rate of (-140°C/min) in spite that itcontains high contents of chromium as wel! as of molybdenum, shows betterresults than SAF2507 and SAF2906. It appears from this investigation that a 20 high PRE does not solely explain the CPT values, without the relationship PREaustenit/PRE ferrite is of extreme weight for the properties of the higher alloyedduplex steels, and a very narrow and exact leveling between the alloyingéléments is required in order to obtain this optimum ratio, which lies between0,9-1,15; preferably 0,9-1,05 and simultaneously obtain PRE values of above 012657 12 46. The relationship PRE austenit/PRE ferrite against CPT in the modifiedASTM G48C test for the test heats is given in Table 3.

The strength at room température (RT), 100°C and 200°C and the 5 impact strength at room température (RT) hâve been determined for ail heatsand is shown as average amount for three tests.

Tensile test specimen (DR-5C50) were manufactured fromextruded bars, 0 20mm, which were heattreated at températures according toTable 2 in 20 minutes followed by cooling down in either air or water (605195, 10 605197,605184). The results of the tests are presented in Table 4 and 5. The results of the tensile test show that the contents of chromium, nitrogen andtungsten strongly influence the impact strength of the material. Besides 605153,ail heats fulfill the requirement of a 25% élongation at tensile testing at roomtempérature (RT).

Table 4. Impact strength

Heat Température Rp0.2 Rpi.o Rm A5 Z (MPa) (MPa) (MPa) (%) (%) 605193 RT 652 791 916 29,7 38 100°C 513 646 818 30,4 36 200°C 511 583 756 29,8 36 605195 RT 671 773 910 38,0 66 100°C 563 637 825 39,3 68 200°C 504 563 769 38,1 64 605197 RT 701 799 939 38,4 66 100°C 564 652 844 40,7 69 200°C 502 577 802 35,0 65 605178 RT 712 828 925 27,0 37 100°C 596 677 829 31,9 45 200°C 535 608 763 27,1 36 605183 RT 677 775 882 32,4 67 100°C 560 642 788 33,0 59 200°C 499 578 737 29,9 52 605184 RT 702 793 915 32,5 60 100°C 569 657 821 34,5 61 200°C 526 581 774 31,6 56 605187 RT 679 777 893 35,7 61 100°C 513 628 799 38,9 64 200°C 505 558 743 35,8 58 605153 RT 715 845 917 20,7 24 100°C 572 692 817 29,3 27 012657 13 200°C 532 611 749 23,7 31 605182 RT 627 754 903 28,4 43 100°C 493 621 802 31,8 42

Table 5. impact Strength

Heat Annealirtg [°C/min] Cooling impact strength [43 Anneaiing [°C/min] Cooling Impact strength [JJ 605193 1100/20 Air 35 1100/20 Water 242 605195 1150/20 Water 223 605197 1100/20 Water 254 1130/20 Water 259 605178 1100/20 Air 62 1100/20 Water 234 605183 1050/20 Air 79 1050/20 Water 244 605184 1100/20 Water 81 1100/20 Air 78 605187 1050/20 Air 51 1100/20 Water 95 605153 1100/20 Air 50 1100/20 Water 246 605182 1100/20 Air 22 1100/20 Water 324

Table 6. 5

This investigation shows very distinct that water quenching is certainlynecessary in order to obtain the best structure and consequently good valuesfor the impact strength. The requirement is 100J at test at room températureand ail heats pass this, except heat 605184 and 605187, where certainly the 10 latter lies very near the requirement.

Table 6 shows the results from the Tungsten-lnert-Gas remeltingtest (henceforth-abbreviated TIG), where the heats 605193,605183,605184 aswell as 605253 show a good structure in the heat affected zone (Heat Affected 15 Zone, henceforth-abbreviated HA2). The Ti- containing heats show Tin in HAZ.A too high chromium- and nitrogen content results in précipitation of Cr2 N,which shall be avoided because it detoriates the properties of the materiai.

Heat Précipitations Protective gas Ar (99,99%) 605193 HAZ: OK 605195 HAZ: Large amounts of TIN and σ-phase 605197 HAZ: Smail amounts of Cr2N in δ-grains, but not much 605178 HAZ: Cr2N in δ-grains, otherwise OK 012657 14 605183 HAZ: OK 605184 HAZ: OK 605187 HAZ: Cr2N quite near the meltingbond,no précipitations farther out 605153 HAZ: OK 605182 HAZ: TiN and decorated grainboundaries δ/δ EXAMPLE 2

In the below-mentioned example the composition of a furthernumber of test heats produced with the purpose to find the optimum 5 composition îs given. These heats are modified starting out from the propertiesof the heats with good structural stability as well as high corrosion résistance,from the results, which were shown in example 1. AH heats in Table 7 areincluded in the composition according to the présent invention, where heats 1-8are included into a statistical test model, while the heats e to n are additiona! 10 test alloys within the scope of this invention. A number of test heats were produced by casting of 270kg ingots, which were hotforged to round bars. Those were extruded to bars, wherefromtest samples were taken. Afterwards the bar was annealed before cold rolling tofiat bars was executed, after that further test material was taken out. Table 7 15 shows the composition for these test heats.

Table 7.

Heat Mn Cr Ni Mo W Co Cu Ru B N 1 605258 1,1 29,0 6,5 4,23 1,5 0,0018 0,46 2 605249 1,0 28,8 7,0 4,23 1,5 0,0026 0,38 3 605259 1.1 29,0 6,6 4,23 0,6 0,0019 0,45 4 605260 1,1 27,5 5,9 4,22 1,5 0,0020 0,44 5 605250 1,1 28,8 7,6 4,24 0,6 0,0019 0,40 6 605251 1,0 28,1 6,5 4,24 1,5 0,0021 0,38 7 605261 1,0 27,8 6,1 4,22 0,6 0,0021 0,43 8 605252 1,1 28,4 6,9 4,23 0,5 0,0018 0,37 e 605254 1.1 26,9 6,5 4,8 1,° 0,0021 0,38 f 605255 1,0 28,6 6,5 4,0 3,0 0,0020 0,31 S 605262 2,7 27,6 6,9 3,9 1,0 1.0 0,0019 0.36 h 605263 1,0 28,7 6,6 4,0 1,0 1,0 0,0020 0,40 in 605253 1,0 28,8 7,0 4,16 1,5 0,0019 0,37 J 605266 1.1 30,0 7,1 4,02 0,0018 0,38 k 605269 1,0 28,5 7,0 3,97 1,0 1,0 0,0020 0,45 I 605268 1,1 28,2 6,6 4,0 1.0 1,0 1,0 0,0021 0,43 m 605270 1,0 28,8 7,0 4,2 1,5 0,1 0,0021 0,41 n 605267 1,1 29,3 6.5 4,23 i,5 0,0019 0,38 012657 15

Table 8. Thermo-Cale

Variant a-formula empirical a T-C PRE total PRE a PREy Tmax sigma Tmax Cr2N 1 46 50 50,2 47,8 50,5 1006 1123 2 52 50 49,1 48,4 49,8 1019 1084 3 45 50 50,2 47,9 52,6 1007 1097 4 46 50 49,2 46,5 49,8 986 1121 5 47 50 49,1 48,5 49,7 1028 1038 6 52 50 48,1 47,1 49,2 998 1086 7 44 50 49,2 46,6 52,0 985 1081 8 46 50 48,1 47,2 49,1 1008 1044 e 46 53 49,3 48,4 49,5 1010 1099 f 65 52 46,7 47,2 46,1 1008 1090 g......... 48 51 48,4 48,4 48,3 1039 979 h 50 53 50,0 48,4 51,7 1035 1087 i 52 50 49,1 48,4 49,8 1019 1084 5

Thermo-Calc-values according to Table 8 (T-C version Nthermodynamîc database for Steel TCFE99) are based on characteristicamounts for ail specified éléments in the different variations. The PRE-numberfor the ferrite and austenite is based on their equilibrium composition at 1100°C. 10 Tmax sigma is the dissolving température for the sigma phase, where highdissolving températures indicate lower structural stability.

The distribution of the alloying éléments in the ferrite- and austenitephase was examined with microprobe analysis, the results appear from Table 9. 15 Table 9.

Heat Phase Cr Mn Ni Mo W Co Cu N 605258 Ferrite 29,8 1,3 4,8 5,0 1,4 0,11 Austenite 28,3 1,4 7,3 3,4 1,5 0,60 605249 Ferrite 29,8 1,1 5,4 5,1 1,3 0,10 Austenite 27,3 1,2 7.9 3,3 1,6 0,53 605259 Ferrite 29,7 1,3 5,3 5,3 0,5 0,10 Austenite 28,1 1.4 7,8 3,3 0,58 0,59 605260 Ferrite 28,4 1,3 4,4 5,0 1,4 0,08 Austenite 26,5 1,4 6,3 3,6 1,5 0,54 605250 Ferrite 30,1 1,3 5,6 5,1 0,46 0,07 Austenite 27,3 1,4 8,8 3,4 0,53 0,52 605251 Ferrite 29,6 1,2 5,0 5,2 1,3 0,08 Austenite 26,9 1,3 7,6 3,5 1.5 0,53 01265? 16 605261 Ferrite 28,0 1,2 4,5 4,9 0,45 0,07 Austenite 26,5 1,4 6,9 3,3 0,56 0,56 605252 Ferrite 29,6 1,3 5,3 5,2 0,42 0,09 Austenite 27,1 1,4 8,2 3,3 0,51 0,48 605254 Ferrite 28,1 1,3 4,9 5,8 0,89 0,08 Austenite 26,0 1,4 7,6 3,8 1,0 0,48 605255 Ferrite 30,1 1,3 5,0 4,7 2,7 0,08 Austenite 27,0 1,3 7,7 3,0 3,3 0,45 605262 Ferrite 28,8 3,0 5,3 4,8 1,4 0,9 0,08 Austenite 26,3 3,2 8,1 3,0 0,85 1,1 0,46 605263 Ferrite 29,7 1,3 5,1 5,1 1,3 0,91 0,07 Austenite 27,8 1,4 7,7 3,2 0,79 1,1 0,51 605253 Ferrite 30,2 1,3 5,4 5,0 1,3 0,09 Austenite 27,5 1,4 8,4 3,1 1,5 0,48 605266 Ferrite 31,0 1,4 5,7 4,8 0,09 Austenite 29,0 1,5 8,4 3,1 0,52 605269 Ferrite 28,7 1.3 5,2 5,1 1,4 0,9 0,11 Austenite 26,6 1,4 7,8 3,2 0,87 1.1 0,52 605268 Fem'te 29,1 1,3 5,0 4,7 1,3 0,91 0,84 0,12 Austenite 26,7 1,4 7,5 3,2 0,97 1,0 1,2 0,51 605270 Ferrite 30,2 1,2 5,3 5,0 1,3 0,11 Austenite 27,7 1,3 8,0 3,2 1,4 0,47 605267 Ferrite 30,1 1.3 5,1 4,9 1,3 0,08 Austenite 27,8 1,4 7,6 3,1 1.8 0,46

The pitting corrosion properties of ail heats hâve been tested in the"Green Death"- solution (1%FeCI3,1%CuCI2,11% H2SO4,1,2% HCl) for 5 ranking. The test procedures are the same as pitting corrosion testing accordingto ASTM G48C, but the testing will be executed in a more aggressive solutionthan 6%FeCI3, the so-ca!led "Green Death'-solution. Aiso the general corrosiontesting in 2%HC! (2 tests per heat) was executed for ranking before thedewpoint testing. The results from ail tests appearfrom Table 10, Figure 2 and 10 Figure 3. Ali tested heats pêrform better than SAF2507 in “Green Death’- solution. Ail heats lie within the identifîed range of 0,9-1,15; preferably 0,9-1,05applicable for the ratio PRE austenit/PRE ferrite at the same time as PRE inboth austenite and ferrite is in excess of 44 and for most of the heats evenconsidérable in excess of 44. Some of the heats attain even the limit of total 15 PRE 50. It is very interesting to note that heat 605251, alloyed with 1,5 weight-% cobalt, performs almost équivalent with heat 605250, alloyed with 0,6 weight-% cobalt, in "Green Death"-solution in spite of the lower chromium content inheat 605251. It is particularly surprising and interesting because heat 605251 012657 17 has a PRE-number of ca. 48, which is in excess of some of today’s commercialsuperduplex alloys simultaneously as the Tmax sigma-value below 1010°Cindicates a good structural stability based on the values in Table 2 in Example 1. 5 In Table 10 even PREW-number (%Cr+3,3%(Mo+0,5%W)+16%N) for the totalcomposition of the alloy and PRE in austenite as well as in the ferrite (roundedoff) based on composition of the phases are specified as measured with microprobe. Content of ferrite was measured after heat-treating at 1100°C followedby water quenching. 0

Table 10

Heat a-halt PREW Total PRE a PRE Y PREy/ PREa CPT°C Green death 605258 48,2 50,3 48,1 49,1 1,021 605249 59,8 48,9 48,3 46,6 0,967 75/80 605259 49,2 50,2 48,8 48,4 0,991 605260 53,4 48,5 46,1 47,0 1,019 605250 53,6 49,2 48,1 46,8 0,974 95/80 605251 54,2 48,2 48,1 46,9 0,976 90/80 605261 50,8 48,6 45,2 46,3 1,024 605252 56,6 48,2 48,2 45,6 0,946 80/75 605254 53,2 48,8 48,5 46,2 0,953 90/75 605255 57,4 46,9 46,9 44,1 0,940 90/80 605262 57,2 47,9 48,3 45,0 0,931 605263 53,6 49,7 49,8 47,8 0,959 605253 52,6 48,4 48,2 45,4 0,942 85/75 605266 62,6 49,4 48,3 47,6 0,986 605269 52,8 50,5 49,6 46,9 0,945 605268 52,0 49,9 48,7 47,0 0,965 605270 . 57,0 49,2 48,5 45,7 0,944 605267 59,8 49,3 47,6 45,4 0,953

In order to examine the structural stability in detail the sampleswere annealed for 20 minutes at 1080°C, 1100°C and 1150°C, whereafter they 15 were quenched in water. The température, where the amount of intermetallicphase became insignificant was determined with help of investigations in alightoptical microscope. A comparison of the structure of the heats afterannealing at 1080°C followed by water quenching indicates which of the heatsare more suspect to contain undesired sigma phase. The results are shown in 20 Table 11. Control of the structure shows that the heats 605249, 605251, 012657 18 605252, 605253, 605254, 605255, 605259, 605260, 605266 as well as 605267are free from unwanted sigma phase. Moreover, heat 605249, alioyed with 1,5weight-% cobalt, is free from sigma phase, while heat 605250, alioyed with 0,6weight-% cobalt, contains a very small amount of sigma phase. Both heats are 5 ailoyed with high contents of chromium, approximateiy 29,0 weight-% and themolybdenum content of approximateiy 4,25 weight-%. If one compares thecompositions of the heats 605249, 605250, 605251 and 605252 with thought onthe content of sigma phase, it is very distinct that the range of composition forthat optimum material is very narrow, in this case with regard to the structural 10 stability. It further shows that the heat 605268 contains only sigma phasecompared to heat 605263, which contains much sigma phase. What mainlydistinguishes these heats from each other is the addition of copper to heat605268. Heat 605266 and also 605267 are free from sigma phase, despite of ahigh content of chromium the later heat is alioyed with copper. Further, the 15 heats 605262 and 605263 with addition of 1,0 weight-% tungsten show astructure with much sigma phase, while it is interesting to note that heat605269, also with 1,0 weight-% tungsten but with higher content of nitrogenthan 605262 and 605263 shows a considérable smaller amount of sigmaphase. Consequently, a very well-leveled balance between the different alloying 20 éléments at these high alloying contents is required of for example chromium and molybdenum in order to obtain good structural properties.

Table 11 shows the results from the light optical examination after annealing at1080°C, 20min followed by water quenching. The amount of sigma phase isspecified with values from 1 to 5, where 1 représenta that no sigma phase was 25 detected in the examination, while 5 represents that a very high content ofsigma phase was detected in the examination.

Table 11.

Heat Sigma phase Cr Mo W Co Cu N Ru 605249 1 28,8 4,23 1,5 0,38 605250 2 28,8 4,24 0,6 0,40 605251 1 28,1 4,24 1,5 0,38 012657 19 605252 1 28,4 4,23 0,5 0,37 605253 1 28,8 4,16 1,5 0,37 605254 1 26,9 4,80 1,0 0,38 605255 1 28,6 4,04 3,0 0,31 605258 2 29,0 4,23 1,5 0.46 605259 1 29,0 4,23 0,6 0,45 605260 1 27,5 4,22 1.5 0,44 605261 2 27,8 4,22 0,6 0,43 605262 4 27,6 3,93 1,0 1,0 0,36 605263 5 28,7 3,96 1,0 1,0 0,40 605266 1 30,0 4,02 0,38 605267 1 29,3 4,23 1,5 0,38 605268 2 28,2 3,98 1,0 1,0 1,0 0,43 605269 3 28,5 3,97 1,0 1,0 0,45 605270 3 28,8 4,19 1,5 0,41 0,1

In Table 12 the results from the impact strength testing of some ofthe heats are shown. The results are very good, which indicates a goodstructure after annealing at 1100°C followed by water quenching and the 5 requirement of 100J will be managed with large margin of ali tested heats.

Table 12.

Heat Annealing [’C/min] Quenching Impact strength M Impact strength U] Impact strength [J] 605249 1100/20 Water >300 >300 >300 605250 1100/20 Water >300 >300 >300 605251 1100/20 Water >300 >300 >300 605252 1100/20 Water >300 >300 >300 605253 1100/20 Water 258 267 257 605254 1100/20 Water >300 >300 >300 605255 1100/20 Water >300 >300 >300 10

Figure 4 shows the results from the hot ductility testing of the mostof the heats. A good workability is of course of vital importance in order to beable to produce the material to product forms such as bars, tubes, such aswelded and seamless tubes, plate, strip, wire, welding wire, constructive 15 éléments, such as for example pumps, valves, flanges and couplings. Theheats 605249,605250, 605251,605252, 605255, 605266 as well as 605267, 012657 the most with nitrogen content around 0,38 weight-% show somewhat improved hot ductility values.

Summary of the test results 5 In order to obtain good corrosion properties, simultaneously as the material shows good structural stability, hotworkability and weldability thematériel should be optimized according to the foliowing: • PRE-number in ferrite should exceed 45, but preferably be at ieast 47. • PRE-number in austenit should exceed 45, but preferably be at Ieast 47. 10 · PRE-number for the entire alloy should preferably be at Ieast 46. • Relationship PRE austenit/PRE ferrite should lie in the range of 0,9-1,15;preferably in the range 0,9-1,05. • The content of ferrite should lie in the range preferably 45-55 volume-%. • Tmax sigma should not exceed 1010°C. 15 · The content of nitrogen should lie in the range 0,28-0,5 weight-%, preferably in the range 0,35-0,48 weight-%, but preferably 0,38-0,40weight-%. • The content of cobalt should lie in the range 0-3,5 weight-%, preferably1,0-2,0 weight-%, but preferably 1,3-1,7 weight-%. 20 · In order to ensure the high nitrogen solubility, Le. if the content of nitrogen is in the range 0,38-0,40 weight-% should at Ieast 29 weight-% Cr beadded as well as at Ieast 3,0 weight-% Mo, thus the total content of theéléments Cr, Mo and N fulfills said requirements on the PRE-number. 25 30

Claims (16)

1. 012657 21 CLAIMS:

   1. Ferritic-austeniticduplexstainlessSteelalloy, characterized in, that it shows the following composition, in weight-%: C max 0,03% Si max 0,5% Mn 0 - 3,0% Cr 24,0 - 30,0% Ni 4,9 -10,0% Mo 3,0-5,0% N 0,28 - 0,5% B 0-0,0030% S max 0,010% Co 0-3,5% W 0-3,0% Cu 0-2,0% Ru 0-0,3% Al 0-0,03% Ca 0-0,010% balance Fe and normal occurring impurities and additions, whereby the contentof ferrite is 40-65 volume-%.
2. 2. Alloy according to claim l.characterized in, that thecontent of manganèse lies between 0,5 and 1,2 weight-%.
3. 3. Alloy according to claim 1 or2, characterized in, that thecontent of chromium lies between 27,0 and 29,0 weight-%.
4. 4. Alloy according to claim 1-3,characterized in, that thecontent of nickel lies between 5,0 and 8,0 weight-%. 012657 22
5. 5. Alloy according to claim 1-4, characterized in, that thecontent of molybdenum lies between 3,6 and 4,7 weight-%.
6. 6. Alloy according to claim 1-5, c h a r a c t e r i z e d in, that thecontent of nitrogen lies between 0,35 and 0,45 weight-%.
7. 7. Alloy according to claim 1-5, c h a r a c t e r i z e d in, that thecontent of ruthénium lies between 0 and 0,3 weight-%, preferably higher than 0and up to 0,1 weight-%.
8. 8. Alloy according to any of antécédent claim, characterizedin, that the content of cobalt lies between 0,5 and 3,5 weight-%, preferablybetween 1,5 and 3,5 weight-%.
9. 9. Alloy according to any of preceding daims, characterizedin, that the content of copper lies between 0,5 and 2,0 weight-%, preferablybetween 1,0 and 1,5 weight-%.
10. 10. Alloy according to any of preceding daims, characterizedin, that the content of ferrite lies between 42 and 60 volume-%, preferablybetween 45 and 55 volume-%.
11. 11. Alloy according to any of preceding daims, characterizedin, that the total PRE- or PREW-value of the alloy exceeds 44, whereby PRE -%Cr + 3,3%Mo + 16N and PREW = %Cr + 3,3(%Mo + 0,5%W) + 16N, wherein% considère weight-%.
12. 12. Alloy according to any of preceding daims, characterizedin, that the PRE- or PREW-value for both ferrite- and austenite phase is higherthan 45 and PRE or PREW-value for the total composition of the alloy is higherthan 46. 012657 23
13. 13. Ailoy according to claim 11 or 12, c h a r a et e r i z e d in, thatthe PRE- or PREW-value for both ferrite- and austenite phase lies between 47 and49.
14. 14. Ailoy according to claim 11, 12 or 13, c h a r a et e r i z e d in, that the ratio between PRE(W)-value for austenite phase and PRE(W)-value fortheferrite phase lies between 0,90 and 1,15; preferably between 0,9 and 1,05.
15. 15. Ailoy according to any of preceding daims for use in chioride- 10 containing environments.
16. 16. Ailoy according to any of preceding daims for use in chloride-containing environments in productforms such as bars, tubes, such as weidedand seamiess tubes, plate, strip, wire, weiding wire, constructive parts, such as 15 for exemple pumps, valves, flanges and couplings.