AU2005301376A1

AU2005301376A1 - Duplex stainless steel

Info

Publication number: AU2005301376A1
Application number: AU2005301376A
Authority: AU
Inventors: Karin Jakobsson; Pasi Kangas
Original assignee: Sandvik Intellectual Property AB
Current assignee: Sandvik Intellectual Property AB
Priority date: 2004-11-04
Filing date: 2005-11-04
Publication date: 2006-05-11
Anticipated expiration: 2025-11-04
Also published as: NO20072275L; EP1812614A4; JP2008519165A; KR20070073870A; AU2005301376B2; CA2586452A1; EP1812614A1; WO2006049572A1; CN101057002A; SE0402698D0; SE0402698L; US20080138232A1; SE528782C2; NO341532B1

Description

WO 2006/049572 PCT/SE2005/001661 Applicant: Sandvik Intellectual Property AB 5 Duplex Stainless Steel TECHNICAL FIELD OF THE INVENTION AND STATE OF THE ART 10 The present invention concerns to a duplex stainless steel alloy having a high Cr-, Mo- and N- content and a ferrite content of 30-70%. 15 Duplex stainless steels are characterised by an austenite-ferrite structure where both phases have different chemical compositions. They are attractive as structural materials where both high mechanical strength and excellent resistance to corrosion are required. Duplex stainless steels are often used as 20 alternatives to austenitic stainless steels and nickel-based alloys due to their lower cost, which is a consequence of the lower nickel content in duplex stainless steels. Duplex stainless steels are extensively used in the onshore and 25 offshore sectors of the oil and gas industry due to their corrosive resistance to the various corrosive media, such as CO2, H 2 S and chlorides, found in such onshore/offshore environments. Umbilical pipes, or "umbilicals", that interconnect units on the land or sea surface with units at the bottom of the sea to 30 transport substances therebetween, such as to crude oil and gas from a source to an oil rig, are often made of duplex stainless steel pipes that are welded together. Downhole tubes, which are grooved tubes that are generally installed within a drill-hole, and integrated production tubes (IPUs), which are composite tubes 35 comprising umbilicals and downhole tubes, are also often made of duplex steel.

WO 2006/049572 PCT/SE2005/001661 2 A downhole tube has to be resistant both to corrosion in the sea water that surrounds it and to corrosion in the substances that it transports. Downhole tubes are supplied in threaded finish and joined to the necessary lengths by means of couplings. Since oil 5 and gas wells are situated at a considerable depth below sea level the length of a downhole tube can be quite considerable. The demands on the material that is used for downhole tubes can be summarised as follows: 10 * Yield point in tension; minimum 110 ksi (kilos per square inch) (760 MPa) * Resistance to corrosion caused by CO 2 or H 2 S. * Good impact strength down to -460C, at least 50J. * The material has to be capable of being manufactured in 15 the shape of seamless tubes and in forms in which threads and fitting couplings for tubes can be produced. US 6749697 discloses a duplex stainless steel alloy with austenite-ferrite structure having a high Cr-, Mo- and N- content. 20 This alloy fulfils the above-mentioned requirements since when in hot extruded and annealed finish the alloy shows high strength, good corrosion resistance in several acids and bases and has especially good pitting resistance in chloride environments, as well as good weldability. The pitting resistance 25 of an alloy is often described as a Pitting Resistance Equivalent number, PRE number = %Cr + 3.3%Mo + 16%N. The alloy is therefore optimised according to the property. The PRE number of this alloy is over 40. The alloy contains in weight-% max 0.05% C, 0-2.0% Si 0-3.0% Mn, 25-35% Cr, 4-10% Ni, 2-6% Mo, 30 0.3-0.6% N, balance Fe and normally occurring impurities whereby the content of ferrite is 30-70%. WO 03/020994 describes an alloy characterised by Mn 0-3%, Cr 24-30%, Ni 4.9-10%, Mo 3-5, Cu 0-2%, W 0-3%, N 0.28-0.5% 35 and Co 0-3.5%. This alloy has a high Cr-, Mo and N content, which increases the alloy's pitting resistance but on the other WO 2006/049572 PCT/SE2005/001661 3 hand increases the risk of poor structural stability. By alloying with Co the alloy is considered to be more structurally stable so at least 0.5% Co, preferably at least 1.5%, max 3.5% Co can be added to increase the corrosion resistance and this is also 5 reported to increase structural stability. Since the alloy may contain W, the PRE number is modified to include the element W having a weight corresponding to half of the weight for Mo, namely PREW = %Cr + 3.3% (%Mo+0.5%W) + 16N. This alloy has a PRE/PREW number over 40. 10 US 6312532 discloses a duplex stainless steel alloy containing Mn 0.3-4%, Cr 27-35%, Ni 3-10%, Mo 0-3%, N 0.3-0.55%, Cu 0.5-3% and W 2-5%.The alloy exhibits a relatively high resistance to corrosion in chloride environments due to alloying 15 with W. Alloying with Cu in combination with high W or Mo contents is stated to decrease the precipitation of intermetallic phases on slow cooling. This property is of great importance when manufacturing stainless steel products of large dimensions where the rate of cooling is relatively slow, which in general 20 increases the risk of intermetallic phases precipitating in the temperature range of about 700-1000 0 C. This alloy has a PREW number over 40. The patent states that at least 2% W should be added for optimal effect and combinations of Mo + 0.5W should not exceed 3.52. When using high contents of Mo and W the Cu 25 content should exceed 1.5% to maximise the structural stability. If large amounts of Cu are used the Mo content should be low to ensure good protection against inter-crystalline corrosion. A disadvantage with duplex stainless steels is that their high 30 alloy content makes them susceptible to the formation of intermetallic phases, such as the sigma and chi phases, from extended exposure to high temperatures. The sigma phase is a hard, brittle and highly corrodible intermetallic compound that is rich in Cr and Mo. The chi phase is an intermetallic compound 35 with a manganese sulphide structure.

WO 2006/049572 PCT/SE2005/001661 4 Significant intermetallic precipitation may lead to a loss of corrosion, resistance and sometimes to a loss of toughness. Furthermore the production of thick and/or long pipes with large diameters is adversely affected because of the precipitation of 5 intermetallic phases inside the products where the cooling rate is relatively slow after annealing. SUMMARY OF THE INVENTION 10 The object of the present invention is to provide a duplex stainless steel that shows high strength, good corrosion resistance, good workability and which is weldable. This object is fulfilled by optimising the alloy described in US 15 6749697 by utilising knowledge of the influence of the elements Cu, W and Co on the structural stability of the alloy and its corrosion properties while retaining or improving the alloy's tensile properties. The object is fulfilled by a duplex stainless steel alloy having the composition disclosed herein namely an 20 alloy that contains (in weight %): Cr 25-35%, Ni 4-10%, Mo 1 6%, N 0.3-0.6%, Mn greater than 0 to 3%, Si max 1.0% and C max 0.06%, Cu and/or W and/or Co 0.1-10%, W 0.1-5%, balance Fe and normally occurring impurities wherein the ferrite content is 30-70%, and which alloy has a yield point in tension 25 being minimum 760 MPa. Such an alloy having high contents of Cr, Mo and N and containing W or W and Cu and/or Co has surprisingly good mechanical and corrosion properties, particularly as regards 30 pitting in a chloride environment. The high contents of Cr, Mo and N give the alloy a very high strength and simultaneously a good workability, especially for hot extrusion into articles such as seamless tubes. The addition of W or W and Cu and/or Co enhances the alloy's corrosion resistance in acid environments, 35 improves its structural stability and its weldability and confers WO 2006/049572 PCT/SE2005/001661 5 greater resistance to some types of corrosion attack by seawater. Besides exhibiting excellent mechanical properties the inventive 5 alloy has a high resistance to stress corrosion cracking caused by hydrogen sulphide. The alloy has good hot workability, is easier to roll and is well suited for applications that require welding, such as the manufacture of butt-welded seamless tubes and seam-welded tubes for various coiled tubing applications. 10 Consequently, the alloy is especially suited for hydraulic tubes, such as umbilicals, downhole tubes and IPUs. However, the most remarkable characteristic of the alloy according to the invention is the extraordinary combination of a high yield point in tension and a high impact toughness. 15 The present inventors has found the following relationship be tween yield point in tension and composition for a duplex stainless steel alloy: 20 Rpo.2 = 31.6% Cr + 34 (% Mo + % W) + 153% N + 10.2% Cu -426. Tungsten, which is similar to molybdenum in function and effect in terms of corrosion chemistry, is used to partly replace the 25 molybdenum in the alloy since tungsten is not as active as molybdenum in promoting the precipitation of intermetallic phases such as the sigma phase. Partly substituting molybdenum with tungsten also increases the alloy's low temperature impact toughness. The utilization of both 30 molybdenum and tungsten improves duplex stainless steel alloy's corrosion resistance. Furthermore since molybdenum is much more expensive than tungsten the substitution of molybdenum with tungsten provides a more cost-effective alloy. 35 An addition of W or W and Cu and/or Co is also essential for suppressing the precipitation of intermetallic phases. The alloy's WO 2006/049572 PCT/SE2005/001661 6 pitting corrosion properties and its resistance to intergranular corrosion are furthermore enhanced by a simultaneous addition of W and Cu, where W at least partly substitutes Mo. However high contents of W in combination with high contents of Cr and 5 Mo increase the risk of intergranular precipitations so the content of W should therefore be limited to max 5 weight %. According to an embodiment of the invention the alloy contains 0.40-0.55% N. It has been found that this high content of 10 nitrogen results in a particularly favourable combination of a high yield point in tension and a high impact toughness. According to another embodiment of the invention, where the inventive duplex stainless steel alloy contains tungsten, the 15 following relationship is satisfied: 0.5(% W)+1(% Mo) =2-10%, or preferably 3-7%. where (% W) and (% Mo) refer to the content of tungsten and 20 molybdenum respectively in weight %. According to another embodiment of the invention the alloy is manufactured using a conventional metallurgical method, such as melting in an arc furnace. The inventive alloy may therefore 25 be readily melted and cast using conventional techniques and equipment. Alternatively the alloy is manufactured by a powder metallurgy method. According to a further embodiment of the invention the alloy 30 comprises a maximum of 1 weight % alloying additions that are added for process metallurgical or hot workability reasons. The present invention also concerns an article in the form of a tube, wire, strip, rod, sheet or bar or any other article having 35 high strength and/or good corrosion resistance, which comprises an alloy according to any of the embodiments disclosed above.

WO 2006/049572 PCT/SE2005/001661 7 Such an article may be a seamless tube, a welding wire, a seam-welded tube, a flange, a coupling, a rotor blade, a fan, a cargo tank, weld material or high strength highly resistant wiring. Said article is either made of the inventive alloy or it 5 comprises a coating of the inventive alloy. Alternatively the article comprises the inventive alloy metallurgically or mechanically bonded (or clad) to a base material such as carbon steel. 10 Due to the good structural stability and weldability of the inventive alloy its field of application is much larger than the fields of application for the alloys constituting the state of the art. 15 The alloy and the article according to any of embodiments described above are intended for use particularly but not exclusively as a construction material or a mechanical or structural component, such as an umbilical, a downhole tube or an integrated production unit (IPU), in sea-water environments, 20 in chloride environments, in corrosive environments, in chemical plants, in the paper industry or as welding wire. Further advantages as well as advantageous features of the in vention appear from the following description and the other de 25 pendent claims. BRIEF DESCRIPTION OF THE DRAWINGS In the appended drawings: 30 Fig 1 is a diagram in the form of a plot of the impact tough ness versus the yield point in tension for test charges of alloys according to embodiments of the invention, and 35 WO 2006/049572 PCT/SE2005/001661 8 Fig 2 is a diagram showing the relation for test charges of alloys according to embodiments of the invention at measured values of yield point in tension and a prediction according to a formula drawn up by the pre 5 sent inventors. DESCRIPTION OF THE INVENTION The principles and advantages of the alloy of the present inven 10 tion and selection of the desired ranges of the constituent ele ments of the alloy which render the unexpected superiority of the alloy can be stated as follows: Chromium (Cr) is a very active element that improves the 15 resistance to a plurality of corrosion types. Moreover chromium increases the strength of the alloy. High chromium content addi tionally implies a very good solubility of N in the material. Consequently it is desirable to keep the Cr-content as high as possible in order to improve the strength and resistance to 20 corrosion. For very good strength properties and resistance to corrosion the content of chromium should be at least 25 weight %, preferably at least 28 weight %. However the content should not exceed 33%. However high contents of Cr increase the risk of forming intermetallic precipitations. For this reason the 25 chromium content preferably not exceed 35 weight %. Nickel (Ni) is used as an austenite-stabilising element and is added to the alloy at a suitable level in order to attain the desir able content of austenite and ferrite, respectively. In order to 30 attain ferrite contents of between 30-70%, the content of nickel should be at least 4 weight %, preferably at least 5 weight % and should not exceed 10 weight %, preferably not exceed 9 weight %. 35 Molybdenum (Mo) is an active element which improves the resistance to corrosion in chloride environments as well as in WO 2006/049572 PCT/SE2005/001661 9 reducing acids. An excessive Mo-content in combination with a high Cr-content means that the risk of forming intermetallic precipitations increases. Since Mo increases the strength of the alloy, the content of Mo should be in the range of at least 1 5 weight %, preferably at least 3%, it should not exceed 6 weight %, preferably not exceed 5 weight %. Nitrogen (N) is a very active element which partly increases the resistance to corrosion and partly increases the structural 10 stability as well as the strength of the material. Furthermore, a high N-content improves the reformation of austenite after welding, which ensures good properties for welded joints. In order to attain a good effect at least 0.3 weight % N should be added. High contents of N increase the risk of precipitation of 15 chromium nitrides, especially when the content of chromium is also high. Furthermore, a high N-content implies that the risk of porosity increases because the solubility of N in the steel melt or weld pool will be exceeded. The N-content should therefore be limited to max 0.60 weight %, it should preferably be at least 20 0.40 weight %, and should not exceed 0.55 weight % N. Manganese (Mn) is added in order to increase the solubility of N in the material, among other things. There are however other elements that have a higher influence on the solubility. Mn in 25 combination with high contents of sulphur can also give rise to the formation of manganese sulphides, which act as initiation points for pitting corrosion. The content of Mn should therefore be limited to being greater than 0 weight %, preferably at least 0.5 weight %, it should not exceed 3 weight %, preferably not 30 exceed 1.5 weight %. Silicon (Si) is utilized as a deoxidiser during steel production and it also increases the floatability under production and welding. It is known that high silicon contents support the 35 precipitation of an intermetallic phase. It has been surprisingly shown that an increased content of silicon favourably reduces WO 2006/049572 PCT/SE2005/001661 10 the precipitation of sigma phase. For this reason a certain content of silicon should be optionally permitted. The content of silicon should however be limited to max 1.0 weight %. Silicon would for example be added up to 0.15% or 0.10%. 5 Carbon (C) strengthens stainless steel but promotes the formation of precipitates harmful to corrosion resistance and therefore has to be considered to be a contaminant in this invention. Carbon has a limited solubility in both ferrite and 10 austenite and this implies a risk of precipitation of chromium carbides. The carbon content should therefore be limited to max 0.05 weight %, preferably to max 0.03 weight % and most preferably to max 0.02 weight %. 15 Copper (Cu) is added in order to improve the duplex stainless steel's resistance to certain corrosive environments such as in acid environments, such as sulphuric acid, and it also decreases the alloy's susceptibility to stress corrosion cracking and provides age-hardening effects. It has been found that Cu 20 decreases the precipitation rate of intermetallic phase on slow cooling in materials with relatively high contents of Mo and/or W. The reason for this is possibly that the precipitation of a copper rich austenite or epsilon phase prevents the precipitation of other intermetallic phases such as the sigma phase. Since 25 precipitation of the epsilon phase should not have the same negative influence on the corrosion properties as the sigma phase, the appearance of small amounts of copper-rich epsilon phase is a positive factor in the inventive alloy. However, high contents of copper mean that the solubility limit is exceeded so 30 the Cu-content should be limited to max 5 weight %. When present, the Cu-content should be at least 0.1 weight %, preferably at least 0.8 weight %, and should not exceed 5 weight %, preferably not exceed 3.5 weight %. 35 Tungsten (W) improves the resistance to corrosion in chloride environments as well as in reducing acids and the alloy's WO 2006/049572 PCT/SE2005/001661 11 resistance to pitting and crevice corrosion. It has been found that alloying with W as a replacement for Mo increases the alloy's low temperature impact strength. At the same time alloying with W and Cu, where W replaces the element Mo in the 5 alloy with the aim of improving pitting resistance properties, can take place with the aim of reducing the risk of worsening the inter-crystalline corrosion resistance. However a too high W content in combination with a high Cr-content increases the risk of precipitation of intermetallic phases, such as the sigma 10 phase. When present, the W-content should therefore be limited to at least 0.1 weight %, it should not exceed 5 weight %, preferably not exceed 3 weight %, and it may be min weight 1%. Cobalt (Co) is added to reduce the precipitation of sigma phase. 15 It increases the alloy's corrosion resistance and structural stability. Cobalt dissolves in the ferrite matrix, like nickel and silicon, and strengthens the ferrite. Cobalt also tends to stabilise austenite. When present, the content of cobalt should be greater than 0%, preferably greater than 0.5% and should not exceed 20 3.5%, preferably not exceed 2% Co. Ferrite: The content of ferrite is important in order to obtain good mechanical properties and corrosion properties as well as good weldability and workability. From a corrosion and welding 25 point of view it is desirable to obtain good properties with a ferrite content between 30-70%. High ferrite contents cause deterioration in low temperature impact toughness and resistance to hydrogen embrittlement. The ferrite content is therefore at least 30%, max 70%, preferably at least 35%, and 30 should not exceed 55%, the remainder being austenite. Alloying additions: Elements added for process metallurgical reasons, in order to obtain melt purification from S or O, for example, or added in order to improve the workability of the 35 material. Examples of such elements are AI, B, Ca, Ce and Mg. In order for such elements not to have a harmful effect on the WO 2006/049572 PCT/SE2005/001661 12 properties of the alloy, the levels of each individual element should be less than 0.1%. The total level of alloying additions should be less than 1%, preferably max 0.1%. 5 Modelling examples Modelling of 21 different compositions was carried out using the thermodynamic calculation program ThermoCalc Version Q. The compositions of the experimental charges are given in Table 1. 10 Table 2 gives the compositions in the ferrite and the austenite phases respectively. Table 3 contains parameters taken from the calculated phase diagrams; such as the amount of sigma phase at 9000C, the maximum temperature for sigma phase 15 (SIGMA) i.e. the temperature at which the sigma phase starts to precipitate at thermodynamic equilibrium, which means that this parameter is a dimension for the structural stability of the alloy, the maximum temperature for chromium nitrides Cr 2 N and the maximum temperature for the precipitation of chromium-rich 20 austenite phase. Observations An increase of the W content in alloys 1-4 increases the balance 25 in the PREW number (PRENW) between austenite and ferrite. The Cr content in the austenite phase also decreases. A high Cr content implies the risk of poor impact strength at low temperatures (-460C) so an increasing W content therefore improves the alloy's impact strength (see Table 2, alloys 1-4). 30 Cu decreases the maximum temperature for sigma phase in alloys with W (see Table 3, compare alloys 3 and 4 with alloys 7 and 8). For each weight % Cu Tmaxsigma decreases by 20-30 0 C. 35 W as a replacement for Mo should give an increased tensile yield limit because W is a bigger atom, which should have a WO 2006/049572 PCT/SE2005/001661 13 greater effect on solution hardening. By replacing Mo with W in the ratio 1:2 the structural stability will be largely unchanged but a better strength will be achieved. 5 Co decreases the risk of sigma phase precipitation by lowering the maximum temperature for sigma phase precipitation. (See Table 3, compare alloy 10 with alloy 11 and alloy 1 with alloy 9.) Test examples 10 Sixteen test charges were produced by casting 170 kg blooms. The blooms were hot-forged to round bars, from which test materials for investigations with respect to corrosion, strength and structural stability were taken. 15 The composition of the sixteen test charges successfully hot forged to round bars with a diameter of 40 mm are given in Table 4. 20 For investigating the structural stability of the test charges test plates from the rods were subjected to solution heat treatment at 7 temperatures between 900-12000C (in steps of 50 0 C). The best possible heat treatment temperature with the lowest degree of intermetallic phase was determined by studies in a light opti 25 cal microscope. The material was then subjected to solution heat treatment at this temperature during 5 minutes before the test material was taken out. The ferrite content was determined by means of point counting in a light optical microscope (LOM). The results are presented in Table 5. 30 For determining the structural stability for the test charges the test material was rapidly heated to the dissolving temperature, were annealed 3 minutes and cooled with a cooling rate of -17.5 0 C/minute and -100 0 C/minute down to room temperature. 35 The amount of sigma phase in the test charges was then determined by picture analysis of pictures from the BSE-detector WO 2006/049572 PCT/SE2005/001661 14 in a Scanning Electron Microscope (SEM). The results are pre sented in Table 6. It has been found that for a good structural stability it is neces 5 sary to restrict the amount of the alloying elements as Cr, Mo and W, while an increased content of N results in an improved structural stability. Two important relations have been observed, namely when there is a requirement of a good structural stability it is advantageous to replace Mo by W. Furthermore, high con 10 tents of N are favourable for the structural stability. It is shown in the example that 5542 has a considerably better structural stability than 5543, where an essential difference is that W re places Mo in a relation 2:1 (2% W for each % Mo). 15 The mechanical strength of the test charges was determined at room temperature and the impact toughness was determined at room temperature and -500C. The results are presented in Table 7. However, a number of the test bars exhibited cracks. The results are also shown in diagram form in Figure 1, which is a 20 plot of the impact toughness versus the yield point in tension. The yield point in tension Rpo 0

.

2 is strongly dependant upon solu tion hardening elements. The relation between yield point in ten sion and the composition satisfies with a comparatively good 25 correlation the formula: Rp0.2 = 31.6% Cr + 34 (% Mo + % W) + 153% N + 10.2% Cu - 426 The appended Figure 2 shows the relation for the test charges 30 at the measured values of Rpo 0

.

2 and the prediction according to this formula. It appears from the formula that for a high yield point in tension N has the strongest influence, while Cr, Mo and W have the same influence. Since W is an element which does not influence the structural stability as negatively as Mo, it is fa 35 vourable to alloy with W while lowering the content of Mo for avoiding problems with the structural stability. However, Mo has WO 2006/049572 PCT/SE2005/001661 15 a greater influence upon the corrosion properties. For a main tained structural stability it is possible to alloy with W that re places Mo by a factor 2, which means that the content of W may be increased with 2% if the content of Mo is lowered by 5 1%, for optimizing the yield point in tension. It appears clearly that for the test charges 5536 in comparison with 5542 and 5548 it is possible to increase the yield point in tension for the materials by lowering the content of Mo and N 10 and at the same time increase the content of W and Cu. A problem for high tensile materials in general is that it is very difficult to obtain a combination of a good impact toughness and a high yield point in tension. It has for the present invention 15 been demonstrated that for charges having a very high yield point in tension, where Rpo.

2 exceeds 800 MPa, it is possible to obtain an acceptable impact toughness at -50 0 C for charges where the content of W and Cu is high at the same time as the content of N has been reduced. It was by that possible to obtain 20 a combination of two important properties for construction materials, which so far has been difficult to obtain for duplex steels. A comparison of these charges 5536 with 5542 and 5548 shows 25 clearly this relationship, where an increase of the content of W and Cu in combination with a lowering of the content of N results in an attractive combination of an acceptable low temperature impact toughness and a high yield point in tension. An optimiza tion of the properties may be obtained by further increasing the 30 content of W and Cu while considering the requirement of a good structural stability. The resistance of the test materials to pitting and crevice corro sion were measured according to ASTM G48C and MTI-2. The 35 critical pitting corrosion temperature (CPT) and the critical crev ice corrosion temperature (CCT) were determined and are shown in Table 8. However, several of the test bars had cracks.

WO 2006/049572 PCT/SE2005/001661 16 The composition in the ferrite and austenite phase, respectively, has been determined by means of microprobe analysis (EPMA), and the results are shown in Table 9. The PRE number may be calculated according to PRE = % Cr + 3.3 (% Mo + 0.5% W) + 5 16% N for the respective phase and the total composition. The PRE number should be as balanced as possible between the austenite and the ferrite phases. The properties (positive/negative+0-) of the test material are 10 compared in Table 10, where also a judgement of the forgea bility of the material has been made on a scale from 0 (the worst) to 5 (the best). It appears that the charge 5548 is the best one with respect to 15 the combination of corrosion resistance, yield point in tension and impact toughness. It appears from Table 4 that this charge has a content of Cu of about 2%, W about 4% and Co about 0.1% in weight. Thus, it is favourable to have all these three elements present in the alloy. 20 An optimum composition of a duplex stainless steel alloy according to the invention where all the properties are consid ered may be as follows: 25 Alloy with high contents of Cr, Cu and W and with a content of N which does not negatively influence the low temperature impact toughness. Restrict the content of Mo so that the requirement of a good structural stability may be met. A high yield point in ten sion is obtained when the content of N is high. It is possible to 30 lower the content of N without lowering the yield point in tension if the content of W or Cu is increased. An acceptable low temperature impact toughness in combination with a high yield point in tension is obtained when the content of N is compara tively low and the content of W and Cu is high. 35

Claims

1. A duplex stainless steel alloy, characterized in that the alloy contains in weight %: 5 * Cr 25-35% * Ni 4-10% * Mo 1-6% * N 0.3-0.6% 10 * Mn greater than 0-3% * Si max 1.0% * C max 0.06% * Cu and/or W and/or Co 0.1-10% * W 0.1-5% 15 balance Fe and normally occurring impurities wherein the ferrite content is 30-70%, and that the alloy has a yield point in tension being min 760 MPa. 20

2. The alloy according to claim 1, characterized in that it contains 0.1-5% Cu.

3. The alloy according to claim 1, characterized in that it contains 0.1-3% Cu. 25

4. The alloy according to claim 2 or 3, characterized in that it contains Cu min 0.8%.

5. The alloy according to any of the preceding claims, 30 characterized in that it contains Si max 0.15% and C max 0.05%.

6. The alloy according to any of the preceding claims, characterized in that the alloy contains Si max 0.1% and C 35 max 0.05%. WO 2006/049572 PCT/SE2005/001661 18

7. The alloy according to any of the preceding claims, characterized in that the alloy contains 0.40-0.55% N.

8. The alloy according any of the preceding claims, 5 characterized in that it contains 1-3 weight % W.

9. The alloy according to any of the preceding claims, characterized in that the following relationship is satisfied: 0.5(% W)+1(% Mo) =2-10%. 10

10.The alloy according to any of the preceding claims, characterized in that the following relationship is satisfied: 0.5(% W)+1(% Mo) =3-7%. 15

11.The alloy according to claim 1, characterized in that it contains greater than 0-3.5% Co.

12.The alloy according to claim 1, characterized in that it contains 28-33 weight % Cr. 20

13.The alloy according to claim 1, characterized in that it contains 0.5-1.5 weight % Mn.

14.The alloy according to claim 13, characterized in that it 25 contains 5-9 weight % Ni.

15.The alloy according to claim 1, characterized in that it contains 35-55% ferrite. 30

16.The alloy according to any of the preceding claims, charac terized in that it is manufactured using a conventional metallurgical method.

17.The alloy according to any of the preceding claims, charac 35 terized in that it comprises a maximum of 1 weight % further alloying additions in total. WO 2006/049572 PCT/SE2005/001661 19

18.An article in the form of a tube, wire, strip, rod, sheet or bar, characterized in that it comprises an alloy according to any of claims 1-17. 5

19.An article according to claim 18, characterized in that it is made of the alloy according to any of claims 1-17.

20.An article according to claim 18, characterized in that it 10 comprises a coating or a cladding of the alloy according to any of claims 1-17.

21.Use of an alloy according to any of claims 1-17, or an article according to any of claims 18-20 as a construction material 15 or a mechanical or structural component, such as an umbilical, a downhole or an integrated production unit (IPU), in sea-water environments, in chloride environments, in corrosive environments, in chemical plants, in the paper industry or as welding wire. 20