NO338090B1 - Ferrite-austenitic duplex stainless 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

The present invention relates to a stainless steel alloy, more specifically a duplex stainless steel alloy with a ferritic-austenitic matrix and with high resistance and corrosion in combination with good structural stability and hot workability, in particular a duplex stainless steel with a ferrite content of 40-65 percent by volume and a well-balanced composition, which gives the material corrosion properties, making it more suitable for use in chloride-containing environments than previously thought possible.

In recent years, the environments in which corrosion-resistant metallic materials were used became more aggressive, the demands on the corrosion properties as well as their mechanical properties increased. Duplex steel alloys, which were established as an alternative to the steel grades that had been used until then, such as high-alloyed austenitic steel, nickel-based alloys or other high-alloyed steels, are not exempt from this development.

An established measure of corrosion resistance in chloride-containing environments is the so-called pitting resistance equivalent (abbreviated PRE), which is defined as

where the percentages for each element refer to percentage by weight. A higher numerical value indicates better corrosion resistance, particularly against pitting corrosion. The essential alloying elements which affect this property are according to the formula Cr, Mo, N. An example of such a steel quality appears in EP 0220141, which is hereby included in this description through this reference. This steel quality with the designation SAF2507 (UNS S32750) was mainly alloyed with high contents of Cr, Mo and N. It has therefore been developed with this property in mind, with above all good resistance to corrosion in chloride environments.

In recent times, the elements Cu and W have also proven to be effective alloy additions for further optimization of the steel's corrosion properties in chloride environments. The element W has therefore been used as a substitute for part of Mo, as for example in the commercial alloy DP3W (UNS S39274) or zeron 100, which contain 2.0% and 0.7% W respectively. The latter contains to and with 0.7% Cu with the aim of increasing the corrosion resistance of the alloy in acidic environments.

The alloying addition of tungsten led to a further development of the measure of corrosion resistance and thus the PRE formula to the PREW formula, which also makes the ratio between the effect of Mo and W on the corrosion resistance of the alloy clearer:

as described for example in EP 0 545 753. This publication refers to a duplex stainless alloy with generally improved corrosion properties. The steel grades described above have a PRE number, regardless of which method is used for the calculation, which is above 40.

Among the alloys that have good corrosion resistance in chloride environments, SAF 2906 should also be mentioned, the composition of which appears from EP 0 708 845. This alloy, which is characterized by a higher content of Cr and N compared to, for example, SAF2507, has proven to be particularly suitable for use in environments where resistance to integral annular corrosion and corrosion in ammonium carbamate is important, but it also has a high corrosion resistance in chloride-containing environments.

Furthermore, EP 534 864 describes a duplex stainless steel with high corrosion resistance produced by means of powder metallurgy, and which is intended for use in environments with a chlorine content.

US-A-4 985 091 describes an alloy intended for use in hydrochloric acid and sulfuric acid environments where integral annular corrosion mainly occurs. It is primarily intended as an alternative to austenitic steels that have been used in recent times.

US-A 6,048,413 describes a duplex stainless alloy as an alternative to austenitic stainless steel, intended for use in chloride-containing environments.

The disadvantage of the alloys described above, which all have high PRE numbers, is the occurrence of hard and brittle intermetallic precipitates in the steel, such as sigma phase, especially after heat treatment, such as for example during welding during later processing. This results in a harder material with poorer machinability and ultimately a reduced corrosion resistance.

In order to further improve, among other things, the pitting corrosion resistance of duplex stainless steel, an increase in the PRE number in both the ferrite phase and the austenite phase is required, without jeopardizing the material's structural stability or machinability. If the composition of the two phases is not equivalent with respect to the active alloy constituents, one phase becomes more sensitive to pitting and crevice corrosion. The phase which is most sensitive to corrosion consequently has its resistance reduced, while the structural stability is reduced for the most highly alloyed phase.

It is therefore an aim of the present invention to provide a duplex stainless steel alloy, which shows high corrosion resistance in combination with improved mechanical properties, and which is very suitable for use in environments where a high resistance to general corrosion and local corrosion is required, so such as in chloride-containing environments.

The present invention relates to a ferritic-austenitic duplex stainless steel alloy. The alloy shows the following composition, in weight percent: 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 0.5-3.5, 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, and the rest Fe and commonly occurring impurities, whereby the ferrite content is 40-65 percent by volume, as PRE - or the PREW value for both the ferrite and austenite phases is higher than 45, the PRE or PREW value for the total composition of the alloy being higher than 46 and the ratio between the PRE or PREW value for the austenite phase and the PRE or PREW the value for the ferrite phase is between 0.90 and 1.15.

It may be another purpose of the present invention to provide a duplex stainless steel alloy with a content of ferrite in the range of 40 to 65 percent by volume and a PRE number of at least between 46 and 50 in both the austenitic and ferritic phases, and with an optimal ratio of PRE austenite to PRE ferrite in the range 0.90 to 1.15; preferably between 0.9 and 1.05.

It may also be a purpose of the present invention to provide a duplex stainless steel alloy with a critical pitting corrosion temperature (Critical Pitting Corrosion Temperature), (from now on abbreviated CPT) value that is higher than 90°C, preferably higher than 95°C , and a critical crevice-corrosion temperature (from now on abbreviated CCT) value of at least 50°C in 6% FeCb, preferably at least 60°C in 6% FeC3.

It may be a further purpose of the present invention to produce an alloy with an impact strength of at least 100 J at room temperature and an elongation after a tensile test of at least 25% at room temperature.

Due to its high alloy content, the material according to the present invention shows strikingly good workability, especially hot workability, and should therefore be very suitable for use, for example, in the production of bars, pipes, such as welded and seamless pipes, plates, bands, wire, welding wire , construction parts, such as pumps, valves, flanges and couplings.

These purposes are fulfilled according to the present invention with duplex stainless steel alloys, which contain (in weight percent) up to 0.03% C, up to 0.5% 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 unavoidable units.

Brief description of the drawings:

Fig. 1 shows CPT values from tests of the test sets in the modified ASTM G48C test in "Green Death" solution compared to the duplex steels SAF2507, SAF2906, as well as the high alloy austenitic steel 654SMO. Fig. 2 shows CPT values obtained using the modified ASTM G48C test in "Green Death" solution for the test batches compared to the duplex steel SAF2507 as well as the austenitic steel 654SMO. Fig. 3 shows the average amount of erosion in mm/year in 2% HCI at a temperature of 75°C. Fig. 4 shows the results from hot ductility testing for the majority of the batches.

Systematic development work has surprisingly shown that with the help of a well-balanced combination of the elements Cr, Mo, Ni, N, Mn and Co, an optimal distribution of the elements in the ferrite and austenite can be achieved, which enables a very corrosion-resistant material with only a negligible amount of sigma phase in the material. The material also achieves good processability, which enables extrusion into seamless pipes. It shows that with the intention of achieving a combination of high corrosion resistance in connection with good structural stability, a very narrow combination of the alloying elements in the material is required. The alloy according to the invention contains (in percentage by weight):

the rest Fe and normally occurring impurities and additives, whereby the content of ferrite is 40-65 percent by volume.

Carbon (C) has limited solubility in both ferrite and austenite. The limited solubility entails a risk of precipitation of carbides, and the content should therefore not exceed 0.03 weight percent, preferably not exceed 0.02 weight percent.

Silicon (Si) is used as a deoxidising agent in steel production, as well as increasing fluidity during production and welding. Too high a content of Si, however, leads to the precipitation of an unwanted intermetallic phase, for which reason the content is limited to a maximum of 0.5% by weight, preferably a maximum of 0.3% by weight.

Manganese (Mn) is added to increase N solubility in the material. However, it has been shown that Mn only has a limited effect on N solubility in the relevant type of alloy. Instead, other elements with a higher impact on solubility have been found. In addition, Mn in combination with a high content of sulfur can give rise to the formation of manganese sulphides, which act as a starting point for pitting corrosion. The content of Mn should therefore be limited to between 0-3.0 weight percent, preferably 0.5-1.2 weight percent.

Chromium (Cr) is a very active element to improve resistance to a large number of corrosion types. Furthermore, a high content of crumb means that you get a very good N solubility in the material. It is therefore desirable to keep the Cr content as high as possible to improve corrosion resistance. For very good values of corrosion resistance, the content of chromium should be at least 24.0% by weight, preferably 27.0-29.0% by weight. High contents of Cr, however, increase the risk of intermetallic precipitation, for which reason the content of chromium must be limited to a maximum of 30.0% by weight

Nickel (Ni) is used as an austenite-stabilizing element, and is added in a suitable content to achieve the desired content of ferrite. In order to achieve the desired ratio between the austenitic and the ferritic phase with between 40-65 volume percent ferrite, an addition of between 4.9-10.0 weight percent nickel, preferably 4.9-8.0 weight percent, is necessary.

Molybdenum (Mo) is an active element that improves corrosion resistance in chloride environments as well as preferably in reducing acids. An excessively high Mo content in combination with high Cr contents means that the risk of intermetallic precipitation increases. The Mo content in the present invention should be in the range 3.0-5.0 weight percent, preferably 3.6-4.7 weight percent, especially 4.0-4.3 weight percent.

Nitrogen (N) is a very active element, which increases the corrosion resistance, the structural stability as well as the firmness of the material. A high N content further improves the recovery of the austenite after welding, which gives good properties inside the welded joint. In order to achieve a good effect of N, at least 0.28% by weight of N should be added. With high contents of N, the risk of precipitation of chromium nitrides increases, especially when the chromium content is also high. Furthermore, a high N content means that the risk of porosity increases due to the exceeded solubility of N in the melt. For these reasons, the N content should be limited to a maximum of 0.5% by weight, preferably >0.35 - 0.45% by weight N is added.

Boron (B) is added to increase the hot workability of the material. If the boron content is too high, the weldability as well as the corrosion resistance can deteriorate. The content of boron should therefore be limited to 0.0030% by weight.

Sulfur (S) negatively affects corrosion resistance by forming soluble sulphides. Furthermore, hot workability deteriorates, and for this reason the content of sulfur should be limited to a maximum of 0.010% by weight.

Cobalt (Co) is added primarily to improve structural stability as well as corrosion resistance. Co is an austenite-stabilizing element. To achieve an effect, at least 0.5% by weight should be added, preferably at least 1.5% by weight. Because cobalt is a relatively expensive element, the addition of cobalt is therefore limited to a maximum of 3.5% by weight.

Tungsten increases resistance to pitting and crevice corrosion. However, the addition of excessive amounts of tungsten in combination with the fact that the Cr content as well as the Mo content is high means that the risk of intermetallic precipitation increases. In the present invention, the W content should be in the range of 0.3.0% by weight, preferably between 0.5 and 1.8% by weight.

Copper is added to improve overall corrosion resistance in acidic environments, such as sulfuric acid. At the same time, Cu affects the structural stability. However, high contents of Cu mean that the solubility in the solid state will be exceeded. The Cu content should therefore be limited to a maximum of 2.0% by weight, preferably between 0.5 and 1.5% by weight.

Ruthenium (Ru) is added to increase corrosion resistance. Because ruthenium is a very expensive element, the content should be limited to a maximum of 0.3 weight percent, preferably more than 0 and up to 0.1 weight percent.

Aluminum (Al) and calcium (Ca) are used as deoxidizers in steel production. The content of Al should be limited to a maximum of 0.03% by weight to limit the formation of nitrides. Ca has a beneficial effect on hot ductility. However, the Ca content should be limited to 0.010% by weight to avoid an undesirable amount of slag.

The content of ferrite is important to achieve good mechanical properties and corrosion properties as well as weldability. From the point of view of corrosion and from the point of view of weldability, a content of ferrite between 40-65% is desirable in order to achieve good properties. Furthermore, high contents of ferrite mean that the impact resistance at low temperatures as well as the resistance to hydrogen-induced brittleness become worse. The content of ferrite is therefore 40-65 volume percent, preferably 42-60 volume percent, especially 45-55 volume percent.

In the examples below, the composition of a number of test batches is presented, which illustrates the effect of different alloying elements on the properties. Rate

605182 represents a reference composition, and is therefore not part of the field of this invention. Nor shall the remaining clauses be considered as limiting the invention, in any other way than only by specifying examples of clauses, which illustrate the invention according to the claims. The specified PRE numbers or values

always considers quantities calculated according to the PRE formula, even if this is not explicitly mentioned.

Example 1

The test batches according to this example were produced by casting casting blocks of 170 kg in the laboratory, which were hot-forged into round bars. These were heat-extruded into bars (round bars as well as flat bars), where the test material was taken out from the round bars. Furthermore, the flat bars were annealed before cold rolling took place, after which additional test material was taken out. Seen from a material engineering point of view, the process can be considered to be representative of production on a larger scale, for example for the production of seamless pipes by the extrusion method, followed by cold rolling. Table 1 shows the composition of the first batch of test batches.

For the purpose of investigating the structural stability, samples from each batch were annealed at 900-1150°C in steps of 50°C, as well as being cured in air and water, respectively. At the lowest temperatures, an intermetallic phase was formed. The lowest temperature, at which the amount of intermetallic phase became negligible, was determined by light-optical microscope studies. New samples from a respective batch were subsequently annealed at the temperature for five minutes, after which the samples were cooled down with a constant cooling rate of -140°C/min to room temperature. Then, the area proportion of the sigma phase in the materials was determined by digital scanning of the images with backscattering of electrons in a scanning electron microscope. The results appear in table 2.

Tmaxsigma was calculated with Termo-Calc (TC version N thermodynamic database for steel TCFE99) based on characteristic quantities for all specified elements in the different variations. Tmaxsigma is the dissolution temperature of sigma phase, where high dissolution temperature indicates low structural stability.

The purpose of this investigation is to be able to rank the material with regard to the structural stability, i.e. that this is not the real content of sigma phase in the samples, which were heat-treated and hardened before, for example, the corrosion testing. One can see that Tmaxsigma, which was calculated with thermo-calc does not directly coincide with the measured amounts of sigma phase. However, it is clear that the test batches with the lowest calculated Tmaxsigma contain the least amount of sigma phase during the investigation.

The pitting corrosion properties of all batches were tested for rank in the so-called "Geen Death" solution, which consisted of 1%FeCl3,1%CuCl2,

11% H 2 SO 4 , 1.2% HCl. The test procedure is equivalent to the pitting corrosion testing according to ASTM G48C, however, it will be performed in the more aggressive "Green Death" solution. Furthermore, some of the batches were tested according to ASTM G48C (two tests per batch). Furthermore, the electrochemical testing in 3% NaCl (six tests per batch) was carried out. The results in terms of the Critical Pitting Temperature (CPT) from all the tests appear in table 3, as well as the PREW number (Cr+3.3(Mo+0.5W)+16N) for the overall composition of the alloy as well as for austenite and ferrite, the indexing alpha refers to the ferrite and gamma refers to the austenite.

It has been established that there is a linear relationship between the lowest PRE 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 alone does not explain the CPT values. In fig. 1, the CPT values from the test in the modified ASTM G48C are shown in diagram form. The duplex steels SAF2507, SAF2906 as well as the high alloy austenitic steel 654SMO are included for reference. It is clear from these results that all the test materials show better CPT in the modified AST G48C than SAF2507, as well as SAF2906. Furthermore, some of the test materials show CPT results in the modified ASTM G48C at the same level as or higher than 654SMO. Test batch 60183, alloyed with cobalt, shows good structural stability at a controlled cooling rate of (-140°C/min) despite containing high contents of chromium as well as molybdenum, showing better results than SAF2507 and SAF2906. It emerges from this investigation that a high PRE does not alone explain the CPT values, without the ratio PRE austenite/PRE ferrite having extreme importance for the properties of the higher alloyed duplex steels, and a very narrow and exact balance between the alloying elements is necessary to achieve this optimal ratio, which lies between 0.9-1.15; preferably 0.9-1.05, and at the same time achieve PRE values above 46. The ratio of PRE-austenite/PRE-ferrite to CPT in the modified ASTM G48C test for the test batches is given in Table 3.

The strength at room temperature (RT), 100°C and 200°C and the impact strength at room temperature (RT) have been determined for all batches and are shown as average values for three tests.

Tensile test specimens (DR-5C50) were prepared from extruded bars, diameter 20 mm, which were heat treated at temperatures according to Table 2 for 20 minutes followed by cooling in either air or water (605195, 605197, 605184). The results of the tests are shown in tables 4 and 5. The results of the tensile test show that the contents of chromium, nitrogen and tungsten strongly influence the impact strength of the material. Apart from 605153, all batches meet the requirement of 25% elongation when tensile tested at room temperature (RT).

The investigation shows very clearly that water hardening is absolutely necessary to achieve the best structure and consequently good values for impact strength. The requirements are 100 J when tested at room temperature, and all the batches meet this, except batch 605184 and 105187, where the latter is certainly very close to the requirement.

Table 6 shows the results from the tungsten-inert gas remelting test (Tungsten-Inert-Gas remelting test, from now on abbreviated TIG), where batches 605193, 605183, 605184 as well as 605253 show a good structure in the heat affected zone (Heat Affected Zone, henceforth abbreviated as HAZ). The Ti-containing batches show Tin in the HAZ. An excessively high crumb and nitrogen content results in the precipitation of C^N, which should be avoided because it impairs the material's properties.

Example 2

In the below-mentioned example, the composition of a further number of test batches which were produced for the purpose of finding the optimum composition is given. These batches have been modified starting from the characteristics of the batches with good structural stability, as well as high corrosion resistance, from the results, which were shown in Table 1. All the batches in Table 7 are included in the composition according to the present invention, where batches 1-8 are included in a statistical test model, while rates e to n are additional test alloys within the scope of the invention.

A number of test batches were produced by casting ingots of 270 kg, which were hot-forged into round bars. These were extruded into bars, from which the test samples were taken. The bar was then annealed before cold rolling into flat bars was carried out, after which additional test material was taken out. Table 7 shows the composition of these test batches.

Thermo-Calc values according to Table 8 (T-C version N thermodynamic database for steel TCFE99) are based on characteristic quantities for all specified elements in the different variations. The PRE number for the ferrite and austenite is based on their equilibrium composition at 1100°C. Tmaxsigma is the dissolution temperature of sigma phase, where high dissolution temperatures indicate lower structural stability.

The distribution of the alloying elements in the ferrite phase and the austenite phase was examined with microprobe analysis, and the results appear in table 9.

The pitting properties of all batches have been tested in "Green Death" solution (1%FeCl3, 1%CuCl2, 11%H2SO4, 1.2% HCI) for rating. The test procedures are the same as pitting corrosion testing according to ASTM G48C, but the testing will be performed in a more aggressive solution than 6%FeCb, the so-called "Green Death" solution. The general corrosion testing in 2%HCI (2 tests per batch) was also carried out for ranking before the dew point testing. The results of all the tests appear in table 10, fig. 2 and fig. 3. All tested rates outperform SAF2507 in "Green Death" solution. All rates are within the identified range of 0.9-1.15; preferably 0.9-1.05, relevant for the ratio PRE austenite/PRE ferrite, at the same time that PRE in both austenite and ferrite is higher than 44, and for the majority of batches even significantly higher than 44. Some of the batches achieve up to and with the limit of total PRE 50. It is very interesting to see that batch 605251, alloyed with 1.5 wt% cobalt, has a performance approximately equivalent to batch 605250, which is alloyed with 0.6 wt% cobalt, in "Green Death » solution, despite the lower crumb content in batch 605251. It is particularly surprising and interesting because batch 605251 has a PRE number of about 48, which is higher than some of today's commercial superduplex alloys, while the Tmaxsigma value below 1010°C indicates a good structural stability based on the values in Table 2 in Example 1.

In Table 10, the PRE number (%Cr+3.3%(Mo+0.5%W)+16%N) is also given for the overall composition of the alloy and the PRE in austenite as well as in the ferrite (rounded) based on composition of the phases shown as measured by microprobe. Ferrite content was measured after heat treatment at 1100°C, followed by hardening in water.

To examine the structural stability in detail, the samples were annealed for 20 minutes at 1080°C, 1100°C and 1150°C, after which they were quenched in water. The temperature at which the amount of intermetallic phase became insignificant was determined by means of investigations in a light optical microscope. A comparison between the structure of the batches after annealing at 1080°C followed by water hardening shows which of the batches is most suspected of containing unwanted sigma phase. The results are shown in Table 11. Inspection of the structure shows that batches 605249, 605251, 605252, 605253, 60524, 605255, 605259, 605260, 605266 as well as 605267 are free of unwanted sigma phase. Furthermore, batch 605249, alloyed with 1.5% by weight cobalt, is free of sigma phase, while batch 605250, alloyed with 0.6% by weight cobalt, contains a very small amount of sigma phase. Both batches are alloyed with a high content of chromium, approx. 29.0% by weight, and a molybdenum content of approx. 4.25% by weight. If one compares the compositions in batches 605249, 605250, 605251 and 605252 with regard to the content of sigma phase, it is very clear that the composition range for the optimal material is very narrow, in this case with regard to structural stability. It further shows that batch 605268 contains only sigma phase, compared to batch 605263, which contains a lot of sigma phase. What mainly distinguishes these batches from each other is the addition of copper to batch 605268. Batch 605266 and also 605267 are free of sigma phase, despite a high content of chromium, the latter batch is alloyed with copper. Furthermore, batches 605262 and 605263 with the addition of 1.0 wt% tungsten show a structure with a lot of sigma phase, while it is interesting to note that batch 605269, also with 1.0 wt% tungsten, but with a higher content of nitrogen than 605262 and 605263, shows a significantly smaller amount of sigma phase. In order to achieve good structural properties, a very carefully balanced balance between the various alloying elements is therefore required at these high alloying contents of, for example, chromium and molybdenum. Table 11 shows the results from the lysoptic examination after annealing at 1080°C, 20 minutes followed by water hardening. The amount of sigma phase is specified with values from 1 to 5, where 1 indicates that no sigma phase was detected during the examination, while 5 indicates that a very high content of sigma phase was detected during the examination.

Table 12 shows the results from the impact resistance testing of some of the batches. The results are very good, indicating a good structure after annealing at 1100°C followed by water hardening, and the requirement of 100J will be met by a large margin for all the batches tested.

Fig. 4 shows the results from the hot ductility testing of most of the batches. A good processability is of course of vital importance to be able to produce the material into product forms such as bars, pipes, such as welded and seamless pipes, plates, bands, wire, welding wire, construction elements, such as for example pumps, valves, flanges and couplings. Batches 605249, 605250, 605251, 605252, 605255, 605266 as well as 605267, mostly with nitrogen content around 0.38% by weight, show somewhat improved hot ductility values.

Summary of test results.

In order to achieve good corrosion properties, while the material shows good structural stability, hot workability and weldability, the material should be optimized according to the following:

• PRE number in ferrite should exceed 45, but preferably be at least 47.

• PRE number in austenite should exceed 45, but preferably be at least 47.

• PRE number for the whole alloy should preferably be at least 46.

• The ratio PRE-austenite/PRE-ferrite should be in the range 0.9-1.15; preferably in the range 0.9-1.05. • The content of ferrite should preferably be in the range of 45-55 percent by volume.

• Tmax sigma should not exceed 1010°C.

• The content of nitrogen should be in the range 0.28-0.5 weight percent, preferably in the range 0.35-0.48 weight percent, but preferably 0.38-0.40

weight percent.

The content of cobalt should be in the range 0-3.5% by weight, preferably

1.0-2.0% by weight, but preferably 1.3-1.7% by weight.

To ensure the high nitrogen solubility, i.e. if the content of nitrogen is in the range of 0.38-0.40% by weight, at least 29% by weight Cr should be added, as well as at least 3.0% by weight Mo, so that the combined content of the elements Cr, Mo and N meets the requirements of PRE -number.

Claims (15)

1. Ferritic-austenitic duplex stainless steel alloy, characterized in that it shows the following composition, in percentage by weight: the rest Fe and commonly occurring impurities, whereby the ferrite content is 40-65% by volume, the PRE or PREW value for both the ferrite and austenite phases being higher than 45, the PRE or PREW value for the total composition of the alloy being higher than 46 and the ratio between the PRE or PREW value for the austenite phase and the PRE or PREW value for the ferrite phase is between 0.90 and 1.15. 2. Alloy as specified in claim 1, characterized by the content of manganese being between 0.5 and 1.2 percent by weight. 3. Alloy as specified in claim 1 or 2, characterized by the content of crumb being between 27.0 and 29.0 percent by weight. 4. Alloy as specified in claims 1-3, characterized in that the nickel content is between 5.0 and 8.0 percent by weight. 5. Alloy as stated in claims 1-4, characterized in that the content of molybdenum is between 3.6 and 4.7 percent by weight. 6. Alloy as specified in claims 1-5, characterized in that the nitrogen content is between 0.35 and 0.45 percent by weight. 7. Alloy as specified in claims 1-5, characterized in that the content of ruthenium is between 0 and 0.3 weight percent, preferably higher than 0 and up to 0.1 weight percent. 8. Alloy as specified in one of the preceding claims, characterized in that the cobalt content is between 0.5 and 3.5 weight percent, preferably between 1.5 and 3.5 weight percent. 9. Alloy as specified in one of the preceding claims, characterized in that the content of copper is between 0.5 and 2.0 weight percent, preferably between 1.0 and 1.5 weight percent. 10. Alloy as specified in one of the preceding claims, characterized in that the content of ferrite is between 42 and 60 volume percent, preferably between 45 and 55 volume percent. 11. Alloy as specified in one of the preceding claims, characterized in that the overall PRE or PREW value for the alloy exceeds 44, where PRE=%Cr + 3.3%Mo + 1.6N and PREW = %Cr + 3.3(%Mo + 0.5%W) + 16N, where % refers to weight percent. 12. Alloy as specified in claim 11, characterized in that the PRE or PREW value for both the ferrite and austenite phases is between 47 and 49. 13. Alloy as specified in claim 11 or 12, characterized in that the ratio between PRE or PREW value for austenite phase and PRE or PREW value for ferrite phase lies between 0.9 and 1.05. 14. Alloy as specified in one of the preceding claims for use in chloride-containing environments. 15. Alloy as specified in one of the preceding claims for use in chloride-containing environments in product forms such as rods, pipes, such as welded and seamless pipes, plates, bands, wire, welding wire, structural parts, such as for example pumps, valves, flanges and connections.