SE524952C2 - 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

nu ..- »ones 10 15 20 25 30 524 952 2 Recently, the elements Cu and W have also been shown to be effective alloy additives for further optimization of the steel's corrosion properties in chloride environments. The element W has then been used as a replacement for a part of Mo, such as in the commercial alloy DP3W (U NS S39274) or Zeron100, which contains 2.0% and 0.7% W respectively.

The latter also contains 0.7% Cu for the purpose of increasing the alloy's corrosion resistance in acid environments. Alloy of tungsten led to a further development of the measure of corrosion resistance and thus the PRE formula to the PREW formula, which also clarifies the relationship between the effect of Mo and W on the corrosion resistance of the alloy: PREW =% Cr + 3.3 (% Mo + 0.5) % W) + 16% N, as described for example in EP 0 545 753. This document relates to a duplex stainless steel alloy with generally improved corrosion properties.

The steels described above have a PRE number, regardless of calculation method, which is over 40.

Of the alloys with good corrosion resistance in chloride environments, SAF 2906 should also be mentioned, the composition of which is stated in EP 0 708 845.

This alloy, which is characterized by higher levels of Cr and N compared to, for example, SAF2507, has been found to be particularly suitable for use in environments where resistance to intercrystalline corrosion and corrosion in ammonium carbamate is important, but it also has a high corrosion resistance in chloride-containing environments.

US-A-4 985 091 describes an alloy intended for use in hydrochloric and sulfuric acid environments, where especially intercrystalline corrosion occurs. It is mainly intended as an alternative to hitherto used austenitic steels.

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

The disadvantage of the alloys described above, all with high PRE numbers, is the appearance 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 leads to a harder material with poorer machinability and finally a deteriorated corrosion resistance.

To further improve i.a. The point corrosion resistance of duplex stainless steels requires an increase in the PRE number in both the ferrite phase and the austenite phase without compromising the structural stability or machinability of the material. If the composition of the two phases is not equivalent with respect to the active alloy components, one phase becomes more susceptible to point and crevice corrosion. Thus, the more corrosion-sensitive phase controls the durability of the alloy, while the structural stability is controlled by the highly alloyed phase.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a duplex stainless steel alloy which exhibits high corrosion resistance in combination with improved mechanical properties and which is most suitable for use in environments where high resistance to general corrosion and local corrosion is required, such as for example in chloride-containing environments.

It is a further object of the present invention to provide a duplex stainless steel alloy having a ferrite content in the range of 40 to 65% by volume and a PRE number of at least between 46 and 50 in both austenite phase and ferrite phase 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 is a further object of the present invention to provide a duplex stainless steel alloy having a Critical Pitting Corrosion Temperature (hereinafter abbreviated CPT) value greater than 90 ° C, preferably greater than 95 ° C and a Critical Crevice Corrosion Temperature (hereinafter abbreviated CCT) value of at least 50 ° C in 6% FeCl 3, preferably at least 60 ° C in 6% FeCl 3.

It is a further object of the present invention to provide an alloy having an impact strength of at least 100 J at room temperature and an elongation at tensile test of at least 25% at room temperature.

Due to its high alloy content, the material according to the present alloy exhibits extremely good machinability, in particular hot machinability and should thus be very suitable for use in, for example, the manufacture of rods, tubes, such as welded and seamless tubes, sheet metal, strip, wire, welding wire, structural parts, such as for example flanges and couplings.

These objects are achieved according to the present invention with duplex stainless steel alloys containing (in% by weight) 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, O-3.0% Mn, 0- 0.0030% B, up to 0.010% S, 0-0, 03% Al, O-0.010% Ca, O-3.0% W, 0-2.0% Cu, 0-3.5% Co, 0-0.3% Ru, the remainder Fe and unavoidable impurities.

Brief Description of the Figures Figure 1 shows CPT values from tests of the test batches in the modified ASTM G48C test in the "green-death" solution compared to the duplex steels SAF2507, SAF 2906 and the high alloy austenitic steel 654SMO.

Figure 2 shows CPT values produced using the modified ASTM G48C test in the "green-death" solution for the test batches compared to the duplex steel SAF2507 and the austenitic steel 654SMO.

Figure 3 shows the mean value of the etching in mm / year in 2% HCl at the temperature 75 ° C.

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

Detailed description of the invention A systematic development work has surprisingly shown that through a well-balanced combination of the elements Cr, Mo, Ni, N, Mn and Co one can obtain optimal distribution of the elements iferrite and in the austenite, which enables a very corrosion-resistant material with only negligible amount of sigma phase in the material. The material also obtains good machinability which enables extrusion into seamless tubes. It turns out that in order to obtain the combination 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% by weight / °): C Max 0.03% Si Max 0.5% Mn O - 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 O-0.03% Ca 0-0.010% the residue Fe together with normally occurring impurities and additives, the ferrite content being 40-65% by volume. 591 (C) has limited solubility in both ferrite and austenite. The limited solubility involves a risk of precipitation of chromium carbides and therefore the content should not exceed 0.03% by weight, preferably not exceed 0.02% by weight. ßjsggl (Si) is used as a deoxidizing agent in steelmaking and increases the flowability in manufacturing and welding. However, too high levels of Si lead to the precipitation of undesired intermetallic phase, so the content should be limited to a maximum of 0.5% by weight, preferably a maximum of 0.3% by weight. Manganese (Mn) is added to increase the N-solubility of the material. However, it has been found that Mn has only a limited effect on the N-solubility of the type of alloy in question. Instead, there are other elements with a higher impact on solubility. In addition, Mn in combination with high sulfur contents can give rise to the formation of manganese sulphides which act as initiation points for point corrosion. The Mn content should therefore be limited to between 0-3.0% by weight, preferably 0.5-1.2% by weight. _l§_r_c¿rg (Cr) is a very active element for improving resistance to most types of corrosion. A high chromium content also 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 the corrosion resistance. For very good values of corrosion resistance, the chromium content should be at least 24.0% by weight, preferably 27.0-29.0% by weight. However, high levels of Cr increase the risk of intermetallic precipitates, so the chromium content must be limited upwards to a maximum of 30.0% by weight. fl i_c_k_el (Ni) is used as an austenite stabilizing element and is added at appropriate levels so that the desired ferrite content is achieved. In order to achieve the desired ratio between the austenitic and the ferritic phase with between 40-65% by volume of ferrite, an addition of between 4.9-10.0% by weight of nickel, preferably 4.9-8.0% by weight, is required. %.

Molybdenum (Mo) is an active element that improves the corrosion resistance in chloride environments and preferably in reducing acids. Too high a Mo content in combination with the Cr contents being high, means that the risk of intermetallic precipitates increases. The mo content of the present invention should be in the range of 3.0-5.0% by weight, preferably 3.6-4.7% by weight, in particular 4.0-4.3% by weight.

Nitrogen (N) is a very active element that increases the corrosion resistance, structural stability and strength of the material. A high N content also improves the regeneration of austenite after welding, which gives good properties of welded joints. To achieve a good effect of N, at least 0.28 oases should be: - neon. 10 15 20 25 30 5 2 4 9 5 2 ° -.- "íš 'f 7% by weight N is alloyed. At high levels of N the risk of precipitation of chromium nitrides increases, especially when the chromium content is at the same time high. In addition, a high N content that the risk of porosity increases due to the solubility of N in the melt being exceeded, the N content should for these reasons be limited to a maximum of 0.5% by weight, preferably alloyed> 0.35 - 0.45% by weight N. low ( B) is added to increase the hot workability of the material, if the boron content is too high, the weldability and corrosion resistance can be impaired, so the boron content should be limited to 0.0030% by weight.

Sulfur (S) has a negative effect on the corrosion resistance by forming easily soluble sulphides. In addition, the hot workability deteriorates, which is why the sulfur content is limited to a maximum of 0.010% by weight. ßgtgg fl (Co) is added mainly to improve the structural stability and corrosion resistance_ Co is an austenite stabilizer. To achieve effect, at least 0.5% by weight, preferably at least 1.5% by weight, should be alloyed. As cobalt is a relatively expensive element, the cobalt addition is therefore limited to a maximum of 3.5% by weight. g increases resistance to point and crevice corrosion. However, the alloying of too high levels of tungsten in combination with the high Cr and Mo levels, means that the risk of intermetallic precipitates increases. The W content of the present invention should be in the range of 0-3.0% by weight, preferably between 0.5 and 1.8% by weight.

Qppg is added to improve the general corrosion resistance in acidic environments such as sulfuric acid. Cu also affects structural stability. However, high levels of Cu mean that the solid solubility is exceeded. The Cu content is therefore limited to a maximum of 2.0% by weight, preferably between 0.5 and 1.5% by weight. nan-n '10 15 20 25 30 524 952 8 Ruthenium (Ru) is alloyed to increase corrosion resistance. As ruthenium is a very expensive element, the content is limited to a maximum of 0.3% by weight, preferably greater than 0 and up to 0.1% by weight / 0.

Aluminum (Al) and Calcium (Ca) are used as deoxidizing agents in steel production. The content of Al should be limited to a maximum of 0.03% by weight to limit nitride formation. Ca has a beneficial effect on hot ductility, but the Ca content should be limited to 0.010% by weight to avoid unwanted amount of slag.

The ferrite content is important to obtain good mechanical and corrosion properties as well as good weldability. From the point of view of corrosion and weldability, it is desirable to have a ferrite content between 40-65% in order to obtain good properties. High ferrite levels also mean that the low-temperature impact strength and the resistance to hydrogen embrittlement risk deteriorating. The ferrite content is therefore 40-65% by volume, preferably 42-60% by volume, in particular 45-55% by volume.

Description of Preferred Embodiments The examples below set forth the composition of a number of test batches, which illustrate the effect of various alloying elements on the properties. Charge 605182 represents a reference composition and thus does not fall within the scope of this invention. Nor should other charges be considered to limit the invention, but only give examples of charges which illustrate the invention according to the claims.

Stated PRE numbers or values always refer to values calculated according to the PREW formula, even if not explicitly stated.

Example 1 Test batches according to this example were manufactured by laboratory casting of 170 kg of ingot which was hot forged into a round bar. This was heat-extruded into a bar (round bar and flat bar), where sample material was taken from a round bar. Furthermore, flat bar was annealed before cold rolling took place, after which additional sample material was taken out. From a technical point of view, the process can be considered representative of production on a larger scale, e.g. for the manufacture of seamless tubes using the extrusion method followed by cold rolling. Table 1 shows the composition of the first round test batches.

Table 1. Composition for test batches,% by weight.

Charge 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 0.01 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 0,0,0 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 In order to examine the structural stability, samples from each batch were annealed. at 900-1150 ° C with steps of 50 ° C and extinguished in air and water, respectively. At the lowest temperatures, intermetallic phase was formed. The lowest temperature at which the amount of intermetallic phase became negligibly small was determined by means of studies under a light optical microscope. New samples from each batch were then annealed at said temperature for five minutes after which the samples were cooled at the constant cooling rate of -140 ° C / min to room temperature. The area fraction sigma phase in the materials was then determined by digital image analysis of images taken with backscattered electrons in a scanning electron microscope. The results are shown in Table 2.

Tmax sigma is calculated with Thermo-Calc (T-C version N thermodynamic database for steel TCFE99) based on guideline values for all specified elements in the different variants. Tmax sigma is the dissolution temperature for sigma phase, where high dissolution temperature indicates lower structural stability. 10 15 20 25 524 952 10 Table 2.

Charge Heat treatment Quantity c [vol-%] Tmax o 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 study is to be able to rank materials with respect to structural stability, ie this is not the actual content of sigma phase in the test pieces that have been heat treated and extinguished before e.g. corrosion test.

It can be seen that Tmax sigma calculated with Thermo-calc does not directly correspond to the measured amount of sigma phase, but it is clear that the experimental charges with the lowest calculated Tmax sigma contain the lowest amount of sigma phase in this study.

The point corrosion properties of all charges have been tested for ranking in the so-called "green-death" solution, which consists of 1% FeCl3, 1% CuCl2, 11% H2SO4, 1.2% HCl. The test procedure corresponds to spot corrosion testing according to ASTM G48C, but is performed in the more aggressive "green-death" solution. In addition, some chargers have been tested according to ASTMG48C (2 trials per charge). Electrochemical testing in 3% NaCl (6 trials per batch) has also been performed. The results in the form of critical point corrosion temperature (CPT) from all experiments are shown in Table 3, such as the PREW number (Cr + 3.3 (Mo + 0.5W) + 16N) for the total alloy composition and for austenite and ferrite. The index alpha refers to ferrite and gamma refers to austenite. 10 15 true 20 years »524 952 11 Table 3.

Charge PRE PRE y PRE y / PRE CPT ° C CPT ° C CPT ° C q PRE a Modified ASTM G48 C 3% NaCl ASTM G48C 6% FeCla Green Death 605193 51.3 49.0 0.9552 46.9 90/90 64 605195 51.5 48.9 09495 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 605153 49.6 51.9 1.0464 48.3 80/85 85 90 605182 54.4 46.2 0.8493 46.6 75/70 85 62 654SMO 90/85 SAF2507 70/70 SAF2609 60/50 It is accepted that there is a linear relationship between the lowest PRE value in austenite or ferrite and the CPT value in duplex steels, but the results in Table 3 show that the PRE number does not only explain the CPT value. Figure 1 presents CPT values from tests in the modified ASTM G48C test in diagrammatic form. The duplex steels SAF2507, SAF2906 and the highly alloyed austenitic steel 654SMO are included as a reference. It is clear from these results that all experimental materials show better CPT in the modified ASTM G48C than SAF2507 and SAF2906. Furthermore, some of the test materials CPT in the modified ASTM G48C show results at or above 654SMO. Experimental charge 605183 Alloyed with cobalt shows good structural stability at controlled cooling rate (-140 ° C / min) despite the fact that it contains high levels of chromium and molybdenum, shows better results than SAF2507 and SAF2906. It turns out in this study that a high PRE not only explains the CPT values, but the PRE austenite / PRE ferrite ratio is of utmost importance for the properties of higher alloyed duplex steels, and a very narrow and careful balance between the alloying elements is required to obtain this optimal ratio ranging from 0.9-1.15; preferably 0.9-1.05 and at the same time obtain PRE values above 46. The ratio of PRE austenite / PRE ferrite to CPT in the modified ASTM G48C test for the test batches is reported in Table 3. were 0000 o on-an oe a. v no os out its; a »n of ununos 4 0 a ß nan 'nu vb IIIOIII o nu nu anno o. nu 10 524 952 12 The strength at room temperature (RT), 100 ° C and 200 ° C and the impact resistance at room temperature (RT) have been determined for all charges and is displayed as the mean of three trials.

Tensile rods (DR-5C50) were made from extruded rods, ø 20mm, 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 study are presented in Tables 4 and 5. The results of the tensile strength test show that the levels of chromium, nitrogen and tungsten strongly affect the tensile strength of the material. All chargers except 605153 meet the requirement for a 25% elongation during tensile testing at room temperature (RT).

Table 4. Tensile strength Charge Temperature RW; Rom Rm A5 Z (MPH) (MPa) (MP2) (%) (° / °) 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 27 200 ° C 532 61 1 749 23.7 31 605182 RT 627 754 903 28.4 43 100 ° C 493 621 802 31.8 42 10 15 524 952 13 Table 5. Impact strength Charge Annealing Cooling Impact annealing Annealing Cooling Impact [° C / min] [Jl [° C / min] [J] 6051 93 1 100/20 Air 35 1 100/20 Water 242 605195 1 150/20 Water 223 605197 1 100/20 Water 254 1 130/20 Water n 259 605178 1100/20 Air 62 1100/20 Water 234 605183 1050/20 Air 79 1050/20 Water 244 605184 1 100/20 Water 81 1 100/20 Air 78 605187 1050/20 Air 51 1 100/20 Water 95 605153 1 100/20 Air 50 1 100/20 Water 246 605182 1 100/20 Air 22 1 100/20 Water 324 This study shows very clearly that water extinguishing is of course required to obtain the best structure and thus good impact strength values. The requirement is 100J when tested at room temperature and this can handle all charges, except charge 605184 and 605187, where admittedly the latter is very close to the requirement.

Table 6 shows results from the Tungsten-lnert-Gas remelting test (hereinafter abbreviated TIG) where the charges 605193, 605183, 605184 and 605253 show a good structure in the heat affected zone (hereinafter abbreviated HAZ). The Ti-containing charges show TiN in HAZ. Too high a chromium and nitrogen content results in the precipitation of Cr2N, which should be avoided as it impairs the properties of the material.

Table 6.

Charge Excretions Shielding gas Ar (99.99%) HAZ: OK HAZ: Large amounts of TIN and c-phase HAZ: Small amounts of CrzN in island barley, but not much HAZ: CrzN in island barley, otherwise OK HAZ: OK HAZ: OK HAZ: CrzN quite close to the melting limit, no precipitates further out HAZ: OK HAZ: TiN and decorated grain boundaries 8/8 605193 605195 605197 605178 605183 605184 605187 605153 605182 10 15 20 524 952 14 Example 2 in the following example is given. number of test batches manufactured with the intention of finding the optimal composition. These charges are modified based on the properties of the charges with good structural stability and high corrosion resistance, from the results shown in Example 1. All charges in Table 7 are included in the composition of the present invention, where charge 1-8 is included in a statistical test plan, while charge e to n are further experimental alloys within the scope of this invention.

A number of experimental batches were produced by casting 270 kg of ingot which was hot forged into a round bar. This was extruded into rods, from which sample material was taken. Then the rod was annealed before cold rolling of the flat rod took place, then additional sample material was taken out. Table 7 shows the composition of these test batches.

Table 7.

Charge Mn Cr Ni Mo W Co Cu Ru BN 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 6.8 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 0.0021 0.38 f 605255 1.0 28.6 6.5 4.0 3.0 0.0020 0.31 g 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 i 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 l 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 1.5 0.0019 0.38 524 952 f * fi ¥ 15 Table 8. Thermo -Calc variant a-formula ot Tc PRE total PRE 8 PRE y Tma Tmax empirical Sigma CrgN 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 n 50 53 50.0 48.4 51.7 1035 1087 i 52 50 49.1 48.4 49 , 8 1019 1084 Thermo-Calc values according to table 8 (TC version N 5 thermodynamic database for steel TCFE99) are based on guide values for elements in the different variants. PRE numbers for ferrite and austenite are based on their equilibrium composition at 1100 ° C. Tma, sigma is the dissolution temperature for sigma phase, where high dissolution temperature indicates lower structural stability.

The distribution of alloying elements in the ferrite and austenite phase 10 was investigated by microprobe analysis, the results are shown in Table 9.

Table 9.

Charge Fas Cr Mn Ni Mo W Co Cu N 605258 Ferm 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 Ferm 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 Ferm 29.7 1.3 5 , 3 5.3 0.5 0.10 N 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 ya: Austenite 26.5 1.4 6.3 3.6 1.5 0.54 _ 3 605250 Femt 30.1 1.3 5.6 5.1 0.46 0.07. Austentt 27.3 1.4 8.8 3.4 0.53 0.52 _Z_, 605251 Ferm 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 ° ', 605261 Ferrrt 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 '2-'2 605252 Ferrit 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 Femt 28.1 1.3 4.9 5.8 0.89 0.08 »nano 10 15 20 524 952 - 16 Austenite 26.0 1.4 7.6 3 .8 1.0 0.48 605255 Ferrit 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 Ferrit 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 Ferrit 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 Ferrit 31.0 1.4 5.7 4.8 0.09 Austenite 29.0 1.5 8.4 3.1 0.52 605269 Ferrit 28.7 1.3 5.2 5.1 1.4 0.9 0.9 0.11 Austenite 26.6 1.4 7.8 3.2 0.87 1, 1 0.52 605268 Ferrit 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 Ferrit 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 Ferrit 30.1 1 , 3 5.1 4.9 1.3 0.08 Austenit 27 , 8 1 4 7.6 3.1 1 8 0.46 The point corrosion properties of all charges have been tested in the "green death" solution (1% FeCl3, 1% CuCl2, 11% H2SO4, 1.2% HCl) for ranking. The test procedure is the same as spot corrosion testing according to ASTM G48C, however, the test is performed in a more aggressive solution than 6% FeCl 2, the so-called "green-death" solution. The results from all experiments are shown in Table 10, Figure 2 and Figure 3. All tested charges perform better than SAF2507 in the green death solution. All charges are within the identified range 0.9-1.15; preferably 0, 9-1.05 in terms of the ratio PRE austenite / PRE ferrite at the same time as the PRE in both austenite and ferrite is higher than 44 and for most charges also significantly higher than 44. Some of the charges even reach the limit total PRE 50. It is very interesting to note that charge 605251, alloyed with 1.5 wt% cobalt, performs almost equivalent to charge 605250, alloyed with 0.6 wt% cobalt, in the "green-death" solution despite the lower chromium content in charge 605251. This is particularly surprising and interesting as charge 605251 has a PRE number of about 48, which is higher than any commercial superduplex alloy today while Tma, another sigma value below 1010 ° C indicates a good structural stability based on the values in Table 2 in Example 1.

Table 10 also shows the PREW number (VoCr + 3.3% (Mo + 0.5% W) + 16% N) for the total alloy composition and PRE in austenite and ferrite (rounded) based on phase composition measured with a microprobe. The ferrite content is measured after heat treatment at 1100 ° C followed by water quenching.

Table 1 0 Charge ot content PREW PRE PRE PREy / CPT ° C Total uy PREO, 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 90180 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 To further investigate the structural stability, the samples were annealed in 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 negligibly small was determined by examinations under a light optical microscope. A comparison of the charge structure after annealing at 1080 ° C followed by water quenching indicates which of the charges are more likely to contain unwanted sigma phase.

The results are shown in Table 11. Structural control shows that the charges 605249, 605251, 605252, 605253, 605254, 605255, 605259, 605260, 605266 and 605267 are free from unwanted sigma phase. Furthermore, charge 605249, alloyed with script sauce 10 15 20 25 524 952 f "18 is 1.5% by weight cobalt, free of sigma phase, while charge 605250, alloyed with 0.6% by weight cobalt, contains some sigma phase. Both charges are alloys with a high content of chromium close to 29.0% by weight and a molybdenum content of close to 4.25% by weight.Comparing the compositions of the charges 605249, 605250, 605251 and 605252 in view of the sigmaphase content, it is very clear that the composition range the material with respect to in this case structural stability is very narrow.Furthermore, it turns out that charge 605268 contains only single sigma phases compared to charge 605263 which contains a lot of sigma phase.The main difference between these charges is the addition of copper to charge 605268. l Charge 605266 and 605267 are sigma phase free despite high chromium content, the latter charge is alloyed with copper.

Furthermore, the batches 605262 and 605263 with the addition of 1.0% by weight of tungsten show a structure with a lot of sigma phase, while it is interesting to note that charge 605269 also with 1.0% by weight of tungsten but with a higher nitrogen content than 605262 and 605263 shows a significantly less amount of sigma phase. A very well-balanced balance is thus required between the various alloying elements at these high alloying levels for e.g. chromium and molybdenum to obtain good structural properties.

Table 11 shows the results of the light optical examination after annealing at 1080 ° C, 20 minutes followed by water quenching. The amount of sigma phase is indicated with values from 1 to 5, where 1 represents that no sigma phase was detected during the examination, while 5 represents that a very high content of sigma phase was detected during the examination.

Table 1 1 _ Chage 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 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 10 15 young; 20 mogna 524 952 19 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 l Table 12 is shown results from impact testing of some of the charges. The results are very good, which indicates fine structure after annealing at 1100 ° C followed by water extinguishing and the requirement of 100J is met by a large margin of all tested charges.

Table 12.

Charge Annealing Cooling Impact strength Impact strength Impact strength FC / mi fl l [J] [J] [Jl 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 Figure 4 shows the results from hot ductility testing of most of the charges. Good machinability is of course crucial to be able to manufacture the material into product shapes such as rods, pipes, such as welded and seamless pipes, sheet metal, strips, wire, welding wire, structural parts, such as flanges and couplings. The charges 605249, 605250, 605251, 605252, 605255, 605266 and 605267 most with a nitrogen content of around 0.38% by weight show slightly better hot ductility values. some: »coon 10 15 20 25 30 524 952 20 Summary of test results In order to obtain good corrosion properties, while the material exhibits good structural stability, hot workability and weldability, the material should be optimized as follows: o The PRE number in ferrite should exceed 45, but preferably be at least 47. o The PRE number in austenite should exceed 45, but preferably be at least 47. o The PRE number for the whole alloy should preferably be at least 46. o The ratio PRE austenite / PRE ferrite should be in the range 0.9-1, 15; preferably in the range 0.9-1.05. The ferrite content should be in the range, preferably 45-55% by volume. o Tma, sigma should not exceed 1010 ° C. The nitrogen content should be in the range 0.28-0.5% by weight, preferably in the range 0.35-0.48% by weight, but preferably 0.38 -0.40% by weight. The cobalt content should be in the range O-3.5% by weight, preferably 1.0-2.0% by weight, but preferably 1.3-1.7% by weight. o In order to ensure high nitrogen solubility, ie if the nitrogen content is in the range 0.38-0.40% by weight, at least 29% by weight Cr should be added and at least 3.0% by weight Mo, so that the total content of the elements Cr, Mo and N meets said requirements of the PRE number.

Claims (15)

524 9 52 '=. =' Ä fi ïš ~ í fi ïii = ~ Zl Patentkrav 1. Ferrite-austenitic duplex stainless steel alloy, made by conventional methods, characterized in that it has the following composition in% 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% co iz o -3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3% Al 0-0.03% Ca 0-0.010% and the rest Fe together with normally occurring impurities and additives, the ferrite content is 40-65% by volume and 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, where% refers to% by weight. 2. An alloy according to claim 1, characterized in that the manganese content is between 0.5 and 1.2% by weight. 3. An alloy according to claim 1 or 2, characterized in that the chromium content is between 27.0 and 29.0% by weight. 4. An alloy according to claims 1-3, characterized in that the nickel content is between 5.0 and 8.0% by weight. . o ø ».n 524 952 2 .. ¥ * ~ â ~~ 2..ëIII = ~" 22. 5. An alloy according to claims 1-4, characterized in that the molybdenum content is between 3.6 and 4.7% by weight. 6. An alloy according to claims 1-5, characterized in that the nitrogen content is between 0.35 and 0.45% by weight. 7. An alloy according to claims 1-5, characterized in that the ruthenium content is between 0 and 0.3% by weight, preferably greater than 0 and up to 0.1% by weight. Alloy according to one of the preceding claims, characterized in that the cobalt content is between 0.5 and 3.5% by weight, preferably between 1.5 and 3.5% by weight. Alloy according to one of the preceding claims, characterized in that the copper content is between 0.5 and 2.0% by weight, preferably between 1.0 and 1.5% by weight. Alloy according to one of the preceding claims, characterized in that the ferrite content is between 42 and 60% by volume, preferably between 45 and 55% by volume. An alloy according to any one of the preceding claims, characterized in that the PRE or PREW value for both the ferrite and austenite phases is greater than 45 and the PRE or PREW value for the total alloy composition is greater than 46. Alloy according to Claim 1 or 11, characterized in that the PRE or PREW value for both the ferrite and austenite phases is between 47 and 49. 13. An alloy according to claim 1, 11 or 12, characterized in that the ratio between the PRE (W) value of the austenite phase and the PRE (W) value of the ferrite phase is between 0.90 and 1.15; preferably between 0.9 and 1.05. . n n n .n nn. ... n n .n n n n n. n n n. n n n n n a n n n .n n n .n n n n n n n. n n. n n n nnn. n n n .nu .nn n. n .... nnn n nn n n n n n n. n .n n n. n n n n n. n L- u n. . .n n n n. n. n n. n. n 23 Alloy according to any one of the preceding claims for use in chloride-containing environments. Alloy according to one of the preceding claims for use in chloride-containing environments in product forms such as rods, pipes, such as welded and seamless pipes, sheet metal, strip, wire, welding wire, structural parts, such as, for example, pumps, valves, flanges and couplings.