ES2266557T3 - DUPLEX STEEL ALLOY. - Google Patents

Abstract

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

Description

Duplex Steel Alloy

Technical Field of the Invention

The present invention relates to an alloy from stainless steel, more specifically, to a duplex alloy of stainless steel with a ferritic-austenitic matrix and with high corrosion resistance in combination with good structural stability and ability to be hot worked. He Duplex stainless steel has a ferrite content of 45-60% by volume and a good composition balanced, which imparts resistant properties to the material corrosion that make it more suitable for use in environments that they contain chloride than what was previously considered to be possible.

Background of the invention

During the last years, the environments in the that metal materials resistant to corrosion have become more aggressive and have increased requirements regarding its properties against corrosion as well as its mechanical properties. Steel alloys duplexes, which were established as an alternative until they were used as steel grades such as, for example, austenitic steels  highly alloyed, nickel base alloys or other high steels alloy, they are not exempt from this development.

An established measure of resistance to corrosion in media containing chloride is called equivalent of bite resistance (abbreviated, PRE) that define as

PRE =% Cr + 3.3% Mo + 16% of N

in which the percentages for each Element refer to percentages by weight. One more numerical value high indicates better corrosion resistance, in particular against pitting corrosion. Alloy elements essential that affect that property are, according to the formula, Cr, Mo and N. You can see an example of such quality of steel in patent EP0220141, which by reference is included in this description. This steel quality with the designation SAF2507 (UNS S32750) is mainly alloyed with high Cr, Mo and N. Consequently it has been developed in consideration of this property with all the good resistance in chloride media previously mentioned.

Recently, the elements Cu and W have proved to be efficient alloying additions to optimize properties of steel against corrosion in chloride media. The element W is then used as a substitute for a part of the Mo as, for example, in DP3W commercial alloy (UNS S39274) or Zeron 100, which contain, respectively, 2% and 0.7% W. last alloy contains even 0.7% Cu in order to increase the corrosion resistance of the alloy in environments acids

The addition of the tungsten alloy element led to develop the measure of corrosion resistance and, therefore, to move from the PRE formula to the PREW formula, which it also makes the relationship between the influence of the Mo and W on the corrosion resistance of alloys:

PREW =% of r + 3.3 (% Mo + 0.5% W) + 16% of N

as described, for example, in EP 0545753. This publication refers to an alloy stainless steel duplex with corrosion properties improved overall. The steel qualities described above have a PRE number that, regardless of the calculation method, is above 40

Of alloys with good resistance to corrosion in chloride environments the SAF will also be mentioned 2906, composition that appears in EP 0 708 845. This alloy, which is characterized by higher Cr and N contents comparatively that those of for example, SF2507, has revealed to be especially suitable for use in media where the resistance to intergranular corrosion and corrosion in carbamate  ammonium, but also has great corrosion resistance in media containing chloride.

In addition, EP 534 864 describes a duplex stainless steel with high corrosion resistance produced by powder metallurgy, and which is intended for use in media containing chloride.

The document US-A-4 985 091 describes an alloy intended for use in media with hydrochloric acid and acid sulfuric acid, in which corrosion mainly occurs intergranular It is primarily intended as an alternative to recently used austenitic steels.

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

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The disadvantage of the alloys described in above, all of them with high PRE numbers, is the appearance of hard and fragile intermetallic precipitates in steel, such as example, the sigma phase, especially after treatment thermal, such as, for example, welding in a subsequent elaboration. This results in a harder material with a worse ability to be worked and, finally, a resistance to deteriorated corrosion

In order to improve, among other aspects, the resistance of stainless steel duplex to corrosion by bites, an increase in the PRE number of the ferrite phase is required and of the austenite phase, without risking the structural stability or the ability of the material to be worked up. If the composition of the two phases is not equivalent in As for the active alloy components, one phase becomes more sensitive to pitting or chink corrosion. Consequently, the phase that is most sensitive to corrosion controls the strength of the alloy, while the stability Structural is controlled by the most alloyed phase.

Summary of the invention

Therefore, it is an objective of the present invention provide a stainless steel duplex alloy that have high corrosion resistance in combination with some improved mechanical properties and that is very appropriate for use in environments where high corrosion resistance is required general and to localized corrosion, such as, for example, in media containing chloride.

It is another objective of the present invention provide a stainless steel duplex alloy with a ferrite content in the range of 40 to 65% by volume and a PRE number of at least 46 to 50 in both the ferrite phase as in the austenite phase, and with an optimal relationship between the PRE of the austenite and the PRE of the ferrite in the range of 0.90 to 1.15; preferably between 0.9 and 1.05.

It is another objective of the present invention provide a stainless steel duplex alloy with a Critical temperature of pitting corrosion (abbreviated CPT) whose value is greater than 90 ° C, preferably greater than 95 ° C, and a critical temperature of corrosion in cracks (abbreviated CCT) whose value is at least 50 ° C in 6% FeCl 3, preferably at least 60 ° C in 6% FeCl 3.

It is also another object of the invention. provide an alloy with an impact resistance of as minimum 100 J at room temperature and a tensile elongation of at least 25% at room temperature.

For this high content of alloying elements, the material according to the present invention has an aptitude to be worked remarkably good, in particular to be hot worked and, therefore, will be very suitable for use in, for example, the production of bars, tubes such as welded and seamless tubes, plate, strip, wire, welding wire, construction parts of, for example, pumps, valves, seals and couplings.
Hundreds

These objectives are met in accordance with the present invention with stainless steel duplex alloys, containing (in% weight) up to 0.03% of C, up to 0.5% of Si, 24.0-30.0% of Cr, 4.9-10.0% of Ni, 3.0-5.0% Mo, 0.28-0.5% N, 0-3.0% of Mn, 0-0.0030% of B, up to 0.010% of S, 0-0.03% of Al, 0-010% of Ca, 0-3.0% of W, 0-2.0% of Cu, 0.5-3.5% Co, 0-0.3% Ru, 0-0.35% of Ti, 0-0.35% of V, remainder Faith and inevitable impurities. The ferrite content of the alloy It is 40-65% by volume, the value of PRE or PREW for both phases, ferrite and austenite, is greater than 45; the value of PRE or PREW for the composition of the alloy is greater than 46 and the relationship 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.

Brief description of the drawings

Figure 1 represents the CPT values of the tests carried out with test casting according to the G48C test of ASTM in "Green Death" solution compared to steels Duplex SAF2507, SAF 2906 as well as with austenitic steel Highly alloy 654SMO.

Figure 2 presents certain CPT values with the help of the ASTM G48C test modified in solution "Green Death "for trial casting, compared to duplex steel SAF2507 as well as 654SMO austenitic steel.

Figure 3 represents the average amount of erosion in mm / year in 2% HCl at the temperature of 75 ° C.

Figure 4 represents the test results hot ductility for most laundry.

Detailed description of the invention

A systematic development work has revealed surprisingly that, by means of a good combination balanced of the elements Cr, Mo, Ni, N, Mn and Co, you can obtain an optimal contribution of the elements in the ferrite and the austenite, which provides a very resistant material to the corrosion with only a negligible amount of sigma phase in the material. The material also has a good aptitude to be worked, allowing to extrude seamless pipes. It gets from stated that, with the intention of obtaining a combination of high corrosion resistance together with good structural stability, a much tighter combination of the alloying materials. The alloy according to the invention Contains (in% by weight):

C

max. 0.03%

Yes

max. 0.5%

Mn

0-3.0%

Cr

24.0-30.0%

Neither

4.9-10.0%

Mo

3.0-5.0%

N

0.28-0.5%

B

0.0-0.0030%

S

max. 0.010%

Co

0.5-3.5%

W

0-3.0%

Cu

0-2.0%

Ru

0-0.3%

To the

0-0.03%

AC

0-0.010%

You

0-0.35%

V

0-0.35%

rest, faith and impurities normally present. He Ferrite content in the alloy is 40-65% in volume, the value of PRE and PREW in the ferrite phase and in the phase austenite is greater than 45, the value of PRE or PREW for the Total alloy composition is greater than 46 and the ratio between the PRE or PREW value for the austenite phase and the PRE or PREW for the ferrite phase is between 0.90 and 1.15.

Carbon (C) has a limited solubility in the ferrite and the austenite. Limited solubility implies a risk of precipitation of chromium carbides and, therefore, their content should not exceed 0.03% by weight, preferably of 0.02% by weight.

Silicon (Si) is used as an agent deoxidant in steel production, as well as because it increases the fluidity during production and welding. But nevertheless, too high contents of Si lead to precipitation of unwanted intermetallic phases, so its content is limited to a maximum of 0.5 by weight, preferably to a maximum of 0.3% in weight.

Manganese (Mn) is added in order to Increase the solubility of nitrogen in the material. But nevertheless, it has been seen that the Mn only has a limited influence on the Nitrogen solubility in the type of alloy in question. Be they have found other elements with a greater influence on the solubility. In addition, the Mn in combination with high contents of Sulfur can lead to the formation of manganese sulphides, which They act as starting points for pitting corrosion. By therefore, manganese content should be limited to between 0 and 3.0% by weight, preferably between 0.5 and 1.2% by weight.

Chromium (Cr) is a very active element for improve resistance against most types of corrosion. In addition, a high chromium content means that you get a good Nitrogen solubility in the material. Thus, it is desirable to maintain Cr content as high as possible in order to improve corrosion resistance To achieve very good amounts of corrosion resistance, the chromium content should be, as minimum, 24.0% by weight, preferably of 27.0-29.0% by weight. However, some content High Cr increases the risk of precipitation of compounds intermetallic, for which reason the chromium content is due limit to a maximum of up to 30.0% by weight.

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Nickel (Ni) is used as an element austenite stabilizer and is added in suitable contents with in order to obtain the desired ferrite content. To obtain the desired relationship between austenitic and ferritic phase with between 40-65 by volume of ferrite, an addition is required between 4.9 and 10.0% by weight nickel, preferably between 4.9 and 8.0% by weight.

Molybdenum is an active element that improves the corrosion resistance in chloride media as well as, preferably, in reducing acids. Too high content of Mo in combination high Cr content implies the risk that the precipitation of intermetallic compounds increases. He Mo content in the present invention should be in the range 3.0-5.0% by weight, preferably of 3.6-4.7% by weight, in particular of 4.0-4.3% by weight.

Nitrogen (N) is a very active element that increases corrosion resistance, structural stability as well as the mechanical strength of the material. In addition, a content High nitrogen improves austenite restoration after welding, which gives good properties in the welded joint. TO high nitrogen content, increases the risk of precipitation of chromium nitrides, especially when simultaneously Chrome content is high. In addition, a high nitrogen content implies that the risk of porosity increases because it exceeds the solubility of nitrogen in the molten material. For these reasons, the nitrogen content should be limited to a maximum of 0.5% by weight, preferably from more than 0.35% to 0.45% is added in weight.

Boron (B) is added in order to increase the ability to hot work the material. For a content too high boron, weldability as well as resistance to corrosion can deteriorate. Therefore, the boron content is should limit to 0.0030% by weight.

Sulfur negatively influences the corrosion resistance by forming sulfides. In addition, fitness to be hot deformed it deteriorates, so the content Boron should be limited to a maximum of 0.010% by weight.

Cobalt (Co) is added in order to improve plus structural stability, as well as resistance to corrosion. Co is a stabilizing element of austenite. For to obtain the effect, at least 0.5% by weight must be added, preferably at least 1.5% by weight. Because the cobalt is a relatively expensive element, the addition of cobalt is limited to a maximum of 3.5% by weight.

Tungsten increases resistance to corrosion by pitting and cracking. But the addition of content  too high W in combination with Cr contents as well as of Mo too high, means the risk that the risk increases of precipitation of intermetallic compounds. The content of W in the present invention must be in the range of 0-3.0% by weight, preferably between 0.5 and 1.8% in weight.

Copper is added in order to improve the general corrosion resistance in acidic media such as from sulfuric acid. At the same time, the Cu has influence on the structural stability However, high Cu content imply that solid state solubility will be exceeded. So, Cu content should 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 the corrosion resistance Because ruthenium is an element  very expensive, the content should be limited to a maximum of 0.3% by weight, preferably at more than 0 and up to 0.1% by weight.

Aluminum (Al) and calcium (Ca) are used as Deoxidants in steel production. Al content is due limit to a maximum of 0.03% by weight in order to limit the nitride formation. Ca exerts a favorable effect on the hot ductility. But the content of Ca should be limited to 0.010% by weight in order to avoid an unwanted amount of human waste.

Ferrite content is important for obtain good mechanical properties and good weldability. From the point of view of corrosion and weldability, it is desirable a ferrite content between 40-65% to Get good properties. In addition, high ferrite contents imply that the impact resistance at lows deteriorates temperatures as well as the risk of resistance to deterioration hydrogen induced fragility. The ferrite content, by therefore, it is 40-65% by volume preferably of 42-60% by volume, in particular of 45-55% by volume.

Description of preferred embodiments

In the following examples the composition of several test washes, which illustrate the effect of Different alloy elements on the properties. Laundry 605182 represents a reference composition and, consequently, it is not part of the field of this invention. The remaining castings should not be considered as limiting the invention, but only specific examples of laundry, which illustrate the invention according to the claims.

PRE numbers or values always consider amounts calculated according to the PREW formula, although This is not explicitly mentioned.

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Example one

The test washes according to this example were produced in the laboratory casting 170 kg ingots that then they were hot forged to round bars. These were extruded to rods (round and flat), drawing from the rounds the test material Flat bars were annealed before perform a cold rolling, after which more was taken test material. From the engineering point of view of materials, the process can be considered representative for the preparation on a larger scale, for example for the production of tubes seamless by extrusion procedure, followed by lamination cold Table 1 presents the composition of the first batch of test washes.

TABLE 1 Composition of test washes,% in weight

Wash Mn Cr Neither Mo W Co V The You N 605193 1.03 27.90 8.80 4.00 0.01 0.02 0.04 0.01 0.01 0.36 605195 0.97 27.90 9.80 4.00 0.01 0.97 0.55 0.01 0.35 0.48 605197 1.07 28.40 8.00 4.00 1.00 1.01 0.04 0.01 0.01 0.44 605178 0.91 27.94 7.26 4.01 0.99 0.10 0.07 0.01 0.03 0.44 605183 1.02 28.71 6.49 4.03 0.01 1.00 0.04 0.01 0.04 0.28 605184 0.99 28.09 7.83 4.01 0.01 0.03 0.54 0.01 0.01 0.44 605187 2.94 27.74 4.93 3.98 0.01 0.98 0.06 0.01 0.01 0.44 605153 2.78 27.85 6.93 4.03 1.01 0.02 0.08 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 investigate stability structural samples were collected at 900-1150 ° C each wash with 50 ° C stages and tempered in air, respectively in water. At the lowest temperatures they formed intermetallic compounds With the help of light microscopy the lowest temperature at which the phase amount was determined Intermetallic was insignificant. They were then annealed to the mentioned temperature for 5 minutes new samples of the corresponding casting, and then the samples were cooled to a cooling rate of 140 ° C / min. Then it was determined the surface fraction of the sigma phase in the materials by digital tracking of images obtained with electrons backscatter in a scanning electron microscope. The Results appear in Table 2.

T_ {mx} sigma was calculated with ThermoCalc (TC N version of databases for TCFE99 steel) on the basis of the characteristic quantities for all elements specified in the different variations. T_ {max} sigma is the dissolution temperature of the sigma phase, indicating some high dissolution temperatures more structural stability low:

TABLE 2

Wash Heat treatment Quantity of \ sigma [% in vol] T_ {max} \ sigma 605193 1100 ° C, 5 min 7.5% 1016 605195 1150 ° C, 5 min 32% 1047 605197 1100 ° C, 5 min 18% 1061 605178 1100 ° C, 5 min 14% 1038 605183 1050 ° C, 5 min 0.4% 997 605184 1100 ° C, 5 min 0.4% 999 605187 1050 ° C, 5 min 0.3% 962 605153 1100 ° C, 5 min 3.5% 1032 605182 1100 ° C, 5 min 2.0% 1028

The purpose of this research is to be able rate the material in terms of its structural stability, this is, the purpose is not the actual sigma phase content in the samples that were treated and tempered before, for example, the corrosion test You can see that T_ {max} sigma, which calculated with Termo-Calc, does not match directly with the measured quantities of sigma phase, but it is clear that the test castings with the calculated T_ {m} sigma values plus low contain the smallest amount of sigma phase during this investigation.

The pitting corrosion properties of all washes were tested to qualify them in the call "Green Death" solution, which is made up of 1% of FeCl 3, 1% CuCl 2, 11% H 2 SO 4, 1.2% HCl. The test procedure is equivalent to the corrosion test by pitting according to ASTM G48C, although made in solution "Green Death", more aggressive. Also, some of the laundry is tested according to ASTM G48C (2 trials per wash). Too The electrochemical test was performed in 3% NaCl (6 tests per wash). The results in the form of the critical temperature of Bite (CTP) of all trials are listed in Table 3, such as the PREW number (Cr + 3.3 (Mo + 0.5W) + 16N) for the composition  Total alloy as well as for austenite and ferrite. The index α refers to ferrite and γ to austenite.

TABLE 3

Wash PRE? PRE γ PREg / PRE? PRE CPTºC CPTºC CPTºC ASTMG48C ASTMG48C NaCl to the 3% modified FeCl 3 to the 6% Greendeath 605193 51.3 49.0 0.9552 46.9 90/90 64 605195 51.5 48.9 0.9495 48.7 90/90 95 605197 53.3 53.7 1,0075 50.3 90/90 > 95 > 95 605178 50.7 52.5 1,0355 49.8 75/80 94 605183 48.9 48.9 1,0000 46.5 85/85 90 93 605184 48.9 51.7 1,0573 48.3 80/80 72 605187 48.0 54.4 1,1333 48.0 70/75 77 605182 54.4 46.2 0.8493 46.6 75/70 85 62 654SMO 90/85 SAF2507 70/70 SAF2609 60/50 605153 49.6 51.9 1.0464 48.3 8085 85 90

It has been established that there is a linear relationship between the lowest PRE number of austenite or ferrite and the value CPT of duplex steel, but the results in Table 3 reveal that the PRE number does not explain only the CPT values. In Fig. 1 The CPT values of the ASTM G48C test are plotted modified. SAF2507 and SAF2906 duplex steels and also steel 654SMO high alloy austenitic are included as reference. Be clearly deduces from these results that all materials tested show a better PCT in the modified ASTM G48C test that both SAF2507 and SAF2906. In addition, some of the Test materials present CPT results according to ASTM G48C modified to the same level, or higher, than 654SMO steel. The test wash 605183, cobalt alloy, presents good structural stability at a controlled cooling rate of (140ºC / min), although it contains high chromium contents and molybdenum; It has better results than AF2507 and SAF2906. This research shows that a high PRE alone does not explain CPT values, but the PRE austenite / PRE ratio of Ferrite has a great influence on the properties of more highly alloyed duplex steels and an adjustment is required narrow and exact between the alloy elements to get this optimal ratio, which is between 0.9 and 1.15, preferably 0.9 and 1.05, and simultaneously obtain PRE values above 46. The PRE ratio of austenite / PRE ferrite versus test CPT Modified ASTM G48C for test washes are given in the Table 3.

They have been determined for all washings the mechanical resistance at room temperature (t.a.), 100ºC and 200ºC and impact resistance at room temperature (t.a.); the Results are presented as an average of three trials.

Test specimens for tensile tests (DR-5C50) were obtained from extruded 20mm bars in diameter, which were heat treated at temperatures according with Table 1 for 20 min and then cooled in air or in water (605195, 605197, 605184). The test results are presented in Tables 4 and 5. The results of the tests a traction reveal that the contents of chromium, nitrogen and tungsten they have a large influence on the impact resistance of material. Apart from 605153, all laundry satisfies the 25% elongation requirement in the tensile test at room temperature (t.a.).

TABLE 4 Impact resistence

Wash Temperature R_ {p0,2} MPa R_1.0 MPa R m MPa TO 5 % Z% 695193 t.a. 652 791 916 29.7 38 100ºC 513 646 818 30.4 36 200ºC 511 583 756 29.8 36 605195 t.a. 671 773 910 38.0 66 100ºC 563 637 825 39.3 68 200ºC 504 563 769 38.1 64 605197 t.a. 701 799 939 38.4 66 100ºC 584 652 844 40.7 69 200ºC 502 577 802 35.0 65 605178 t.a. 712 828 925 27.0 37 100ºC 596 677 829 31.9 Four. Five 200ºC 535 608 763 27.1 36 605183 t.a. 677 775 882 32.4 67 100ºC 560 642 788 33.0 59 200ºC 499 678 737 29.9 52 605184 t.a. 702 793 915 32.5 60 100ºC 569 657 82 34.5 61 200ºC 526 581 774 31.6 56 605187 t.a. 679 777 893 35.7 61 100ºC 513 628 799 38.9 64 200ºC 505 558 743 35.8 58 605153 t.a. 715 845 917 20.7 24 100ºC 572 692 817 29.3 27 200ºC 532 611 749 23.7 31 605182 t.a. 627 754 903 28.4 43 100ºC 493 621 802 31.8 42

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TABLE 5 Impact resistence

Wash Annealing Cooling Resistance Annealing Cooling Resistance (ºC / min) to the impact J (ºC / min) to impact, J 605193 1100/20 To the air 35 1100/20 In Water 242 605195 1150/20 In Water 223 605197 1100/20 In Water 254 1130/20 In Water 259 605178 1100/20 To the air 62 1100/20 In Water 2. 3. 4 605183 1050/20 To the air 79 1050/20 In Water 244 605184 1100/20 In Water 81 1100/20 To the air 78 605187 1050/20 To the air 51 1100/20 In Water 95 605153 1100/20 To the air fifty 1100/20 In Water 246 605182 1100/20 To the air 22 1100/20 In Water 324

This research shows very clearly that water quenching is certainly necessary in order to obtain the better structure and, consequently, good values of the impact resistence. The requirement is 100 J at temperature atmosphere and all the laundry passed this value, except the laundry 605184 and 605187, of which the latter is very close to the value required.

Table 6 presents the results obtained in the tungsten-inert gas refusion test (abbreviated TIG), in which the washes 605193, 605183, 605184 and also 605253 have a good structure in the area thermally affected (abbreviated HAZ). The laundry that contain Ti present TiN in the BEAM. Too high content of chromium and nitrogen results in the precipitation of Cr2N, that should be avoided because it deteriorates the properties of the material.

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TABLE 6

Wash Precipitation Protective gas Ar (99.99%) 605193 MAKE: good 605195 BEAM: large amounts of TiN and phase \ sigma 605197 BEAM: great amounts of Cr 2 N in grains δ, but not too many BEAM: Cr 2 N in grains \ delta, otherwise, good 605178 MAKE: good 605183 MAKE: good 605184 BEAM: Cr_ {2} N quite close of the fusion line, there are no more precipitates there 605187 MAKE: good 605153 BEAM: TiN and grain edges \ delta / \ delta decorated 605182

Example 12

In the example mentioned below, it is given the composition of several other test castings produced with the In order to find the optimal composition. These washes are modified starting from the properties of the laundry with a good structural stability as well as high corrosion resistance according to the results presented in Example 1. All Castings from Table 7 are included in the composition according with the present invention, the laundry being included 1-8 in the statistical test model, while that casts e to n are additional test alloys within of the scope of this invention.

Several test castings were produced by casting 270 kg ingots that were forged into round bars. These were extruded to bars from which the test samples were taken. The bar was then annealed before cold rolling to flat bars, after which more test material was taken. Table 7 presents the composition of these
laundry

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TABLE 7

Wash Mn Cr Neither Mo W Co Cu Ru B N one 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 and 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 1.5 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

TABLE 8 Thermo-Calc

Variant Empirical Formula of α α T-C PRE total PRE? PRE γ T_ {mx} of sigma T_ {max} of Cr2 {N} one 46 fifty 50.2 47.8 50.5 1006 1123 2 52 fifty 49.1 48.4 49.8 1019 1084 3 Four. Five fifty 50.2 47.9 52.6 1007 1097 4 46 fifty 49.2 46.5 49.8 986 1121 5 47 fifty 49.1 48.5 49.7 1028 1038 6 52 fifty 48.1 47.1 49.2 998 1086 7 44 fifty 49.2 46.6 52.0 985 1081 8 46 fifty 48.1 47.2 49.1 1008 1044 and 46 53 49.3 48.4 49.5 1010 1099 F 65 52 46.7 47.2 46.1 1008 1090 g 48 51 48.4 48.4 48.3 1039 979 h fifty 53 50.0 48.4 51.7 1035 1087 j 52 60 49.1 48.4 49.8 1019 1084

Termo-Calc values of a according to Table 8 (N version of T-C base of thermodynamic data for TCFE99 steel) are based on characteristic quantities for all items specified in The different variants. The PRE number for ferrite and Austenite is based on its equilibrium composition at 1100 ° C. Tmax of sigma is the dissolution temperature of the sigma phase, of which high dissolution temperatures indicate a lower structural stability

The distribution of alloy elements in The ferrite and austenite phases were examined by analysis of micro samples, whose results appear in Table 9 ...

TABLE 9

Wash Phase Cr Mn Neither Mo W Co Cu N 605258 Ferrite 29.8 1.3 4.8 5.0 1.4 0.11 Austenite 28.3 1.4 7.3 3.4 1.5 0.60 605249 Ferrite 29.8 1.1 5.4 5.1 1.3 0.10 Austenite 27.3 1.2 7.9 3.3 1.6 0.53 605259 Ferrite 29.7 1.3 5.3 5.3 0.5 0.10 Austenite 28.1 1.4 7.8 3.3 0.58 0.59 605260 Ferrite 28.4 1.3 4.4 5.0 1.4 0.08 Austenite 26.5 1.4 6.3 3.6 1.5 0.54 605250 Ferrite 30.1 1.3 5.6 5.1 0.46 0.07 Austenite 27.3 1.4 8.8 3.4 0.53 0.52 605251 Ferrite 29.6 1.2 5.0 5.2 1.3 0.08 Austenite 26.9 1.3 7.6 3.5 1.5 0.53 605261 Ferrite 28.0 1.2 4,5 4.9 0.45 0.07 Austenite 26.5 1.4 6.9 3.3 0.56 0.56 605252 Ferrite 29.6 1.3 5.3 5.2 0.42 0.09 Austenite 27.1 1.4 8.2 3.3 0.51 0.48 605254 Ferrite 28.1 1.3 4.9 5.8 0.89 0.08 Austenite 26.0 1.4 7.6 3.8 1.0 0.48 605256 Ferrite 30.1 1.3 5.0 4.7 2.7 0.08 Austenite 27.0 1.3 7.7 3.0 3.3 0.48 605262 Ferrite 28.8 3.0 5.3 4.8 1.4 0.9 0.08 Austenite 26.3 3.2 8.1 3.0 0.85 1.1 0.46 605263 Ferrite 29.7 1.3 5.1 5.1 1.3 0.91 0.07 Austenite 27.8 1.4 7.7 3.2 0.79 1.1 0.51

TABLE 9 (continued)

Wash Phase Cr Mn Neither Mo W Co Cu N 605253 Ferrite 30.2 1.3 5.4 5.0 1.3 0.09 Austenite 27.5 1.4 8.4 3.1 1.5 0.48 605266 Ferrite 31.0 1.4 5.7 4.8 0.09 Austenite 29.9 1.5 8.4 3.1 0.52 605269 Ferrite 28.7 1.3 5.2 5.1 1.4 0.9 0.11 Austenite 26.6 1.4 7.8 3.2 0.87 1.1 0.52 605268 Ferrite 29.1 1.3 5.0 4.7 1.3 0.91 0.84 0.12 Austenite 28.7 1.4 7.5 3.2 0.97 1.0 1.2 0.51 605270 Ferrite 30.2 1.2 5.3 5.0 1.3 0.11 Austenite 27.7 1.3 8.0 3.2 1.4 0.47 605267 Ferrite 30.1 1.3 5.1 4.9 1.3 0.08 Austenite 27.8 1.4 7.6 3.1 1.8 0.48

The pitting corrosion properties of all the washings have been determined for evaluation in the solution "Green Death" (1% of FeCl_3, 1% of CuCl_2, 11% of H 2 SO 4, 1.2% HCl). The test procedure is the same corrosion test according to ASTM G48C, but is performed in a more aggressive solution than 6% FeCl 3, the so-called "Green Death" solution. The evaluation was also carried out on general corrosion test in 2% HCl (2 tests per wash) before the dew point test. The results of all Assays are presented in Table 10, Fig. 2 and Fig. 3. All tested washings behave in the "Green Death" solution better than SAF2507. All washings are within the range identified from 0.9-1.15 preferably from 0.9-1.05, applicable for the PRE ratio of austenite / PRE ferrite at the same time as PRE in austenite and in the ferrite it is more than 44 and, for most laundry, even considerably more than 44. Some of the laundry They even reach the total PRE limit of 50. It's very interesting note that laundry 605251, alloyed with 1.5% by weight of cobalt, is behaves in the "Green Death" solution almost equivalent to laundry 605250, alloyed with 0.6% by weight of cobalt, despite the lower chromium content of the wash 605251. This It is particularly surprising and interesting because the laundry 605251 has a PRE number of approximately 48, which is greater than that of some of the commercial duplex alloys today, to the since the sigma value T_ {m} below 1010ºC indicates a good structural stability based on the values in Table 2 of Example 1.

In Table 10, even the PREW number (% of Cr + 3.3% (Mo + 0.5% W) + 16% N) for the total composition of the alloy and the PRE in austenite and also in the ferrite (rounded) is Specify as measured with micro samples. Ferrite content it was measured after heat treatment at 1100 ° C followed by quenching in water

TABLE 10

Wash Α cont. PREW total PRE? PRE γ PRE γ / PREA CPTºC Green Death 605258 48.2 50.3 48.1 49.1 1,021 605249 59.8 48.9 48.3 46.8 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 0.076 90/80 605261 50.8 48.6 45.2 46.3 1,024 605252 56.6 48.2 48.2 45.6 0.946 80/75 605254 53.2 48.8 48.5 46.2 0.953 90/75 605255 57.4 46.9 46.9 44.1 0.940 90/80 605262 57.2 47.9 48.3 45.0 0.931 605263 53.6 49.7 49.8 47.8 0.959 605253 52.6 48.4 48.2 45.4 0.942 85/75 605266 62.6 49.4 48.3 47.6 0.986 605269 52.8 50.5 49.6 46.9 0.945 605268 52.0 49.9 48.7 47.0 0.965 605270 57.0 49.2 48.5 45.7 0.944 605267 59.8 49.3 47.6 45.4 0.953

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In order to examine stability in detail structurally, samples were annealed for 20 minutes at 1080 ° C, 1100 ° C and 1150 ° C, and then they were quenched in water. With help of investigations by light optical microscopy, the temperature at which the amount of intermetallic phase was insignificant. A comparison of the structure of the washes after annealing at 1080 ° C, then tempering in water indicates which washings are more suspicious of containing the unwanted phase sigma. The results are presented in Table 11. The control of structure reveals that the washes 605249, 605251, 605252, 605253, 605254, 605255, 605259, 605260, 605266 and 605267 are exempt from the Unwanted sigma phase. In addition, laundry 605249, alloyed with 1.5% in Cobalt weight, is exempt from sigma phase, while laundry 605250, alloyed with 0.6% by weight of cobalt, contains an amount Very small sigma phase. Both casts are high alloyed chromium contents, approximately 29.0% by weight, and the Molybdenum content is approximately 4.25% by weight. Whether compare wash compositions 605249, 605250, 605251 and 605252 considering the sigma phase content, it is very clear that the composition range for that optimal material is very narrow, in this case in terms of structural stability. Be it further deduces that laundry 605268 contains only sigma phase in comparison with wash 605263, which contains a lot of sigma phase. The which mainly distinguishes these alloys from each other is the addition of copper to the wash 605268. The wash 605266 and also the 605267 are exempt from sigma phase; despite the high content of chrome, the last one is alloyed with copper. In addition, washes 605262 and 605263, with an addition of 1.0% by weight of tungsten, have a structure with a lot of sigma phase, although it is interesting to note that wash 605269, also with 1.0% by weight of tungsten but with a nitrogen content greater than that of 605262 and 605263, presents a considerably smaller amount of sigma phase. Consequently, a very well balanced balance is required between the different alloy elements to these high contents of alloys, for example, of chromium and molybdenum, in order to obtain Good structural properties

Table 11 presents the test results optical with light after annealing at 1080 ° C for 20 minutes, followed by quenching in water. The amount of sigma phase is indicated by values from 1 to 5, representing 1 that sigma phase was not detected in the exam, and meaning 5 that in the exam a content was detected Sigma phase too high.

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TABLE 11

Wash Phase sigma Cr Mo W Co Cu N Ru 605249 one 28.8 4.23 1.5 0.38 605250 2 28.8 4.24 0.6 0.40 605251 one 28.1 4.24 1.5 0.38 605252 one 28.4 4.23 0.5 0.37 605253 one 28.8 4.16 1.5 0.37 605254 one 26.9 4.80 1.0 0.38 605255 one 28.6 4.04 3.0 0.31 605258 2 29.0 4.23 1.5 0.46 605259 one 29.0 4.23 0.6 0.45 605260 one 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 one 30.0 4.02 0.38 605267 one 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

Table 12 shows the results of the impact resistance test of some of the washings. The results are very good, indicating a good structure after annealing at 1100 ° C followed by quenching in water, and the 100 J requirement will be controlled with a high margin of all tested washings.

TABLE 12

Wash Annealing (ºC / min) Quenching Resistance to Resistance to the Resistance to the impact (J) impact (J) impact (J) 605249 1100/20 In Water > 300 > 300 > 300 605250 1100/20 In Water > 300 > 300 > 300 605251 1100/20 In Water > 300 > 300 > 300 605252 1100/20 In Water > 300 > 300 > 300 605253 1100/20 In Water 258 267 257 605254 1100/20 In Water > 300 > 300 > 300 605255 1100/20 In Water > 300 > 300 > 300

The results of the hot ductility test of most of the laundry. Obviously, a good aptitude to be worked is vital importance to produce a material with which to conform products such as bars, tubes, construction elements such such as pumps, valves, gaskets and couplings. The washings 605249, 605250, 605251, 605252, 605255, 605266 and 605267, the most with a nitrogen content in the environment of 0.38% in weight, have some hot ductility values somewhat improved

Summary of test results

In order to get good properties against corrosion, while the material is good structural stability, ability to be hot worked and good weldability, the material must be optimized according to next:

?

The ferrite PRE number must exceed 45, but preferably it will be at least 47

?

The PRE number of the austenite must exceed 45, but preferably it will be at least 47

?

The PRE number for the alloy integer should preferably be at least 46.

?

PRE austenite / PRE ratio of ferrite should be in the range of 0.9-1.5, preferably in the range of 0.9-1.05.

?

Ferrite content must be preferably in the range of 45-55% in volume.

?

T_ {max} sigma must not exceed 1010 ° C.

?

The nitrogen content must be in the range of 0.28-0.5% by weight, preferably in the range of 0.38-0.48% in weight.

?

Cobalt content must be in the range of 0-3.5% by weight, preferably 1.0-2.0% by weight but, preferably, 1.3-1.7% by weight ...

?

In order to ensure high nitrogen solubility, that is, if the nitrogen content It is in the range of 0.38-0.40% by weight, it is due add at least 29% by weight of Cr as well as at least 3.0% in Mo weight, so that the total content of the elements Cr, Mo and N meet the requirements mentioned on the number PRE.

Claims (15)

1. A stainless steel duplex alloy ferritic-austenitic, which has the composition Next in% by weight:

C

max. 0.03%

Yes

max. 0.5%

Mn

0-3.0%

Cr

24.0-30.0%

Neither

4.9-10.0%

Mo

3.0-5.0%

N

0.28-0.5%

B

0.0-0.0030%

S

max. 0.010%

Co

0.5-3.5%

W

0-3.0%

Cu

0-2.0%

Ru

0-0.3%

To the

0-0.03%

AC

0-0.010%

You

0-0.35%

V

0-0.35%

rest, faith and impurities normally present, by what the ferrite content in the alloy is of 40-65% by volume, the value of PRE and PREW in the ferrite phase and in the austenite phase is greater than 45, the value of PRE or PREW for the total alloy composition is greater than 46 and the relationship 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 according to claim 1, characterized in that the manganese content is between 0.5 and 1.2% by weight. 3. Alloy according to claim 1 or 2, characterized in that the chromium content is between 27.0 and 29.0% by weight. 4. Alloy according to claim 1-3, characterized in that the nickel content is between 5.0 and 8.0% by weight. 5. Alloy according to claim 1-4, characterized in that the molybdenum content is between 3.6 and 4.7% by weight. 6. Alloy according to claim 1-5, characterized in that the nitrogen content is between 0.35 and 0.45% by weight. 7. Alloy according to claim 1-5, characterized in that the ruthenium content is between 0 and 0.3% by weight, preferably between more than 0 and 0.1% by weight. 8. Alloy according to any 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. 9. Alloy according to any 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. 10. Alloy according to any of the preceding claims, characterized in that the ferrite content is between 42 and 60% by volume, preferably between 45 and 55% by volume. 11. Alloy according to any of the preceding claims, characterized in that the total PRE or PREW value of the alloy exceeds 44, with PRE =% Cr + 3.3% Mo + 16N and PREW =% Cr + 3.3 (% Mo + 0.5% W) + 16N, considering% as% by weight. 12. Alloy according to claim 11, characterized in that the PRE or PREW value for both the austenite phase and the ferrite phase is between 47 and 49. 13. Alloy according to claim 11 or 12, characterized in that the ratio of the PRE or PREW value for the austenite phase and the PRE or PREW value for the ferrite phase is between 0.9 and 1.05. 14. Alloy according to any of the preceding claims for use in media containing chloride. 15. Alloy according to any of the preceding claims for use in media containing chloride, in product forms such as bars and tubes such as welded and seamless pipes, sheet, wire, wire for welding, construction parts such as, for example, pumps, valves, joints and couplings.