SE527178C2 - Use of a duplex stainless steel alloy - Google Patents

Abstract

The present invention relates to a stainless steel alloy, more precisely a duplex stainless steel alloy having ferritic-austenitic matrix and having high corrosion resistance in combination with good structural stability and hot-workability, in particular a duplex stainless steel having a ferritic content of 40-65% and a well-balanced composition, which gives the material corrosion properties making it more suitable for use in chloride-containing environments than what has been found possible previously. The material according to the present invention has, in view of the high alloy content thereof, extraordinarily good workability, in particular hot-workability, and should thereby be very suitable to be used for, for instance, the manufacture of bars: pipes, such as welded and weld less pipes, weld material, construction parts, such as, for instance, flanges and couplings. These objects are met according to the present invention with duplex stainless alloy, which contain (in % by weight): C more than 0 and up to max 0.03 Si up to max 0.5 Mo 0-3.0 20 Cr 24.0-30.0 Ni 4.9-10.0 Mo 3.0-5.0 N 0.28-0.5 B 0-0.0030 25 S up to max 0.010 Co 0-3.5 W 0-3.0% Cu 0-2.0 Ru 0-0.3 3 0 Al 0-0.03 Ca 0-0.010%, balance Fe together with inevitable contaminations.

Description

25 30 527 178 2 degree of movement in the water and on the surface. Examples of such movements are currents in the water, wave movements, and movements of the platform and the production ship.

The requirements placed on the pipes in an umbilical are mainly related to corrosion and mechanical properties. The pipe material must be resistant to corrosion in seawater, which encloses the outer surface of the pipes. This property is considered to be the most important because seawater has a very corrosive effect on stainless steels.

In addition, the material must have high corrosion resistance to any corrosive solutions injected into the oil well. The material must be compatible with hydraulic fluids without contaminating the fluid. Any contaminants can have a very negative effect on the service function of the control unit at the seabed.

The mechanical properties of the pipe material used are very important for the umbilical pipe application. Since the depth can be considerable at the site of oil extraction, the dynamic part of the umbilical generally becomes long, and thus heavy. The weight must be carried by the platform or the surface of the production vessel. In practice, there are two ways to reduce the weight of an umbilical with a given configuration. You can choose a lighter material or a material with the same density but with higher yield and yield strengths. By choosing a material with a higher strength, pipes with a thinner wall can be used, and thereby the total mass of the umbilical is reduced. The deeper the sea at the extraction site, the more important the total weight per unit length becomes umbilical of the material.

In recent years, as the environments in which corrosion-resistant metallic materials are used have become more stressful, the demands on the materials' corrosion properties as well as on their mechanical properties have increased.

Duplex steel alloys, which were established as an alternative to hitherto used steels, such as Ferritic steels previously used in this application, nickel base alloys or other high alloy steels, are not excluded from this development. 10 15 20 25 30 527 178 3 The latest developments in the market for umbilical tubes also entail further increased demands on the performance of the materials. The requirements that have hitherto applied with regard to strength and corrosion resistance have been met by existing alloys. However, the new requirements placed on future construction materials for umbilicals entail considerably stricter requirements for corrosion resistance, partly due to the fact that plants are designed in warmer water and partly because process solutions in the umbilical will maintain higher temperatures. The new requirements that are set may mean that the alloy must have resistance to crevice corrosion in seawater at temperatures up to 70-90 ° C. Today's construction materials do not meet these requirements with sufficient corrosion resistance. It is this problem that needs to be solved.

So far, however, all possible alloys evaluated have had a weak point. An alloy with higher resistance to chloride-induced corrosion that also meets other requirements such as increased strength and good structural stability would, on the other hand, mean greater opportunities to meet the new requirements placed on umbilical pipes.

An accepted measure of corrosion resistance in chloride-containing environments is the so-called Pitting Resistance Equivalent (abbreviated PRE) as they are P niered PRE =% Cr + 3.3% Mo + 16% N where the percentages for each element aim at weight percent.

A higher numerical value indicates a better corrosion resistance, especially against point corrosion. The main alloying elements which affect this property are according to the formula Cr, Mo, N. An example of such a steel grade appears from EP0220141, which by this reference is hereby included in this description. This steel grade with the designation SAF2507 (UNS 832750) has mainly been alloyed with high levels of Cr, Mo and N. It is thus developed towards this property with above all good corrosion resistance in chloride environments. Recently, the elements Cu and W have also been shown to be effective alloying 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, as for example in the commercial alloys DP3W (UNS 839274) or Zeron100, which contain 2.0% and 0.7% W, respectively.

The latter also contains 0.7% Cu with the aim of increasing the alloy's corrosion resistance in acid environments. The alloying of tungsten led to a further development of the measure of corrosion resistance and 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, which relates to a duplex stainless steel alloy with generally improved corrosion properties.

The steel types described above have a PRE number, regardless of calculation method, which is above 40, but the PRE number is limited upwards to about 43 because higher values mean that the alloys have substandard structural stability. A higher alloying degree increases the risk of intermetallic phase precipitation, which is why the alloying level in duplex steels is considered limited to achieving PRE values around a maximum of approximately 43, regardless of the calculation method.

Of the alloys with good corrosion resistance in chloride environments, mention should also be made of SAF 2906, the composition of which is shown 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. This alloy has a corrosion resistance in chloride environment corresponding to the alloy UNS S3275O but at the same time a higher yield strength Rp0.2. This means that this alloy has advantages over UNS S32750 as an umbilical material as lower weight of the umbilical can be obtained.

However, the corrosion resistance does not provide any improvements compared to UNS 832750, which means considerable limitations in umbilical pipes that are exposed to higher temperatures in future plants. 10 15 20 25 30 527 178 5 The alloy 19D (UNS 832001) is a duplex alloy characterized by the composition 19.5-21.5% Cr, 0.05-0.17% N and max 0.6% Mo. This alloy has a PRE number of about 22 which is why the alloy is unsuitable in seawater applications such as umbilicals. Therefore, in order to obtain sufficient corrosion resistance in this alloy, a cathodic protection must be applied in the form of a zinc layer on the outer surface of the umbilical tube. However, if the zinc layer is consumed or if a large surface is damaged, the corrosion protection is destroyed and a rapid corrosion process can occur, which means costly repairs and downtime.

A problem with 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 during welding during later processing. This leads to a harder material with poorer machinability and ultimately impaired corrosion resistance.

Another group of alloys with good corrosion resistance are austenitic steels where PRE numbers up to 55 have been made possible by alloying high levels of Cr, Mo and N combined with high Ni levels. These alloys should work very well against the new tougher corrosion conditions in umbillcals. The disadvantage of these alloys is that they have a considerably lower yield strength than duplex steels and are also considerably more expensive to manufacture, mainly due to their high content of Ni, which is an expensive alloying substance. Examples of austenites with good durability in a chloride environment are UNS S32654 with a PRE number of about 55, and UNS S34565 with a PRE number of about 45. However, these have too low a strength and a high cost to be a realistic alternative for umbilical tubes.

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 alloying 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.

The requirements that may be placed on an alloy that will meet the future requirements for umbilical tubes can be summarized in Table 1, where examples of the best various alternative alloys available on the market are currently included. It is clear that all existing alloys at at least one point do not meet the new stricter requirements imposed on umbilical tubes. Table 1 Property Requirements = UNS UNS UNS UNS alloy according to S32750 S32906 S32654 S32001 invention PRE Min 46 42.5 42 55 22 Tensile strength 720 550 650 430 450 Rpo, (N / mmz) Point corrosion> 90 ° C 50 50> 95 <20 CPTi ° C Crevice corrosion 260 ° C 35 35 60 <20 CCTi ° C Structure- Max 0.5% OK OK OK OK stability sigma phase Manufacturing Weldable OK OK OK OK with conventional technology 10 15 20 25 30 527 178 Summary of the invention It is therefore a The object of the present invention is to provide a duplex stainless steel alloy which exhibits high corrosion resistance in combination with improved mechanical properties and at the same time good structural stability and which is most suitable for use in environments where high resistance to general corrosion and local corrosion is required, such as t. ex. in chloride-containing environments.

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 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 a yield strength RPog of at least 720 N / mmz and an elongation at tensile test of at least 25% at room temperature.

The material according to the present invention has for its high alloy content extremely good machinability, in particular heat machinability and should thus be very suitable for use in, for example, the manufacture of rods, pipes, such as welded and seamless pipes, welding materials, structural parts, such as grooves and couplings.

These objects are fulfilled according to the present invention with duplex stainless steel alloys containing (in% by weight) C greater than 0 up to a maximum of 0.03% Si up to a maximum of 0.5% Mn 0 - 3.0% Cr 24.0 - 30, 0% Ni 4.9 - 10.0% 10 15 20 25 30 527 178 8 Mo 3.0 - 5.0% N 0.28 - 0.5% B 0-0.0030% S up to max 0.010% Co 0-3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3% Al 0-0.03% Ca 0-0.010% residue Fe plus unavoidable impurities.

Brief description of the ur gurema Figure 1 Figure 2 Figure 3 Figure 4 shows CPT values from tests of experimental chargers in the modified ASTM G48C test in the “green-death” solution compared to the duplex steels SAF2507, SAF 2906. shows CPT values produced using the modified The ASTM G48C test in the "green-death" solution for test soharms compared to the duplex steels SAF2507 and SAF 2906. shows the mean value of the milling in mm / year in 2% HC | at a temperature of 75 ° C. chargerna.

Detailed description of the invention A systematic development work has surprisingly shown that a well-balanced combination of the elements Cr, Mo, Ni, N, Mn and Co can obtain an optimal distribution of the elements in the ferrite and in the austenite, which enables a very corrosion-resistant material with only negligible amount of sigma phase in the material. The material also receives good machinability that enables extrusion into seamless pipes. In order to obtain the combination of high corrosion resistance in connection with good structural stability, a very tight combination of the alloying elements in the material is required. The alloy according to the invention therefore contains (in% by weight): C greater than 0 up to a maximum of 0.03% Si up to a maximum of 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 up to max 0.010% Co 0-3.5% W 0-3.0% Cu 0-2.0% Ru 0-0.3% Al 0-0.03% Ca 0-0.010.

The effect of the alloying elements is described in the following: lšgl (C) has limited solubility in both ferrite and austenlt. 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.

Silicon (Si) is used as a deoxidizing agent in steel production and increases the surface area during 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 in the current alloy type. 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 solids 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. Year green (Cr) is a very active element to improve resistance to various 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 odd 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 icïel (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 4.9-9.0% by weight, is required. %, in particular 6.0-9.0% by weight.

Molybdenum (Mo) is an active element that improves 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 precipitations increases. The Mo content of the present invention should be in the range of 3.0-5.0% by weight, preferably 3.6-4.9% by weight, in particular 4.4-4.9% by weight. _l§v_tá_v_e_ (N) is a very active element that increases the corrosion resistance, structural stability and strength of the material. In addition, a high N content 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% by weight of N should be alloyed. At high levels of N, the risk of precipitation of chromium nitrides increases, especially when the chromium content is high at the same time. In addition, a high N content means that the risk of porosity increases due to that the solubility of N in the melt is 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.

An excessively high chromium and nitrogen content results in the precipitation of CrzN, which should be avoided as it impairs the properties of the material, especially in heat treatment, e.g. welding. ä '(B) is added to increase the water processability of the material. If the boron content is too high, the weldability and corrosion resistance may deteriorate. The boron content should therefore be greater than 0 and up to 0.0030% by weight.

Sulfur (S) has a negative effect on corrosion resistance by forming easily soluble solids. In addition, the heat workability deteriorates, which is why the sulfur content is limited to a maximum of 0.010% by weight.

K_ot fl t (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 1.0% by weight, should be alloyed. Since cobalt is a relatively expensive element, the cobalt addition is therefore limited to a maximum of 3.5% by weight.

Tungsten increases resistance to point and crevice corrosion. However, the alloying of too high levels of tungsten in combination with the Cr contents and the Mo contents being high, 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 0-1.8% by weight. 10 15 20 25 30 527 178 12 @p_p_a_r 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 0.1 and 1.5% by weight.

Ruthenium (Ru) is alloyed to increase corrosion resistance. Ruthenium is a very expensive element, so the content is limited to a maximum of 0.3% by weight, preferably greater than 0 and up to 0.1% by weight.

Aluminum (Al) and Calcium (Ca) are used as deoxidizing agents in steel application. 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, a ferrite content between 40-65% is desirable 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 In the examples below, the composition of a number of test batches is stated, which illustrates the effect of different alloying elements on the properties. Charge 605182 represents a reference composition and is thus not included in the scope of this invention. Nor should other charges be considered to limit the acquisition, but only give examples of charges that illustrate the acquisition according to the patent claims. 10 15 20 25 527 178 13 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 forged into a round bar. This was extruded into a bar (round bar and flat bar), where sample material was taken out of a round bar. Furthermore, flat bars were 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 pipes using the extrusion method followed by cold rolling. Table 2 shows the composition of the first round test batches.

Table 2.

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 0.04 OQ1 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 0.01 .44 605178 0.91 27.94 7.26 4.01 0.99 0.10 0.07 0.01 003 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 27.85 693 4.03 131 0.02 0.06 092 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 l In order to examine the structural stability, samples from each batch were annealed at 900-1150 ° C with steps of 50 ° C and quenched in air and water, respectively. At the lowest temperatures, an internal 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 3. 10 15 20 25 527 178 14 Tmax sigma is calculated with Thermo-Calc (T -C version N thermodynamic database for steel TCFEQQ) based on guideline values for all listed elements in the different variants. Tmax sigma is the dissolution temperature for sigma phase, where high dissolution temperature indicates lower structural stability.

Table 3.

Charge Heat treatment Many o [vol-%] Tmax o 605193 1100 ° C, 5min 73% 1016 605195 1 150 ° C, 5min 32% 1047 605197 1 100 ° C, 5min 18% 1061 605178 1100 ° C, 5min 14% 1038 605183 1050 ° C, 5min 0J4% 997 605184 1 100 ° C, 5min 0.4% 999 605187 1050 ° C, 5min 0.3% 962 605153 1100 ° C, 5min 3.5% 1032 605182 1 100 ° 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 Tm, sigma calculated with Therrno-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% FeC | 3, 1% CuCl2, 11% H2SO4, 1.2% HCl. however, the idea of the more aggressive “green-death” solution is carried out.

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 (CP1 ') from all experiments are shown in Table 4, 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 20 527 178 15 Table 4 Charge PRE PRE y PRE y / PRE CPT ° C CPT ° C CPT ° CO, PRE ,,, modified ASTM G48 c 3% Nacl ASTM G48C 6% FeClg (600mv Green Death SCE) 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 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 SAF2507 39.4 42.4 1.0781 41.1 70/70 80 95 SAF2906 39.6 48.4 1.1717 41.0 60/50 75 75 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 4 show that the PRE number does not only explain the CPT value.

It is clear from these results that all test materials show better CPT in the modified ASTM G48C than SAF2507 and SAF2906.

Experimental charge 605183 alloyed with cobalt shows good structural stability at controlled cooling rate (-140 ° Clmin) 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 ratio PRE austenite / PRE ferrite 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 test charges is reported in Table 4.

The strength at room temperature (RT), 100 ° C and 200 ° C and the impact strength at room temperature (RT) have been determined for all charges and are shown as an average of three experiments. 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 5 and 6. 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 5.

Charge Temperature RPM Rpm Rm A5 Z (MPa) (MP (MP2) (%) (%) 605193 RT 652 791 916 29.7 38 100 ° C 513 646 818 30.4 36 200 ° C 51 1 583 756 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 596 677 629 31.9 45 200 ° C 535 608 763 27.1 36 605183 RT 677 775 882 32.4 67 1 00 ° C 560 642 786 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 81 7 29.3 27 200 ° C 532 61 1 749 23.7 31 6051 82 RT 627 754 903 28.4 43 100 ° C 493 621 802 31.8 42 10 15 527 178 17 Table 6.

Charge Annealing Cooling Impact strength Glowing Cooling Impact rc / min] LIL rc / mim [J] 605193 1100/20 Air 35 1100/20 Water 242 605195 1150/20 Water 223 605197 1 100/20 Water 254 1 130/20 Water 259 605178 1 100/20 Air 62 1 100/20 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 1100/20 Water 95 605153 1100/20 Air 50 1100/20 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 the latter is admittedly very close to the requirement.

Table 7 shows results from the Tungsten-lnert-Gas remelting test (hereinafter abbreviated TIG) where charges 605193, 605183, 605184 and 605253 show a stable structure in the heat affected zone (Heat Affected Zone, hereinafter abbreviated HAZ). The Ti-containing charges show TiN in HAZ.

Table 7.

Charge Excretions Protective gas Ar (99.99%) 6051 93 HAZ: OK 6051 95 HAZ: Large amounts of TlN and o-phase 605197 HAZ: Small amounts of CrzN in island grain, but not much 605178 HAZ: CrzN in island grain, otherwise OK 6051 83 HAZ: OK 605184 HAZ: OK 605187 HAZ: CrgN fairly close to the melting point, no precipitates further out 605153 HAZ: OK 6051 82 HAZ: TiN and decorated grain boundaries 5/8 527 178 18 Example 2 In the example below, the composition of further a number of test batches manufactured with the intention of achieving the optimal composition. These charges are modified based on the properties of charges with good structural stability and high corrosion resistance, from the results shown in Example 1. All charges in Table 8 are included in the composition according to the present invention, where charge 1-8 is included in a statistical test plan, while charge e to n are additional test alloys within the scope of this invention.

A number of experimental hairs were produced by casting 270 kg of ingot which was 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 8 shows the composition of these test batches.

Table 8 Chaérgg 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 6.8 433 0.6 0.0019 0¿5 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 Og? e 605254 1.1 26.9 6.5 4.8 1.0 0.0021 038 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 LO 0.0020 0.40 i 605253 1.0 28 .8 7.0 4.16 1.5 03019 0.37 j 605266 1.1 30.0 7.1 4.02 0.0018 0.38 k 605269 1.0 28.5 7.0 3.97 1 , 0 1, 0 0.0020 0.45 I 605268 1, 1 28.2 6.6 4.0 1.0 1, 0 1.0 0.0021 0.43 m 605270 1.0 28.8 7, 0 4.2 1.5 0.1 03021 0.41 n 605267 1.1 29.3 6.5 4.23 1.5 0.0019 0.36 19 The distribution of alloying elements in the ferrlt and austenite phases was examined by microprobe analysis, the result is shown in Table 9.

Table 9 Cha e Fas Cr Mn Ni Mo W Co Cu N 805258 Ferm 29.8 1.8 4.8 5.0 1.4 0.11 Austenite 28.8 1.4 7.8 8.4 1.5 0.60 805249 Ferm 29.8 1.1 5.4 5.1 1.8 0.10 Aueienir 27.8 1.2 7.9 8.8 1.8 0.58 805259 Ferm 29.7 1.8 5.8 5, 8 0.5 0.10 Auetenit 28.1 1.4 7.8 8.8 0.58 0.59 805280 Ferm 28.4 1.8 4.4 5.0 1.4 0.08 Ausrenir 28.5 1.4 8.8 8.8 1.5 0.54 805250 Ferm 80.1 1.8 5.8 5.1 0.48 0.07 Aueteni: 27.8 1.4 8.8 8.4 0.58 0.52 805251 Ferm 29.8 1.2 5.0 5.2 1.8 0.08 Auetenii 28.9 1.8 7.8 8.5 ¿5 058 805281 Ferm 28.0 1.2 4.5 4.9 0.45 0.07 Auerenil 28.5 1.4 8.9 8.8 0.58 0.58 805252 Ferm 29.8 1.8 5.8 52 0.42 0.09 Austenir 27.1 1.4 8.2 8.8 0.51 0.48 805254 Ferm 28.1 1.8 4.9 5.8 0.89 0.08 Auetenir 28.0 1.4 7.8 8.8 1, 0 0.48 805255 Ferm 80.1 1.8 5.0 4.7 2.7 0.08 Austeni: 27.0 1.8 7.7 8.0 8.8 0.45 805282 Ferm 28.8 8 .0 5.8 4.8 1.4 0.9 0.08 Auetenir 28.8 8.2 8.1 8.0 0.85 1.1 0.48 805288 Ferm 29.7 1.8 5.1 5.1 1.8 0.91 0.07 Ausienii 27.8 1.4 7.7 8.2 0.79 1.1 0.51 805258 Ferm 80.2 1.8 5.4 5.0 1, 8 0.09 Aueienrr 27.5 1.4 8.4 8.1 1.5 0.48 805288 Ferm 81.0 1.4 5.7 4.8 0.09 Auetenn 29.0 1.5 8.4 8.1 0.52 805289 Ferm 28.7 1.8 5.2 5.1 1.4 0.9 0.11 Austenir 28.8 1.4 7.8 8.2 0.87 1.1 0.52 805288 Ferm 29.1 1.8 5.0 4, 7 1.8 0.91 0.84 0.12 Austenite 26.7 1.4 7.5 § .2 0.97 1.0 1.2 0.51 805270 Ferm 803 1.2 5.8 5.0 1.8 0.11 Auslenn 27.7 1.8 8 0 8.2 1.4 0.47 805287 Ferm 80.1 1.8 5.1 4.9 1.8 0.08 Auetenir 27.8 1, 4 7.8 8.1 1.8 0.48 The point corrosion properties of all charges have been tested in the "green death" solution (1% FeC | _ ~ ,, 1% CuC | 2, 11% H2SO4, 1.2% HCl ) for ranking.

The test procedure is the same as point corrosion testing according to ASTM 10 G48C, however, the test is performed in a more aggressive solution than 6% FeCl3, the so-called 'green-dead' solution. General corrosion testing in 2% HCl (2 10 15 20 527 178 20 tests per charge) has also been performed for ranking before dew point testing.

The results from all experiments are shown in Table 10, Figure 2 and Figure 3. All tested charges perform better than SAF2507 in 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 the 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% by weight of cobalt. performs almost equivalent to charge 605250, alloyed with 0.6% by weight of cobalt. in the "green-death" solution despite the lower chromium content in charge 605251. It 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 Tm, sigma value below 1010 ° C indicates a good structural stability based on the values in Table 2 in Example 1. Table 10 also indicates the PREW number (% Cr + 3.3% (Mo + 0.5% VV) + 16% N) for the total alloy composition and PRE in austenite and ferrite (rounded) based on phase composition measured with microprobe. The ferrite content is measured after heat treatment at 1,100 ° C followed by water quenching.

Table 1 0 Charge ol content PREW PRE PRE PREy / CPT ° C Total <1 y PREQ Green death 605258 48.2 50.3 48.1 49.1 1.021 65/70 605249 59.8 48.9 48Q 46.6 0.967 75/80 605259 49.2 50.2 48.8 48.4 0.991 75/75 605260 53.4 48.5 46.1 47.0 1.019 75/80 605250 53.6 49.2 48.1 46, 8 0974 95/80 605251 54.2 48.2 48.1 46.9 0.976 90/80 605261 50.8 48.6 45.2 46.3 1024 80/70 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 48.9 44.1 0.940 90/80 605262 57.2 4L9 48.3 45, 0 0931 70/85 605263 53.6 49.7 49.8 47.8 0.959 80/75 605253 52.6 48.4 483 45.4 0.942 85/75 605266 62.6 49.4 48.3 47.6 0.986 70/65 605269 52.8 50.5 49.6 46.9 0.945 80/90 605268 52.0 49.9 48.7 47.0 0.965 85/75 10 15 20 5 2 7 1 7 8 21 605270 57 .0 49.2 48.5 45.7 0.944 80/85 605267 59.8 49.3 47.6 45L4 0.953 60/65 Table 1 1 Charge CPT CCT Rp0.12 Rm AZ Average Average RT RT RT RT 605258 84 68 725 929 40 73 605249 74 78 706 922 38 74 605259 90 85 722 928 39 73 605260 93 70 709 917 40 73 605250 89 83 698 923 38 75 6 05251 95 65 700 909 37 74 605261 93 78 718 918 40 73 605252 87 70 704 909 38 74 605254 93 80 695 909 39 73 605255 84 65 698 896 37 74 605262 80 83 721 919 36 75 605263 83 75 731 924 37 73 605253 96 75 707 908 38 73 605266 63 78 742 916 34 71 605269 95 90 732 932 39 73 605268 75 85 708 926 38 73 605270 95 80 71 1 916 38 74 605267 58 73 759 943 34 71 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 antenna metal phase became negligibly small was determined by examinations under a light optical microscope. A comparison of the structure of the charges 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 1.5% by weight of cobalt, is free of sigma phase, while charge 605250, alloyed with 0.6% by weight of cobalt, contains some sigma phase. Both charges are alloyed with a high content of chromium close to 29.0% by weight and a molybdenum content of close to 4.25% by weight. If one compares the compositions of charges 605249, 605250, 605251 and 605252 in view of the sigma phase content, it is very clear that the composition range of the optimum material with respect to in this case structural stability is very narrow. Furthermore, it is found that charge 605268 contains only single sigma phases compared to charge 605263 which contains a lot of sigma phase. What mainly distinguishes these charges is the addition of copper to charge 605268. In Charge 605266 and 605267, sigma phase is free despite high chromium content, the latter charge is alloyed with copper. Furthermore, charges 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. Thus, a very well-balanced balance between the various alloying elements is required at these high alloying levels for e.g. chromium and molybdenum to obtain good structural properties.

Table 12 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 by 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 12 Chaljgg 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 1.5 038 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 605256 2 29.0 4.23 1.5 0.46 605259 1 29.0 4.23 0.6 0.45 605260 1 27.5 4.22 1.5 0.44 605261 2 2L8 4.22 0.6 0.43 605262 4 27.6 3.93 1 .0 1.0 0.36 605263 5 28.7 3.96 g) 1.0 0.40 605266 1 30.0 432 0.38 605267 1 29.3 4.23 1, 5 0.38 605268 2 28.2 3.98 1.0 10 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 10 15 20 25 527 178 23 l Table 13 shows results from impact testing of some of the charges.

The results are very good, which indicates a 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 13.

Charge Annealing Cooling Impact strength Impact strength Impact strength [jC / min] 1.1] [J] [JJ 605249 1 100/20 Water> 300> 300> 300 605250 1 100/20 Water> 300> 300> 300 605251 1 100/20 Water> 300 > 300> 300 605252 1 100/20 Water> 300> 300> 300 605253 1 100120 Water 258 267 257 605254 1 100/20 Water> 300> 300> 300 605255 1 100/20 Water> 300> 300> 300 Figure 4 shows the results of the hot ductility test of most of the charges.

Good machinability is of course crucial in order to be able to manufacture the material for product shapes such as rods, pipes, such as welded and seamless pipes, wire, welding materials, structural parts, such as, for example, grooves and couplings.

Chargema 605249, 605250, 605251, 605252, 605255, 605266 and 605267 the most with nitrogen content around 0.38% by weight show slightly better hot ductility values.

The elongation fatigue properties provide information on how much, and how many times, a material can be stretched, before elongation fatigue cracks occur in the material. Since umbilical tubes are welded together to long lengths, reeled on drums before being twisted into the umbilical, it is not uncommon for a number of operations to take place where some plastic deformation occurs before the umbilical comes into operation. The strain fatigue data produced underline that the risk of fracture due to strain fatigue in an umbilical tube borders on non-existence.

Summary The requirements placed on the umbilical tubes of the future and which are met by an optimized alloy as above are that PRE min 46 in the alloy combined with PRE in 10 15 20 25 527 178 24 austenite or ferrite exceeds 45 is required to obtain sufficiently good point and crevice corrosion properties. Thus it is required that: CPT in 6% FeC | 3 CCT in 6% FeC | 3> 90 ° C z 60 ° C The strength required to be able to significantly reduce the weight of an umbilical is: Tensile strength Rp0.2 min 720 N / mm2 For to be able to manufacture umbilical pipes and to ensure that point and crevice corrosion resistance and mechanical properties are maintained, the following is required regarding structural stability: o The alloy must be weldable with conventional welding methods o Maximum 0.5% sigma phase in the structure o Maximum dissolution temperature for sigma phase is 1010 ° C The present invention exhibits extremely good machinability for its high alloy content, in particular heat machinability and should thus be very suitable for use in, for example, the manufacture of rods, pipes, such as welded and seamless pipes, welding materials, structural parts, such as grooves and couplings.

Claims (2)

10 15 20 25 Patent claim 527 178 25 Use of a ferrite-austenitic duplex stainless steel alloy having the following composition (% by weight): C Si Mn Cr Ni Mo NBS Co W Cu Ru Al Ca greater than 0 and up to a maximum of 0.03% up to a maximum of 0 .5% 0 - 3.0% 24.0 - 30.0% 4.9 - 10.0% 3.0 - 5.0% 0.28 - 0.5% 0-0.0030% up to max 0.010% 0-3.5% 0-3.0% 0-2.0% 0-0.3% 0-0.03% 0-0.010% and the remainder Fe together with normally occurring impurities and additives, the ferrite content being 40 -65% by volume and the correlation PRE =% Cr + 3.3% Mo + 16% N exceeds 46 for the total composition of the alloy and that the PRE in austenite and ferrite phase exceeds 45 and that the yield strength RPM for the alloy exceeds 720 N / mmz, and that CPT> 90 ° C and CCT z 60 ° C as umbilical cord tubes in chloride-containing environments, in particular seawater environments. 10 15 20 25 30 527 178 26 Use of a ferrite-austenitic duplex stainless steel alloy having the following composition (% by weight): C Si Mn Cr Ni Mo NBS Co W Cu Ru Al Ca greater than 0 and up to a maximum of 0.03% up to a maximum of 0 .5% 0 - 3.0% 24.0 - 30.0% 4.9 - 10.0% 3.0 - 5.0% 0.28 - 0.5% 0-0.0030% up to max 0.010% 0-3.5% 0-3.0% 0-2.0% 0-0.3% 0-0.03% 0-0.010% and the remainder Fe together with normally occurring impurities and additives, the ferrite content being 40 -65% by volume and the correlation PRE =% Cr + 3.3% Mo + 16 ° / 0N exceeds 46 for the total composition of the alloy and that the PRE in austenite and ferrite phase exceeds 45 and that the yield strength RPG; for the alloy exceeds 720 N / mmz, and that CPT> 90 ° C and CCT z 60 ° C for the manufacture of welding materials.