Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the present invention relates to a stainless steel alloy, closer determined a duplex stainless steel alloy with ferritic-austenitic matrix and with high resistance to corrosion in combination with good structural stability and hotworkability, particularly a duplex stainless steel with a content of ferrite of 40- 65 volume-% and a well balanced composition, which imparts the material corrosion properties, which make it more suitable for use in chloride-containing environments than earlier been considered being possible.
• the element W has by then been used as substitute for a portion of Mo, as for example in the commercial alloy DP3W (UNS S39274) or ZeronlOO, which contain 2,0% respectively 0,7% W.
• the later contains even 0,7% Cu with the purpose to increase the corrosion resistance of the alloy in acid environments.
• PREW %Cr+3,3(%Mo+0,5%W)+16%N, such as described for example in EP 0 545 753.
• This publication refers to a duplex stainless alloy with generally improved corrosion properties.
• the above-described described steel grades have a PRE-number, irrespective method of calculation, which lies above 40.
• US-A-4 985 091 describes an alloy intended for use in hydrochloric and sulfuric acid environments, where mainly intergranular corrosion arises. It is primarily intended as alternative to recently used austenitic steels.
• US-A-6 048 413 describes a duplex stainless alloy as alternative to austenitic stainless steels, intended for use in chloride-containing environments.
• CPT Critical Pitting Corrosion Temperature
• CCT Critical Crevice-corrosion Temperature
• the material according to the present invention shows remarkably good workability, in particular hotworkability and shall thereby be very suitable to be used for example the production of bars, tubes, such as welded and seamless tubes, plate, strip, wire, welding wire, constructive parts, such as for example pumps, valves, flanges and couplings.
• duplex stainless steel alloys which contain (in 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, 0-3,0% Mn, 0-0,0030% B, up to 0,010% S, 0-0,03% Al, 0-0,010% Ca, 0-3,0% W, 0-2,0% Cu, 0-3,5% Co, 0-0,3% Ru, balance Fe and inevitable impurities.
• Figure 1 shows CPT-values from tests of the test heats in the modified ASTM G48C test in "Green Death"-solution compared with the duplex steels SAF2507,
• Figure 2 shows CPT-values attained with the help of the modified ASTM G48C test in " Green Death"-solution for the test heats compared with the duplex steel
• Figure 3 shows the average amount of erosion in mm/year in 2%HCI at a temperature of 75°C.
• Figure 4 shows the results from hot ductility testing for most of the heats.
• Carbon (C) has limited solubility in both ferrite and austenite.
• the limited solubility implies a risk of precipitation of chromium carbides and the content should therefore not exceed 0,03 weight-%, preferably not exceed 0,02 weight- %.
• Silicon (Si) is utilized as desoxidation agent in the steel production as well as it increases the flowability during production and welding.
• too high contents of Si lead to precipitation of unwanted intermetallic phase, wherefore the content is limited to max 0,5 weight-%, preferably max 0,3 weight-%.
• Manganese (Mn) is added in order to increase the N-solubility in the material.
• Mn only has a limited influence on the N-solubility in the type of alloy in question. Instead there are found other elements with higher influence on the solubility.
• Mn in combination with high contents of sulfur can give rise to formation of manganese sulfides, which act as initiation- points for pitting corrosion.
• the content of Mn should therefore be limited to between 0-3,0 weight-%, preferably 0,5-1 ,2 weight-%.
• Chromium (Cr) is a much active element in order to improve the resistance to a majority of corrosion types. Furthermore, a high content of chromium implies that one gets a very good N-solubility in the material. Thus, it is desirable to keep the Cr-content as high as possible in order to improve the corrosion resistance. For very good amounts of corrosion resistance the content of chromium should be at least 24,0 weight-%, preferably 27,0 -29,0 weight-%. However, high contents of Cr increase the risk for intermetallic precipitations, for what reason the content of chromium must be limited up to max 30,0 weight-%.
• Nickel (Ni) is used as austenite stabilizing element and is added in suitable contents in order to obtain the desired content of ferrite.
• Molybdenum (Mo) is an active element which improves the resistance to corrosion in chloride environments as well as preferably in reducing acids. A too high Mo-content in combination with that the Cr-contents are high, implies that the risk for intermetallic precipitations increases.
• the Mo-content in the present invention should lie in the range of 3,0-5,0 weight-%, preferably 3,6-4,7 weight- %, in particular 4,0-4,3 weight-%.
• N Nitrogen
• N is a very active element, which increases the corrosion resistance, the structural stability as well as the strength of the material. Further, a high N- content improves the recovering of the austenite after welding, which gives good properties within the welded joint.
• at least 0,28 weight-% N should be added.
• N the risk for precipitation of chromium nitrides increases, especially when simultaneously the chromium content is high.
• a high N-content implies that the risk for porosity increases because of the exceeded solubility of N in the smelt For these reasons the N-content should be limited to max 0,5 weight-%, preferably >0,35 - 0,45 weight-% N is added.
• Boron (B) is added in order to increase the hotworkability of the material. At a too high content of Boron the weldability as well as the corrosion resistance could deteriorate. Therefore, the content of boron should be limited to 0,0030 weight-%.
• S Sulfur influences the corrosion resistance negatively by forming soluble sulfides. Further, the hot workability detoriates, for what reason the content of sulfur is limited to max 0,010 weight-%.
• Co Co is added in order to improve foremost the structural stability as well as the corrosion resistance.
• Co is an austenite-stabilizing element.
• Tungsten increases the resistance to pitting- and crevice corrosion. But the addition of too high contents of tungsten in combination with that the Cr- contents as well as Mo-contents are high, means that the risk for intermetallic precipitations increases.
• the W-content in the present invention should lie in the range of 0-3,0 weight-%, preferably between 0,5 and 1 ,8 weight-%.
• Copper is added in order to improve the general corrosion resistance in acid environments such as sulfuric acid. At the same time Cu influences the structural stability. However, high contents of Cu imply that the solid solubility will be exceeded. Therefor the Cu-content should be limited to max 2,0 weight- %, preferably between 0,5 and 1 ,5 weight-%.
• Ruthenium is added in order to increase the corrosion resistance. Because ruthenium is a very expensive element, the content should be limited to max 0,3 weight-%, preferably more than 0 and up to 0,1 weight-%.
• Aluminum (Al) and Calcium (Ca) are used as desoxidation agents at the steel production.
• the content of Al should be limited to max 0,03 weight-% in order to limit the forming of nitrides.
• Ca has a favorable effect on the hotductility.
• the Ca-content should be limited to 0,010 weight-% in order to avoid an unwanted amount of slag.
• the content of ferrite is important in order to obtain good mechanical properties and corrosion properties as well as good weldability. From a corrosion point of view and a point of view of weldability a content of ferrite between 40-65% is desirable in order to obtain good properties. Further, high contents of ferrite imply that the impact strength at low temperatures as well as the resistance to hydrogen-induced brittleness risks deteriorating.
• the content of ferrite is therefore 40-65 volume-%, preferably 42-60 volume-%, in particular 45-55 volume-%.
• test heats according to this example were produced by casting of 170kg ingots in the laboratory, which were hotforged to round bars. Those were hotextruded to bars (round bars as well as flat bars), where test material was taken out from the round bars. Further on the flat bars were annealed before cold rolling took place, whereafter further test material was taken out. From a material engineering point of view, the process can be considered to be representative for the preparation in bigger scale, for example for the production of seamless tubes by the extrusion method, followed by cold rolling. Table 1 shows the composition of the first batch of test heats.
• Tmax sigma was calculated with Thermo-Calc (TC version N thermodynamic database for steel TCFE99) based on characteristic amounts for all specified elements in the different variations.
• T max sigma is the dissolving temperature for the sigma phase, where high dissolving temperatures indicate lower structural stability.
• the pitting corrosion properties of all heats were tested for ranking in the so-called "Green Death"-solution, which consists of 1%FeCl 3 , 1%CuCI 2 , 11% H 2 SO 4 , 1 ,2% HCI.
• the test procedure is equivalent to the pitting corrosion testing according to ASTM G48C, however, it will be carried out in the more aggressive "Green Death"-solution.
• some of the heats were tested according to ASTMG48C (2 tests per heat). Also the electrochemical testing in 3%NaCI (6 tests per heat) was carried out.
• CPT Critical Pitting Temperature
• the test heat 605183, alloyed with cobalt shows good structural stability at a controlled cooling rate of (-140°C/min) in spite that it contains high contents of chromium as well as of molybdenum, shows better results than SAF2507 and SAF2906. It appears from this investigation that a high PRE does not solely explain the CPT values, without the relationship PRE austenit/PRE ferrite is of extreme weight for the properties of the higher alloyed duplex steels, and a very narrow and exact leveling between the alloying elements is required in order to obtain this optimum ratio, which lies between 0,9-1 ,15; preferably 0,9-1 ,05 and simultaneously obtain PRE values of above 46.
• the relationship PRE austenit/PRE ferrite against CPT in the modified ASTM G48C test for the test heats is given in Table 3.
• the strength at room temperature (RT), 100°C and 200°C and the impact strength at room temperature (RT) have been determined for all heats and is shown as average amount for three tests.
• Tensile test specimen (DR-5C50) were manufactured from extruded bars, 0 20mm, which were heattreated at temperatures according to Table 2 in 20 minutes followed by cooling down in either air or water (605195, 605197, 605184). The results of the tests are presented in Table 4 and 5. The results of the tensile test show that the contents of chromium, nitrogen and tungsten strongly influence the impact strength of the material. Besides 605153, all heats fulfill the requirement of a 25% elongation at tensile testing at room temperature (RT).
• Table 6 shows the results from the Tungsten-Inert-Gas remelting test (henceforth-abbreviated TIG), where the heats 605193, 605183, 605184 as well as 605253 show a good structure in the heat affected zone (Heat Affected Zone, henceforth-abbreviated HAZ).
• TIG Tungsten-Inert-Gas remelting test
• HAZ Heat Affected Zone
• test heats were produced by casting of 270kg ingots, which were hotforged to round bars. Those were extruded to bars, wherefrom test samples were taken. Afterwards the bar was annealed before cold rolling to flat bars was executed, after that further test material was taken out. Table 7 shows the composition for these test heats.
• Thermo-Calc-values according to Table 8 are based on characteristic amounts for all specified elements in the different variations.
• the PRE-number for the ferrite and austenite is based on their equilibrium composition at 1100°C.
• Tm a x sigma is the dissolving temperature for the sigma phase, where high dissolving temperatures indicate lower structural stability.
• the pitting corrosion properties of all heats have been tested in the "Green Death"- solution (1%FeCI 3 , 1%CuCI 2 , 11% H 2 SO 4 , 1 ,2% HCI) for ranking.
• the test procedures are the same as pitting corrosion testing according to ASTM G48C, but the testing will be executed in a more aggressive solution than 6%FeCl 3 , the so-called “Green Death”-solution.
• the general corrosion testing in 2%HCI (2 tests per heat) was executed for ranking before the dewpoint testing. The results from all tests appear from Table 10, Figure 2 and Figure 3. All tested heats perform better than SAF2507 in "Green Death"- solution.
• heats lie within the identified range of 0,9-1 ,15; preferably 0,9-1 ,05 applicable for the ratio PRE austenit/PRE ferrite at the same time as PRE in both austenite and ferrite is in excess of 44 and for most of the heats even considerable in excess of 44. Some of the heats attain even the limit of total PRE 50. It is very interesting to note that heat 605251 , alloyed with 1 ,5 weight- % cobalt, performs almost equivalent with heat 605250, alloyed with 0,6 weight- % cobalt, in "Green Death"-solution in spite of the lower chromium content in heat 605251. It is particularly surprising and interesting because heat 605251 has a PRE-number of ca. 48, which is in excess of some of today's commercial superduplex alloys simultaneously as the T max sigma-value below 1010°C indicates a good structural stability based on the values in Table 2 in Example 1.
• heat 605249 alloyed with 1 ,5 weight-% cobalt
• heat 605250 alloyed with 0,6 weight-% cobalt
• Both heats are alloyed with high contents of chromium, approximately 29,0 weight-% and the molybdenum content of approximately 4,25 weight-%. If one compares the compositions of the heats 605249, 605250, 605251 and 605252 with thought on the content of sigma phase, it is very distinct that the range of composition for that optimum material is very narrow, in this case with regard to the structural stability. It further shows that the heat 605268 contains only sigma phase compared to heat 605263, which contains much sigma phase. What mainly distinguishes these heats from each other is the addition of copper to heat
• Heat 605266 and also 605267 are free from sigma phase, despite of a high content of chromium the later heat is alloyed with copper. Further, the heats 605262 and 605263 with addition of 1 ,0 weight-% tungsten show a structure with much sigma phase, while it is interesting to note that heat
• 605269 also with 1 ,0 weight-% tungsten but with higher content of nitrogen than 605262 and 605263 shows a considerable smaller amount of sigma phase. Consequently, a very well-leveled balance between the different alloying elements at these high alloying contents is required of for example chromium and molybdenum in order to obtain good structural properties.
• Table 11 shows the results from the light optical examination after annealing at 1080°C, 20min followed by water quenching. The amount of sigma phase is specified with values from 1 to 5, where 1 represents that no sigma phase was detected in the examination, while 5 represents that a very high content of sigma phase was detected in the examination.
• Table 12 the results from the impact strength testing of some of the heats are shown. The results are very good, which indicates a good structure after annealing at 1100°C followed by water quenching and the requirement of 100J will be managed with large margin of all tested heats.
• Figure 4 shows the results from the hot ductility testing of the most of the heats.
• a good workability is of course of vital importance in order to be able to produce the material to product forms such as bars, tubes, such as welded and seamless tubes, plate, strip, wire, welding wire, constructive elements, such as for example pumps, valves, flanges and couplings.
• the heats 605249,605250, 605251, 605252, 605255, 605266 as well as 605267, the most with nitrogen content around 0,38 weight-% show somewhat improved hot ductility values.
• PRE-number in ferrite should exceed 45, but preferably be at least 47.
• PRE-number in austenit should exceed 45, but preferably be at least 47.
• PRE-number for the entire alloy should preferably be at least 46.
• Relationship PRE austenit/PRE ferrite should lie in the range of 0,9-1 ,15; preferably in the range 0,9-1 ,05.
• the content of ferrite should lie in the range preferably 45-55 volume-%.
• Tm a x sigma should not exceed 1010°C.
• the content of nitrogen should lie in the range 0,28-0,5 weight-%, preferably in the range 0,35-0,48 weight-%, but preferably 0,38-0,40 weight-%.
• the content of cobalt should lie in the range 0-3,5 weight-%, preferably 1 ,0-2,0 weight-%, but preferably 1 ,3-1 ,7 weight-%. • In order to ensure the high nitrogen solubility, i.e. if the content of nitrogen is in the range 0,38-0,40 weight-% should at least 29 weight-% Cr be added as well as at least 3,0 weight-% Mo, thus the total content of the elements Cr, Mo and N fulfills said requirements on the PRE-number.

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