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
  • C22C38/44—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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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

Definitions

• 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 ferrite content of 40-65 % by volume and a well-balanced composition that gives the material corrosion properties making it more suit- able for use in chloride-containing environments than what has previously been found possible.
• Valves and couplings on the unit at the bottom of the sea are controlled hydraulically and electrically from a platform, a production ship or another unit on the surface of the sea or on land.
• An umbilical cord pipe a so-called umbilical, couples together the guiding unit with the units on the bottom of the sea.
• the part of the umbilical that lies on the bottom of the sea, for instance, between two underwater units on different extraction sites, is called static umbilical since the same only to a relatively small extent is effected by the motions of the sea.
• the part of the umbilical, that is situated between the bottom of the sea and the surface is called dynamic umbilical and is effected to a large extent by motions in the water and on the surface. Examples of such motions are flows in the water, wave motions as well as motions of the platform and the production ship.
• the demands that are made on the pipes in an umbilical are foremost related to corrosion and mechanical properties.
• the pipe material has to be resistant to corrosion in sea water, which surrounds the outer surface of the pipes. This property is what is regarded as being most important, since sea water has a very corrosive impact on stainless steel. Furthermore, the material has to have high corrosion resistance to the possible corrosive solutions that are injected in the oil well. The material has to be compatible with hydraulic liquids without contaminating the liquid. Possible contamination may affect the service function of the control unit at the bottom of the sea very
• the mechanical properties of the used pipe material are very important for the application of umbilical pipes. Since the depth may be considerable on the site of the oil production, the dynamic part of the umbilical generally becomes long, and thereby heavy. The weight has to be carried by the platform or the floating production ship. In practice, there is two ways to decrease the weight of an umbilical having a given configuration. It is possible to choose a lighter material or a material having the same density but having higher tensile yield limit and ultimate tensile strength. By choosing a material having higher strength, pipes having thinner wall may be used, and thereby the total mass of the umbilical is reduced. The deeper the sea at the site of extraction, the more important the total weight per unit of length of umbilical of the material will be.
• Duplex steel alloys that were established as an alternative to hitherto used types of steel, such as, for instance, ferritic steel that previously were used in this application, nickel base alloys or other high-alloy steels, are not excepted from this development.
• PRE Pitting Resistance Equivalent
• the principal alloying elements that affect this property are, according to the formula, Cr, Mo, N.
• An example of such a steel grade is seen in EP 0 220 141 , which through this reference hereby is included in this description.
• This steel grade, having the trade mark of SAF 2507 (UNS S32750), has essentially been alloyed with high contents of Cr, Mo and N. Thus, it is developed towards this property with, above all, good corrosion resistance in chloride environments.
• the elements Cu and W have turned out to be efficient alloying additives for additional optimization of the corrosion properties of the steel in chloride environments.
• the element W has, on that occasion, been used as substitution for a part of Mo, as for instance in the commercial alloys DP3W (UNS S39274) or ZeronlOO, which contain 2,0 % and 0,7 % of W, respectively.
• the latter also contains 0,7 % of Cu with the purpose of increasing the alloy's corrosion resistance in acid environments.
• PREW % Cr + 3,3(% Mo + 0,5 % W) + 16 % N, as described, for instance, in EP 0 545 753, which relates to a duplex stainless alloy having generally improved corrosion properties.
• the above-described steel grades have a PRE number, irrespective of method of calculation, which is above 40 but the PRE number is limited upwards to about 43 since higher values mean that the alloys obtain inferior structural stability.
• a higher degree of alloying increases the risk of precipitation of intermetallic phase, and therefore the level of alloying in duplex steel is regarded as limited to achieve PRE values around a maximum of about 43, irrespective of method of calculation.
• SAF 2906 should also be mentioned, the composition of which is seen in EP 0 708 845.
• This alloy which is characterized by higher contents of Cr and N in comparison with, for instance, SAF 2507, has turned out to be especially suitable for use in environments 5 where the resistance to intercrystalline corrosion and corrosion in ammonium carba- mate is of importance, but it has also a high corrosion resistance in chloride-containing environments.
• the alloy has a corrosion resistance in chloride environment corresponding to the o alloy UNS S32750, but simultaneously a higher yield point in tension Rpo,2- This makes that this alloy has advantages in comparison with UNS S32750 as umbilical material, since lower weight of the umbilical can be obtained.
• the corrosion resistance gives, however, no improvements in comparison with UNS S32750, which means considerable limitations in umbilical pipes that are exposed to higher tem- 5 peratures in future plants.
• the alloy 19D (UNS S32001 ) is a duplex alloy characterized by the composition 19,5-21 ,5 % of Cr, 0,05-0,17 % of N and max 0,6 % of Mo.
• This alloy has a PRE number of about 22, and therefore the alloy is unsuitable in sea-water applications o such as umbilicals. Accordingly, in order to achieve a sufficient corrosion resistance in this alloy, a cathode protection has to be applied in the form of a zinc layer on the outer surface of the umbilical pipe. If the zinc layer is consumed or if a greater surface becomes damaged, the corrosion protection is, however, ruined and a fast corrosion process may occur, which means expensive repairs and down periods. 5
• austenitic steels with PRE numbers of up to 55 having been made possible by the addition of high contents of Cr, Mo and N combined with high contents of Ni.
• Said alloys should work 5 very well to the new tougher corrosion conditions in umbilicals.
• the disadvantage of the same alloys is that they have considerably lower yield point in tension than duplex steel and are, furthermore, considerably more expensive to manufacture, foremost by virtue of their high percentage of Ni, which is an expensive alloying material.
• austenites having good resistance in chloride environment are UNS S32654 having a PRE number of about 55, and UNS S34565 having a PRE number of about 45. These have, however, too low a strength and high a cost in order to be a realistic alternative for umbilical pipes.
• the pitting resistance of duplex stainless steel an increase of the PRE number is required in both the ferrite phase and the austenite phase without, because of this, jeopardizing the structural stability or the workability of the material. If the composition in the two phases is not equivalent in respect of the active alloying components, one of the phases becomes more susceptible to pitting and crevice corrosion. Thus, the more corrosion-susceptible phase controls the resistance of the alloy, while the structural stability is controlled by the highest alloyed phase.
• CPT Critical Pitting Corrosion Temperature
• CCT Critical Crevice Corrosion Temperature
• 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 seamless pipes, weld material, construction parts such as, for instance, flanges and couplings.
• duplex stain- less steel alloys which contain (in % by weight)
• FIG 1 shows CPT values from test of the experimental charges in the modified
• the alloy according to the invention contains (in % by weight):
• Carbon (C) has limited solubility in both ferrite and austenite.
• the limited solubility means 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.
• Si Silicon
• Si is utilized as deoxidizer in the steel production and increases the flowabil- ity in production and upon welding.
• too high contents of Si lead to precipi- tation of undesired intermetallic phase, and therefore the content should be limited to max 0,5 % by weight, preferably max 0,3 % by weight.
• Manganese (Mn) is added in order to increase the solubility of N in the material.
• Mn only has a limited impact on the solubility of N in the alloy type in question. Instead, there are other elements having higher impact on the solubility.
• Mn may in combination with high sulphur contents give rise to the formation of manganese sulphides, which work as initiation spots for pitting. Therefore, the content of Mn should be limited to between 0-3,0 % by weight, preferably 0,5-1 ,2 % by weight.
• Chromium (Cr) is a very active element in order to improve the resistance to the majority of corrosion types. Furthermore, a high chromium content means that a very good solubility of N is obtained in the material. Thus, it is desirable to hold the content of Cr as high as possible in order to improve the corrosion resistance. For very good values of the corrosion resistance, the chromium content should be at least
• Nickel (Ni) is used as austenite-stabilizing element and is added in suitable contents so that the desired ferrite content is attained.
• an addition of between 4,9-10,0 % by weight of nickel is required, preferably 4,9-9,0 % by weight, in particular 6,0-9,0 % by weight.
• Molybdenum (Mo) is an active element that improves the corrosion resistance in chloride environments as well as preferably in reducing acids. Too high a content of Mo, in combination with the contents of Cr being high, means that the risk of inter- metallic precipitations increases.
• the content of Mo in the present invention should be in the interval of 3,0-5,0 % by weight, preferably 3,6-4,9 % by weight, in particular 4 ,4-4,9 % by weight.
• N Nitrogen
• N is a very active element that increases the corrosion resistance, the structural stability as well as the strength of the material. Furthermore, a high content of N improves the reformation of austenite after welding, which gives good properties of welded joints. In order to achieve a good effect from N, at least 0,28 % by weight of N should be added. At high contents of N, the risk of precipitation of chromium nitrides increases, especially when the chromium content simultaneously is high. Furthermore, a high content of N means that the risk of porosity increases by virtue of the solubility of N in the charge being exceeded. The content of N should, for these reasons, be limited to max 0,5 % by weight, preferably is >0, 35-0,45 % by weight of N added.
• Boron (B) is added in order to increase the hot workability of the material. At too high a boron content, the weldability and the corrosion resistance may be deteriorated.
• the boron content should be greater than 0 and up to 0,0030 % by weight.
• S Sulphur
• S Sulphur
• Co Co
• Co is added foremost in order to improve the structural stability as well as the corrosion resistance.
• Co is an austenite stabilizer.
• at least 0,5 % by weight preferably at least 1 ,0 % by weight should be added. Since cobalt is a relatively expensive element, the cobalt addition is therefore limited to max 3,5 % by weight.
• Tungsten increases the resistance to pitting and crevice corrosion. But addition of too high contents of tungsten in combination with the contents of Cr and contents of Mo being high, means that the risk of intermetallic precipitations increases.
• the content of W in the present invention should be in the interval of 0-3,0 % by weight, preferably between 0-1 ,8 % by weight.
• Copper is added in order to improve the corrosion resistance in acid environments such as sulphuric acid. Cu also affects the structural stability. However, high contents of Cu means that the solid solubility is exceeded. Therefore, the content of Cu is limited to max 2,0 % by weight, preferably between 0,1 and 1 ,5 % by weight.
• Ruthenium (Ru) is added in order to increase the corros on resistance. Ruthenium is a very expensive element, and therefore the content is I mited to max 0,3 % by weight, preferably greater than 0 and up to 0,1 % by we ght.
• Al aluminium
• Ca Calcium
• the ferrite content is important in order to obtain good mechanical properties and corrosion properties as well as good weldability. From a corrosion and a weldability point of view, it is desirable having a ferrite content of between 40-65 % in order to obtain good properties. Furthermore, high ferrite contents means that the low-temperature impact resistance as well as the resistance to hydrogen embrittlement risk being deteriorated. Therefore, the ferrite content is 40-65 % by volume, preferably
• Experimental charges according to this example were produced by laboratory casting of 170 kg of ingot that was hot-forged into round bar. The same was hot extruded into bar (round bar as well as flat bar), where test material was sampled from round bar. Furthermore, flat bar was annealed before cold rolling took place, and then additional test material was sampled. The process may, from a material technology point of view, be regarded as representative for the manufacture on a larger scale, for instance for the manufacture of seamless pipes by means of the extrusion method followed by cold rolling. Table 2 shows composition of experimental charges of the first batch.
• Tmax sigma is calculated by means of Thermo-Calc (T-C version N the thermody- namic database of steel TCFE99) based on guiding values of all stated elements in the different variants.
• T max sigma is the resolution temperature of the sigma phase, with high resolution temperature indicating lower structural stability. Table 3
• the object of this investigation is to be able to rank materials in respect of the structural stability, i.e. this is not the actual content of sigma phase in the test pieces that have been heat treated and quenched before, for instance, corrosion test. It is evident that T max sigma that has been calculated by means of Thermo-calc does not directly corresponds with measured quantity of sigma phase, but in this investigation it is, however, clear that the experimental charges having the lowest calculated T max sigma contain the lowest quantity of sigma phase.
• the pitting properties of all charges have been tested for ranking in the so-called "Green Death” solution that consists of 1 % FeCI 3 , 1 % CuCI 2) 11 % H 2 S0 4 , 12 % HCI .
• the test procedure corresponds to the pitting testing according to ASTM G48C, but is carried out in the more aggressive "Green Death” solution. Furthermore, some charges have been tested according to ASTM G48C (2 experiments per charge). Also electrochemical testing in 3 % NaCI (6 experiments per charge) has been carried out.
• the results in the form of critical pitting temperature (CPT) from all experiments are seen in Table 4, such as the PREW number (Cr + 3,3(Mo + 0,5 W) + 16 N) of the total alloy composition as well as of austenite and ferrite.
• the indexing alpha relates to ferrite and gamma relates to austenite.
• Test charge 605 183 alloyed with cobalt shows good structural stability at controlled cooling rate (-140 °C/min), in spite of it containing high contents of chromium as well as molybdenum, has better results than SAF 2507 as well as SAF 2906.
• Table 7 shows results from Tungsten Inert Gas remelting test (henceforth abbreviated TIG), with the charges 605 193, 605 183, 605 184 as well as 605 253 having a stable structure in the heat affected zone (henceforth abbreviated HAZ).
• TIG Tungsten Inert Gas remelting test
• HAZ heat affected zone
• Example 2 In the example below, the composition is given of an additional number of experimental charges manufactured with the intention of finding the optimal composition. Said charges are modified, based on the properties of the charges having good structural stability as well as high corrosion resistance, from the results that were shown in Example 1. All charges in Table 8 are comprised of the composition according to the present invention, with charges 1-8 being included in a statistical experimental plan, while charges e to n are additional experimental alloys within the scope of this invention.
• the pitting properties of all charges have been tested in the "Green Death” solution (1 % FeCI 3 , 1 % CuCI 2 , 11 % H 2 S0 4 , 1 ,2 % HCI) for ranking.
• the test procedure is the same as pitting testing according to ASTM G48C, but the testing is carried out in a more aggressive solution than 6 % FeC , the so-called "Green Death” solution.
• general corrosion test in 2 % HCI (2 experiments per charge) has been carried out for ranking before dew point testing. The results from all experiments are seen in Table 10, Figure 2 and Figure 3. All tested charges perform better than SAF 2507 in the Green Death solution.
• the PREW number (% Cr + 3,3 %(Mo + 0,5 % W) + 16 % N) is given for the total alloy composition and PRE in austenite as well as ferrite (rounded) based on phase composition being measured by means of micro probe.
• the ferrite content is measured after heat treatment at 1100 °C followed by water quenching.
• Structural control shows that the charges 605 249, 605 251 , 605 252, 605253, 605 254, 605 255, 605 259, 605 260, 605 266 as well as 605 267 are free from undesired sigma phase. Furthermore, charge 605 249, alloyed with 1 ,5 % by weight of cobalt, is free from sigma phase, while charge 605 250, alloyed with 0,6 % by weight of cobalt, contains a little sigma phase. 0 Both charges are alloyed with high percentage of chromium, almost 29,0 % by weight, as well as molybdenum content of almost 4,25 % by weight.
• compositions of the charges 605 249, 605 250, 605 251 and 605 252 are compared considering the sigma phase content, it is very clear that the composition interval for the optimal material in respect of, in this case, structural stability, is very narrow. 5 Furthermore, it is evident that charge 605 268 contains only occasional sigma phase in comparison with charge 605 263, which contains much sigma phase. What essentially separates these charges, is addition of copper to charge 605 268. In charge 605 266 as well as 605 267, the sigma phase is free in spite of high chromium content, the later charge is alloyed with copper.
• the charges o 605 262 and 605 263, having the addition of 1 ,0 % by weight of tungsten have a structure with much sigma phase, while it is interesting to note that charge 605 269, also having 1 ,0 % by weight of tungsten but of a higher nitrogen content than 605 262 and 605 263, has a considerably smaller quantity of sigma phase.
• charge 605 269 also having 1 ,0 % by weight of tungsten but of a higher nitrogen content than 605 262 and 605 263, has a considerably smaller quantity of sigma phase.
• a very well-adjusted balance between the various alloying elements is required at 5 these high alloy contents for, e.g. , chromium and molybdenum, in order to obtain good structural properties.
• Table 12 shows the results from the light optical investigation after annealing at 1080 °C, 20 min, followed by water quenching.
• the amount of sigma phase is indi- o cated by means of values from 1 to 5, with 1 representing that no sigma phase has been detected upon the investigation, while 5 representing that a very high percentage of sigma phase has been detected upon the investigation.
• results are shown from impact resistance testing of some of the charges. The results are very good, which indicates a fine structure after annealing at 1100 °C followed by water quenching and the requirement of 100 J is met by a large margin by all tested charges.
• Figure 4 shows the results from hot ductility test of most of the charges.
• a good workability is naturally crucial in order to be able to manufacture the material into product shapes such as bars, pipes, such as welded and seamless pipes, thread, weld material, construction parts such as, for instance, flanges and couplings.
• the strain controlled fatigue properties give information about how much, and how many times, a material may be elongated, before strain controlled fatigue cracks arise in the material. Since umbilical pipes are welded together into long lengths, are reeled on drums before the are twisted into the umbilical, it is not unusual that a number of operations occurs where certain plastic deformation arises before the umbilical starts function.
• the strain controlled fatigue data that has been established emphasize that the risk of rupture as a consequence of strain controlled fatigue in an umbilical pipe borders on zero.
• the strength that is required for being able to substantially reduce the weight of an umbilical is:
• 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 weldless pipes, weld material, construction parts, such as, for instance, flanges and couplings.

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