US20200123629A1 - Method of producing tube of duplex stainless steel - Google Patents

Method of producing tube of duplex stainless steel Download PDF

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US20200123629A1
US20200123629A1 US16/655,377 US201916655377A US2020123629A1 US 20200123629 A1 US20200123629 A1 US 20200123629A1 US 201916655377 A US201916655377 A US 201916655377A US 2020123629 A1 US2020123629 A1 US 2020123629A1
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tube
stainless steel
duplex stainless
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cold working
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Daniel SVEDBERG
Per OLSSON ARTBERGER
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Alleima AB
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Sandvik Materials Technology AB
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C1/00—Manufacture of metal sheets, metal wire, metal rods, metal tubes by drawing
    • B21C1/003—Drawing materials of special alloys so far as the composition of the alloy requires or permits special drawing methods or sequences
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C23/00—Extruding metal; Impact extrusion
    • B21C23/02—Making uncoated products
    • B21C23/04—Making uncoated products by direct extrusion
    • B21C23/08—Making wire, bars, tubes
    • B21C23/085—Making tubes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/12—Both compacting and sintering
    • B22F3/14—Both compacting and sintering simultaneously
    • B22F3/15—Hot isostatic pressing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C21D1/30—Stress-relieving
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C21D1/32—Soft annealing, e.g. spheroidising
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
    • C22C33/0278—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5%
    • C22C33/0285—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5% with Cr, Co, or Ni having a minimum content higher than 5%
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
    • F16L9/00—Rigid pipes
    • F16L9/02—Rigid pipes of metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2241/00—Treatments in a special environment
    • C21D2241/01—Treatments in a special environment under pressure
    • C21D2241/02—Hot isostatic pressing

Definitions

  • the present disclosure relates to a method of producing a tube of duplex stainless steel.
  • the tube is subjected to cold working to reach the desired geometry and tolerances.
  • the cold working may also be used to deformation harden the tube to a specified minimum yield strength. Since duplex steel deformation hardens rapidly along with area reduction during cold working, it is difficult to achieve both a specified geometry and the specified minimum yield strength in one cold working operation.
  • the duplex stainless steel tube is cold worked and thereafter annealed at approximately 1100° C. causing a full recrystallisation of the duplex stainless steel. Thereafter, the duplex stainless steel tube is subjected to a second cold working step with a limited area reduction. The limited area reduction is chosen such that the specific minimum yield strength is achieved.
  • EP 2853614 discloses a duplex stainless steel pipe and a manufacturing method thereof. Cold working of the duplex material introduces anisotropy in the pipe. A low temperature heat treatment is performed at a heat treatment temperature of 350° C. to 450° C. Thus, the anisotropy of the yield strength in the pipe axis direction and in the pipe circumferential direction of the stainless steel pipe is decreased after the cold working.
  • WO 2017/114847 discloses a process of producing a duplex stainless steel tube. The process is based around formulas taking into account the constituents of the duplex stainless steel as well as parameters of the area reduction during cold working in order to provide a target yield strength.
  • a method of producing a tube of duplex stainless steel comprising the following composition, in weight %:
  • the method comprises steps of:
  • the tube of stainless steel which has been cold worked is soft annealed by subjecting the tube to a temperature within a range of 500-750° C. for a time period of 0.5-5 minutes, recrystallisation of the cold worked tube is substantially avoided.
  • the yield strength of a duplex stainless steel tube produced in accordance with this method is improved over that of the duplex stainless steel material before the step of cold working while ductility remains at an adequate level, without becoming too brittle. As a result, the above mentioned object is achieved.
  • a particular minimum yield strength may be targeted without having to anneal the duplex stainless steel tube to full recrystallisation and without having to subject the duplex stainless steel tube to a second cold working step.
  • the present disclosure is based around one single step of cold working providing a high area reduction followed by soft annealing.
  • duplex stainless steel materials When subjected to temperatures within a range of 450-1000° C., duplex stainless steel materials are prone to embrittlement and reduced corrosion resistance. Therefore, commonly this temperature range has been avoided during the manufacturing and use of duplex stainless steel tubes.
  • the inventors have realised that subjecting a duplex stainless steel tube to the temperature range of 500-750° C. for a short controlled time period of 0.5-5 minutes will improve the ductility of the duplex stainless steel tube without causing embrittlement or reduced corrosion resistance. Moreover, the inventors have discovered that a target minimum yield strength may be achieved in a controlled manner.
  • the target minimum yield strength may be achieved by the step of soft annealing at the above mentioned temperature range and within the above mentioned time period. In contrast to one of the above described prior art methods, this is achieved without requiring annealing at a high temperature causing full recrystallisation followed by a second cold working step.
  • the step of soft annealing results in a partial reduction of the yield strength achieved in the step of cold working, to arrive at a target minimum yield strength, see further below.
  • duplex stainless steel material also goes under the name, Sandvik SAF2507®.
  • One common use of such duplex stainless steel tubes is within the oil and gas industry.
  • the step of forming the tube of the duplex stainless steel results in a tube ready for the cold working step.
  • the step of forming the tube may comprise the producing of an ingot or a billet from a melt. The ingot or billet is then hot extruded into a tube.
  • the step of forming the tube may comprise the production of a duplex stainless steel powder, which is formed and hot isostatic pressed into a tube shape.
  • the step of cold working the tube may be performed in a known manner such as by cold rolling, cold pilgering, cold drawing, or combinations thereof.
  • the step of cold working may be the only cold working step in the method of producing a tube of duplex stainless steel.
  • the step of cold working may comprise cold working to a final dimension of the tube, or to substantially the final dimension of the tube. Only a minor diameter adjustment may take place subsequent to the step of cold working, as a result of e.g. peel turning, grinding, and/or polishing.
  • Rp0.2 target minimum yield strength
  • the unit of Rp0.2 is MPa.
  • a target minimum yield strength Rp0.2 within a range of 868.25-1156.25 MPa may be achieved with a standard deviation of 10 MPa for a duplex stainless steel tube of the above defined material, Sandvik SAF2507®.
  • a particular minimum yield strength may be easily targeted without having to anneal the duplex stainless steel tube to full recrystallisation and without having to subject the duplex stainless steel tube to a second cold working step.
  • the step of soft annealing the tube may be followed by a step of:
  • FIG. 1 illustrates a tube of duplex stainless steel
  • FIG. 2 illustrates a method of producing a tube of duplex stainless steel.
  • FIG. 3 illustrates a diagram over the yield strength of test specimens of a tube of duplex stainless steel produced in accordance with the method.
  • FIG. 1 illustrates a duplex stainless steel tube 2 according to embodiments.
  • the duplex stainless steel tube 2 has an outer diameter D, and a wall thickness w.
  • the duplex stainless steel tube 2 has been produced by a method of producing a tube of duplex stainless steel according to aspects and/or embodiments discussed herein.
  • the outer diameter D of the tube 2 may be within a range of 50-250 mm.
  • a wall thickness w of the tube 2 may be within a range of 5-35 mm.
  • FIG. 2 illustrates a method 10 of producing a tube of duplex stainless steel.
  • the tube may be a tube 2 as discussed in connection with FIG. 1 .
  • the duplex stainless steel comprises the following composition, in weight %:
  • composition of the duplex stainless steel As to the composition of the duplex stainless steel, the following is to be noted regarding the individual alloying elements therein:
  • Carbon, C is a representative element for stabilizing austenitic phase and an important element for maintaining mechanical strength. However, if a large content of carbon is used, carbon will precipitate as carbides and thus reduces corrosion resistance. According to one embodiment, the carbon content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is 0 to 0.03 wt % such as 0 to 0.02 wt %.
  • Chromium, Cr has strong impact on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter, especially pitting corrosion. Cr improves the yield strength, and counteracts transformation of austenitic structure to martensitic structure upon deformation of the duplex stainless steel. However, an increasing content of Cr will result in for the formation of unwanted stable chromium nitride and sigma phase and a more rapid generation of sigma phase. According to one embodiment, the chromium content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is of from 24 to 26 wt %.
  • Manganese, Mn has a deformation hardening effect on the duplex stainless steel as defined hereinabove or hereinafter. Mn is also known to form manganese sulphide together with sulphur present in the steel, thereby improving the hot workability. However, at too high levels, Mn tends to adversely affect both corrosion resistance and hot workability. According to one embodiment, the manganese content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is 0 to 1.5 wt % such as 0 to 1.2 wt %.
  • Molybdenum, Mo has a strong influence on the corrosion resistance of the duplex stainless steel as defined hereinabove or hereinafter and it heavily influences the pitting resistance equivalent, PRE. Mo has also a positive effect on the yield strength and increases the temperature at which the unwanted sigma-phases are stable and further promotes generation rate thereof. Additionally, Mo has a ferrite-stabilizing effect. According to one embodiment, the molybdenum content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is of from 3.0 to 4.5 wt % such as 3.5 to 4.0 wt %.
  • Nickel, Ni has a positive effect on the resistance against general corrosion. Ni also has a strong austenite-stabilizing effect. According to one embodiment, the nickel content of the duplex stainless steel used in the method disclosed hereinbefore and hereinafter is of from 6 to 8 wt %, such as 6.5 to 7.5 wt %.
  • the silicon content of the duplex stainless steel used in the method disclosed hereinabove or hereinafter is of 0 to 1.0 wt % such as 0 to 0.8 wt %.
  • Phosphorous, P may be present as an impurity in the stainless steel used in the method disclosed hereinabove or hereinafter, and will result in deteriorated workability of the steel if at too high level, thus, P ⁇ 0.05 wt % such as P ⁇ 0.03 wt %.
  • S may be present as an impurity in the stainless steel used in the method disclosed hereinabove or hereinafter and will result in deteriorated workability of the steel if at too high level, thus, S ⁇ 0.03 wt % such as S ⁇ 0.02 wt %.
  • Oxygen, O may be present as an impurity in the stainless steel used in the method disclosed hereinabove or hereinafter, wherein O ⁇ 0.010 wt %.
  • the duplex stainless steel as defined hereinabove or hereinafter may also comprise small amounts other alloying elements which may have been added during the method, e.g. Ca ( ⁇ 0.01 wt %), Mg ( ⁇ 0.01 wt %), Cu (max 0.5 wt %), and rare earth metals REM ( ⁇ 0.2 wt %).
  • the method 10 comprises steps of:
  • the step of forming 12 a tube may comprise steps of:
  • a tube of the duplex stainless steel may be provided for the subsequent step of cold working 14 the tube.
  • the method 10 may include further known intermediate and subsequent production steps such as cutting, peel turning, deep boring, straightening, and acid pickling.
  • the step of forming 12 a tube may comprise steps of:
  • a tube based on a powder duplex stainless steel material may be produced and provided for the subsequent step of cold working 14 the tube.
  • the method 10 may include further known intermediate and subsequent production steps such as cutting, peel turning, straightening, and acid pickling.
  • the step of soft annealing 16 the tube may be performed e.g. in a furnace under controlled temperature conditions.
  • a further alternative may be to induction heat the tube to a specified temperature.
  • the step of soft annealing 16 the tube may be performed in accordance with a particular formula in order to achieve a particular target minimum yield strength, Rp0.2, in the tube of the duplex stainless steel.
  • the formula reads:
  • T represents the temperature in degrees Celsius
  • t represents the time period in minutes, which results in that the targeted minimum yield strength, Rp0.2, has the unit MPa.
  • a standard deviation of 10 MPa is achieved in the target minimum yield strength, Rp0.2.
  • a target minimum yield strength Rp0.2 within a range of 868.25-1156.25 MPa may be achieved with a standard deviation of 10 MPa when the step of soft annealing 16 the tube is performed in accordance with the formula.
  • the step of soft annealing 16 the tube may be followed by a step of:
  • the step of soft annealing 16 the tube may comprise subjecting the tube to a temperature within a range of 600-750° C. for a time period of 1-3 minutes.
  • the step of cold working 14 the tube includes an area reduction of the tube within a range of 25-70%.
  • the duplex stainless steel may be strain hardened in the step of cold working 14 the tube.
  • the area reduction relates to a reduction of the cross sectional area of the tube.
  • the difference in cross sectional area before and after the step of cold working is divided by the cross sectional area before the step of cold working. Only the area of the tube where there is material is taken account of in the calculation of the area reduction, i.e. the cross sectional area of the inside of the tube is excluded.
  • a duplex stainless steel tube of the above defined material, Sandvik SAF2507®, having an outer diameter, D of 173.8 mm and a wall thickness of 29.7 mm has been produced by the steps of forming 12 the tube, including producing an ingot or casted billet, and cold working 14 the tube.
  • the step of cold working 14 was performed by cold pilgering with an area reduction of 30.7%.
  • the step of soft annealing 16 was performed by subjecting different lengthwise sections of the tube to different temperatures over varying time periods, followed by water quenching. More specifically, the different sections of the tube were subjected to induction heating using effect ranges 40-100 kW at a frequency of 30 kHz for different time periods utilizing different feeding rates through the induction heating apparatus.
  • test specimens which have been subjected to the same soft annealing temperature and time have a yield strength within a limited yield range, as clearly visible in the diagram of FIG. 3 .
  • the eighth test samples within each circle have been subjected to the same temperature and time period.
  • the microstructure of the soft annealed tube had not recrystallised and was similar to, or almost similar to, that of the cold worked tube. Concerning the hardness of the soft annealed tube, it could be concluded that the hardness is on same level as low reduction cold worked Sandvik SAF2507® material, i.e. a hardness comparable to that of a tube made of Sandvik SAF2507®, which has been produced according to a prior art method including a second step of cold working with an area reduction of less than 25%.

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US16/655,377 2018-10-17 2019-10-17 Method of producing tube of duplex stainless steel Abandoned US20200123629A1 (en)

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