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EP3913103A1 - Ni-based alloy tube and welded joint - Google Patents

Ni-based alloy tube and welded joint Download PDF

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Publication number
EP3913103A1
EP3913103A1 EP21175398.3A EP21175398A EP3913103A1 EP 3913103 A1 EP3913103 A1 EP 3913103A1 EP 21175398 A EP21175398 A EP 21175398A EP 3913103 A1 EP3913103 A1 EP 3913103A1
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EP
European Patent Office
Prior art keywords
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content
good
based alloy
reason
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Application number
EP21175398.3A
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German (de)
French (fr)
Inventor
Hiroyuki Hirata
Mitsuru Yoshizawa
Takahiro Osuki
Masaaki Terunuma
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Nippon Steel Corp
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Nippon Steel Corp
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Publication of EP3913103A1 publication Critical patent/EP3913103A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C30/00—Alloys containing less than 50% by weight of each constituent
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C19/00—Alloys based on nickel or cobalt
    • C22C19/03—Alloys based on nickel or cobalt based on nickel
    • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
    • C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
    • C22C19/055—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 20% but less than 30%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C30/00—Alloys containing less than 50% by weight of each constituent
    • C22C30/02—Alloys containing less than 50% by weight of each constituent containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C30/00—Alloys containing less than 50% by weight of each constituent
    • C22C30/04—Alloys containing less than 50% by weight of each constituent containing tin or lead
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/10—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of nickel or cobalt or alloys based thereon

Definitions

  • the present invention relates to a Ni-based alloy tube and a welded joint.
  • Some kind of plant equipment is produced by constructing by welding.
  • a weld bead shape may influence on corrosion behavior.
  • some kind of heat exchanger consists of Ni-base alloy tubes serving as passage of coolant and constructed by butt-welding.
  • various kinds of corrosive fluid flow inside the tubes.
  • the corrosive fluid accumulates and concentrates at a toe of weld, where a surface of the bead and a surface of a base metal intersect with each other.
  • corrosion is liable to proceed at the toe of weld, which raises a problem.
  • Patent Documents 1 to 8 have no discussions about these problems.
  • Ni-based alloy having high corrosion resistance is used as a material to produce tubes, it is difficult to form an internal bead having an appropriate shape during butt welding and making corrosion not liable to proceed between the tubes.
  • a problem is that it is difficult to obtain a Ni-based alloy tube that allows an internal bead to be stably formed inside during welding and prevents a reinforcement with excessive height.
  • an objective of the present invention is to solve the problems described above and to provide a Ni-based alloy tube that allows an internal bead having good properties of its weldment to be formed stably, and a welded joint.
  • the present invention is made to solve the above problems, and the gist of the present invention is the following Ni-based alloy tube and welded joint.
  • the present invention makes it possible to obtain a Ni-based alloy tube that allows an internal bead having good properties of a weldment to be formed stably.
  • Figure 1 is a diagram illustrating a bevel shape in EXAMPLE.
  • the present inventors studied weldability of a Ni-based alloy tube and obtained the following findings (a) to (d).
  • the content of C is set at 0.050% or less.
  • the content of C is to be preferably 0.025% or less, more preferably 0.020% or less, and still more preferably 0.015% or less. From the viewpoint of corrosion resistance, the content of C is desirably reduced as much as possible, but an excessive reduction of the content of C increases production costs. For this reason, the content of C is to be 0.001% or more.
  • the content of C is to be preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more.
  • Si has a deoxidation effect.
  • a content of Si is to be 0.05% or more.
  • the content of Si is to be preferably 0.08% or more, more preferably 0.10% or more, and still more preferably 0.12% or more.
  • the content of Si is to be 0.50% or less.
  • the content of Si is to be preferably 0.48% or less, more preferably 0.45% or less, and still more preferably, the content of Si is to be 0.40% or less.
  • Mn has a deoxidation effect.
  • Mn has an effect of increasing a phase stability of metal and makes no small contribution to stable formation of the internal bead.
  • a content of Mn is to be 0.05% or more.
  • the content of Mn is to be preferably 0.10% or more, more preferably 0.15% or more, and still more preferably 0.20% or more.
  • the content of Mn is to be 1.00% or less.
  • the content of Mn is to be preferably 0.90% or less, and more preferably 0.80% or less. Still more preferably, the content of Mn is to be 0.70% or less.
  • a content of P is to be 0.025% or less.
  • the content of P is to be preferably 0.023% or less, and more preferably 0.020% or less.
  • the content of P is preferably reduced as much as possible, but an excessive reduction of the content of P increases production costs. For this reason, the content of P is to be preferably 0.001% or more, and more preferably 0.002% or more.
  • S is normally contained in the Ni-based alloy as an impurity, but in the alloy tube according to the present invention, S has an effect of increasing capability of forming the internal bead together with O during welding. For this reason, a content of S is to be 0.0001% or more.
  • the content of S is to be preferably 0.0002% or more, and more preferably 0.0003% or more.
  • the content of S is to be 0.0030% or less.
  • the content of S is to be preferably 0.0025% or less, and more preferably 0.0020% or less. Note that S is required to satisfy Formula (i) described below together with O and Sn.
  • Fe is useful for increasing hot workability.
  • Fe contributes to reduction of an alloy cost.
  • a content of Fe is to be 18.0% or more.
  • the content of Fe is to be preferably 18.5% or more, and more preferably 19.0% or more. Still more preferably, the content of Fe is to be 25.0% or more.
  • the content of Fe is to be 40.0% or less.
  • the content of Fe is to be preferably 39.5% or less, and more preferably 39.0% or less. Still more preferably, the content of Fe is to be 35.0% or less.
  • Cu increases phase stability of metal and has an effect of increasing corrosion resistance under an environment where non-oxidizing acid and chloride are present. For this reason, a content of Cu is to be 1.50% or more.
  • the content of Cu is to be preferably 1.52% or more, more preferably 1.55% or more, and still more preferably 1.57% or more.
  • the content of Cu is to be 3.00% or less.
  • the content of Cu is to be preferably 2.98% or less, and more preferably 2.95% or less. Still more preferably, the content of Cu is to be 2.80% or less.
  • Co has an effect of increasing phase stability of metal.
  • a content of Co is to be 0.01% or more.
  • the content of Co is to be preferably 0.02% or more, and more preferably 0.03% or more. Still more preferably, the content of Co is to be 0.05% or more.
  • the content of Co is to be 0.80% or less.
  • the content of Co is to be preferably 0.78% or less, more preferably 0.75% or less, and still more preferably 0.70% or less.
  • Cr is an element that is essential in keeping corrosion resistance.
  • Cr causes formation of a passivation film on a surface, increasing corrosion resistance under an acid environment of oxidative properties.
  • a content of Cr is to be 19.5% or more.
  • the content of Cr is to be preferably 19.7% or more, more preferably 20.0% or more, and still more preferably 20.2% or more.
  • the content of Cr is to be 23.5% or less.
  • the content of Cr is to be preferably 23.0% or less, more preferably 22.5% or less, and still more preferably 22.3% or less.
  • Mo increases corrosion resistance under an environment where non-oxidizing acid and chloride are present. For this reason, a content of Mo is to be 2.4% or more.
  • the content of Mo is to be preferably 2.6% or more, more preferably 2.8% or more, and still more preferably 2.9% or more.
  • the content of Mo is to be 3.6% or less.
  • the content of Mo is to be preferably 3.4% or less, more preferably 3.2% or less, and still more preferably 3.1% or less.
  • W increases corrosion resistance under an environment where non-oxidizing acid and chloride are present.
  • a content of W is to be 0.01% or more.
  • the content of W is to be preferably 0.02% or more, more preferably 0.03% or more, and still more preferably 0.04% or more.
  • the content of W is to be 0.80% or less.
  • the content of W is to be preferably 0.70% or less, more preferably 0.60% or less, and still more preferably 0.50% or less.
  • Ti forms its carbide to contribute to strengthening and to prevent or reduce formation of Cr carbide, so as to mitigate deterioration in corrosion resistance of grain boundaries.
  • a content of Ti is to be 0.50% or more.
  • the content of Ti is to be preferably 0.55% or more, more preferably 0.60% or more, and still more preferably 0.65% or more.
  • the content of Ti is to be 1.20% or less.
  • the content of Ti is to be preferably 1.00% or less, more preferably 0.80% or less, and still more preferably 0.75% or less.
  • N contributes to phase stability of metal and has an effect of increasing pitting resistance. For this reason, a content of N is to be 0.0010% or more.
  • the content of N is to be preferably 0.0020% or more, more preferably 0.0030% or more, and still more preferably 0.0040% or more.
  • the content of N is to be 0.0200% or less.
  • the content of N is to be preferably 0.0180% or less, more preferably 0.0150% or less, and still more preferably, the content of N is to be 0.0130% or less.
  • Al has a deoxidation effect. Al contributes to increase in oxidation resistance at high temperature. For this reason, a content of Al is to be 0.001% or more.
  • the content of Al is to be preferably 0.005% or more, and more preferably 0.010% or more. Still more preferably, the content of Al is to be 0.020% or more.
  • the content of Al is to be 0.200% or less.
  • the content of Al is to be preferably 0.180% or less, and more preferably 0.160% or less. Still more preferably, the content of Al is to be 0.150% or less.
  • O is normally contained in the Ni-based alloy as an impurity, but in the alloy tube according to the present invention, O has an effect of increasing capability of forming the internal bead together with S during welding. For this reason, a content of O is to be 0.0004% or more.
  • the content of O is to be preferably 0.0006% or more, and more preferably 0.0008% or more.
  • the content of O is set at 0.0120% or less.
  • the content of O is to be preferably 0.0100% or less, and more preferably 0.0080% or less. Note that O is required to satisfy Formula (i) described below together with S and Sn.
  • Sn may be contained within the range described below.
  • Sn has an effect of increasing a penetration depth during welding, increasing a capability of forming the internal bead. Therefore, it may be contained when necessary. However, if Sn is contained excessively, hot workability is decreased, and weld crack susceptibility is increased. In addition, the internal bead is liable to have a convex shape. For this reason, a content of Sn is to be 0.030% or less.
  • the content of Sn is to be preferably 0.028% or less, and more preferably 0.025% or less.
  • the content of Sn is to be preferably 0.001% or more, more preferably 0.002% or more, and still more preferably 0.003% or more. Note that Sn is required to satisfy Formula (i) described below together with S and O.
  • S and O are surface-active elements and have an action of strengthening inward convection in a molten pool during welding.
  • Sn contributes to formation of an arc energizing path and has an effect of increasing a degree of concentration of an arc. Welding heat is thereby transmitted in a depth direction at a center of the molten pool.
  • the middle value of Formula (i) is to be 0.0010% or more.
  • the middle value of Formula (i) is to be preferably 0.0012% or more, and more preferably 0.0015% or more.
  • the middle value of Formula (i) is more than 0.0220%, a surface tension of molten metal is decreased, or melting of the center of the molten pool is accelerated, which causes sagging. As a result, the internal bead is made to have a convex shape, and it is no longer possible to form the internal bead stably inside the tube. Therefore, the middle value of Formula (i) is to be 0.0220% or less.
  • the middle value of Formula (i) is to be preferably 0.0215% or less, and more preferably 0.0210% or less.
  • one or more elements selected from V, Nb, Ta, Ca, Mg, B, and REM may be contained within the ranges described below. Reasons for limiting a content of each element will be described.
  • V 0.40% or less
  • V combines with carbon to form its carbide, preventing or reducing formation of Cr carbide, so as to mitigate deterioration in corrosion resistance of grain boundaries. Therefore, it may be contained when necessary. However, if V is contained excessively, carbide and carbo-nitride of V precipitate in a large quantity, decreasing ductility. For that reason, the content of V is to be 0.40% or less. The content of V is to be preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, to obtain the effect described above, the content of V is to be preferably 0.01% or more, and more preferably 0.02% or more.
  • Nb combines with carbon to form its carbide, preventing or reducing formation of Cr carbide, so as to mitigate deterioration in corrosion resistance of grain boundaries. Therefore, it may be contained when necessary. However, if Nb is contained excessively, carbide and carbo-nitride of Nb precipitate in a large quantity, decreasing ductility. In addition, weld crack susceptibility is increased. For that reason, a content of Nb is to be 0.40% or less. The content of Nb is to be preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, to obtain the effect described above, the content of Nb is to be preferably 0.01% or more, and more preferably 0.02% or more.
  • Ta accelerates formation of a passivation film of Cr and has an effect of increasing corrosion resistance. Therefore, it may be contained when necessary. However, if Ta is contained excessively, carbide of Ta precipitates in a large quantity, decreasing ductility. For that reason, a content of Ta is to be 0.40% or less.
  • the content of Ta is to be preferably 0.35% or less, and more preferably 0.30% or less.
  • the content of Ta is to be preferably 0.01% or more, and more preferably 0.02% or more.
  • Ca has an effect of improving hot workability. Therefore, it may be contained when necessary. However, if Ca is contained excessively, Ca combines with oxygen, significantly decreasing cleanliness. As a result, hot workability is rather decreased. For this reason, a content of Ca is to be 0.0100% or less.
  • the content of Ca is to be preferably 0.0080% or less, and more preferably 0.0060% or less.
  • the content of Ca is to be preferably 0.0005% or more, and more preferably 0.0010% or more.
  • Mg has an effect of improving hot workability. Therefore, it may be contained when necessary. However, if Mg is contained excessively, Mg combines with oxygen, significantly decreasing cleanliness. As a result, hot workability is rather decreased. For this reason, a content of Mg is to be 0.0100% or less.
  • the content of Mg is to be preferably 0.0080% or less, and more preferably 0.0060% or less.
  • the content of Mg is to be preferably 0.0005% or more, and more preferably 0.0010% or more.
  • B segregates in grain boundaries at high temperature and has effects of strengthening the grain boundaries and increasing hot workability. Therefore, it may be contained when necessary. However, if B is contained excessively, weld crack susceptibility is increased. For this reason, a content of B is set at 0.0100% or less.
  • the content of B is to be preferably 0.0080% or less, and more preferably 0.0060% or less.
  • the content of B is to be preferably 0.0002% or more, and more preferably 0.0005% or more.
  • REM has an effect of improving hot workability in production. Therefore, it may be contained when necessary. However, if REM is contained excessively, REM combines with oxygen, significantly decreasing cleanliness. As a result, hot workability is rather decreased. For this reason, a content of REM is set at 0.0800% or less.
  • the content of REM is to be preferably 0.0600% or less, and more preferably 0.0500% or less.
  • the content of REM is to be preferably 0.0005% or more, and more preferably 0.0010% or more.
  • REM refers to Sc, Y, and lanthanoids
  • the content of REM refers to the amount of these elements.
  • the balance is Ni and impurities.
  • impurities means elements that are not added intentionally but are mixed in the Ni-based alloy in producing the Ni-based alloy industrially due to various factors such as raw materials and a production process, and that are allowed to be within ranges in which the impurities have no adverse effect on the present invention.
  • An internal bead is formed when end portions of alloy tubes are welded together. To form a favorable internal bead, it is preferable to control, an arithmetic average roughness Ra on an internal surface of the alloy tube in a longitudinal direction.
  • An internal surface roughness of the alloy tube herein refers to a surface roughness after a final process in a production process. In other words, although the internal surface roughness of the alloy tube fluctuates in a course of the production, a surface roughness in a middle of the production does not matter in obtaining an advantageous effect of the present invention, and it will suffice that the internal surface roughness of the tube in the longitudinal direction after the final process satisfies a range specified in the present invention.
  • the arithmetic average roughness Ra of the tube in the longitudinal direction is more than 9.0 ⁇ m, weld metal is hindered from wetting on the internal surface of the tube, and thus the weld metal resists spreading along a width direction, that is, a circumference of the tube.
  • the internal bead is liable to have a convex shape, and a height of reinforcement is liable to increase.
  • the arithmetic average roughness Ra of the tube in the longitudinal direction is to be preferably 9.0 ⁇ m or less.
  • the arithmetic average roughness Ra is to be preferably 7.0 ⁇ m or less, and more preferably 5.0 ⁇ m or less.
  • a lower limit value of the arithmetic average roughness Ra is not limited to a specific value; however, in many cases, the arithmetic average roughness Ra is normally 0.1 to 1.0 ⁇ m or more in a case where a production method described below is used.
  • the arithmetic average roughness Ra is specified in JIS B 0601:2001 and can be measured by using a contact-type surface roughness instrument.
  • a welded joint of Ni-based alloy tubes described above can be obtained by performing butt welding on tube ends of the Ni-based alloy tubes under predetermined conditions.
  • the welded joint of the Ni-based alloy tubes includes weld metal which is to be a weldment by solidification, and base material portion.
  • the base material portions each include a weld heat affected zone that is influenced by heat input of welding.
  • the base material portion excluding the weld heat affected zone inherits the chemical composition, the surface roughness, and other properties of the Ni-based alloy tube described in Sections 1 and 2 described above.
  • the weldment refers to the weld metal and the weld heat affected zone.
  • Ni-based alloy tube according to the present invention provides its advantageous effects irrespective of its production method as long as the Ni-based alloy tube has the requirement described above; nonetheless, the Ni-based alloy tube can be produced stably by a production method described below, for example.
  • Ni-based alloy ingot being a starting material of the Ni-based alloy tube is produced.
  • the Ni-based alloy ingot is preferably produced by melting an alloy having the chemical composition described above using an electric furnace, refining the alloy to remove impurities, and casting the alloy. Subsequently, the obtained ingot is preferably subjected to hot forging to be formed into a billet having a columnar shape. Thereafter, the obtained billet is worked to be shaped into a tube.
  • the billet is preferably subjected to hot extrusion and then to cold rolling or cold drawing. Softening heat treatment and intermediate pickling may be performed in the middle of the working when necessary. Thereafter, the alloy tube is preferably subjected to solution treatment as heat treatment. After the solution treatment, pickling or working may be performed when necessary.
  • the solution treatment is preferably performed under such conditions of heating in a temperature region of 950°C to 1230°C for 1 to 15 minutes and performing water cooling. Additionally, it is preferable to perform one of machining such as grinder processing and grinding, and shotblast or shotpeening treatment, on the internal surface of the tube.
  • an advantageous effect of the present invention does not pertain to the surface roughness in the middle of the course and is influenced only by the surface roughness of the tube in the longitudinal direction after the final process.
  • a welded joint can be obtained by welding together end portions of Ni-based alloy tubes according to the present invention as starting materials.
  • a method for the welding is not limited to a specific method; however, the welding can be performed by arc welding, for example.
  • a heat input is preferably set to be within the range from 4 to 20 kJ/cm.
  • Ar gas as shielding gas and back shielding gas during welding.
  • a flow rate of the gas flowed to a welding location is preferably adjusted as appropriate.
  • a chemical composition of a welding material (filler material) to be used is not limited to a specific material, either; however, the chemical composition is preferably a composition described below.
  • the composition it is preferable for the composition to contain, in mass%, C: 0.150% or less, Si: 1.00% or less, Mn: 3.50% or less, P: 0.030% or less, S: 0.0001 to 0.0100%, Fe: 38.0% or less, Cu: 3.00% or less, Co: 15.0% or less, Cr: 14.0 to 26.0%, Mo: 17.0% or less, W: 4.5% or less, at least one of Nb and Ta: 4.20% or less in total, Ti: 1.50% or less, V: 0.35% or less, N: 0.0500% or less, Al: 1.50% or less, and O: 0.0004 to 0.0120%, with the balance being Ni and impurities, and to make a relation between contents of S and O satisfy Formula (a) shown below.
  • Formula (a) shown below.
  • the alloy plates were performed solution treatment that the alloy plates were retained in a hydrogen furnace at 1150°C for 10 minutes and cooled by water cooling. Thereafter, test plates each having a width of 50 mm and a length of 100 mm were cut from the alloy plates.
  • shotpeening was conducted on only one side, assuming the internal surface of the alloy tube, as shown in Table 2.
  • one side of test plates were machine-ground or polished 1 to 5 times using a whetstone with granularity of #40 or #60.
  • polishing (#40 ⁇ 1 time) indicates that a test plate was polished once using a whetstone with a granularity of #40.
  • welded joints each of which a back bead was formed across an overall length of its weld line were determined to have no problem in capability of forming an internal bead of the alloy tube and were rated as "Good".
  • welded joints each of which a width of a back bead was 2 mm or more across an overall length of its weld line were rated as "Excellent”, and welded joints each of which a width of a back bead formed was less than 2 mm but 1 mm or more were rated as "Acceptable”.
  • a back bead in this example corresponds to an internal bead that is formed when the weld is made from the outside of the alloy tube.
  • Test pieces made of alloy types A to H and L to N all satisfied the specifications according to the present invention, and their capabilities of forming a back bead and shapes of their back beads were good.
  • the test piece N1 made of the alloy type N satisfied the range specified by Formula (i), and therefore, its capability of forming a back bead and a height of its back bead were both satisfactory.
  • test pieces I1 and K1 made of the alloy types I and K their contents of S and O did not satisfy Formula (i) and were higher than the respective specified ranges. Therefore, sagging of its molten metal was significant, and a height of its back bead did not satisfy a target of the height.
  • a test piece J1 made of the alloy type J a relation between its contents of S and O did not satisfy Formula (i), and the contents were lower than the respective specified ranges. Therefore, melting in a plate thickness direction was not sufficient, and a capability of forming the back bead as targeted was not obtained.
  • test piece O1 made of the alloy type O its contents of S, O, and Sn were lower than the range specified by Formula (i). Therefore, melting in a plate thickness direction was not sufficient, and a capability of forming the back bead did not satisfy a target of the capability.
  • test pieces P1 and Q1 made of the alloy types P and Q1 their contents of S, O and Sn were higher than the range specified by Formula (i), or their contents of Sn were more than the specified range. Therefore, sagging of its molten metal was large, and a back bead height as targeted was not satisfied.
  • the present invention makes it possible to obtain a Ni-based alloy tube that allows an internal bead to be formed stably during butt welding and prevents a reinforcement with excessive height.

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Abstract

A Ni-based alloy tube includes a chemical composition containing, in mass%, C: 0.001 to 0.050%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0001 to 0.0030%, Fe: 18.0 to 40.0%, Cu: 1.50 to 3.00%, Co: 0.01 to 0.80%, Cr: 19.5 to 23.5%, Mo: 2.4 to 3.6%, W: 0.01 to 0.80%, Ti: 0.50 to 1.20%, N: 0.0010 to 0.0200%, Al: 0.001 to 0.200%, O: 0.0004 to 0.0120%, and Sn: 0 to 0.030%, with the balance: Ni and impurities, and satisfying [0.0010 ≤ S + 2O + 0.2Sn ≤ 0.0220].

Description

    TECHNICAL FIELD
  • The present invention relates to a Ni-based alloy tube and a welded joint.
    • BACKGROUND ART
  • In a chemical plant and a power plant, various kinds of plant equipment such as flue gas treatment equipment and seawater treatment equipment are installed. An inside of a plant is a harsh corrosive environment where substances accelerating corrosion, such as chlorides and hydrogen sulfide are present in a large amount. For this reason, materials used for plant equipment are required to have a strength as well as corrosion resistance. Therefore, as disclosed in Patent Documents 1 to 8, Ni-based alloys with enhanced corrosion resistance, which are assumed to be used for plant equipment, are developed.
    • LIST OF PRIOR ART DOCUMENTS PATENT DOCUMENT
    • Patent Document 1: JP54-110918A
    • Patent Document 2: JP63-89637A
    • Patent Document 3: JP2-156034A
    • Patent Document 4: JP3-173732A
    • Patent Document 5: JP5-271832A
    • Patent Document 6: JP9-87786A
    • Patent Document 7: JP10-30140A
    • Patent Document 8: JP2012-72446A
    SUMMARY OF INVENTION TECHNICAL PROBLEM
  • Some kind of plant equipment is produced by constructing by welding. In such equipment, a weld bead shape may influence on corrosion behavior.
  • For example, some kind of heat exchanger consists of Ni-base alloy tubes serving as passage of coolant and constructed by butt-welding. When the heat exchanger is used, various kinds of corrosive fluid flow inside the tubes. At this point, if a height of an internal bead that is formed inside the tubes by welding, that is, a reinforcement of an internal bead is excessively high, the corrosive fluid accumulates and concentrates at a toe of weld, where a surface of the bead and a surface of a base metal intersect with each other. As a result, corrosion is liable to proceed at the toe of weld, which raises a problem.
  • On the other hand, if an amount of heat input during welding is reduced with an excessive decrease in a height of reinforcement intended, a butting surface between tubes is not melted completely, which makes it difficult to form a stable internal bead. As a result, a weld defect occurs, and corrosive fluid accumulates and concentrates in the weld defect, making corrosion liable to proceed, which raises a problem. However, Patent Documents 1 to 8 have no discussions about these problems.
  • Therefore, even when Ni-based alloy having high corrosion resistance is used as a material to produce tubes, it is difficult to form an internal bead having an appropriate shape during butt welding and making corrosion not liable to proceed between the tubes. In other words, a problem is that it is difficult to obtain a Ni-based alloy tube that allows an internal bead to be stably formed inside during welding and prevents a reinforcement with excessive height.
  • In view of the above, an objective of the present invention is to solve the problems described above and to provide a Ni-based alloy tube that allows an internal bead having good properties of its weldment to be formed stably, and a welded joint.
    • SOLUTION TO PROBLEM
  • The present invention is made to solve the above problems, and the gist of the present invention is the following Ni-based alloy tube and welded joint.
    1. (1) A Ni-based alloy tube having a chemical composition consisting of, in mass%:
      • C: 0.001 to 0.050%,
      • Si: 0.05 to 0.50%,
      • Mn: 0.05 to 1.00%,
      • P: 0.025% or less,
      • S: 0.0001 to 0.0030%,
      • Fe: 18.0 to 40.0%,
      • Cu: 1.50 to 3.00%,
      • Co: 0.01 to 0.80%,
      • Cr: 19.5 to 23.5%,
      • Mo: 2.4 to 3.6%,
      • W: 0.01 to 0.80%,
      • Ti: 0.50 to 1.20%,
      • N: 0.0010 to 0.0200%,
      • Al: 0.001 to 0.200%,
      • O: 0.0004 to 0.0120%,
      • Sn: 0 to 0.030%, and
      • optionally,
      • V: 0.40% or less,
      • Nb: 0.40% or less,
      • Ta: 0.40% or less,
      • Ca: 0.0100% or less,
      • Mg: 0.0100% or less,
      • B: 0.0100% or less, and
      • REM: 0.0800% or less,
      • with the balance: Ni and impurities, and
      • satisfying Formula (i) below: 0.0010 ≤ S + 2 O + 0.2 Sn ≤ 0.0220
        Figure imgb0001
      • where each symbol of an element in the formula indicates a content of the element (mass%) contained in Ni-based alloy, and a content being zero means that the element is not contained.
    2. (2) The Ni-based alloy tube according to (1) above, wherein on an internal surface of the Ni-based alloy tube, an arithmetic average roughness Ra of the tube in a longitudinal direction is 9.0 µm or less.
    3. (3) A welded joint including the Ni-based alloy tube according to (1) or (2) above.
    ADVANTAGEOUS EFFECTS OF INVENTION
  • The present invention makes it possible to obtain a Ni-based alloy tube that allows an internal bead having good properties of a weldment to be formed stably.
    • BRIEF DESCRIPTION OF DRAWING
  • [Figure 1]
    Figure 1 is a diagram illustrating a bevel shape in EXAMPLE.
    • DESCRIPTION OF EMBODIMENTS
  • The present inventors studied weldability of a Ni-based alloy tube and obtained the following findings (a) to (d).
    1. (a) A shape of an internal bead of a Ni-based alloy tube formed during butt welding is influenced by contents of S and O contained in the tube. The present inventors revealed that, if the contents of S and O are small, the internal bead is not formed stably, and a butting surface partly remains unmelted.
      On the other hand, if the contents of S and O are excessive, although the internal bead can be formed stably, a reinforcement of the internal bead becomes excessively high. Therefore, in this case, corrosive fluid accumulates in a vicinity of a bead, making corrosion liable to proceed. Hence, in order to allow an internal bead to be formed stably and prevent a reinforcement with excessive height, it is necessary to adjust the content of S and the content of O within their prescribed ranges.
    2. (b) A possible reason that S and O have an influence on formation of a bead is as follows. S and O are surface-active elements and therefore strengthen inward convection in a molten pool during welding. As a result, welding heat is liable to be transmitted in a depth direction, which allows an internal bead to be formed stably. On the other hand, if S and O are contained excessively, a surface tension of molten metal is decreased excessively, which makes molten metal liable to suffer sagging. As a result, the internal bead is liable to have a convex shape where a shape of the internal bead excessively swells, (hereinafter, simply referred to as "convex shape"), and a height of reinforcement of the internal bead is liable to increase.
    3. (c) Additionally, a shape of an internal bead is influenced by a surface roughness of an internal surface of the tube in a longitudinal direction. The present inventors revealed that, when a surface roughness is large, a height of reinforcement of an internal bead is increased, which makes an internal bead liable to have a convex shape. For this reason, it is desirable to control the surface roughness within a prescribed range. In particular, when a surface roughness of an internal surface of a tube in a longitudinal direction is large, molten metal is prevented from spreading in a width direction, which makes an internal bead have a convex shape, and a height of reinforcement of an internal bead is liable to increase.
    4. (d) Moreover, the present inventors revealed that a shape of an internal bead is also influenced by a content of Sn. When Sn is contained, a penetration depth is increased, which allows an internal bead to be formed stably. On the other hand, if Sn is contained excessively, penetration becomes excessive, and an internal bead is liable to have a convex shape. A reason for this is considered that Sn vaporizes from a surface of a molten pool during welding and increases a degree of concentration of an arc. For this reason, when Sn is contained, it is necessary to control a content of Sn within a prescribed range and to make a relation among a content of S, a content of O, and the content of Sn satisfy a prescribed range so as to obtain an internal bead having an appropriate shape.
  • The present invention is made based on the findings described above. Requirements of the present invention will be described below in detail.
    • 1. Chemical Composition of Alloy Tube
  • Reasons for limiting a content of each element are as follows. In the following description, a symbol "%" for each content means "mass%".
    • C: 0.001 to 0.050%
  • If C is contained excessively, C combines with Cr through weld thermal cycle to form carbide in grain boundaries in weld heat affected zone. Cr depleted zones are formed to develop in vicinities of the grain boundaries, decreasing corrosion resistance. For this reason, the content of C is set at 0.050% or less. The content of C is to be preferably 0.025% or less, more preferably 0.020% or less, and still more preferably 0.015% or less. From the viewpoint of corrosion resistance, the content of C is desirably reduced as much as possible, but an excessive reduction of the content of C increases production costs. For this reason, the content of C is to be 0.001% or more. The content of C is to be preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more.
    • Si: 0.05 to 0.50%
  • Si has a deoxidation effect. For this reason, a content of Si is to be 0.05% or more. The content of Si is to be preferably 0.08% or more, more preferably 0.10% or more, and still more preferably 0.12% or more. However, if Si is contained excessively, a phase stability of the alloy is decreased, and a weld crack susceptibility of the alloy is increased. In addition, the excessive containing of Si may make it difficult for the internal bead to be formed stably. For this reason, the content of Si is to be 0.50% or less. The content of Si is to be preferably 0.48% or less, more preferably 0.45% or less, and still more preferably, the content of Si is to be 0.40% or less.
    • Mn: 0.05 to 1.00%
  • As with Si, Mn has a deoxidation effect. In addition, Mn has an effect of increasing a phase stability of metal and makes no small contribution to stable formation of the internal bead. For this reason, a content of Mn is to be 0.05% or more. The content of Mn is to be preferably 0.10% or more, more preferably 0.15% or more, and still more preferably 0.20% or more. However, if Mn is contained excessively, hot workability is decreased. For this reason, the content of Mn is to be 1.00% or less. The content of Mn is to be preferably 0.90% or less, and more preferably 0.80% or less. Still more preferably, the content of Mn is to be 0.70% or less.
    • P: 0.025% or less
  • P is contained in the Ni-based alloy as an impurity and significantly increases weld crack susceptibility. For this reason, a content of P is to be 0.025% or less. The content of P is to be preferably 0.023% or less, and more preferably 0.020% or less. The content of P is preferably reduced as much as possible, but an excessive reduction of the content of P increases production costs. For this reason, the content of P is to be preferably 0.001% or more, and more preferably 0.002% or more.
    • S: 0.0001 to 0.0030%
  • S is normally contained in the Ni-based alloy as an impurity, but in the alloy tube according to the present invention, S has an effect of increasing capability of forming the internal bead together with O during welding. For this reason, a content of S is to be 0.0001% or more. The content of S is to be preferably 0.0002% or more, and more preferably 0.0003% or more. However, if S is contained excessively, the internal bead of the tube is made to have a convex shape, and weld crack susceptibility is increased. For this reason, the content of S is to be 0.0030% or less. The content of S is to be preferably 0.0025% or less, and more preferably 0.0020% or less. Note that S is required to satisfy Formula (i) described below together with O and Sn.
    • Fe: 18.0 to 40.0%
  • Fe is useful for increasing hot workability. In addition, Fe contributes to reduction of an alloy cost. For this reason, a content of Fe is to be 18.0% or more. The content of Fe is to be preferably 18.5% or more, and more preferably 19.0% or more. Still more preferably, the content of Fe is to be 25.0% or more. However, if Fe is contained excessively, phase stability of metal is decreased. For this reason, the content of Fe is to be 40.0% or less. The content of Fe is to be preferably 39.5% or less, and more preferably 39.0% or less. Still more preferably, the content of Fe is to be 35.0% or less.
    • Cu: 1.50 to 3.00%
  • Cu increases phase stability of metal and has an effect of increasing corrosion resistance under an environment where non-oxidizing acid and chloride are present. For this reason, a content of Cu is to be 1.50% or more. The content of Cu is to be preferably 1.52% or more, more preferably 1.55% or more, and still more preferably 1.57% or more. However, if Cu is contained excessively, hot workability is decreased. For this reason, the content of Cu is to be 3.00% or less. The content of Cu is to be preferably 2.98% or less, and more preferably 2.95% or less. Still more preferably, the content of Cu is to be 2.80% or less.
    • Co: 0.01 to 0.80%
  • As with Cu, Co has an effect of increasing phase stability of metal. For this reason, a content of Co is to be 0.01% or more. The content of Co is to be preferably 0.02% or more, and more preferably 0.03% or more. Still more preferably, the content of Co is to be 0.05% or more. However, if Co is contained excessively, production costs are significantly increased because Co is a very expensive element. For this reason, the content of Co is to be 0.80% or less. The content of Co is to be preferably 0.78% or less, more preferably 0.75% or less, and still more preferably 0.70% or less.
    • Cr: 19.5 to 23.5%
  • Cr is an element that is essential in keeping corrosion resistance. In particular, Cr causes formation of a passivation film on a surface, increasing corrosion resistance under an acid environment of oxidative properties. For this reason, a content of Cr is to be 19.5% or more. The content of Cr is to be preferably 19.7% or more, more preferably 20.0% or more, and still more preferably 20.2% or more. However, if Cr is contained excessively, phase stability of metal is decreased. For this reason, the content of Cr is to be 23.5% or less. The content of Cr is to be preferably 23.0% or less, more preferably 22.5% or less, and still more preferably 22.3% or less.
    • Mo: 2.4 to 3.6%
  • Mo increases corrosion resistance under an environment where non-oxidizing acid and chloride are present. For this reason, a content of Mo is to be 2.4% or more. The content of Mo is to be preferably 2.6% or more, more preferably 2.8% or more, and still more preferably 2.9% or more. However, if Mo is contained excessively, phase stability of metal is decreased. Moreover, Mo is an expensive element, and thus production costs are increased. For this reason, the content of Mo is to be 3.6% or less. The content of Mo is to be preferably 3.4% or less, more preferably 3.2% or less, and still more preferably 3.1% or less.
    • W: 0.01 to 0.80%
  • As with Mo, W increases corrosion resistance under an environment where non-oxidizing acid and chloride are present. For this reason, a content of W is to be 0.01% or more. The content of W is to be preferably 0.02% or more, more preferably 0.03% or more, and still more preferably 0.04% or more. However, if W is contained excessively, phase stability of metal is decreased. Moreover, the containing of W increases production costs because W is an expensive element. For this reason, the content of W is to be 0.80% or less. The content of W is to be preferably 0.70% or less, more preferably 0.60% or less, and still more preferably 0.50% or less.
    • Ti: 0.50 to 1.20%
  • Ti forms its carbide to contribute to strengthening and to prevent or reduce formation of Cr carbide, so as to mitigate deterioration in corrosion resistance of grain boundaries. For this reason, a content of Ti is to be 0.50% or more. The content of Ti is to be preferably 0.55% or more, more preferably 0.60% or more, and still more preferably 0.65% or more. However, if Ti is contained excessively, carbide and carbo-nitride of Ti precipitate in a large quantity, decreasing ductility. For this reason, the content of Ti is to be 1.20% or less. The content of Ti is to be preferably 1.00% or less, more preferably 0.80% or less, and still more preferably 0.75% or less.
    • N: 0.0010 to 0.0200%
  • N contributes to phase stability of metal and has an effect of increasing pitting resistance. For this reason, a content of N is to be 0.0010% or more. The content of N is to be preferably 0.0020% or more, more preferably 0.0030% or more, and still more preferably 0.0040% or more. However, if N is contained excessively, nitride precipitates, decreasing ductility. For this reason, the content of N is to be 0.0200% or less. The content of N is to be preferably 0.0180% or less, more preferably 0.0150% or less, and still more preferably, the content of N is to be 0.0130% or less.
    • Al: 0.001 to 0.200%
  • Al has a deoxidation effect. Al contributes to increase in oxidation resistance at high temperature. For this reason, a content of Al is to be 0.001% or more. The content of Al is to be preferably 0.005% or more, and more preferably 0.010% or more. Still more preferably, the content of Al is to be 0.020% or more. However, if Al is contained excessively, Al combines with oxygen, decreasing cleanliness and hot workability. In addition, the excessive containing of Al may make it difficult for the internal bead to be formed stably. For this reason, the content of Al is to be 0.200% or less. The content of Al is to be preferably 0.180% or less, and more preferably 0.160% or less. Still more preferably, the content of Al is to be 0.150% or less.
    • O: 0.0004 to 0.0120%
  • O is normally contained in the Ni-based alloy as an impurity, but in the alloy tube according to the present invention, O has an effect of increasing capability of forming the internal bead together with S during welding. For this reason, a content of O is to be 0.0004% or more. The content of O is to be preferably 0.0006% or more, and more preferably 0.0008% or more. However, if O is contained excessively, the internal bead is made to have a convex shape, and hot workability is decreased. For this reason, the content of O is set at 0.0120% or less. The content of O is to be preferably 0.0100% or less, and more preferably 0.0080% or less. Note that O is required to satisfy Formula (i) described below together with S and Sn.
  • In the chemical composition, in addition to the elements described above, Sn may be contained within the range described below.
    • Sn: 0 to 0.030%
  • Sn has an effect of increasing a penetration depth during welding, increasing a capability of forming the internal bead. Therefore, it may be contained when necessary. However, if Sn is contained excessively, hot workability is decreased, and weld crack susceptibility is increased. In addition, the internal bead is liable to have a convex shape. For this reason, a content of Sn is to be 0.030% or less. The content of Sn is to be preferably 0.028% or less, and more preferably 0.025% or less. On the other hand, to obtain the effects described above, the content of Sn is to be preferably 0.001% or more, more preferably 0.002% or more, and still more preferably 0.003% or more. Note that Sn is required to satisfy Formula (i) described below together with S and O.
  • As described above, since S, O, and Sn effectively contribute to formation of an internal bead of the tube, the following Formula (i), which is a numerical relation among the content of S, the content of O, and the content of Sn, needs to be satisfied for the Ni-based alloy tube according to the present invention.
    0.0010 ≤ S + 2 O + 02 Sn ≤ 0.0220
    Figure imgb0002
     where each symbol of an element in the formula indicates a content of the element (mass%) contained in Ni-based alloy, and a content being zero means that the element is not contained. When the content of Sn is less than 0.001%, Sn is treated as Sn = 0 in the above formula.
  • S and O are surface-active elements and have an action of strengthening inward convection in a molten pool during welding. In addition, Sn contributes to formation of an arc energizing path and has an effect of increasing a degree of concentration of an arc. Welding heat is thereby transmitted in a depth direction at a center of the molten pool. As a result, although these elements have an effect of stable formation of the internal bead, the middle value of Formula (i) being less than 0.0010% fails to provide this effect. Therefore, the middle value of Formula (i) is to be 0.0010% or more. The middle value of Formula (i) is to be preferably 0.0012% or more, and more preferably 0.0015% or more.
  • On the other hand, if the middle value of Formula (i) is more than 0.0220%, a surface tension of molten metal is decreased, or melting of the center of the molten pool is accelerated, which causes sagging. As a result, the internal bead is made to have a convex shape, and it is no longer possible to form the internal bead stably inside the tube. Therefore, the middle value of Formula (i) is to be 0.0220% or less. The middle value of Formula (i) is to be preferably 0.0215% or less, and more preferably 0.0210% or less.
  • In the chemical composition, in addition to the elements described above, one or more elements selected from V, Nb, Ta, Ca, Mg, B, and REM may be contained within the ranges described below. Reasons for limiting a content of each element will be described.
    • V: 0.40% or less
  • V combines with carbon to form its carbide, preventing or reducing formation of Cr carbide, so as to mitigate deterioration in corrosion resistance of grain boundaries. Therefore, it may be contained when necessary. However, if V is contained excessively, carbide and carbo-nitride of V precipitate in a large quantity, decreasing ductility. For that reason, the content of V is to be 0.40% or less. The content of V is to be preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, to obtain the effect described above, the content of V is to be preferably 0.01% or more, and more preferably 0.02% or more.
    • Nb: 0.40% or less
  • As with V and Ti, Nb combines with carbon to form its carbide, preventing or reducing formation of Cr carbide, so as to mitigate deterioration in corrosion resistance of grain boundaries. Therefore, it may be contained when necessary. However, if Nb is contained excessively, carbide and carbo-nitride of Nb precipitate in a large quantity, decreasing ductility. In addition, weld crack susceptibility is increased. For that reason, a content of Nb is to be 0.40% or less. The content of Nb is to be preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, to obtain the effect described above, the content of Nb is to be preferably 0.01% or more, and more preferably 0.02% or more.
    • Ta: 0.40% or less
  • Ta accelerates formation of a passivation film of Cr and has an effect of increasing corrosion resistance. Therefore, it may be contained when necessary. However, if Ta is contained excessively, carbide of Ta precipitates in a large quantity, decreasing ductility. For that reason, a content of Ta is to be 0.40% or less. The content of Ta is to be preferably 0.35% or less, and more preferably 0.30% or less. On the other hand, to obtain the effect described above, the content of Ta is to be preferably 0.01% or more, and more preferably 0.02% or more.
    • Ca: 0.0100% or less
  • Ca has an effect of improving hot workability. Therefore, it may be contained when necessary. However, if Ca is contained excessively, Ca combines with oxygen, significantly decreasing cleanliness. As a result, hot workability is rather decreased. For this reason, a content of Ca is to be 0.0100% or less. The content of Ca is to be preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, to obtain the effect described above, the content of Ca is to be preferably 0.0005% or more, and more preferably 0.0010% or more.
    • Mg: 0.0100% or less
  • As with Ca, Mg has an effect of improving hot workability. Therefore, it may be contained when necessary. However, if Mg is contained excessively, Mg combines with oxygen, significantly decreasing cleanliness. As a result, hot workability is rather decreased. For this reason, a content of Mg is to be 0.0100% or less. The content of Mg is to be preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, to obtain the effect described above, the content of Mg is to be preferably 0.0005% or more, and more preferably 0.0010% or more.
    • B: 0.0100% or less
  • B segregates in grain boundaries at high temperature and has effects of strengthening the grain boundaries and increasing hot workability. Therefore, it may be contained when necessary. However, if B is contained excessively, weld crack susceptibility is increased. For this reason, a content of B is set at 0.0100% or less. The content of B is to be preferably 0.0080% or less, and more preferably 0.0060% or less. On the other hand, to obtain the effects described above, the content of B is to be preferably 0.0002% or more, and more preferably 0.0005% or more.
    • REM: 0.0800% or less
  • As with Ca and Mg, REM has an effect of improving hot workability in production. Therefore, it may be contained when necessary. However, if REM is contained excessively, REM combines with oxygen, significantly decreasing cleanliness. As a result, hot workability is rather decreased. For this reason, a content of REM is set at 0.0800% or less. The content of REM is to be preferably 0.0600% or less, and more preferably 0.0500% or less. On the other hand, to obtain the effect described above, the content of REM is to be preferably 0.0005% or more, and more preferably 0.0010% or more. Here, REM refers to Sc, Y, and lanthanoids, and the content of REM refers to the amount of these elements.
  • In the chemical composition of the Ni-based alloy according to the present invention, the balance is Ni and impurities. The term "impurities" as used herein means elements that are not added intentionally but are mixed in the Ni-based alloy in producing the Ni-based alloy industrially due to various factors such as raw materials and a production process, and that are allowed to be within ranges in which the impurities have no adverse effect on the present invention.
    • 2. Internal Surface Roughness of Alloy Tube
  • An internal bead is formed when end portions of alloy tubes are welded together. To form a favorable internal bead, it is preferable to control, an arithmetic average roughness Ra on an internal surface of the alloy tube in a longitudinal direction. An internal surface roughness of the alloy tube herein refers to a surface roughness after a final process in a production process. In other words, although the internal surface roughness of the alloy tube fluctuates in a course of the production, a surface roughness in a middle of the production does not matter in obtaining an advantageous effect of the present invention, and it will suffice that the internal surface roughness of the tube in the longitudinal direction after the final process satisfies a range specified in the present invention.
  • On the internal surface of the Ni-based alloy tube, if the arithmetic average roughness Ra of the tube in the longitudinal direction is more than 9.0 µm, weld metal is hindered from wetting on the internal surface of the tube, and thus the weld metal resists spreading along a width direction, that is, a circumference of the tube. As a result, the internal bead is liable to have a convex shape, and a height of reinforcement is liable to increase. For this reason, on the internal surface of the Ni-based alloy tube, the arithmetic average roughness Ra of the tube in the longitudinal direction is to be preferably 9.0 µm or less. The arithmetic average roughness Ra is to be preferably 7.0 µm or less, and more preferably 5.0 µm or less. A lower limit value of the arithmetic average roughness Ra is not limited to a specific value; however, in many cases, the arithmetic average roughness Ra is normally 0.1 to 1.0 µm or more in a case where a production method described below is used.
  • Here, the arithmetic average roughness Ra is specified in JIS B 0601:2001 and can be measured by using a contact-type surface roughness instrument.
    • 3. Welded Joint
  • A welded joint of Ni-based alloy tubes described above can be obtained by performing butt welding on tube ends of the Ni-based alloy tubes under predetermined conditions. The welded joint of the Ni-based alloy tubes includes weld metal which is to be a weldment by solidification, and base material portion. The base material portions each include a weld heat affected zone that is influenced by heat input of welding. The base material portion excluding the weld heat affected zone inherits the chemical composition, the surface roughness, and other properties of the Ni-based alloy tube described in Sections 1 and 2 described above. The weldment refers to the weld metal and the weld heat affected zone.
    • 4. Production Method
  • A preferable production method for the Ni-based alloy tube according to the present invention will be described. The Ni-based alloy tube according to the present invention provides its advantageous effects irrespective of its production method as long as the Ni-based alloy tube has the requirement described above; nonetheless, the Ni-based alloy tube can be produced stably by a production method described below, for example.
    • 4-1. Ni-Based Alloy Tube
  • First, a Ni-based alloy ingot being a starting material of the Ni-based alloy tube is produced. The Ni-based alloy ingot is preferably produced by melting an alloy having the chemical composition described above using an electric furnace, refining the alloy to remove impurities, and casting the alloy. Subsequently, the obtained ingot is preferably subjected to hot forging to be formed into a billet having a columnar shape. Thereafter, the obtained billet is worked to be shaped into a tube.
  • Specifically, the billet is preferably subjected to hot extrusion and then to cold rolling or cold drawing. Softening heat treatment and intermediate pickling may be performed in the middle of the working when necessary. Thereafter, the alloy tube is preferably subjected to solution treatment as heat treatment. After the solution treatment, pickling or working may be performed when necessary.
  • Here, in order to bring the arithmetic average roughness Ra of the internal surface of tube in the longitudinal direction to 9.0 µm or less, it is preferable to perform the following process. Specifically, the solution treatment is preferably performed under such conditions of heating in a temperature region of 950°C to 1230°C for 1 to 15 minutes and performing water cooling. Additionally, it is preferable to perform one of machining such as grinder processing and grinding, and shotblast or shotpeening treatment, on the internal surface of the tube.
  • Although the arithmetic average roughness Ra fluctuates in a course of the production, an advantageous effect of the present invention does not pertain to the surface roughness in the middle of the course and is influenced only by the surface roughness of the tube in the longitudinal direction after the final process.
    • 4-2. Welded Joint of Ni-Based Alloy Tubes
  • A welded joint can be obtained by welding together end portions of Ni-based alloy tubes according to the present invention as starting materials. A method for the welding is not limited to a specific method; however, the welding can be performed by arc welding, for example. As a condition for performing arc welding, for example, a heat input is preferably set to be within the range from 4 to 20 kJ/cm. In addition, it is preferable to use Ar gas as shielding gas and back shielding gas during welding. A flow rate of the gas flowed to a welding location is preferably adjusted as appropriate.
  • A chemical composition of a welding material (filler material) to be used is not limited to a specific material, either; however, the chemical composition is preferably a composition described below. In other words, it is preferable for the composition to contain, in mass%, C: 0.150% or less, Si: 1.00% or less, Mn: 3.50% or less, P: 0.030% or less, S: 0.0001 to 0.0100%, Fe: 38.0% or less, Cu: 3.00% or less, Co: 15.0% or less, Cr: 14.0 to 26.0%, Mo: 17.0% or less, W: 4.5% or less, at least one of Nb and Ta: 4.20% or less in total, Ti: 1.50% or less, V: 0.35% or less, N: 0.0500% or less, Al: 1.50% or less, and O: 0.0004 to 0.0120%, with the balance being Ni and impurities, and to make a relation between contents of S and O satisfy Formula (a) shown below.
    0.0010 ≤ S + 2 O ≤ 0.0220
    Figure imgb0003
     where each symbol of an element in the formulas indicates a content of the element (mass%) contained in the welding material, and a content being zero means that the element is not contained.
  • The present invention will be described below more specifically with reference to examples, but the present invention is not limited to these examples.
    • EXAMPLE
  • Alloys having chemical compositions shown in Table 1 were melted and produced into ingots. Thereafter, the ingots were subjected to hot forging and hot rolling to have a thickness of 10 mm. Subsequently, scales formed on their surfaces were removed by pickling. At this point, arithmetic average roughnesses Ra of all steel types were about 12 µm. Thereafter, with a production process of an alloy tube assumed, cold rolling was performed while softening heat treatment and intermediate pickling were performed, by which 3-mm alloy plates were obtained.
  • Subsequently, the alloy plates were performed solution treatment that the alloy plates were retained in a hydrogen furnace at 1150°C for 10 minutes and cooled by water cooling. Thereafter, test plates each having a width of 50 mm and a length of 100 mm were cut from the alloy plates. For some of the test plates, shotpeening was conducted on only one side, assuming the internal surface of the alloy tube, as shown in Table 2. For the rest of test plates which were not subjected to the shotpeening, one side of test plates were machine-ground or polished 1 to 5 times using a whetstone with granularity of #40 or #60. In Table 2, for example, polishing (#40 × 1 time) indicates that a test plate was polished once using a whetstone with a granularity of #40.
    • [Table 1]
  • Figure imgb0004
    • [Table 2]
  • Table 2
    Test piece Alloy type Condition of production Property
    Treatment of back-side of plate Arithmetic average roughness Ra (µm)
    A1 A Shotpeening 2.1
    A2 Machine grinding 6.6
    A3 Polishing (#40× 1 time) 9.2**
    A4 Polishing (#40×2 times) 7.2
    A5 Polishing (#60×1 time) 6.5
    A6 Polishing (#60×2 times) 5.2
    A7 Polishing (#60×3 times) 3.1
    A8 Polishing (#60×4 times) 1.3
    A9 Polishing (#60×5 times) 1.0
    B1 B Shotpeening 2.1
    B2 Machine grinding 6.5
    B3 Polishing (#40×1 time) 9.3 **
    B4 Polishing (#40×2 times) 7.0
    B5 Polishing (#60×1 time) 6.7
    B6 Polishing (#60×2 times) 5.5
    B7 Polishing (#60×3 times) 2.8
    B8 Polishing (#60x4 times) 1.6
    B9 Polishing (#60×5 times) 1.2
    C1 C Shotpeening 22
    D1 D Shotpeening 2.0
    E1 E Shotpeening 1.9
    F1 F Shotpeening 18
    G1 G Shotpeening 1.9
    H1 H Shotpeening 2.0
    I1 I * Shotpeening 2.2
    J1 J * Shotpeening 2.3
    K1 K * Shotpeening 2.0
    L1 L Shotpeening 2.3
    M1 M Shotpeening 2.0
    N1 N Shotpeening 2.1
    O1 O * Shotpeening 2.1
    P1 P * Shotpeening 2.3
    Q1 Q * Shotpeening 2.0
    * indicates that the condition fell out of the range defined in the present invention.
    ** indicates that the condition fell out of the preferable range defined in the present invention.
  • An arithmetic average roughness of each test plate was measured using a contact type roughness gauge. In addition, two test plates of each of the alloy types were prepared, and an end face of each test plates in a rolling direction was beveled as illustrated in Figure 1. Beveled end faces of these test plates were butted against each other and subjected to root pass welding using a filler material having a chemical composition shown in Table 3 and an outer diameter of 1.0 mm, by which a welded joint was obtained. In the welding, a heat input was set at about 5 kJ/cm, and Ar gas was used as shielding gas and back shielding gas and was flowed to a welding location at a flow rate of 10 L/min.
    • [Table 3]
  • Table 3
    Chemical Composition (mass%, Balance: Ni and impurities)
    C Si Mn P S Fe Cu Cr Mo Ti N Al O Others S+2O
    0.010 0.25 0.28 0.010 0.0022 23.0 1 95 22.3 3.1 0.78 0.0035 0.095 0.0062 Co:0.02, W:0.02 0.0146
  • Of the obtained welded joints, welded joints each of which a back bead was formed across an overall length of its weld line were determined to have no problem in capability of forming an internal bead of the alloy tube and were rated as "Good". Of such welded joints, welded joints each of which a width of a back bead was 2 mm or more across an overall length of its weld line were rated as "Excellent", and welded joints each of which a width of a back bead formed was less than 2 mm but 1 mm or more were rated as "Acceptable". A back bead in this example corresponds to an internal bead that is formed when the weld is made from the outside of the alloy tube.
  • Thereafter, three transverse sections were made to appear from each welded joint, and welded joints each of which the sections all had a back bead height of 1.0 mm or less were determined to be good in shape of an internal bead of an alloy tube and were rated as "Good". Of such welded joints, welded joints each of which the sections all had a back bead height of 0.8 mm or less were rated as "Excellent", and the other welded joints were rated as "Acceptable". Results are collectively shown in Table 4 below.
    • [Table 4]
  • Table 4
    Test piece Alloy type Welded joint
    Capability of forming back bead Shape of back bead
    A1 A Good (Excellent) Good (Excellent) Inventive example
    A2 Good (Excellent) Good (Excellent)
    A3 Good (Excellent) Good (Acceptable)
    A4 Good (Excellent) Good (Excellent)
    A5 Good (Excellent) Good (Excellent)
    A6 Good (Excellent) Good (Excellent)
    A7 Good (Excellent) Good (Excellent)
    A8 Good (Excellent) Good (Excellent)
    A9 Good (Excellent) Good (Excellent)
    B1 B Good (Excellent) Good (Excellent)
    B2 Good (Excellent) Good (Excellent)
    B3 Good (Excellent) Good (Acceptable)
    B4 Good (Excellent) Good (Excellent)
    B5 Good (Excellent) Good (Excellent)
    B6 Good (Excellent) Good (Excellent)
    B7 Good (Excellent) Good (Excellent)
    B8 Good (Excellent) Good (Excellent)
    B9 Good (Excellent) Good (Excellent)
    C1 C Good (Excellent) Good (Excellent)
    D1 D Good (Excellent) Good (Excellent)
    E1 E Good (Excellent) Good (Excellent)
    F1 F Good (Excellent) Good (Excellent)
    G1 G Good (Excellent) Good (Acceptable)
    H1 H Good (Acceptable) Good (Excellent)
    I1 I * Good (Excellent) Bad Comparative example
    J1 J * Bad Good (Excellent)
    K1 K * Good (Excellent) Bad
    L1 L Good (Excellent) Good (Excellent) Inventive example
    M1 M Good (Excellent) Good (Excellent)
    N1 N Good (Excellent) Good (Excellent)
    O1 O * Bad Good (Excellent) Comparative example
    P1 P * Good (Excellent) Bad
    Q1 Q * Good (Excellent) Bad
    * indicates that the condition fell out of the range defined in the present invention.
    The underlined indicates that the property fell out of the target property in the present invention.
  • Test pieces made of alloy types A to H and L to N all satisfied the specifications according to the present invention, and their capabilities of forming a back bead and shapes of their back beads were good. Of these, the test piece N1 made of the alloy type N satisfied the range specified by Formula (i), and therefore, its capability of forming a back bead and a height of its back bead were both satisfactory.
  • In contrast, for both of test pieces I1 and K1 made of the alloy types I and K, their contents of S and O did not satisfy Formula (i) and were higher than the respective specified ranges. Therefore, sagging of its molten metal was significant, and a height of its back bead did not satisfy a target of the height. For a test piece J1 made of the alloy type J, a relation between its contents of S and O did not satisfy Formula (i), and the contents were lower than the respective specified ranges. Therefore, melting in a plate thickness direction was not sufficient, and a capability of forming the back bead as targeted was not obtained.
  • Similarly, for a test piece O1 made of the alloy type O, its contents of S, O, and Sn were lower than the range specified by Formula (i). Therefore, melting in a plate thickness direction was not sufficient, and a capability of forming the back bead did not satisfy a target of the capability. In addition, for test pieces P1 and Q1 made of the alloy types P and Q1, their contents of S, O and Sn were higher than the range specified by Formula (i), or their contents of Sn were more than the specified range. Therefore, sagging of its molten metal was large, and a back bead height as targeted was not satisfied.
    • INDUSTRIAL APPLICABILITY
  • The present invention makes it possible to obtain a Ni-based alloy tube that allows an internal bead to be formed stably during butt welding and prevents a reinforcement with excessive height.

Claims (3)

  1. A Ni-based alloy tube having a chemical composition consisting of, in mass%:
    C: 0.001 to 0.050%,
    Si: 0.05 to 0.50%,
    Mn: 0.05 to 1.00%,
    P: 0.025% or less,
    S: 0.0001 to 0.0030%,
    Fe: 18.0 to 40.0%,
    Cu: 1.50 to 3.00%,
    Co: 0.01 to 0.80%,
    Cr: 19.5 to 23.5%,
    Mo: 2.4 to 3.6%,
    W: 0.01 to 0.80%,
    Ti: 0.50 to 1.20%,
    N: 0.0010 to 0.0200%,
    Al: 0.001 to 0.200%,
    O: 0.0004 to 0.0120%,
    Sn: 0 to 0.030%, and
    optionally,
    V: 0.40% or less,
    Nb: 0.40% or less,
    Ta: 0.40% or less,
    Ca: 0.0100% or less,
    Mg: 0.0100% or less,
    B: 0.0100% or less, and
    REM: 0.0800% or less,
    with the balance: Ni and impurities, and
    satisfying Formula (i) below:
    0.0010 ≤ S + 2 O + 0.2 Sn ≤ 0.0220
    Figure imgb0005
     where each symbol of an element in the formula indicates a content of the element (mass%) contained in Ni-based alloy, and a content being zero means that the element is not contained.
  2. The Ni-based alloy tube according to claim 1, wherein on an internal surface of the Ni-based alloy tube, an arithmetic average roughness Ra of the tube in a longitudinal direction is 9.0 µm or less.
  3. A welded joint including the Ni-based alloy tube according to claim 1 or 2.
EP21175398.3A 2020-05-22 2021-05-21 Ni-based alloy tube and welded joint Pending EP3913103A1 (en)

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KR20240141289A (en) * 2022-01-28 2024-09-26 닛폰세이테츠 가부시키가이샤 Ni-Fe-Cr alloy welded joint
KR20240168410A (en) * 2022-03-31 2024-11-29 닛폰세이테츠 가부시키가이샤 alloy material
EP4589039A1 (en) * 2022-09-16 2025-07-23 Nippon Steel Corporation Austenite alloy material
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