Ni-based alloy for welding material and welding wire, rod and power

Abstract

A Ni-based alloy for a welding material including, by mass, 0.001 to 0.1% of C, 18 to 25% of Co, 16 to 20% of Cr, 2.5 to 3.5% of Al, 9.0 to 15.0% of Mo+W, 0.001 to 0.03% of B and the balance being Ni and inevitable impurities.

Claims

1. A product having a welded portion formed of a Ni-based alloy for a welding material used at a temperature of 700 to 800 C. comprising, by mass, 0.01 to 0.04% of C, 18 to 25% of Co, 16 to 20% of Cr, 2.5 to 3.5% of Al, more than 11.5% and up to 14.0% or less of Mo+W, 0.001 to 0.03% of B and the balance being Ni and inevitable impurities, wherein the welded portion has a y (Ni.sub.3Al) strengthening phase, a solid solution temperature of the y (Ni.sub.3Al) strengthening phase being in a range of 850 to 900 C., and a precipitation amount of the y (Ni.sub.3Al) strengthening phase at 800 C. being 10 to 25% by volume.

2. The product according to claim 1, wherein the Ni-based alloy contains, by mass, 20 to 23% of Co, 17 to 19% of Cr, 2.8 to 3.2% of Al, 10.0 to 12.0% of Mo+W and 0.003 to 0.01% of B.

3. The product according to claim 1, wherein a creep rupture time of the welded portion under conditions of 800 C. and 294 MPa is 200 hours or longer.

4. A welding wire for forming the welded portion of the product according to claim 1.

5. A welding rod for forming the welded portion of the product according to claim 1.

6. A welding powder for forming the welded portion of the product according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph of 0.2% yield strength at 800 C. of alloys, through a tensile test of welded joints;

(2) FIG. 2 is a graph of a creep rupture time; and

(3) FIG. 3 is a graph of a correlation between a solid solution temperature and a precipitation amount of a phase.

DETAILED DESCRIPTION OF THE INVENTION

(4) The present inventors have studied to improve properties of a welding material through a material test and a thermodynamic calculation. As a result, the present inventors have come to develop a welding material that can be used at a temperature of 700 to 800 C., by selecting an alloy composition.

(5) In this description, carbon, cobalt, chromium, aluminum, molybdenum, tungsten, boron and nickel are represented by element symbols of C, Co, Cr, Al, Mo, W, B and Ni, respectively. In addition, Mo+W represents a total amount of molybdenum and tungsten.

(6) The Ni-based alloy for the welding material, and a welding wire, a welding rod and a welding powder including the material according to an embodiment of the present invention will be described below.

(7) The Ni-based alloy for the welding material includes, by mass, 0.001 to 0.1% C, 18 to 25% Co, 16 to 20% Cr, 2.5 to 3.5% Al, 9.0 to 15.0% Mo+W, 0.001 to 0.03% B and the balance being Ni and inevitable impurities.

(8) The Ni-based alloy preferably includes, by mass, 0.001 to 0.05% C, 18 to 25% Co, 16 to 20% Cr, 2.5 to 3.5% Al, 9.0 to 15.0% Mo+W, 0.001 to 0.03% B and the balance being Ni and inevitable impurities.

(9) The Ni-based alloy preferably includes, by mass, 0.001 to 0.05% C, 18 to 25% Co, 16 to 20% Cr, 2.5 to 3.5% Al, 10.0 to 14.0% Mo+W, 0.001 to 0.03% B and the balance being Ni and inevitable impurities.

(10) The Ni-based alloy preferably includes, by mass, 0.01 to 0.04% C, 20 to 23% Co, 17 to 19% Cr, 2.8 to 3.2% Al, 10.0 to 12.0% Mo+W, 0.003 to 0.01% B and the balance being Ni and inevitable impurities.

(11) For the Ni-based alloy, it is preferable that a solid solution temperature of a (Ni.sub.3Al) strengthening phase is in a range of 850 to 900 C. and a precipitation amount at 800 C. is 10 to 25% by volume.

(12) For the Ni-based alloy, a creep rupture time of a welded portion under conditions of 800 C. and 294 MPa is preferably 200 hours or longer.

(13) The Ni-based alloy can be supplied in forms of a welding wire, a welding rod, a welding powder or the like.

(14) Compositional range of elements constituting the Ni-based alloy and reason for the limitation will be described below. The following percentage is by mass unless otherwise described.

(15) C dissolves in a matrix to improve a tensile strength at a high temperature, and forms a carbide such as MC and M.sub.23C.sub.6. The precipitates mainly precipitate in a grain boundary of a dendrite structure which is formed during solidification of the alloy and have an effect of strengthening the grain boundary by preventing the grain boundary from being linearized. This effect is confirmed from an amount of approximately 0.001%. The grain boundary strengthening due to the carbide is particularly important when the alloy is used at a high temperature exceeding 1,000 C. as disclosed in JP-A-2010-84167, and it is desirable to increase the amount of precipitates as well by increasing the carbon content.

(16) However, C easily forms segregation during the solidification. When C is excessively added, a weld crack due to the segregation is promoted. If the segregation remains, a coarse carbide may precipitate and embrittle the material when the alloy is held at high temperature for a long time period.

(17) When used at a temperature of 800 C. or lower, diffusion does not occur so much and the linearization of the grain boundary is hard to occur, as compared to the case of 1,000 C. Accordingly, C may be decreased as compared with an amount in the welding material of a gas turbine moving blade and an upper limit of the C content is set at 0.1%. It becomes possible to suppress the weld crack and the embrittlement which originate in the segregation, by decreasing the C content. A preferable range is 0.001 to 0.05%, more preferably 0.01 to 0.05%.

(18) Co substitutes for Ni and dissolves in a matrix. Thus, Co has an effect of increasing a strength of a substrate at high temperature. This effect becomes remarkably when 18% or more of Co is substituted. Co forms a harmful phase more easily than Ni from a viewpoint of phase stability. When the Co content exceeds 25%, precipitation of a harmful phase such as a phase or a phase generates. Therefore, a preferable range is 18 to 25%. Further preferable range is 2.0 to 23%.

(19) Cr forms a dense oxide film of Cr.sub.2O.sub.3 on a surface of the alloy, and has an effect of increasing oxidation resistance and corrosion resistance at a high temperature. It is necessary to contain at least 16% of Cr from a viewpoint of the oxidation resistance and the corrosion resistance of a weld portion. However, when the content exceeds 20%, the phase precipitates, and the ductility and fracture toughness of the material are deteriorated. Accordingly, the content is determined to be 20% or less. Therefore, a preferable range is 16 to 20%. A particularly suitable range is 17 to 19%.

(20) Al forms a (Ni.sub.3Al) phase, and is indispensable for increasing a strength of the Ni based alloy. In order to obtain a sufficient strength, an amount of a precipitated (Ni.sub.3Al) phase needs to be 10% or more by volume. In order to precipitate the amount at 700 to 800 C. which is an assumed service temperature, an Al content needs to be 2.5% or more by mass. As the Al content increases, a strength increases while weldability decreases. The present invention includes controlling of a solid solution temperature of the (Ni.sub.3Al) phase to 900 C. or lower from a viewpoint of suppressing of a weld crack. For this reason, an upper limit of the Al content is be 3.5%. A preferable range is 2.5 to 3.5%. A further preferable range is 2.8 to 3.2% from the balance between the strength and the weldability.

(21) Mo and W have an effect of strengthening a matrix by solid solution strengthening. For the alloy of the present invention, an upper limit of the Al content is determined from a viewpoint of weldability. Therefore, the alloy desirably includes a large amount of Mo and W so as to increase its strength. In order to obtain a sufficient strength at 800 C., a total amount of Mo and W needs to be 9.0% or more.

(22) JP-A-2010-84167 also discloses that large amounts of elements such as Mo and W are added. A substrate which is an object to be welded in the present invention is a Ni based alloy which is hard to form a harmful phase in a weld dilution zone as compared with a ferritic steel. Therefore, Mo and W in a total amount of 15.0% can be added at maximum. When an amount of Mo and W exceeds 15.0%, a hard and brittle intermetallic compound phase is generated in a weld metal itself. Therefore, a preferable range is 9.0 to 15.0%. A further preferable range is 10.0 to 14.0%, and the particularly preferable range is 10.0 to 12.0%.

(23) B has an effect of increasing a strength of a grain boundary similarly to C, and ductility at a high temperature can be expected to be improved by an addition of B This effect can be obtained by 0.001% of B. However, when an B content exceeds 0.03%, B causes partial melting of the grain boundary and a precipitation of a harmful phase. Therefore, a preferable range is 0.001 to 0.03%. Further preferable properties of the alloy are obtained in a range of 0.003 to 0.015%.

(24) Examples and comparative examples will be described in detail below.

(25) Table 1 shows chemical compositions of alloy samples for test. In the Table, Nos. 1 to 10 are examples of the invention, and No. 11 to 15 are comparative examples.

(26) TABLE-US-00001 TABLE 1 Alloy sample compositions for test phase Solid solution Precipitation Alloy composition (mass %) temperature amount Classification No. Ni C Co Cr Al Mo W Mo + W B ( C.) (%) Example of 1 Balance 0.10 23.0 18.0 3.0 0.0 10.0 10.0 0.012 867 15.5 the invention 2 Balance 0.10 20.0 18.0 3.2 0.0 10.0 10.0 0.005 885 18.3 3 Balance 0.05 20.0 16.0 3.5 0.0 10.0 10.0 0.005 897 23.2 4 Balance 0.045 18.0 16.0 2.8 2.5 12.0 14.5 0.005 876 13.7 5 Balance 0.045 18.0 20.0 2.5 2.5 12.0 14.5 0.010 854 10.5 6 Balance 0.045 23.0 20.0 3.0 0.0 12.0 12.0 0.010 878 17.2 7 Balance 0.01 23.0 16.0 3.2 2.5 9.0 11.5 0.015 872 18.4 8 Balance 0.01 24.5 16.0 3.2 5.0 9.0 14.0 0.015 891 20.0 9 Balance 0.005 23.0 18.0 2.8 2.5 9.0 11.5 0.015 863 13.4 10 Balance 0.005 23.0 18.0 2.8 5.0 9.0 14.0 0.015 860 15.2 Comparative 11 Balance 0.20 20.0 18.0 4.0 0.0 8.0 8.0 0.006 940 27.3 Example 12 Balance 0.10 20.0 18.0 3.8 2.5 7.5 10.0 0.012 932 24.8 13 Balance 0.05 20.0 18.0 3.6 6.0 10.0 16.0 0.012 916 25.1 14 Balance 0.03 20.0 18.0 3.0 0.0 5.0 5.0 0.006 835 9.1 15 Balance 0.03 20.0 18.0 2.4 0.0 10.0 10.0 0.006 810 6.2

(27) An 10 kg ingot of each alloy sample was produced by vacuum melting. An oxide film and a casting defect on its surface has been removed from the produced ingot, and the resultant ingot was worked, through a hot forging process and a cold drawing process, into a welding wire with a diameter of 1.2 mm. With use of this welding wire, an Ni-based alloy material having a tubular form with an outer diameter of 34 mm and an inner diameter of 18 mm was TIG welded. Thus, a welded joint for evaluating properties was produced. Here, the TIG welding is an abbreviation of Tungsten Inert Gas welding.

(28) While the alloy was worked into a wire shape and used for welding in the examples, the alloy may be in a rod or powder shape. A test piece was taken from the welded joint, and its high-temperature strength properties were measured by a tensile test and a creep test. In addition, a microstructure of a cross section of the weld portion was observed, and the weldability of each alloy was evaluated by confirming presence or absence of a weld crack or a harmful phase.

(29) Table 2 shows test results on the tensile test, the creep test, and the weldability.

(30) TABLE-US-00002 TABLE 2 Evaluation result of properties of each alloy Result of tensile Result of creep test at high tem- test (800 C. perature (800 C.) and 294 MPa) Classifi- 0.2% yield rupture time cation No. strength (MPa) (h) Weldability Example 1 512 229 2 519 246 3 531 303 4 524 245 5 509 224 6 520 260 7 535 249 8 532 237 9 518 217 10 526 234 Comparative 11 320 95 x Example 12 374 89 x 13 420 103 14 448 146 15 456 108 : good, : possible, x: impossible

(31) FIG. 1 is a graph showing the 0.2% yield strength of each alloy at 800 C., obtained by the tensile test of the welded joint.

(32) When the alloy is used for a high-temperature member, the alloy is desired to have the 0.2% yield strength of approximately 500 MPa at 800 C., which is an assumed service temperature. All examples of the invention Nos. 1 to 10 have the strength exceeding 500 MPa.

(33) Since the examples are broken at a weld portion in any test, it is considered that the values show the strength of the weld metal itself. The result is almost equivalent to a result of tensile test of forged alloys. In addition, a defect such as a crack was not observed in the weld portion. From this result, it can be determined that all examples of the invention have excellent weldability.

(34) On the other hand, in Comparative Examples 11 to 13, a crack was observed in the weld metal or a boundary portion between the weld metal and the substrate. Therefore, it is considered that breakage has progressed starting from the crack in Comparative Examples 11 to 13. Accordingly, the material has yielded under a stress of 500 MPa or less in the case of Comparative Examples 11 to 13, which stress is lower than the normal strength of the alloy. A factor of the crack is considered that an Al content is large and the precipitation temperature of phase is 900 C. or higher.

(35) In Comparative Examples 14 and 15, the weldability was adequate and the weld crack was not observed. However, since the Al content is small and the precipitation amount of the phase is not sufficient, or since an amount of solid solution strengthening elements Mo+W is not sufficient, the strength is insufficient as the joint although there is no weld defects.

(36) FIG. 2 shows a result of the creep test. Similar tendency as the tensile test is obtained. It may be considered that the alloy is satisfactorily durable at a temperature of 800 C. if the rupture time is 200 hours or longer under the test conditions (800 C. and 294 MPa) of this time.

(37) The examples of the invention include desirable amounts of a precipitation strengthening element and a solid solution strengthening element. They have adequate weldability, and a welded joint thereof has no defect. Accordingly, the examples of the invention has the creep strength exceeding targeted 200 hours.

(38) On the other hand, the comparative examples have only an approximately half of the creep strength due to the weld crack and/or the insufficient strength of the weld metal itself.

(39) FIG. 3 shows a correlation between a solid solution temperature of the phase and a precipitation amount of the phase at 800 C. of each alloy.

(40) In the figure, a relationship therebetween is generally proportional, and there is a tendency that the solid solution temperature is high and the precipitation amount increases as the amount of Al is large and the stability of the phase is high.

(41) The alloy of the present invention needs to have a solid solution temperature in a range of 850 to 900 C. from a viewpoint of the weldability, and have the precipitation amount of the phase at 800 C. of 10 to 25% from a viewpoint of the strength at the service temperature. Specifically, the solid solution temperature of the phase and the precipitation amount of the phase need to be within the range shown by a dotted line in the figure.

(42) As long as the alloy satisfies the above solid solution temperature of the phase and precipitation amount of the phase, a welded joint having both weldability and strength can be obtained, and the alloy can be used for a weld member such as a boiler, a steam turbine and a gas turbine which are used at a high temperature of approximately 800 C. As for a form of the alloy to be used as the welding material, any of a wire shape, a rod shape and a powder shape is considered to provide a similar effect.

(43) It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.