Classifications

• • 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
  • C22C19/056—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 10% but less than 20%
• • 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%

Definitions

• the present invention relates to a material making a casting part of a steam turbine in which a high temperature steam as a working fluid is flowed.
• the present invention relates to a Ni-based alloy of a casting part of the steam turbine with excellent high temperature strength, castability and weldability, and a turbine casing of the steam turbine, a valve casing of the steam turbine, a nozzle box of the steam turbine and a pipe of the steam turbine which are made of the Ni-based alloy for the casting part of the steam turbine.
• the temperature of the steam to be employed in the steam turbine is increased.
• the steam temperature is increased to 600° C. or more.
• the steam temperature is likely to be increased up to 650° C. or 700° C.
• the turbine casings, valve casings, nozzle boxes, pipes and the like of the steam turbine which are to be exposed to a high temperature steam, may cause large stresses therein as the temperature of the steam flowing around the turbine casings, valve casings, nozzle boxes, pipes and the like of the steam turbine is increased.
• these parts of the steam turbine are required to resist against such a high temperature condition and such a high stress condition and thus, to be made of respective materials with excellent strength, ductility and toughness within a temperature range of room temperature through high temperature.
• the Ni-based alloy Since the Ni-based alloy has its excellent high temperature strength and high corrosion resistance, the Ni-based alloy would be employed mainly for jet engines and gas turbines. As the Ni-based alloy may be typically exemplified Inconel Alloy 617 (made by Special Metals Corporation and Inconel Alloy 706 (made by Special Metals Corporation).
• the mechanism of enhancement in high temperature strength of the Ni-based alloy is originated from a precipitated phase such as a gamma prime phase (Ni 3 (Al, Ti) and/or gamma double prime phase in the matrix phase of the Ni-based alloy by adding Al and Ti to the Ni-based alloy.
• a precipitated phase such as a gamma prime phase (Ni 3 (Al, Ti) and/or gamma double prime phase in the matrix phase of the Ni-based alloy by adding Al and Ti to the Ni-based alloy.
• a precipitated phase such as a gamma prime phase (Ni 3 (Al, Ti) and/or gamma double prime phase in the matrix phase of the Ni-based alloy by adding Al and Ti to the Ni-based alloy.
• Co and Mo are solid-solved (i.e., the use of solute strengthening) in the matrix phase of the Ni-based alloy so as to develop the high temperature strength thereof.
• the high temperature strength is not enough for the Ni-based alloy to be employed under such a high temperature condition. Moreover, it is required that the high temperature strength of the Ni-based alloy is developed by the modification of the composition of the Ni-based alloy while the castability and weldability of the Ni-based alloy are maintained.
• an aspect of the present invention relates to a Ni-based alloy for a casting part of a steam turbine having excellent high temperature strength, castability and weldability, including, in percentage by mass, 0.01 to 0.15 of C, 18 to 28 of Cr, 10 to 15 of Co, 8 to 12 of Mo, 1.5 to 2 of Al, 0.1 to 3 of Ti, 0.001 to 0.006 of B, 0.1 to 0.7 of Ta, and the balance of Ni plus unavoidable impurities.
• Another aspect of the present invention relates to a
• Ni-based alloy for a casting part of a steam turbine having excellent high temperature strength, castability and weldability, including, in percentage by mass, 0.01 to 0.15 of C, 18 to 28 of Cr, 10 to 15 of Co, 8 to 12 of Mo, 1.5 to 2 of Al, 0.1 to 3 of Ti, 0.001 to 0.006 of B, 0.1 to 0.4 of Nb, and the balance of Ni plus unavoidable impurities.
• Still another aspect of the present invention relates to a Ni-based alloy for a casting part of a steam turbine having excellent high temperature strength, castability and weldability, including, in percentage by mass, 0.01 to 0.15 of C, 18 to 28 of Cr, 10 to 15 of Co, 8 to 12 of Mo, 1.5 to 2 of Al, 0.1 to 3 of Ti, 0.001 to 0.006 of B, 0.1 to 0.7 of Ta+2Nb (Ta:Nb in mole ratio is 1:2) , and the balance of Ni plus unavoidable impurities.
• a further aspect of the present invention relates to a turbine casing of a steam turbine, including at least a portion made of any one of the Ni-based alloys as described above through casting.
• a still further aspect of the present invention relates to a valve casing of a steam turbine, including at least a portion made of any one of the Ni-based alloys as described above through casting.
• Another aspect of the present invention relates to a nozzle box of a steam turbine, including at least a portion made of any one of the Ni-based alloys as described above through casting.
• Still another aspect of the present invention relates to a pipe of a steam turbine, including at least a portion made of any one of the Ni-based alloys as described above through casting.
• the high temperature strength, the castability and the weldability in the Ni-based alloy and these parts of the present invention can be enhanced in comparison with the conventional ones.
• a Ni-based alloy of a casting part of a steam turbine with excellent high temperature strength, castability and weldability according to an embodiment of the present invention has a composition as described below.
• denomination “%” means “% by mass” unless otherwise specified.
• M1 C: 0.01 to 0.15%, Cr: 18 to 28%, Co: 10 to 15%, Mo: 8 to 12%, Al: 1.5 to 2%, Ti: 0.1 to 3%, B: 0.001 to 0.006%, Ta: 0.1 to 0.7%, and the balance of Ni plus unavoidable impurities.
• Ta+2 Nb means that Ta:Nb in mole ratio is 1:2.
• the unavoidable impurities of the Ni-based alloy numbered as (M1) to (M3), it is desired that the content of Si is set to 0.1% or less and the content of Mn is set to 0.1% or less.
• the unavoidable impurities can be exemplified Cu, Fe and S in addition to Si and Mn.
• the Ni-based alloy having such a composition as described above is preferable for a material making a casting part of a steam turbine which is operated within a temperature range of 680° C. to 750° C.
• the casting part of the steam turbine may be exemplified a turbine casing of the steam turbine, a valve casing of the steam turbine, a nozzle box of the steam turbine and a pipe of the steam turbine.
• the nozzle box is a ring-shaped steam flow path, which is provided around the turbine rotor, for introducing the high-temperature and pressure steam supplied in the steam turbine into the first section of the steam turbine with first nozzles and first turbine rotor blades.
• the pipe is a main steam pipe for introducing the steam from a boiler into a high pressure turbine or a high temperature-reheat steam pipe.
• the turbine casing, the valve casing, the nozzle box and the pipe are provided under the environment where these parts are exposed to a high-temperature and pressure steam.
• the Ni-based alloy may be applied for every portion of the casting part of the steam turbine or a portion of the casting part thereof.
• the casting parts of the steam turbine arranged over the high pressure steam turbine are likely to be disposed under the high-temperature and pressure atmosphere.
• the casting parts of the steam turbine arranged in the area from the high pressure steam turbine bridging to a part of the medium pressure turbine are also likely to be disposed under the high-temperature and pressure atmosphere.
• the casting part of the steam turbine to be disposed under the high-temperature and pressure atmosphere is not limited to the above-exemplified ones.
• the phrase of “the casting part of the steam turbine to be disposed under the high-temperature and pressure atmosphere” means a casting part of the steam turbine disposed and exposed to the temperature atmosphere within a temperature range of 680° C. to 750° C.
• the Ni-based alloy as described above has a high temperature strength, castability and weldability superior than those of a conventional Ni-based alloy. Therefore, if such a casting part of a steam turbine as the turbine casing, the valve casing, the nozzle box and the pipe may be made of the Ni-based alloy of this embodiment according to the present invention, the casting part can have a higher reliability under the high temperature atmosphere. Namely, the turbine casing, the valve casing, the nozzle box and the pipe with the respective high reliabilities under the high temperature atmosphere can be manufactured.
• Carbon (C) is effective as a constituent element of M 23 C 6 carbide functioning as reinforcing phase.
• the precipitation of the M 23 C 6 carbide during the operation of the steam turbine is one of main factors for maintaining the creep strength of an alloy (i.e., the Ni-based alloy) under a high temperature atmosphere of 650° C. or more.
• carbon has an effect of ensuring the fluidity of a hot melt during casting.
• the carbon content is set less than 0.01%, the mechanical strength (hereinafter, often means a high temperature strength) of the Ni-based alloy may be reduced because the carbide cannot be sufficiently precipitated, and the fluidity of the hot melt of the Ni-based alloy during casting is reduced conspicuously.
• the carbon content is set more than 0.15%, the composition segregation of the hot melt of the Ni-based alloy at the production of a large ingot of the Ni-based alloy is inclined to be increased and the creation of M 6 C carbide as brittle phase is promoted. In this point of view, the carbon content is set within a range of 0.01 to 0.15%.
• Chromium (Cr) is inevitable element for developing the oxidation resistance, the corrosion resistance and the mechanical strength of the Ni-based alloy, and inevitable as a constituent element of M 23 C 6 carbide.
• the precipitation of the M 23 C 6 carbide during the operation of the steam turbine is one of main factors for maintaining the creep strength of an alloy (i.e., the Ni-based alloy) under a high temperature atmosphere of 650° C. or more.
• chromium has an effect of enhancing the oxidation resistance of the Ni-based alloy under a high temperature steam atmosphere. When the chromium content is set less than 18%, the oxidation resistance of the Ni-based alloy may be reduced.
• the chromium content is set more than 28%, the precipitation of M 23 C 6 carbide is remarkably promoted so as to increase the inclination of the coarsening of the precipitated M 23 C 6 carbide.
• the chromium content is set within a range of 18 to 28%.
• Co Co
• Co Co
• the cobalt content is set more than 15%, such intermetallic compound phases as lowering the mechanical strength of the Ni-based alloy are generated so that the mechanical strength of the Ni-based alloy is reduced.
• the cobalt content is set less than 10%, the processability (castability) of the Ni-based alloy is reduced and the mechanical strength of the Ni-based alloy is also reduced.
• the carbon content is set within a range of 10 to 15%.
• Molybdenum (Mo) is solid-solved into the matrix phase of the Ni-based alloy to enhance the mechanical strength of the matrix phase thereof. Moreover, a part of the constituent elements of the M 23 C 6 carbide is substituted with Mo elements to enhance the stability of the M 23 C 6 carbide.
• Molybdenum content is set less than 8%, the above-described effect/function cannot be exhibited.
• Molybdenum content is set more than 12%, the composition segregation of the hot melt of the Ni-based alloy at the production of a large ingot of the Ni-based alloy is inclined to be increased and the creation of M 6 C carbide as brittle phase is promoted. In this point of view, the molybdenum content is set within a range of 8 to 12%.
• Titanium (Ti) generates a ⁇ ′ phase (gamma prime phase: Ni 3 Al) with nickel in the same manner as aluminum so as to develop the mechanical strength of the Ni-based alloy.
• ⁇ ′ phase gamma prime phase: Ni 3 Al
• the titanium content is set less than 0.1%, the hot workability of the Ni-based alloy is deteriorated.
• the titanium content is set more than 3%, the notch sensitivity of the Ni-based alloy is increased. In this point of view, the titanium content is set within a range of 0.1 to 3%.
• Boron (B) is solid-solved into the matrix phase of the Ni-based alloy to enhance the mechanical strength of the matrix phase thereof.
• the boron content is set less than 0.001%, the mechanical strength of the matrix phase thereof cannot be developed.
• the boron content is set more than 0.006%, grain boundary embrittlement may be caused in the Ni-based alloy. In this point of view, the boron content is set within a range of 0.001 to 0.006%.
• Niobium (Nb) is solid-solved into the ⁇ ′ phase (gamma prime phase (Ni 3 (Al, Ti)) so as to stabilize the precipitation strengthening thereof.
• gamma prime phase Ni 3 (Al, Ti)
• the niobium content is set less than 0.1%, the stability of the precipitation strengthening cannot be enhanced in comparison with a conventional steel.
• the niobium content is set more than 0.4%, the mechanical strength of the Ni-based alloy is developed, but the processability (castability) is reduced. In this point of view, the niobium content is set within a range of 0.1 to 0.4%.
• the precipitation strengthening of the ⁇ ′ phase (gamma prime phase (Ni 3 (Al, Ti) can be developed by setting the total content represented by the expression (Ta+2 Nb) within a range of 0.1 to 0.7%.
• the total content of (Ta+2 Nb) is set less than 0.1%, the precipitation strengthening may not be developed sufficiently in comparison with a conventional steel.
• the total content of (Ta+2 Nb) is set more than 0.7%, the mechanical strength of the Ni-based alloy is developed, but the processability (castability) of the Ni-based alloy may be reduced.
• the tantalum content and the niobium content are set at least to 0.01% or more, respectively.
• the specific gravity of niobium is about half as large as the specific gravity of tantalum (specific gravity of tantalum: 16.6, specific gravity of niobium: 8.57), the total solid solubility into the matrix phase of the Ni-based alloy can be increased by adding tantalum and niobium in combination into the matrix phase thereof in comparison with the addition of tantalum. Moreover, since tantalum is a strategic substance, it is difficult to obtain it stably. On the other hand, since the reserve of niobium is about one hundred times as much as the reserve of tantalum, niobium can be stably supplied.
• the ⁇ ′ phase is strengthened under a higher temperature condition.
• the oxidation resistance of tantalum is superior than the oxidation resistance of niobium.
• silicon (Si), manganese (Mn), copper (Cu), iron (Fe) and sulfur (S) are classified as unavoidable impurities. It is desired that the remaining contents of these impurities are reduced to zero % as possible. It is desired that the remaining contents of at least silicon (Si) and manganese (Mn) among these impurities are set to 0.1% or less, respectively.
• silicon (Si) is added thereto for compensating the poor corrosion resistance thereof.
• the Ni-based alloy contains a relatively large amount of chromium (Cr) to ensure the corrosion resistance of the Ni-based alloy, the remaining content of silicon (Si) in the Ni-based alloy is set to 0.10 or less and then, desirably reduced to zero % as possible.
• manganese (Mn) constitutes manganese sulfide (MnS) with sulfur (S) so as to suppress the brittleness of the Ni-based alloy because sulfur (S) may cause the brittleness for the plain carbon steel.
• S sulfur
• the remaining content of sulfur (S) in the Ni-based alloy is extremely low, it is not required to add manganese (Mn) into the Ni-based alloy. In this point of view, the remaining content of manganese (Mn) is set to 0.1% or less and then, desirably reduced to zero % as possible.
• the Ni-based alloy for a casting part of a steam turbine according to the present invention which is used for the turbine casing, the valve casing and the nozzle box, can be produced as follows: First of all, the composition of the Ni-based alloy is melted by means of vacuum induction melting (VIM) and the thus obtained hot melt is injected into a molding box to form an ingot. Then, the ingot is treated by means of solution treatment.
• VIM vacuum induction melting
• the composition of the Ni-based alloy is melted by means of vacuum induction melting (VIM) and the thus obtained hot melt is injected into a cylindrical molding box under the condition that the cylindrical molding box is rotated at high rotation speed.
• VIM vacuum induction melting
• the hot melt is pressurized by the centrifugal force originated from the rotation of the cylindrical molding box, the thus obtained ingot is formed in a predetermined pipe shape, and then, treated by means of solution treatment. In this way, the pipe of the steam turbine can be manufactured, which is called as centrifugal casting method.
• the solution treatment is preferably conducted for 4 to 15 hours within a temperature range of 1100 to 1200° C.
• the solution treatment is conducted in order to solid-solve the ⁇ ′ precipitated phase uniformly.
• the temperature in the solution treatment is set less than 1100° C., the solid-solution cannot be conducted sufficiently.
• the temperature in the solution treatment is set more than 1200° C., the strength of the Ni-based alloy is reduced due to the coarsening of crystal grains thereof.
• the turbine casing, the valve casing and the nozzle box as casting parts according to the present invention may be also manufactured as follows: First of all, the composition of the Ni-based alloy for the casting part of the steam turbine according to the present invention is melted by means of electric furnace (EF), and decarburized by means of argon-oxygen decarburization (AOD). The thus obtained hot melt is injected into a corresponding molding box and then, casted under atmosphere. The thus obtained ingot is treated by means of solution treatment.
• EF electric furnace
• AOD argon-oxygen decarburization
• the manufacturing methods for the turbine casing, the valve casing, the nozzle box and the pipe are not limited to the above-described ones.
• Table 1 shows the chemical compositions of Sample 1 to Sample 28 which are supplied for the evaluation of high temperature strength, the castability and the weldability.
• the chemical compositions of Sample 1 to Sample 6 are belonging to the chemical composition range defined in the present invention.
• the chemical compositions of Sample 7 to Sample 28 are not belonging to the chemical composition range defined in the present invention. Therefore, Sample 7 to Sample 28 correspond to Comparative Examples, respectively.
• Sample 7 has a chemical composition equal to the chemical composition of a conventional Inconel Alloy 617.
• the Ni-based alloy of each of Samples contains iron (Fe), copper (Cu) and sulfur (S) in addition to silicon (Si) and manganese (Mn) as unavoidable impurities.
• the high temperature strength was evaluated by tensile strength test.
• 20 kg of the Ni-based alloy was melted in vacuum induction melting furnace to form an ingot per Sample (i.e., Sample 1 to Sample 28).
• Sample 1 to Sample 28 have the corresponding chemical composition listed in Table 1.
• solution treatment was conducted for the ingot for four hours at 1180° C. to form a cast steel.
• each of Samples was prepared in a predetermined size from the cast steel.

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• 2008
  • 2008-12-24 JP JP2008328459A patent/JP2010150585A/ja not_active Withdrawn
• 2009
  • 2009-10-19 EP EP09013152A patent/EP2206795A3/en not_active Withdrawn
  • 2009-11-10 CN CN200910212116A patent/CN101845573A/zh active Pending
  • 2009-12-08 US US12/633,468 patent/US20100158682A1/en not_active Abandoned

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