TECHNICAL FIELD

The present invention relates to a high temperature component made of a γ′ (gamma prime) precipitation strengthening-type Ni-based alloy and a method for producing the same.

BACKGROUND ART

Conventionally, for example, high temperature components such as a turbine component of a gas turbine engine are made of a superalloy material that exhibits predetermined mechanical properties in a high-temperature environment. As such a superalloy material, a γ′ precipitation strengthening-type Ni-based alloy intended for finely precipitating an intermetallic compound called a γ′ phase to improve high-temperature strength is known. The γ′ precipitation strengthening-type Ni-based alloy contains, for example, at least one of Cr (chromium), W (tungsten), Mo (molybdenum), Re (rhenium) and Co (cobalt) as a main element that forms a solid solution with a parent phase (γ matrix phase) and strengthens the parent phase, and contains at least one of Al (aluminum), Ti (titanium), Ta (tantalum), Nb (niobium) and V (vanadium) as a main element forming a γ′ phase (mainly, Ni3(Al,Ti)) by combining with Ni (nickel). PTLs 1 and 2 disclose this kind of γ′ precipitation strengthening-type nickel-based alloy and components made thereof.

The manufacturing process of a component made of a γ′ precipitation strengthening-type nickel-based alloy described in PTL1 includes the steps of obtaining a billet in which alloy powder is consolidated by hot isostatic pressing (HIP) and/or extrusion consolidation, obtaining an intermediate product molded by net shape forging the billet at a temperature slightly lower than a γ′ solvus temperature of the alloy (solid solution temperature of the γ′ phase), and obtaining a product in which crystal grains are uniformly coarsened by performing a solution heat treatment (supersolvus heat treatment) at a temperature higher than the γ′ solvus temperature of the alloy. In the solution heat treatment, a crystal grain structure of the intermediate product is recrystallized at a temperature higher than the γ′ solvus temperature of the alloy and lower than an initial melting temperature, and a γ′ precipitate is dissolved (solid solution) in the alloy, and then undergoes age hardening treatment for reprecipitation of the γ′ phase inside the matrix or at the grain boundary.

The nickel-based alloy described in PTL2 is set to have a total content of Al, Ti, and Nb of 10.5% or more and 13% or less by atomic percent, in order to set a volume ratio of the γ′ phase of 40 to 50%. A component made of this alloy is obtained by solidifying an alloy powder by hot isostatic pressing and/or drawing, molding it into a component by isothermal forging, subjecting the molded component to a recrystallization heat treatment, and cooling it. In the recrystallization heat treatment, a component having a coarse crystal grain microstructure exceeding 15 pin is obtained by treating at a temperature higher than the solvus temperature of the γ′ phase of the alloy and lower than a melting start temperature of the alloy.

Incidentally, as one of the powder metallurgy methods, metal injection molding (hereinafter referred to as “MIM”), which is a combination of a resin molding technique and a powder metallurgy technique, is known. The MIM manufacturing process generally includes the steps of obtaining a compound by uniformly kneading metal powder and a binder (plastic+wax), obtaining an intermediate compact by injecting the compound into a mold and releasing it from the mold, removing (debindering) the binder from the intermediate compact with heating, a catalyst, a solvent, or the like, and sintering the debindered intermediate compact to obtain a compact (powder compact).

CITATION LIST

Patent Literature

PTL1: JP 2016-532777 APTL2: JP 2007-277721 A

SUMMARY OF INVENTION

Technical Problem

MIM can mold three-dimensional shapes with near net shape, and has excellent points that a material yield is high and material costs and post-processing costs can be reduced, production running time is relatively short and productivity is high, and the like. Therefore, when MIM is applied to a production method for a high temperature component, there are many advantages such as being capable of providing a high temperature component at low cost.

It has been confirmed that a high temperature component having predetermined high-temperature characteristics can be produced by MIM with compositions such as IN718 (IN: Inconel is a registered trademark, hereinafter the same), which is a γ″ precipitation strengthening-type Ni-based alloy. However, it is known that high temperature components produced by MIM with γ′ precipitation strengthening-type Ni-based alloys are inferior in high-temperature characteristics.

The inventors of the present application have adopted an alloy having a typical composition of IN713C (hereinafter referred to as “IN713C-MIM”) as an example of a γ′ precipitation strengthening-type Ni-based alloy that constitutes a high temperature component to be produced by MIM, and investigated high-temperature characteristics of IN713C-MIM. IN713C is one of γ′ precipitation strengthening-type Ni-based alloys with excellent creep resistance.

According to previous research on IN713C-MIM, IN713C-MIM has low creep resistance as compared to components produced by casting, and has not reached the high-temperature characteristics that can be adopted as high temperature components such as turbine components.

The inventors of the present application consider that the high-temperature characteristics of IN713C-MIM do not improve due to the fine crystal grain structure depending on the particle diameter of the metal powder as the raw material, and it is effective to coarsen the crystal grains in order to improve the high-temperature characteristics. In general, it is known that creep resistance is improved by the coarsening of crystal grains in a Ni-based alloy having high temperature resistance, and in the γ′ precipitation strengthening-type Ni-based alloy described in PTL1, high-temperature characteristics including creep resistance are improved by the coarsening of crystal grains.

However, when the inventors of the present application conducted a heat treatment (supersolvus heat treatment) of IN713C-MIM in accordance with the technique described in PTL1, coarsening of crystal grains to a degree sufficient to improve creep resistance was not confirmed between before and after the heat treatment.

Also, PTLs 1 and 2 disclose a powder forging method in which a sintered body of alloy powder is forged. As disclosed in PTLs 1 and 2, it is known that the crystal grains can be coarsened by recrystallization and grain growth by heat treatment after straining the components before heat treatment by isothermal forging or cold forging. This is because, when a free energy of a material is increased due to dislocation accumulated in the crystal grains due to the applied plastic strain, the recrystallized crystal grains generated using this free energy as a driving force become fine, and a grain boundary energy to be a driving force for grain growth is higher as the crystal grains are finer. However, both MIM and forging are material processing technologies, and forging a powder compact molded by MIM is not usually performed, and thus MIM and forging are incompatible.

The present invention has been made in view of the above circumstances, and an object of the present invention is to suggest a technique for coarsening crystal grains of a structure of a high temperature component, when producing, from a metal powder, the high temperature component made of a γ′ precipitation strengthening-type Ni-based alloy, using a powder shaping method other than a method including plastic working such as powder forging.

Solution to Problem

A typical composition of IN713C contains 0.08 to 0.20% by mass of C (carbon), and a powder compact obtained by shaping alloy powder of this composition by MIM further increases the content of C. The inventors of the present application have presumed that carbides (metal carbides) present at grain boundaries of the powder compact hinders grain boundary migration and inhibits crystal grain growth, and have reached an idea that one of the factors that inhibit coarsening of the IN713C-MIM crystal grains is the carbon content of the powder compact of IN713C-MIM.

Thus, a method for producing a high temperature component according to one embodiment of the present invention includes:a shaping step of shaping a powder compact of a desired high temperature component shape using a specific powder shaping method, from an alloy powder of γ′ precipitation strengthening-type Ni-based alloy; anda crystal grain coarsening step of coarsening a crystal grain size of the powder compact by heat treatment,in which the powder compact contains 0.002% or more and 0.07% or less of C, and 5.40% or more and 8.40% or less of Al+Ti by mass percentage.

It has been found that, according to the method for producing a high temperature component, the content of C that is present at the grain boundaries of the powder compact and generates carbides that inhibit crystal grain growth is limited to 0.002% by mass or more and 0.07% or less in the powder compact, whereby the crystal grain size of the high temperature component obtained has grown from the particle diameter of the alloy powder. A high temperature component having a crystal structure coarsened by such crystal grain growth is expected to have high creep resistance.

Then, according to the method for producing a high temperature component, it is possible to produce a high temperature component including a γ′ precipitation strengthening-type Ni-based alloy containing 0.002% or more and 0.07% or less of C, and 5.40% or more and 8.40% or less of Al+Ti by mass percentage, in which the average size of crystal grains is 150 μm or more, and the structure of the crystal grains is equiaxed in all cross sections in three orthogonal directions and non-dendritic structure. In the present specification and claims, a metal structure in which the average of the dimensional ratios (aspect ratios) between the major axis and the minor axis of each crystal grain is less than 2 is defined as “equiaxed structure”.

The high temperature component may have a content of C of greater than 0.03% and 0.07% or less by mass percentage.

In the high temperature component, the γ′ precipitation strengthening-type Ni-based alloy may contain, in addition to C, Al, and Ti, 4.60% or less of Nb+Ta, 5.00% or more and 22.80% or less of Cr, 19.50% or less of Co, 1.80% or more and 13.75% or less of Mo+W, 0.10% or less of B, 1.0% or less of Zr, and 2.0% or less of Hf by mass percentage.

Further, in the high temperature component, the γ′ precipitation strengthening-type Ni-based alloy may contain 0.03% or more and 0.07% or less of C, 6.00% or more and 7.50% or less of Al+Ti, 1.50% or more and 3.00% or less of Nb+Ta, 11.00% or more and 15.00% or less of Cr, 3.80% or more and 5.20% or less of Mo, 0.005% or more and 0.020% or less of B, and 0.05% or more and 0.20% or less of Zr by mass percentage, with the balance being made up of Ni and inevitable impurities.

The method for producing a high temperature component may further include a porosity reduction step of reducing a porosity by applying an isotropic pressure to the powder compact using a gas pressure, which is performed between the shaping step and the crystal grain coarsening step, or simultaneously with the crystal grain coarsening step.

Also, in the method for producing a high temperature component, it is preferred that the crystal grain coarsening step includes heating the powder compact at a predetermined coarsening temperature in a vacuum atmosphere or an inert gas atmosphere, and the coarsening temperature is a temperature in the range of a pinning effect disappearance temperature specific to the powder compact or higher and a solidus temperature of the powder compact or lower. However, the solidus temperature may be a value obtained by adding a predetermined α° C. to a solidus temperature obtained by experiment.

Moreover, in the method for producing a high temperature component, the content of C may be greater than 0.03% and 0.07% or less by mass percentage.

Further, in the method for producing a high temperature component, the powder compact may include, in addition to C, Al, and Ti, 4.60% or less of Nb+Ta, 5.00% or more and 22.80% or less of Cr, 19.50% or less of Co, 1.80% or more and 13.75% or less of Mo+W, 0.10% or less of B, 1.0% or less of Zr, and 2.0% or less of Hf by mass percentage.

Furthermore, in the method for producing a high temperature component, the powder compact may contain 0.03% or more and 0.07% or less of C, 6.00% or more and 7.50% or less of Al+Ti, 1.50% or more and 3.00% or less of Nb+Ta, 11.00% or more and 15.00% or less of Cr, 3.80% or more and 5.20% or less of Mo, 0.005% or more and 0.020% or less of B, and 0.05% or more and 0.20% or less of Zr by mass percentage, with the balance being made up of Ni and inevitable impurities.

Also, in the method for producing a high temperature component, the specific powder shaping method is other than a powder forging method, and the shaping step may include collecting the alloy powder into the high temperature component shape and sintering the collected alloy powder.

Moreover, in the method for producing a high temperature component, the shaping step may include injecting a compound obtained by kneading the alloy powder and a resin binder into a mold to shape an intermediate compact, debindering the intermediate compact, and sintering the debindered intermediate compact to obtain the powder compact.

Thus, a high temperature component with high shape accuracy can be obtained by using MIM to obtain a powder compact shaped into a shape of a high temperature component. Furthermore, by using MIM, the material yield is high, material costs and post-processing costs can be reduced, and production running time is relatively short, and thus an improvement in productivity can be expected.

In the method for producing a high temperature component, it is preferred that the average particle diameter of the alloy powder is 20 μm or more and 60 μm or less.

When the alloy powder has the above average particle diameter, properties for removing the resin binder from gaps between the powders when the intermediate compact is debindered are expected to be improved.

Further, in the method for producing a high temperature component, it is preferred that the alloy powder contains 0.002% or more and 0.02% or less of C by mass percentage.

Advantageous Effects of Invention

According to the present invention, when producing, from a metal powder, a high temperature component made of a γ′ precipitation strengthening-type Ni-based alloy having excellent high-temperature characteristics using a shaping method other than forging such as MIM, crystal grains of a structure of the high temperature component can be coarsened.

