NI-BASE ALLOY FOR TURBINE ROTOR OF STEAM TURBINE AND TURBINE ROTOR OF STEAM TURBINE

Abstract

An Ni-base alloy for a turbine rotor of a steam turbine contains in percent by weight C: 0.05 to 0.15, Cr: 22 to 28, Co: 10 to 22, Mo: 8 to 12, Al: 0.8 to less than 1.5, Ti: 0.1 to 0.6, B: 0.001 to 0.006, Re: 0.1 to 2.5, and the balance of Ni and unavoidable impurities.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2008-092782 filed on Mar. 31, 2008; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a material configuring a turbine rotor of a steam turbine into which high-temperature steam flows as a working fluid, and more particularly to an Ni-base alloy for a turbine rotor of a steam turbine excelling in high-temperature strength and the like, and a turbine rotor of a steam turbine made of the Ni-base alloy.

2. Description of the Related Art

For a thermal power plant including a steam turbine, a technology for suppression of the emission of carbon dioxide is being watched with interest in view of the global environmental protection, and needs for highly efficient power generation are increasing.

To increase the power generation efficiency of the steam turbine, it is effective to raise the turbine steam temperature to a high level, and the recent thermal power plant having the steam turbine has its steam temperature raised to 600° C. or higher. There is a tendency that the steam temperature will be increased to 650° C., and further to 700° C. in future.

A turbine rotor, in which moving blades rotated by high-temperature steam are implanted, has a high temperature by circulation of high-temperature steam and generates a high stress by rotating. Therefore, the turbine rotor is required to withstand a high temperature and a high stress, and a material configuring the turbine rotor is demanded to have excellent strength, ductility and toughness in a range of room temperature to a high temperature.

Particularly, if the steam temperature exceeds 700° C., a conventional iron-based material is poor in high-temperature strength, so that the application of the Ni-base alloy is considered in for example JP-A 7-150277(KOKAI).

The Ni-base alloy has been applied extensively as a material mainly for jet engines and gas turbines because it is excellent in high-temperature strength and corrosion resistance. As its typical examples, Inconel 617 alloy (manufactured by Special Metals Corporation) and Inconel 706 alloy (manufactured by Special Metals Corporation) have been used.

As a mechanism to enhance the high-temperature strength of the Ni-base alloy, Al and Ti are added to secure the high-temperature strength by precipitating a precipitated phase called as a gamma prime phase (Ni₃(Al, Ti)) or a gamma double prime phase, or both of them within the mother phase material of the Ni-base alloy. There is for example Inconel 706 alloy which secures high-temperature strength by precipitating both the gamma prime phase and the gamma double prime phase.

Meanwhile, the high-temperature strength of Inconel 617 alloy is secured by reinforcing (solid-solution strengthening) the mother phase of Ni group by adding Co and Mo. For example, JP-A 2002-88455(KOKAI) and JP-A 2001-247942(KOKAI) disclose a Ni-base alloy having high-temperature strength characteristic improved by adjusting added element components based on the components of Inconel alloy. The Ni-base alloy of JP-A 2002-88455(KOKAI) is provided with improved sulfidation corrosion at a high temperature. JP-A 2001-247942(KOKAI) describes a rotor shaft using a Ni-base alloy which suppresses a fragile intermetallic compound formed when used for a long time.

Since the above-described conventional Ni-base alloys are poor in productivity, they were used for relatively small high-temperature parts and the like only.

Therefore, in a case where the conventional Ni-base alloy is applied to, for example, jet engine or gas turbine members, portions where the Ni-base alloy is used are limited to small blades having a length of less than 1 m, a disk material having a gross weight of less than 1 ton or the like.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention provides an Ni-base alloy for a turbine rotor of a steam turbine that workability such as forgeability is excellent, and a large-size forged product turbine rotor can be produced, and a turbine rotor of a steam turbine.

According to an aspect of the invention, there is provided an Ni-base alloy for a turbine rotor of a steam turbine, which contains in percent by weight C: 0.05 to 0.15, Cr: 22 to 28, Co: 10 to 22, Mo: 8 to 12, Al: 0.8 to less than 1.5, Ti: 0.1 to 0.6, B: 0.001 to 0.006, Re: 0.1 to 2.5, and the balance of Ni and unavoidable impurities.

According to an aspect of the invention, there is also provided a turbine rotor which is disposed through a steam turbine into which high-temperature steam is introduced, wherein at least a predetermined portion is formed of the Ni-base alloy for the turbine rotor of a steam turbine described above.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described below.

An Ni-base alloy for a turbine rotor of a steam turbine in an embodiment according to the present invention is composed of the compositional component ranges shown below. In the following description, percentages indicating the compositional components are by weight unless otherwise indicated.

(Ml) Ni-base alloy which contains C: 0.05% to 0.15%, Cr: 22% to 28%, Co: 10% to 22%, Mo: 8% to 12%, Al: 0.8% to less than 1.5%, Ti: 0.1% to 0.6%, B: 0.001% to 0.006%, Re: 0.1% to 2.5%, and the balance of Ni and unavoidable impurities.

In the unavoidable impurities in the Ni-base alloy of the above (Ml), it is preferably suppressed that at least Si is 1% or less, and Mn is 1% or less.

The Ni-base alloy having the compositional component ranges described above is suitable as a material configuring a turbine rotor of a steam turbine which has a temperature in a range of 680 to 750° C. during its operation. All portions of the turbine rotor of the steam turbine may be made of the Ni-base alloy, and some portions, which have a particularly high temperature, of the turbine rotor of the steam turbine may be made of this Ni-base alloy. As some portions of the turbine rotor of the steam turbine which have a high temperature, there are specifically all regions of a high-pressure steam turbine section, or regions ranging from the high-pressure steam turbine section to some portions of an intermediate-pressure steam turbine section.

The Ni-base alloys of the compositional component ranges described above can improve workability such as forgeability. In other words, the Ni-base alloy is used to configure the turbine rotor of the steam turbine, so that the workability such as forgeability of the turbine rotor can be improved, and the turbine rotor having high reliability can be produced without generating a crack or the like when manufacturing.

The reasons of limiting the individual compositional component ranges of the Ni-base alloy according to the present invention described above will be described below.

(1) C (Carbon)

C is useful as a component element of M₂₃C₆ type carbide which is a strengthening phase, and particularly, the creep strength of the alloy is maintained by precipitating the M₂₃C₆ type carbide during the operation of the steam turbine in a high-temperature environment of 650° C. or higher. And, it also has an effect of securing the fluidity of a molten metal at the time of casting. If the C content is less than 0.05%, a sufficient precipitation amount of carbide cannot be secured, so that mechanical strength is degraded, and the fluidity of the molten metal at the time of casting lowers considerably. Meanwhile, if the C content exceeds 0.15%, the tendency of segregation of components increases at the time of producing a large ingot, the generation of M₆C type carbide which is an embrittlement phase is promoted, and mechanical strength is improved, but forgeability is degraded. Therefore, the C content is determined to be 0.05% to 0.15%.

(2) Cr (Chromium)

Cr is an indispensable element to improve oxidation resistance, corrosion resistance and mechanical strength of the Ni-base alloy. Besides, it is indispensable as a component element of the M₂₃C₆ type carbide, and particularly, the creep strength of the alloy is maintained by precipitating the M₂₃C₆ type carbide during the operation of the steam turbine in a high-temperature environment of 650° C. or higher. And, Cr improves the oxidation resistance in a high-temperature steam environment. If the Cr content is less than 22%, the oxidation resistance decreases. Meanwhile, if the Cr content exceeds 28%, precipitation of the M₂₃C₆ type carbide is accelerated considerably, resulting in increasing the tendency of coarsening. Therefore, the Cr content is determined to be 22% to 28%.

(3) Co (Cobalt)

In the Ni-base alloy, Co improves the mechanical strength of a mother phase by forming a solid solution in the mother phase. But, if the Co content exceeds 22%, an intermetallic compound phase which degrades the mechanical strength is generated, and forgeability is degraded. Meanwhile, if the Co content is less than 10%, workability is degraded, and the mechanical strength is lowered. Therefore, the Co content is determined to be 10% to 22%.

(4) Mo (Molybdenum)

Mo provides an effect of forming a solid solution into an Ni mother phase to enhance the mechanical strength of the mother phase, and its partial substitution in M₂₃C₆ type carbide enhances the stability of the carbide. If the Mo content is less than 8%, the above effect is not exerted, and if the Mo content exceeds 12%, a tendency of segregation of components increases when a large ingot is produced, and the generation of M₆C type carbide which is an embrittlement phase is accelerated. Therefore, the Mo content is determined to be 8% to 12%.

Mo is common with the above-described Co on a point that they have an effect to improve the mechanical strength of the mother phase. And, to exhibit effectively the common characteristic and their other characteristics, when the Mo content is for example 8 to less than 10%, it is desirable that the Co content is larger than 15% and not larger than 22%. When the Mo content is for example 10 to 12%, it is desirable that the Co content is 10% to 15%.

(5) Al (Aluminum)

Al generates a γ′ phase (gamma prime phase: Ni₃Al) with Ni and improves the mechanical strength of the Ni-base alloy based on the precipitation. If the Al content is less than 0.8%, the mechanical strength is not improved in comparison with a conventional steel, and if the Al content is 1.5% or more, the mechanical strength is improved, but forgeability is degraded. Therefore, the Al content is determined to be 0.8% to less than 1.5%.

(6) Ti (Titanium)

Similar to Al, Ti generates a γ′ phase (gamma prime phase: Ni₃Ti) with Ni and improves the mechanical strength of the Ni-base alloy. If the Ti content is less than 0.1%, the above effect is not exerted, and if the Ti content exceeds 0.6%, hot workability is degraded, and notch sensitivity becomes high. Therefore, the Ti content is determined to be 0.1% to 0.6%.

(7) B (Boron)

B segregates in the grain boundary to affect the high-temperature characteristics. And, B has an effect to improve the mechanical strength of an Ni mother phase by precipitating in the mother phase. If the B content is less than 0.001%, the effect to improve the mechanical strength of the mother phase is not exerted, and if the B content exceeds 0.006%, there is a possibility that the grain boundary is embrittled. Therefore, the B content is determined to be 0.001% to 0.006%.

(8) Re (Rhenium)

Re has an effect to improve the mechanical strength of an Ni mother phase by forming a solid solution in the mother phase. If the Re content is less than 0.1%, an effect to improve the mechanical strength of the mother phase is not exerted, and if the Re content exceeds 2.5%, a fragile phase is formed. Therefore, the Re content is determined to be 0.1% to 2.5%.

Similar to the Re, Co and Mo have an effect to improve the mechanical strength of the Ni mother phase by forming a solid solution in the mother phase. But, when the content is same, the Re is most excellent in improvement of the mechanical strength and can improve the mechanical strength without largely varying the chemical component composition of a base metal.

(9) Si (Silicon), Mn (Manganese), Cu (Copper), Fe (Iron) and S (Sulfur)

Si, Mn, Cu, Fe and S are classified to unavoidable impurities in the Ni-base alloy according to the present invention. The residual contents of the unavoidable impurities are desired to be decreased toward 0% as much as possible. It is desirable that at least Si and Mn in the unavoidable impurities are suppressed to 1% or below.

Si is added to the ordinary steel to supplement the corrosion resistance. But, since the Ni-base alloy has a large Cr content to secure sufficient corrosion resistance, a residual content of Si in the Ni-base alloy according to the present invention is determined to be 1% or less, and it is desired that the residual content is reduced to 0% as much as possible.

In the ordinary steel, Mn prevents brittleness, which results from S (sulfur), in a form of MnS. But, since the S content in the Ni-base alloy is very small, it is not necessary to add Mn. Therefore, the residual content of Mn in the Ni-base alloy according to the present invention is determined to be 1% or below, and it is desired that the residual content is reduced to 0% as much as possible.

The above-described Ni-base alloy according to the present invention is produced by melting the compositional components configuring the Ni-base alloy by a vacuum induction melting furnace, subjecting the obtained ingot to a soaking treatment, forging it, and conducting a solution treatment.

It is preferable that the soaking treatment is maintained at a temperature range of 1050 to 1250° C. for 5 to 72 hours, and the solution treatment is maintained at a temperature range of 1100 to 1200° C. for 4 to 5 hours. Here, the solution treatment temperature is determined to form a homogeneous solid solution of the γ′ phase precipitates, and if the temperature is lower than 1100° C., a solid solution is not formed adequately. If the temperature exceeds 1200° C., crystal grains are coarsened and the strength is degraded. And, forging is performed at a temperature range of 950 to 1150° C.

In a case where the above-described Ni-base alloy according to the present invention is used to configure a turbine rotor of a steam turbine, for example, as one method (double melt), the raw material is subjected to vacuum induction melting (VIM) and electro-slag remelting (ESR) and then poured into a prescribed mold. Subsequently, a forging treatment and a heat treatment are performed to produce the turbine rotor. As another method (double melt), the raw material is subjected to vacuum induction melting (VIM) and vacuum arc remelting (VAR) and then poured into a prescribed mold. Subsequently, a forging treatment and a heat treatment are performed to produce the turbine rotor. As still another method (triple melt), the raw material is subjected to vacuum induction melting (VIM), electro-slag remelting (ESR) and vacuum arc remelting (VAR) and then poured into a prescribed mold. Subsequently, a forging treatment and a heat treatment are performed to produce the turbine rotor. The turbine rotors produced by the above methods are inspected by ultrasonic inspection or the like.

It is described below that the Ni-base alloy according to the present invention is excellent in forgeability.

(Evaluation of Forgeability)

It is described below that the Ni-base alloy having the chemical composition ranges of the present invention has excellent forgeability. Table 1 shows chemical compositions of Sample 1 to Sample 5 used for evaluation of the forgeability. And, Sample 1 to Sample 4 are Ni-base alloys with the chemical composition ranges of the present invention, and Sample 5 is an Ni-base alloy with its composition not within the chemical composition ranges of the present invention and used as a comparative example. Sample 5 has a chemical composition corresponding to a conventional steel Inconel 617. The Ni-base alloy with the chemical composition ranges of the present invention contains Fe (iron), Cu (copper) and S (sulfur) as unavoidable impurities in addition to Si and Mn.

TABLE 1
(Wt %)
	Ni	C	Si	Mn	Cr	Fe	Al	Mo	Co	Cu	Ti	B	S	Re
Example	Sample 1	Balance	0.099	0.55	0.57	23	1.56	1.21	8.9	19.6	0.24	0.36	0.0039	0.001	0.12
	Sample 2	Balance	0.099	0.55	0.57	27.4	1.56	1.21	8.9	14.5	0.24	0.36	0.0039	0.001	2.48
	Sample 3	Balance	0.096	0.53	0.57	25.7	1.55	1.2	10.3	12.2	0.24	0.35	0.0041	0.0009	0.13
	Sample 4	Balance	0.096	0.53	0.57	23.2	1.55	1.2	11.9	12.2	0.24	0.35	0.004	0.0009	2.47
Comparative	Sample 5	Balance	0.076	0.51	0.55	22.9	1.57	1.21	8.9	12.2	0.25	0.36	0.0038	0.0009	0
Example

For evaluation of forgeability, Ni-base alloys of Sample 1 to Sample 5 having the chemical compositions shown in Table 1 each in 10 kg were melted in a vacuum induction melting furnace, and test specimens made of cylindrical ingots having a diameter of 87 mm and a length of 140 mm were produced. Subsequently, the ingots were undergone a soaking treatment at 1050° C. for five hours. Forging treatment was conducted by a 500-kgf hammer forging machine at a temperature range of 950 to 1100° C. (reheating at 1100° C.). For the forgeability, the above-described forging treatment was performed until the test specimens came to have a diameter of 30 mm. Evaluation was performed based on a forging ratio of the above treatment and the presence or not of a forging crack at that time.

The forging ratio is defined by the division of a length of the test specimen which is a forged object stretched by the forging treatment by a length of the test specimen which is the forged object before the forging treatment. According to the forging treatment, if the temperature of the test specimen lowers, namely if the test specimen becomes hardened, the forging treatment is repeated by reheating up to a reheating temperature of 1100° C. And, for the presence or not of a forging crack, the test specimens undergone the forging treatment are visually checked. If there is no crack, it is indicated as “None”, and the forgeability is evaluated as “0” to indicate that the forgeability is excellent. Meanwhile, if there is a crack, it is indicated as “Yes”, and the forgeability is evaluated as “X” to indicate that the forgeability is inferior.

Table 2 shows results obtained by evaluating the forgeability of the respective samples.

	TABLE 2
	Forging
	ratio	Crack	Forgeability
Example	Sample 1	6.6	NONE	∘
	Sample 2	6.3	NONE	∘
	Sample 3	6.7	NONE	∘
	Sample 4	6.4	NONE	∘
Comparative	Sample 5	5.4	YES	x
Example

As shown in Table 2, it was found that Sample 1 to Sample 4 have excellent forgeability in comparison with Sample 5.

Although the invention has been described above by reference to the embodiments of the invention, the invention is not limited to the embodiments described above. It is to be understood that modifications and variations of the embodiments can be made without departing from the spirit and scope of the invention.

Claims

1. An Ni-base alloy for a turbine rotor of a steam turbine, the Ni-base alloy contains in percent by weight C: 0.05 to 0.15, Cr: 22 to 28, Co: 10 to 22, Mo: 8 to 12, Al: 0.8 to less than 1.5, Ti: 0.1 to 0.6, B: 0.001 to 0.006, Re: 0.1 to 2.5, and the balance of Ni and unavoidable impurities.

2. The Ni-base alloy for a turbine rotor of a steam turbine according to claim 1,

wherein the unavoidable impurities are suppressed in percent by weight to Si: 1 or below and Mn: 1 or below.

3. A turbine rotor configured to dispose through a steam turbine into which high-temperature steam is introduced,

wherein at least a predetermined portion is formed of the Ni-base alloy for a turbine rotor of a steam turbine according to claim 1.

4. A turbine rotor configured to dispose through a steam turbine into which high-temperature steam is introduced,

wherein at least a predetermined portion is formed of the Ni-base alloy for a turbine rotor of a steam turbine according to claim 2.