US20150132144A1

US20150132144A1 - Precipitation hardening martensitic stainless steel, turbine component formed of said martensitic stainless steel, and turbine including said turbine component

Info

Publication number: US20150132144A1
Application number: US14/535,353
Authority: US
Inventors: Shinji Oikawa; Shinya Imano; Hiroyuki Doi
Original assignee: Mitsubishi Hitachi Power Systems Ltd
Current assignee: Mitsubishi Power Ltd
Priority date: 2013-11-08
Filing date: 2014-11-07
Publication date: 2015-05-14
Also published as: JP6317566B2; JP2015093991A

Abstract

It is an objective of the invention to provide a precipitation-hardening martensitic stainless steel having a far better balance between a high mechanical strength and a high toughness than conventional ones as well as having good corrosion resistance properties. There is provided a precipitation-hardening martensitic stainless steel throughout which precipitates of intermetallic compounds are dispersed, the martensitic stainless steel including: 0.1 mass % or less of C; 11 to 13 mass % of Cr; 7.5 to 11 mass % of Ni; 0.9 to 1.7 mass % of Al; 0.85 to 1.35 mass % of Mo; 1.75 to 2.75 mass % of W; and the balance including Fe and inevitable impurities, in which “[Mo content]+0.5×[W content]” is from 1.9 mass % to 2.5 mass %, and “[Mo content]/[W content]” is from 0.4 to 0.6.

Description

CLAIM OF PRIORITY

The present application claims priority from Japanese patent application serial no. 2013-231943 filed on Nov. 8, 2013, the content of which is hereby incorporated by reference into this application.

1. FIELD OF THE INVENTION

The present invention relates to steels having high mechanical properties and a high corrosion resistance, and particularly to a precipitation-hardening martensitic stainless steel. The invention also particularly relates to a turbine component formed of such a precipitation-hardening martensitic stainless steel of the invention, and a turbine including such a turbine component of the invention.

2. DESCRIPTION OF RELATED ART

Because of the recent trend toward the conservation of energies (such as fossil fuel energy) and the global environment conservation (such as suppression of CO₂gas emission), a strong demand exists to increase the efficiencies of apparatuses (such as steam turbines) used in thermal power plants. An effective means to improve the efficiency of steam turbines is to increase length of the blades (such as long blades) of the turbine. By using longer turbine long blades, more steam energy can be converted into rotational energy of a turbine. Such an increased conversion efficiency has an additional effect of reducing the number of turbine casings, thereby leading to a reduction in construction time and cost.
Currently, long blades of steam turbines in ultra super critical (USC) power plants are formed mainly of martensitic stainless steels. A problem with such a steam turbine long blade is that the longer a steam turbine long blade is, the much stronger centrifugal force the blade receives (because, generally, the centrifugal force is proportional to the product of the mass of the blade and the rotational radius of the turbine (the length of the turbine blade)). Therefore, there is a strong need for steam turbine long blade materials having a higher mechanical strength than conventional materials. Such long blade materials also require a high toughness in order to prevent sudden rupture.
Various structural materials having both a high mechanical strength and a high toughness have been proposed. For example, JP 2005-194626 A discloses a precipitation-hardening martensitic stainless steel including: 12.25 to 14.25 wt. % of Cr, 7.5 to 8.5 wt. % of Ni; 1.0 to 2.5 wt. % of Mo; 0.05 wt. % or less of C; 0.2 wt. % or less of Si; 0.4 wt. % or less of Mn; 0.03 wt. % or less of P; 0.005 wt. % or less of S; 0.008 wt. % or less of N; 0.90 to 2.25 wt. % of Al; and the balance practically Fe, wherein the total amount of Cr and Mo is from 14.25 to 16.75 wt. %.
JP 2011-225913 A discloses a precipitation-hardening martensitic stainless steel including: 0.05 to 0.10 mass % of C, 12.0 to 13.0 mass % of Cr; 6.0 to 7.0 mass % of Ni; 1.0 to 2.0 mass % of Mo; 0.01 to 0.05 mass % of Si; 0.06 to 1.0 mass % of Mn; 0.3 to 0.5 mass % of Nb; 0.3 to 0.5 mass % of V; 1.5 to 2.5 mass % of Ti; 1.0 to 2.3 mass % of Al; and the balance including Fe and inevitable impurities.
JP 2012-102638 A discloses a precipitation-hardening martensitic stainless steel including: 0.10 mass % or less of C, 13.0 to 15.0 mass % of Cr; 7.0 to 10.0 mass % of Ni; 2.0 to 3.0 mass % of Mo; 0.5 to 2.5 mass % of Ti; 0.5 to 2.5 mass % of Al; 0.5 mass % or less of Si; 0.1 to 1.0 mass % of Mn; and the balance including Fe and inevitable impurities.
In spite of the growing worldwide responsibility towards global environment conservation, the world's energy demand is continuing to rise. In order to meet both of these conflicting demands, there is a strong need to further increase the efficiency of thermal power plants (in particular steam turbines). As already described, an effective way to increase the efficiency of steam turbines is to increase the length of turbine long blades. A material used to form such longer-than-conventional turbine long blades needs to have both a higher mechanical strength and a higher toughness than conventional martensitic stainless steels (such as the ones disclosed in JP 2005-194626 A, JP 2011-225913 A and JP 2012-102638 A). In addition, steam turbine long blades are used in a harsh corrosive environment because they are exposed to a severe dry and wet cycle. Therefore, materials used for steam turbine long blades also require a high corrosion resistance (such as a high stress corrosion cracking (SCC) resistance).
Generally, there is a trade off between the mechanical strength and corrosion resistance of steels. Common martensitic stainless steels have a high mechanical strength, but have a relatively poor corrosion resistance. In contrast, precipitation-hardening martensitic stainless steels, which contain a relatively large amount of Cr and a relatively small amount of C, are excellent in corrosion resistance, but are relatively poor in mechanical strength.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an objective of the present invention to provide a precipitation-hardening martensitic stainless steel having a far better balance between a high mechanical strength and a high toughness than conventional ones as well as having good corrosion resistance properties (such as a high SCC resistance and a high pitting potential). Another objective of the invention is to provide a turbine component formed of such a precipitation-hardening martensitic stainless steel of the invention, and a turbine including such a turbine component of the invention.
(I) According to one aspect of the present invention, there is provided a precipitation-hardening martensitic stainless steel throughout which precipitates of intermetallic compounds are dispersed, the martensitic stainless steel including: 0.1 mass % or less of C (carbon); 11 to 13 mass % of Cr (chromium); 7.5 to 11 mass % of Ni (nickel); 0.9 to 1.7 mass % of Al (aluminum); 0.85 to 1.35 mass % of Mo (molybdenum); 1.75 to 2.75 mass % of W (tungsten); and the balance including Fe (iron) and inevitable impurities, in which “[content of the Mo]+0.5×[content of the W]” is from 1.9 mass % to 2.5 mass %, and “[content of the Mo]/[content of the W]” is from 0.4 to 0.6.
In the above aspect (I) of the invention, the following modifications and changes can be made.
i) The precipitation-hardening martensitic stainless steel further includes 0.4 mass % or less of Ti (titanium).
ii) Part of the Ni is substituted by 3 mass % or less of Co (cobalt).
iii) The precipitation-hardening martensitic stainless steel further includes one or both of Nb (niobium) and V (vanadium) in total amount of 0.5 mass % or less.
iv) The precipitation-hardening martensitic stainless steel further includes 0.1 mass % or less of Si (silicon) and/or 1 mass % or less of Mn (manganese).
v) The inevitable impurities include one or more of 0.5 mass % or less of P (phosphorus), 0.5 mass % or less of S (sulfur), 0.1 mass % or less of Sb (antimony), 0.1 mass % or less of Sn (tin), 0.1 mass % or less of As (arsenic), and 0.1 mass % or less of N (nitrogen).
vi) One of the intermetallic compounds is β-NiAl phase.
vii) The precipitation-hardening martensitic stainless steel is solution heat treated at 850 to 950° C. followed by aging heat treatment at 450 to 650° C.
(II) According to another aspect of the present invention, there is provided a turbine component formed of the precipitation-hardening martensitic stainless steel of the invention.
(III) According to still another aspect of the present invention, there is provided a turbine rotor including the turbine component of the invention, in which the turbine component is a steam turbine long blade.
(IV) According to yet another aspect of the present invention, there is provided a steam turbine including the turbine rotor of the invention.
(V) According to a further aspect of the present invention, there is provided a thermal power plant including the steam turbine of the invention.

Advantages of the Invention

According to the present invention, it is possible to provide a precipitation-hardening martensitic stainless steel having a far better balance between a high mechanical strength and a high toughness than conventional ones as well as having good corrosion resistance properties (such as a high SCC resistance and a high pitting potential). Also possible is to provide a turbine component formed of such a precipitation-hardening martensitic stainless steel of the invention, a turbine including such a turbine component of the invention, and a thermal power plant including such a turbine of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a perspective view of an exemplary steam turbine long blade formed of the invention's stainless steel;

FIG. 2 is a schematic illustration showing a longitudinal sectional view of an example of a turbine according to the invention;

FIG. 3 is a system diagram of an example of a thermal power plant according to the invention;

FIG. 4 is a graph showing a relationship between a “[Mo content]/[W content]” ratio and tensile strength; and

FIG. 5 is a graph showing a relationship between a “[Mo content]/[W content]” ratio and impact energy absorption.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. However, the invention is not limited to the specific embodiments described below, but various combinations and modifications are possible without departing from the spirit and scope of the invention.
(Composition of Precipitation-Hardening Martensitic Stainless Steel)
The composition of the precipitation-hardening martensitic stainless steel of the invention will be described below.
C Component:
The C suppresses formation of δ-ferrite phase which has an adverse effect on the mechanical properties and corrosion resistance of the stainless steel. Also, carbon forms a carbide with Cr or Ti, thereby precipitation-hardening the steel. However, when the C content exceeds 0.1 mass %, the toughness of the steel decreases due to excessive precipitation of the carbides; the corrosion resistance decreases due to decreased Cr concentration near the grain boundaries; and the martensitic transformation temperature lowers. Therefore, the C content is preferably 0.1 mass % or less, more preferably 0.05 mass % or less and even more preferably 0.025 mass % or less.
Cr Component:
The Cr forms a passivation film at the surface of the stainless steel, thereby improving the corrosion resistance. When the Cr content is less than 11 mass %, sufficient corrosion resistance cannot be achieved. When the Cr content is more than 13 mass %, δ-ferrite phase is prone to form, thereby degrading the mechanical properties and corrosion resistance. Therefore, the Cr content is preferably from 11 to 13 mass %, more preferably from 11.5 to 12.5 mass % and even more preferably from 11.75 to 12.25 mass %.
Ni Component:
The Ni suppresses δ-ferrite phase formation and enhances the tensile strength of the stainless steel by the dispersion/precipitation hardening effect of Ni-based intermetallic compounds (e.g., Ni—Al compounds). The Ni also has an effect of increasing the quench hardening properties and the toughness of the stainless steel. These effects are insufficient at Ni contents less than 7.5 mass %. At Ni contents more than 11 mass %, some of austenite phase is retained and precipitates, thereby degrading the mechanical strength (such as tensile strength) of the steel. Accordingly, the Ni content is preferably from 7.5 to 11 mass %, more preferably from 8.5 to 10.5 mass % and even more preferably from 9 to 10 mass %.
Al Component:
The Al, too, forms Ni—Al intermetallic compounds, thereby enhancing the precipitation hardening effect on the steel. This effect is insufficient at Al contents less than 0.9 mass %. At Al contents more than 1.7 mass %, Ni—Al intermetallic compounds precipitate excessively and δ-ferrite phase is prone to form, thereby degrading the steel characteristics. Accordingly, the Al content is preferably from 0.9 to 1.7 mass %, more preferably from 1.1 to 1.5 mass % and even more preferably from 1.25 to 1.4 mass %.
Mo Component:
The Mo improves the corrosion resistance of the steel and also increases the mechanical strength (by, for example, solid solution strengthening). These effects are insufficient at Mo contents less than 0.85 mass %. At Mo contents more than 1.35 mass %, δ-ferrite phase formation and/or the excessive formation of Fe-based intermetallic compounds (such as Laves phases) is promoted, thereby degrading the mechanical properties and/or corrosion resistance of the steel. Accordingly, the Mo content is preferably from 0.85 to 1.35 mass %, more preferably from 1 to 1.3 mass % and even more preferably from 1.1 to 1.2 mass %.
W Component:
The W, like the Mo, improves the corrosion resistance of the steel and also increases the mechanical strength (by, for example, solid solution strengthening). These effects are insufficient at W contents less than 1.75 mass %. At W contents more than 2.75 mass %, δ-ferrite phase formation and/or the excessive formation of Fe-based intermetallic compounds (such as Laves phases) is promoted, thereby degrading the mechanical properties and/or corrosion resistance of the steel. Accordingly, the W content is preferably from 1.75 to 2.75 mass %, more preferably from 2 to 2.5 mass % and even more preferably from 2.2 to 2.5 mass %.
The compositional balance between the Mo and the W is the most important parameter for the invention. The preferable Mo—W balance according the invention is as follows: The weighted combined content expressed by “[M content]+0.5×[W content]” is an important parameter and preferably from 1.9 to 2.5 mass % and more preferably from 2 to 2.4 mass %. The Mo/W content ratio is also important and preferably from 0.4 to 0.6. By adjusting the Mo and W contents to this preferable range, a precipitation-hardening martensitic stainless steel providing a more excellent balance among several important mechanical properties than conventional martensitic stainless steels can be achieved. For example, a high mechanical strength (a tensile strength of 1550 MPa or more) and a high toughness (an impact energy absorption of 30 J or more) can be both achieved.
Ti Component:
The Ti forms carbides and intermetallic compounds (e.g., Ni—Ti—Al compounds), thereby precipitation-hardening the stainless steel. Ti carbides are preferentially formed over Cr carbides. As a result, the formation of Cr carbides is suppressed, thereby increasing the corrosion resistance of the stainless steel. The addition of Ti is not essential for the invention, but is preferable because it has some positive effects on the stainless steel of the invention. However, the addition of Ti exceeding 0.4 mass % causes the excessive precipitation of intermetallic compounds and the formation of undesirable phases (such as σ-ferrite phase), thereby degrading the mechanical properties (such as toughness) of the stainless steel. Accordingly, the Ti content is preferably 0.4 mass % or less, more preferably 0.35 mass % or less, and even more preferably 0.3 mass % or less.
Co Component:
The Co suppresses the δ-ferrite phase formation. The Co also adjusts the martensitic transformation temperature, thereby increasing the uniformity of the martensite matrix. The addition of Co is not essential for the invention, but, preferably, the Ni is partially replaced by the Co because the Co has some positive effects on the stainless steel of the invention. In the case when the Co is added, the total content of the Ni and Co is preferably from 7.5 to 11 mass %. However, when the Co content exceeds 3 mass %, some of austenite phase is prone to be retained and the amount of the precipitation of Ni—Al intermetallic compounds decreases, thereby degrading the mechanical strength (such as tensile strength) of the steel. Therefore, the Co content is preferably 3 mass % or less, and more preferably 2.8 mass % or less.
Nb Component:
The Nb forms precipitable carbides, thereby improving the mechanical strength of the stainless steel. The addition of Nb is not essential for the invention, but is preferable because the Nb has some positive effects on the stainless steel. However, when the Nb content exceeds 0.5 mass %, the δ-ferrite phase formation is promoted. Therefore, the Nb content is preferably 0.5 mass % or less, and more preferably 0.45 mass % or less.
V Component:
The V may be substituted for part or all of the Nb. In this case, it is preferable that the total content of V and/or Nb is the same as the preferable Nb content when added alone. Therefore, the total content of Nb and/or V is preferably 0.5 mass % or less, and more preferably 0.45 mass % or less. Although the addition of V is not essential for the invention, a combined addition of V and Nb has an effect of enhancing the precipitation hardening.
Si Component:
The Si functions as a deoxidizing agent when melting the stainless steel. Even a small addition of Si is effective in providing such deoxidizing function. The addition of Si is not essential for the invention, but is preferable because the Si has some positive effects on the stainless steel. Si contents more than 1 mass % result in a relatively strong tendency to form δ-ferrite phase, thus deteriorating the characteristics of the stainless steel. Accordingly, the Si content is preferably 1 mass % or less, more preferably 0.5 mass % or less, and even more preferably 0.25 mass % or less. When the stainless steel is melted by vacuum carbon deoxidation or electroslag remelting, no intentional Si addition is required.
Mn Component:
The Mn functions as deoxidizing and desulfurizing agents when melting the stainless steel. Even a small addition of Mn is effective in providing such deoxidizing and desulfurizing functions. The Mn also has an effect of suppressing the δ-ferrite phase formation. The addition of Mn is not essential for the invention, but is preferable because the Mn has some positive effects on the stainless steel. However, at Mn contents exceeding 1 mass %, some of austenite phase is prone to be retained. Accordingly, the Mn content is preferably 1 mass % or less, more preferably 0.5 mass % or less, and even more preferably 0.25 mass % or less. When melting the stainless steel by vacuum induction melting (VIM) or vacuum arc remelting (VAR), no intentional Mn addition is required.
Inevitable Impurities:
The term “inevitable impurity”, as used herein and in the appended claims, refers to an unintentionally contained material such as one originally contained in a starting material and one contaminated during manufacture. Examples of inevitable impurities are P, S, Sb, Sn, As and N. The precipitation-hardening martensitic stainless steel of the invention unavoidably contains one or more such inevitable impurities.
Reduction of the P and/or S improves the toughness of the stainless steel without sacrificing the mechanical strength; therefore, it is preferable that each of the P and S contents is suppressed to as low as possible. Each of the P and S contents is preferably 0.5 mass % or less and more preferably 0.1 mass % or less in order to increase the toughness.
Reduction of the Sb, Sn and/or As, too, improves the toughness. Therefore, it is also preferable that each of the Sb, Sn and As contents is suppressed to as low as possible. Specifically, each of the Sb, Sn and As contents is preferably 0.1 mass % or less and more preferably 0.05 mass % or less.
The N has a strong affinity for the Al and the Ti and easily forms nitrides (such as AlN and TiN), thereby decreasing the toughness of the stainless steel. Such nitride formation has another adverse effect of reducing the formation of precipitation strengthening intermetallic compounds (such as Ni—Al and Ni—Ti—Al compounds), thereby reducing the mechanical strength of the stainless steel. Therefore, the N content is preferably suppressed to as low as possible. Specifically, the N content is preferably 0.1 mass % or less and more preferably 0.05 mass % or less.
(Manufacturing Method)
Except for heat treatment, there is no particular limitation on the method for manufacturing the precipitation-hardening martensitic stainless steel of the invention, but any conventional method may be used. The preferred heat treatment according to the invention will be described below.
First, an un-heat-treated stainless steel is solution heat treated by heating the steel to a temperature from 800 to 1000° C. (more preferably from 850 to 950° C.), maintaining it at this temperature, and then quenching it. By this solution heat treatment, constituent chemical elements (such as Ni, Al and Ti) of precipitable compounds are dissolved in the austenitic matrix to form solid solutions during the heating, and the austenitic matrix is then transformed to the martensitic matrix by the quenching.
Preferably, the solution heat-treated steel is aging heat treated by heating it to a temperature from 450 to 650° C. (more preferably from 500 to 600° C.), maintaining it at this temperature, and then cooling it slowly. By this aging heat treatment, intermetallic compounds (such as β-NiAl phase) and carbides are formed and precipitated. By these solution and aging heat treatments, a precipitation-hardening martensitic stainless steel having a desirable fine structure, in which fine precipitates are dispersed throughout a homogeneous martensite matrix, can be achieved.
More preferably, after the solution and aging heat treatments, a subzero treatment is performed in order to reduce the retained austenite. The subzero treatment of the invention involves cooling a steel to a temperature lower than room temperature and maintaining it at this temperature in order to transform any retained austenite to martensite. The subzero treatment of the invention involves, for example, cooling a steel to −70° C. or lower using a coolant (such as dry ice and liquid nitrogen) and an organic solvent (such as isopentane), and maintaining it at this temperature for 4 hours or longer.
When forming a turbine component from the invention's martensitic stainless steel, the component forming may be performed after carrying out all of the above heat treatments. Alternatively and preferably, the component forming may be conducted at an intermediate process stage between the solution heat treatment and the aging heat treatment. This is because at such a process stage, any precipitable materials (such as Ni—Al compounds) have not yet been precipitated, and therefore the stainless steel is easier to be deformed and handled. In the latter case, the aging heat treatment can be carried out after the component forming.
(Turbine Component)
Because the precipitation-hardening martensitic stainless steel of the invention has both good mechanical properties and a good corrosion resistance, it can be advantageously applied to turbine components (e.g., steam turbine long blades of 50 inches or more and gas turbine compressor blades). FIG. 1 is a schematic illustration showing a perspective view of an exemplary steam turbine long blade formed of the invention's stainless steel. As illustrated in FIG. 1, a steam turbine long blade 10 is of an axial entry type. The steam turbine long blade 10 includes a blade profile section 11 (on which high-speed steam impinges) and a blade root section 12. In order to connect adjacent blades 10, a stub 14 is formed at a central position of the profile section 11 and a shroud 15 is formed along the end edge of the profile section 11. An erosion shield 13 is formed on a leading edge portion of the profile section 11 in order to protect the profile section 11 from erosion caused by impingement of high-speed steam containing liquid water particles. The erosion shield 13 is optional depending on the severity of the erosive environment. That is, the erosion shield 13 may not necessarily be equipped when the erosion is not very severe, because the invention's steel has a sufficient erosion resistance.
An example of the erosion shield 13 is a Co based alloy plate (e.g., a stellite (registered trademark) plate). The stellite plate can be bonded to the profile section 11 by TIG welding, electron beam welding, brazing or the like. Preferably, after the bonding of the erosion shield 13, a stress removal (SR) heat treatment is performed at 550 to 650° C. (more preferably 570 to 630° C.) to remove residual stresses potentially causing cracks. Another way to protect the profile section 11 from erosion is a surface hardening technique, which involves hardening the surface of a leading edge portion of the profile section 11 by local heating using a high heat input laser or the like.
(Turbine)
FIG. 2 is a schematic illustration showing a longitudinal sectional view of an example of a turbine according to the invention. As illustrated in FIG. 2, a low-pressure stage steam turbine 20 mainly includes a turbine rotor 21 rotatable by an operating fluid (steam) passing therethrough and a turbine casing 25 housing the turbine rotor 21. The turbine rotor 21 includes a rotation shaft 22 and a plurality of axially spaced rotor disks 23 on the rotation shaft 22. Each rotor disk 23 has, on its circumference, a plurality of circumferentially spaced radially oriented turbine long blades 10. The turbine long blades 10 are typically so designed as to increase in length with increasing distance downstream of the flow of the operating fluid. The turbine casing 25 includes a plurality of vanes 26, an operating fluid inlet 27, and an operating fluid outlet 28. The vanes 26 are bonded to the inner wall of the turbine casing 25 in such a way that each vane 26 is positioned between adjacent turbine long blades 10.
(Thermal Power Plant)
FIG. 3 is a system diagram of an example of a thermal power plant according to the invention. As shown in FIG. 3, in a thermal power plant 30, high-temperature, high-pressure steam (operating fluid) is produced in a boiler 31, generates mechanical power in a high-pressure stage turbine 32, and is reheated in the boiler 31. Then, the reheated steam generates an additional mechanical power in an intermediate-pressure stage turbine 33, and generates a yet additional mechanical power in a low-pressure stage turbine 20. The mechanical power generated in these steam turbines is converted into an electrical power by an electric generator 34. The steam exiting the low-pressure stage turbine 20 is introduced into a steam condenser 35 to condense the steam back into water, which is returned to the boiler 31.

Examples

The present invention will be described in more detail below by way of examples. However, the invention is not limited to the specific examples below.
(Preparation of Inventive Stainless Steels 1 to 12 and Comparative Stainless Steels 1 to 17)
First, various steel ingots having different compositions were prepared by melting different sets of starting materials in a vacuum high-frequency induction melting furnace (5.0×10⁻³Pa or lower) at a temperature of 1600° C. or higher. Each steel ingot was hot-forged into a rectangular bar (100 mm wide, 30 mm thick, 1000 mm long) using a 1000-ton forging machine and a 250-kgf hammer forging machine. Next, the rectangular bar was further cut into a starting (un-heat-treated) sample bar (50 mm wide, 30 mm thick, 120 mm long).
The sample bars were subjected to the following heat treatments using a box furnace: First, each sample bar was solution heat treated by maintaining the sample bar at 900° C. for 1 hour and then dipping it into room temperature water (water-quenching). Then, the sample bar was aging heat treated by maintaining it at 538° C. for 2 hours and then air-cooling it in room temperature air. Any subzero treatment was not performed.
Tables 1 to 5 show the results of the chemical composition analysis of the above-described stainless steel ingots. Although, except for the N, the contents of inevitable impurities (the P, S, Sb, Sn and As) are not shown in Tables, these inevitable impurities contents were within the range specified by the invention.

TABLE 1

Chemical Composition of Inventive Stainless Steels 1 to 6.

(Unit: mass %)

	Inventive	Inventive	Inventive	Inventive	Inventive	Inventive
	Stainless	Stainless	Stainless	Stainless	Stainless	Stainless
	Steel
1	Steel 2	Steel 3	Steel 4	Steel 5	Steel 6

C	0.01	0.01	0.01	0.01	0.03	0.01
Cr	12.08	11.08	12.90	12.14	12.02	12.10
Ni	9.51	9.52	9.49	7.61	10.91	9.44
Al	1.30	1.32	1.31	1.34	1.31	0.92
Mo	1.15	1.14	1.16	1.15	1.13	1.13
W	2.27	2.24	2.27	2.25	2.26	2.25
Ti	—	—	—	—	—	—
Co	—	—	—	—	—	—
Nb + V	—	—	—	—	—	—
Si	—	—	—	—	—	—
Mn	—	—	—	—	—	0.15
N	—	—	—	—	0.01	—

Fe +	Balance
Inevitable
Impurities

Mo + 0.5W	2.29	2.26	2.30	2.28	2.26	2.26
Mo/W	0.51	0.51	0.51	0.51	0.50	0.50

Note 1:
Mark “—” means that the element was not intentionally added or the element was below detection limit.
Note 2:
“Inevitable Impurities” are P, S, Sb, Sn and As.

TABLE 2

Chemical Composition of Inventive Stainless Steels 7 to 12.

(Unit: mass %)

Inventive	Inventive	Inventive	Inventive	Inventive	Inventive
Stainless	Stainless	Stainless	Stainless	Stainless	Stainless
Steel 7	Steel 8	Steel 9	Steel 10	Steel 11	Steel 12

C	0.01	0.01	0.01	0.01	0.01	0.01
Cr	11.94	12.04	11.89	12.01	12.01	12.04
Ni	9.45	9.50	9.53	9.52	8.01	9.38
Al	1.63	1.28	1.30	1.31	1.28	1.33
Mo	1.14	1.01	1.24	1.13	1.14	1.11
W	2.25	2.02	2.44	2.23	2.22	2.22
Ti	—	—	—	0.30	—	—
Co	—	—	—	—	1.52	—
Nb + V	—	—	—	—	—	0.44
Si	0.23	—	—	—	—	—
Mn	—	—	—	—	—	—
N	—	—	—	—	—	—

Fe +	Balance
Inevitable
Impurities

Mo + 0.5W	2.27	2.02	2.46	2.25	2.30	2.22
Mo/W	0.51	0.50	0.51	0.51	0.53	0.50

Note 1:
Mark “—” means that the element was not intentionally added or the element was below detection limit.
Note 2:
“Inevitable Impurities” are P, S, Sb, Sn and As.

TABLE 3

Chemical Composition of Comparative Stainless Steels 1 to 6.

(Unit: mass %)

	Comparative	Comparative	Comparative	Comparative	Comparative	Comparative
	Stainless	Stainless	Stainless	Stainless	Stainless	Stainless
	Steel
1	Steel 2	Steel 3	Steel 4	Steel 5	Steel 6

C	0.16	0.01	0.01	0.01	0.01	0.01
Cr	12.11	10.48	13.58	12.00	11.98	11.88
Ni	9.51	9.44	9.47	7.03	11.51	9.53
Al	1.32	1.33	1.31	1.35	1.30	0.75
Mo	1.10	1.13	1.14	1.15	1.15	1.15
W	2.25	2.25	2.26	2.27	2.27	2.26
Ti	—	—	—	—	—	—
Co	—	—	—	—	—	—
Nb + V	—	—	—	—	—	—
Si	—	—	—	—	—	—
Mn	—	—	—	—	—	—
N	—	—	—	—	—	—

Fe +	Balance
Inevitable
Impurities

Mo + 0.5W	2.23	2.26	2.27	2.29	2.29	2.28
Mo/W	0.49	0.50	0.50	0.51	0.51	0.51

Note 1:
Mark “—” means that the element was not intentionally added or the element was below detection limit.
Note 2:
“Inevitable Impurities” are P, S, Sb, Sn and As.

TABLE 4

Chemical Composition of Comparative Stainless Steels 7 to 12.

(Unit: mass %)

Comparative	Comparative	Comparative	Comparative	Comparative	Comparative
Stainless	Stainless	Stainless	Stainless	Stainless	Stainless
Steel 7	Steel 8	Steel 9	Steel 10	Steel 11	Steel 12

C	0.01	0.01	0.01	0.01	0.01	0.01
Cr	12.04	12.08	12.03	12.01	12.05	12.10
Ni	9.50	9.55	9.52	9.50	9.53	4.44
Al	2.02	1.28	1.33	1.34	1.25	1.27
Mo	1.11	2.21	1.33	0.73	—	1.15
W	2.23	—	1.75	2.92	4.43	2.25
Ti	—	—	—	—	—	—
Co	—	—	—	—	—	5.03
Nb + V	—	—	—	—	—	—
Si	—	—	—	—	—	—
Mn	—	—	—	—	—	—
N	—	—	—	—	—	—
Fe +	Balance
Inevitable
Impurities
Mo + 0.5W	2.23	2.21	2.21	2.19	2.22	2.28
Mo/W	0.50	Mo only	0.75	0.25	W only	0.51

Note 1:
Mark “—” means that the element was not intentionally added or the element was below detection limit.
Note 2:
“Inevitable Impurities” are P, S, Sb, Sn and As.

TABLE 5

Chemical Composition of Comparative Stainless Steels 13 to 17.

(Unit: mass %)

	Comparative	Comparative	Comparative	Comparative	Comparative
	Stainless	Stainless	Stainless	Stainless	Stainless
	Steel
13	Steel 14	Steel 15	Steel 16	Steel 17

C	0.01	0.01	0.01	0.01	0.01
Cr	11.97	11.88	12.08	12.01	12.04
Ni	9.47	9.52	9.53	9.44	9.50
Al	1.31	1.32	1.31	1.31	1.32
Mo	1.16	1.13	1.14	1.15	1.17
W	2.26	2.25	2.24	2.24	2.20
Ti	—	0.51	—	—	—
Co	—	—	—	—	—
Nb + V	1.0	—	—	—	—
Si	—	—	1.24	—	—
Mn	—	—	—	1.23	—
N	—	—	—	—	0.15

Fe +	Balance
Inevitable
Impurities

Mo + 0.5W	2.29	2.26	2.26	2.27	2.27
Mo/W	0.51	0.50	0.51	0.51	0.53

Note 1:
Mark “—” means that the element was not intentionally added or the element was below detection limit.
Note 2:
“Inevitable Impurities” are P, S, Sb, Sn and As.

(Steel Property Measurement and Evaluation)
Each of the above samples (Inventive Stainless Steels 1 to 12 and Comparative Stainless Steels 1 to 17) was observed or measured for: the microtexture; the room temperature 0.02% proof stress and the room temperature tensile strength (as representatives of the mechanical strength); the percent elongation and the percent reduction in area (as representatives of the ductility); the room temperature impact energy absorption (as a representative of the toughness); and the pitting potential (as a representative of the corrosion resistance). These observation and measurement methods will be briefly explained below.
The microtexture of the starting sample bars was observed with an optical microscope. When a starting sample bar has a martensite matrix in which the content of δ-ferrite phase precipitation does not exceed 1.0% and the content of retained austenite phase precipitation does not exceed 10%, the sample bar is rated as “Pass” in terms of the microtexture. A sample bar that does not satisfy the above criterion is rated as “Fail”. The content of δ-ferrite phase precipitation was measured according to the inclusion rating defined in JIS G 0555. The content of retained austenite phase precipitation was measured by X ray diffraction method.
For the 0.02% proof stress and tensile strength measurements, the above-described staring sample bars were further machined to form a tensile test rod (gauge length of 30 mm; outer diameter of 6 mm). Each tensile test rod was measured for the 0.02% proof stress and the tensile strength at room temperature according to the test methods defined in JIS Z 2241. A tensile test rod having a 0.02% proof stress of 1000 MPa or more is rated as “Pass” in terms of the 0.02% proof stress; a tensile test rod that does not satisfy this criterion is rated as “Fail”. A tensile test rod having a tensile strength of 1550 MPa or more is rated as “Pass” in terms of the tensile strength; a tensile test rod that does not satisfy this criterion is rated as “Fail”. A tensile test rod having an elongation of 10% or more is rated as “Pass” in terms of the percent elongation; a tensile test rod that does not satisfy this criterion is rated as “Fail”. A tensile test rod having a reduction in area of 30% or more is rated as “Pass” in terms of the percent reduction in area; a tensile test rod that does not satisfy this criterion is rated as “Fail”.
For the impact energy absorption measurement, the above-described starting sample bars were further machined to form a Charpy test piece having a 2 mm V-notch. Each test piece was subjected to a Charpy impact test at room temperature according to JIS Z 2242. A test piece having an impact energy absorption of 30 J or more is rated as “Pass” in terms of the impact energy absorption; a test piece that does not satisfy this criterion is rated as “Fail”.
For the pitting potential measurement, the above-described starting sample bars were further machined to form a flat-plate test piece (15 mm long, 15 mm wide and 3 mm thick). Then, the entire surface of each flat-plate test piece was insulation coated except for the measurement surface (area of 1.0 cm²). The pitting potential measurement was performed in a 30° C., 3.0% aqueous NaCl solution at a sweep rate of 20 mV/min. A flat-plate test piece having a pitting potential of 150 mV or more is rated as “Pass” in terms of the pitting potential; a test piece that does not satisfy this criterion is rated as “Fail”.
The observation and measurement results are summarized in Tables 6 to 10.

TABLE 6

Observation and Measurement Results of Inventive Stainless Steels 1 to 6.

Microtexture	Pass	Pass	Pass	Pass	Pass	Pass
0.02% Proof	Pass	Pass	Pass	Pass	Pass	Pass
Stress
Tensile	Pass	Pass	Pass	Pass	Pass	Pass
Strength
Percent	Pass	Pass	Pass	Pass	Pass	Pass
Elongation
Percent	Pass	Pass	Pass	Pass	Pass	Pass
Reduction
in Area
Impact	Pass	Pass	Pass	Pass	Pass	Pass
Energy
Absorption
Pitting	Pass	Pass	Pass	Pass	Pass	Pass
Potential

TABLE 7

Observation and Measurement Results of Inventive Stainless Steels 7 to 12.

TABLE 8

Observation and Measurement Results of Comparative Stainless Steels 1 to 6.

Microtexture	Fail	Pass	Fail	Pass	Fail	Pass
0.02% Proof	Fail	Pass	Pass	Pass	Fail	Fail
Stress
Tensile	Fail	Pass	Pass	Pass	Fail	Fail
Strength
Percent	Pass	Pass	Fail	Fail	Pass	Pass
Elongation
Percent	Pass	Pass	Fail	Fail	Pass	Pass
Reduction
in Area
Impact	Pass	Pass	Fail	Fail	Pass	Pass
Energy
Absorption
Pitting	Fail	Fail	Pass	Pass	Pass	Pass
Potential

TABLE 9

Observation and Measurement Results of Comparative Stainless Steels 7 to 12.

Microtexture	Pass	Pass	Pass	Pass	Pass	Fail
0.02% Proof	Pass	Fail	Fail	Pass	Pass	Fail
Stress
Tensile	Pass	Fail	Fail	Pass	Pass	Fail
Strength
Percent	Fail	Pass	Pass	Pass	Pass	Fail
Elongation
Percent	Fail	Pass	Pass	Pass	Pass	Fail
Reduction
in Area
Impact	Fail	Pass	Pass	Fail	Fail	Fail
Energy
Absorption
Pitting	Pass	Pass	Pass	Pass	Pass	Pass
Potential

TABLE 10

Observation and Measurement Results of Comparative Stainless Steels 13 to 17.

	Comparative	Comparative	Comparative	Comparative	Comparative
	Stainless	Stainless	Stainless	Stainless	Stainless
	Steel
13	Steel 14	Steel 15	Steel 16	Steel 17

Microtexture	Pass	Fail	Fail	Fail	Fail
0.02%	Pass	Pass	Fail	Fail	Fail
Proof Stress
Tensile	Pass	Pass	Fail	Fail	Fail
Strength
Percent	Fail	Pass	Fail	Pass	Pass
Elongation
Percent	Fail	Pass	Fail	Pass	Pass
Reduction
in Area
Impact Energy	Fail	Fail	Fail	Pass	Pass
Absorption
Pitting	Pass	Pass	Pass	Pass	Pass
Potential

As shown in Tables 6 and 7, Inventive Stainless Steels 1 to 12 have a martensite matrix containing only small amounts of δ-ferrite phase precipitates and retained austenite phase precipitates. In addition, for all of Inventive Stainless Steels 1 to 12, fine β-NiAl phase precipitates of 10 nm or smaller in particle size are uniformly dispersed in each martensite grain. Also, all of Inventive Steels are rated as “Pass” in terms of: the mechanical properties including the mechanical strength (0.02% proof stress and tensile strength); the ductility (the percent elongation and the percent reduction in area); and the toughness (the impact energy absorption). Further, all of Inventive Steels have a good corrosion resistance (a high pitting potential). It is thus demonstrated from the above results that the precipitation-hardening martensitic stainless steel of the invention has, compared with conventional ones, a far better balance among a high mechanical strength, a high ductility, a high toughness and a high corrosion resistance.
By contrast, Comparative Stainless Steels 1 to 17 are rated as “Fail” in terms of at least one of the micro texture, mechanical strength, ductility, toughness and corrosion resistance. That is, Comparative Stainless Steels do not satisfy all of these performances. Specifically, Comparative Stainless Steel 1 is rated as “Fail” in terms of the microtexture, mechanical strength and corrosion resistance, because the C content falls out of the invention's specification range.
Comparative Stainless Steel 2 is rated as “Fail” in terms of the corrosion resistance, because the Cr content falls below the invention's specification. Comparative Stainless Steel 3 is rated as “Fail” in terms of the microtexture, ductility and toughness, because the Cr content exceeds the invention's specification.
Comparative Stainless Steel 4 is rated as “Fail” in terms of the ductility and toughness, because the Ni content falls below the invention's specification. Comparative Stainless Steel 5 is rated as “Fail” in terms of the microtexture and mechanical strength, because the Ni content exceeds the invention's specification.
Comparative Stainless Steel 6 is rated as “Fail” in terms of the mechanical strength, because the Al content falls below the invention's specification. Comparative Stainless Steel 7 is rated as “Fail” in terms of the ductility and toughness, because the Al content exceeds the invention's specification.
Comparative Stainless Steel 12 is rated as “Fail” in terms of the microtexture, mechanical strength, ductility and toughness, because the amount of substitution of Ni by Co exceeds the invention's specification. Comparative Stainless Steel 13 is rated as “Fail” in terms of the ductility and toughness, because the total content of Nb and V exceeds the invention's specification. Comparative Stainless Steel 14 is rated as “Fail” in terms of the microtexture and toughness, because the Ti content exceeds the invention's specification. Comparative Stainless Steel 15 is rated as “Fail” in terms of the microtexture, mechanical strength, ductility and toughness, because the Si content exceeds the invention's specification. Comparative Stainless Steel 16 is rated as “Fail” in terms of the microtexture and mechanical strength, because the Mn content exceeds the invention's specification. Comparative Stainless Steel 17 is rated as “Fail” in terms of the microtexture and mechanical strength, because the N content exceeds the invention's specification.
Next, the invention's preferred compositional balance between the Mo and the W will be discussed by comparing the measurement results of Inventive Stainless Steel 1 and Comparative Stainless Steels 8 to 11. FIG. 4 is a graph showing a relationship between the “[Mo content]/[W content]” ratio and the tensile strength. FIG. 5 is a graph showing a relationship between the “[Mo content]/[W content]” ratio and the impact energy absorption.
FIG. 4 shows that as the “[Mo content]/[W content]” ratio decreases (i.e., the W content increases relative to the Mo content), the tensile strength increases. This is probably due to the positive effect of solid solution strengthening by the W and formation of Laves (Fe₂W) phase. Microtexture observation of Comparative Stainless Steel 11 showed that Laves phase was actually formed and precipitated.
As for the impact energy absorption, as shown in FIG. 5, as the “[Mo content]/[W content]” ratio decreases (i.e., the W content increases relative to the Mo content), the impact energy absorption decreases. In this case, however, an excessive formation of Laves phase probably has an adverse effect of significantly degrading the toughness of the W-rich stainless steels. The results shown in FIGS. 4 and 5 confirm that “[Mo content]/[W content]” ratios from 0.4 to 0.6 are desirable in order to achieve an excellent balance between a high mechanical strength and a high toughness.
(Steam Turbine Long Blade)
A 51-inch steam turbine long blade (see FIG. 1) was formed from Invention Stainless Steel 1 as follows: First, Inventive Stainless Steel 1 was subjected to a vacuum carbon deoxidation, which involves melting and deoxidizing the steel in a high vacuum of 5.0×10⁻³Pa by utilizing the chemical reaction of “C+O→CO”. Next, the deoxidized steel was formed into an electrode rod by extend forging. Then, the electrode rod was subjected to an electroslag remelting, which involves immersing the rod in a molten slag, melting it by the Joule's heat generated by current flow through it, and resolidifying it in a water cooled mold. By this electroslag remelting, a high-quality steel ingot was obtained.
The steel ingot was hot-forged, and then closed-die forged into a 51-inch long blade. The die-formed long blade was solution heat treated by maintaining it at 900° C. for 2 hours and quenching it by forced cooling using a blower, and was then formed into a final shape by cutting. Next, the final shaped blade was aging heat treated by maintaining it at 538° C. for 4 hours and cooling it in air. Finally, finish processing (such as straightening and surface polishing) was performed to complete the formation of the 51-inch long blade.
A test specimen was cut out from each of an end portion, a center portion and a root portion of the thus formed steam turbine long blade in such a manner that the length direction of the test specimen was parallel to the length direction of the blade. Then, each test specimen was subjected to the above-described observation and measurements. All of the blade portions are rated as “Pass” in terms of all of the microtexture; the mechanical strength (the 0.02% proof stress and the tensile strength); the ductility (the percent elongation and the percent reduction in area); the toughness (the impact energy absorption); and the corrosion resistance (the pitting potential).
As has been described, the precipitation-hardening martensitic stainless steel of the present invention has a homogeneous martensite matrix throughout which intermetallic compound precipitates are uniformly dispersed. Also, the invention's stainless steel has a far better balance between a high mechanical strength and a high corrosion resistance than conventional stainless steels. Thus, the invention's stainless steel can be advantageously applied to longer-than-conventional steam turbine long blades.
Also, there can be provided: a high performance turbine rotor including such a longer-than-conventional steam turbine long blade; a high performance steam turbine including such a high performance turbine rotor; and a high performance thermal power plant including such a high performance steam turbine. Further, the precipitation-hardening martensitic stainless steel of the invention can be used for components (such as blades) of other type turbines such as gas turbine compressors.
Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.

Claims

What is claimed is:

1. A precipitation-hardening martensitic stainless steel throughout which precipitates of intermetallic compounds are dispersed, the martensitic stainless steel comprising: 0.1 mass % or less of C; 11 to 13 mass % of Cr; 7.5 to 11 mass % of Ni; 0.9 to 1.7 mass % of Al; 0.85 to 1.35 mass % of Mo; 1.75 to 2.75 mass % of W; and the balance including Fe and inevitable impurities, wherein “[content of the Mo]+0.5×[content of the W]” is from 1.9 mass % to 2.5 mass %, and “[content of the Mo]/[content of the W]” is from 0.4 to 0.6.

2. The precipitation-hardening martensitic stainless steel according to claim 1, further including 0.4 mass % or less of Ti.

3. The precipitation-hardening martensitic stainless steel according to claim 1, wherein part of the Ni is substituted by 3 mass % or less of Co.

4. The precipitation-hardening martensitic stainless steel according to claim 1, further including one or both of Nb and V in total amount of 0.5 mass % or less.

5. The precipitation-hardening martensitic stainless steel according to claim 1, further including 0.1 mass % or less of Si and/or 1 mass % or less of Mn.

6. The precipitation-hardening martensitic stainless steel according to claim 1, wherein the inevitable impurities include one or more of 0.5 mass % or less of P, 0.5 mass % or less of S, 0.1 mass % or less of Sb, 0.1 mass % or less of Sn, 0.1 mass % or less of As, and 0.1 mass % or less of N.

7. The precipitation-hardening martensitic stainless steel according to claim 1, wherein one of the intermetallic compounds is β-NiAl phase.

8. The precipitation-hardening martensitic stainless steel according to claim 1, wherein the precipitation-hardening martensitic stainless steel is solution heat treated at 850 to 950° C. followed by aging heat treatment at 450 to 650° C.

9. A turbine component formed of the precipitation-hardening martensitic stainless steel according to claim 1.

10. A turbine rotor including the turbine component according to claim 9, wherein the turbine component is a steam turbine long blade.

11. A steam turbine including the turbine rotor according to claim 10.

12. A thermal power plant including the steam turbine according to claim 11.

13. A turbine component formed of the precipitation-hardening martensitic stainless steel according to claim 2.

14. A turbine component formed of the precipitation-hardening martensitic stainless steel according to claim 3.

15. A turbine component formed of the precipitation-hardening martensitic stainless steel according to claim 4.

16. A turbine component formed of the precipitation-hardening martensitic stainless steel according to claim 5.