JP2004052673A - Erosion shield method for turbine blade, and erosion shielded turbine blade - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an erosion shield method for a low-cost and long-life turbine blade for reinforcing a part easily subjected to erosion without stress erosion cracks and delay breakage cracks, and an erosion-shielded turbine blade. <P>SOLUTION: This erosion shield method includes: a standard heat treatment process of performing solution heat treatment for the whole of the turbine blade formed of precipitation hardening type martensitic stainless steel and then performing aging treatment; and a local hardening heat treatment process of locally rapid-heating only the blade tip part of the turbine blade above the point Ar<SB>1</SB>after the standard heat treatment process, holding the same in a complete austenitic range for a designated time and then rapid-quenching the same to the room temperature, and subsequently aging the same above the point Ar<SB>3</SB>to generate residual stress of compression in the surface layer of the blade tip part. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a turbine blade of a low pressure turbine of a steam turbine, and more particularly to an erosion shield method and an erosion shielded turbine blade in a steam turbine.
[0002]
[Prior art]
Turbine blades of a steam turbine are placed in a harsh environment in direct contact with steam, and suffer significant heat damage such as erosion due to dew condensation. When the erosion causes the turbine blades to lose thickness, not only the strength is insufficient, but also cracks may be generated from the damaged erosion. Therefore, an erosion shield material is attached to a portion that is susceptible to erosion, thereby increasing the resistance to erosion and extending the life of the entire turbine blade.
[0003]
A plate-like stellite is attached to the erosion shield of a conventional low-pressure turbine blade (base material: 17-4PH steel) by silver brazing.
[0004]
[Problems to be solved by the invention]
However, the quality of the erosion shield formed by pasting a stellite plate has a large variation in quality.
[0005]
In addition, since stellite (Co—Cr—W alloy) is an expensive material, the manufacturing cost increases.
[0006]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and is a low-cost and long-life erosion of a turbine blade capable of strengthening an erosion-sensitive portion without causing stress corrosion cracking and delayed fracture cracking. It is an object to provide a shielding method and an erosion shielded turbine blade.
[0007]
[Means for Solving the Problems]
The present inventors have conducted intensive studies on hardening the erosion shield target part itself for the purpose of improving the erosion resistance of the turbine blade material. As a result, precipitation hardening type martensite such as 17-4PH steel was obtained. In the case of system stainless steel, when the hardness is increased without any limitation, the following two problems occur. Therefore, attention was paid to the fact that these problems need to be solved.
[0008]
(1) First, when gradually increasing the hardness of this type of steel, 7 ^{(Source; M.Tsubota et.al.; 4 th International Symposium} on Environmental Degradationof Materials in Nuclear Power Systems-Water Reactor, ( 1989), 9-66), there is a problem that the rate of occurrence of stress corrosion cracking (SCC) sharply increases in a region where the hardness of the material exceeds 340 in Vickers hardness Hv. The vertical axis in the figure indicates the SCC crack depth (μm). In the figure, the black triangle (shown as 630) is 17-4PH steel, the black circle (shown as 431) is 17Cr steel, the cross (shown as CW304) is SUS304 stainless steel, and the others (403, 420J1, CA40, F6NM). Indicates 12Cr steel. Incidentally, the SCC test is an accelerated test in which the sample is immersed in 40 ppm DO ₂ water at a temperature of 288 ° C. for 500 hours under a tensile stress condition by a CBB method (Created Bent Beam test). From this figure, it is clear that when the hardness of the material exceeds Hv340, the incidence of stress corrosion cracking increases significantly.
[0009]
(2) Secondly, as the hardness of this type of steel is increased, as shown in FIG. 8 (Source: Shinsaku Matsumoto, “Delayed Crack Fracture”, Nikkan Kogyo Shimbun (1989)) However, there is a problem that the delayed fracture strength sharply decreases in a region where the Vickers hardness exceeds 350 Hv. In addition, the vertical axis | shaft of a figure shows the delay fracture strength ratio which made the intensity | strength of the defect-free material which does not have a crack the reference value 1.0, and made the intensity | strength of the test material the index with respect to this. In the figure, white circles indicate 0.2C-Cr steel, black circles indicate 0.2C-Si-Mn-Cr steel, white triangles indicate 0.4C-Cr steel, and black triangles indicate 0.37C-Cr-Mo steel. , And the open squares indicate 0.36C-Cr-Mo-V steel, respectively. Incidentally, in the delayed fracture cracking test, an acceleration test was performed while a 0.1% NaCl aqueous solution was dropped on a φ6 mm diameter round bar having a notch of 1 mm. From this figure, it is apparent that when the hardness of the material exceeds 350 Hv, the delayed fracture strength ratio sharply decreases, and the occurrence rate of delayed fracture increases significantly.
[0010]
All of these stress corrosion cracks and delayed crack fractures occur under tensile stress. For the above reasons, precipitation hardening martensitic stainless steel, for example, 17-4PH is generally used with a Vickers hardness Hv of 340 or less. Such a 17-4PH steel is a precipitation-hardening stainless steel whose strength is enhanced by a Cu precipitation phase, and therefore is an aging heat treatment essential steel type. The composition of a typical 17-4PH steel is, by mass%, C; 0.05%, Si; 0.3%, Mn; 0.5%, Cu; 4%, Ni; 4%, Cr; , Nb + Ta; 0.3%, with the balance being Fe.
[0011]
Therefore, the present inventors have taken measures to improve the erosion resistance by increasing the hardness of the surface layer without inducing stress corrosion cracking or delayed cracking in precipitation-hardened martensitic stainless steel. Various studies were made. As a result, the present invention described below has been completed.
[0012]
An erosion shield method for a turbine blade according to the present invention is an erosion shield method for a turbine blade for improving the resistance of the turbine blade to erosion, wherein the entire turbine blade made of precipitation-hardened martensitic stainless steel is solution-solutioned. A standard heat treatment step of aging treatment after the treatment, and after the standard heat treatment step, only the blade tip portion of the turbine blade is locally heated to ₁ point or more of Ar, kept for a predetermined time, rapidly cooled to room temperature, and subsequently aged at ₁ point or less of Ar And a local hardening heat treatment step of generating a compressive residual stress on the surface layer of the blade tip portion.
[0013]
In the above-described local hardening heat treatment step, it is preferable to locally heat the tip portion of the blade using a high-frequency heating method, a laser heating method, or a burner heating method. This is because these heating means have a high energy density and a large amount of heat input per unit area, so that they are suitable for rapid heating and can reach the target temperature in a short time. The heat input amount is adjusted by controlling the power supply amount. However, it is desirable to actually measure the temperature of the heating target portion using a contact type or non-contact type temperature sensor and manage the temperature. For such a temperature sensor, it is preferable to use a thermocouple and a Pt resistance thermometer in the case of a contact type, and it is preferable to use a radiation thermometer in the case of a non-contact type.
[0014]
In the above-mentioned local hardening heat treatment step, it is preferable that the cooling rate of the blade tip portion after the heating and holding is made higher than that of the surrounding portions. This is because the greater the cooling rate, the greater the amount of martensitic phase transformation, the greater the volume expansion, and the greater the residual stress in compression. Here, it is desirable that the residual compressive stress generated on the surface layer at the tip portion of the blade be an appropriate magnitude according to the strength of the base material. This is because stress corrosion cracking and delayed fracture cracking substantially do not occur in the precipitation hardening type martensitic stainless steel in the presence of a compressive residual stress of an appropriate size.
[0015]
In addition, the holding time of the local heating is set to a time sufficient for the portion to completely undergo austenite transformation. 5 seconds or more is recommended as such an appropriate holding time.
[0016]
The erosion shielded turbine blade according to the present invention is made of precipitation-hardened martensitic stainless steel, and after solution treatment, after the whole is aged, locally heats only the blade tip portion to _one or more Ar points, It is characterized by having a hardened part in which a residual stress of compression exists in the surface layer at the tip of the blade by quenching to room temperature after holding for a predetermined time, and then aging at a point of Ar ₁ point or less.
[0017]
Next, the erosion shield method of the present invention and its operation will be described with reference to FIGS.
[0018]
As shown in FIG. 1, the wing material made of a forged product of precipitation-hardened martensitic stainless steel (17-4PH steel) is subjected to standard heat treatment, and then only the wing tip portion has a temperature of Ar ₁ point or more ( Local heating to about 1000 ° C.), rapid cooling after holding for a predetermined time, followed by rapid cooling after heating to a temperature immediately below Ar ₁ point (about 450 ° C.), holding for a predetermined time, and then rapidly cooling.
[0019]
Since the part subjected to the local hardening heat treatment undergoes a martensitic phase transformation, as shown in FIGS. 5 and 6, the volume after heating is larger than that before heating, and as a result, residual stress due to compression is generated. For this reason, even if the hardness of the erosion shield portion is set to Vickers hardness Hv of 340 or more (for example, Hv = 400), stress corrosion cracking and delayed fracture cracking do not occur.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the present embodiment, a case where an erosion shield is performed on a turbine rotor blade used in a low-pressure steam turbine engine will be described as an example.
First, the steam turbine engine will be described with reference to FIG. The steam turbine engine mainly includes a rotor 6, a casing (casing) 7, and a turbine blade 10.
[0021]
The stationary blades 8 and the moving blades 9 constituting the turbine blade 10 are alternately arranged in a plurality of stages. The moving blades 9 are mounted around the rotor 6, and the stationary blades 8 are fixed around the casing (vehicle compartment) 7 side. Have been. The casing 7 is provided so as to surround the gap between the stationary blade 8 and the moving blade 9 from outside so that steam does not leak to the outside through the gap between the stationary blade 8 and the moving blade 9. The inflowing steam flows through a passage formed by the stationary blades 8 and the moving blades 9, expands and rotates the rotor 6 via the moving blades 9, and uses the rotating power for operation of a generator or the like.
[0022]
As shown in FIG. 3, the turbine rotor blades 9 are provided on a blade main body 91 extending in the circumferential direction, a platform 92 provided on a base on an inner peripheral side of the blade main body 91, and provided on an inner peripheral side of the platform 92. This is a forged product in which the fixed part 93 is integrated. At a part of the tip of the wing body 91, an erosion shield part harder than the surroundings is formed.
[0023]
As shown in FIGS. 3 and 4, an erosion shield portion 96 is formed at the tip of the wing body 91. The erosion shield portion 96 is harder (more than Vickers hardness Hv400) than the surrounding portion (less than Vickers hardness Hv340) and has a compressive residual stress of an appropriate magnitude.
[0024]
Next, the case where the above-mentioned erosion shield portion is formed will be described with reference to the thermal history diagram of FIG.
[0025]
First, the turbine blade 9 made of a forged product of a precipitation hardening type martensitic stainless steel (17-4PH steel) is subjected to a standard heat treatment.
[0026]
Furthermore, only the wing tip portion 96 of the wing body 91 is locally and rapidly heated to a temperature of about Ar ₁ point or more (about 1000 ° C.) using a high-frequency induction heater, and rapidly cooled after being kept for a predetermined time T4 (for example, 10 minutes). Then, the mixture was heated to a temperature immediately below Ar ₁ point (about 450 ° C.), maintained for a predetermined time T5 (for example, 4 hours), and then rapidly cooled. As a result, the portion 96 is hardened by Vickers hardness of about 100 Hv or more than the surrounding portions.
[0027]
Since the part subjected to the local hardening heat treatment undergoes a martensitic phase transformation, as shown in FIGS. 5 and 6, the volume after heating is larger than that before heating, and as a result, residual stress due to compression is generated. For this reason, even if the hardness of the erosion shield part 96 is set to Vickers hardness Hv of 340 or more (for example, Hv = 400), stress corrosion cracking and delayed fracture cracking do not occur.
[0028]
【The invention's effect】
As described in detail above, according to the present invention, a residual stress of compression can be generated in a portion which is susceptible to erosion damage using a relatively simple method, so that stress corrosion cracking and delayed fracture cracking do not occur. The target part can be hardened and strengthened. Therefore, the turbine blade of the steam turbine can be erosion shielded at low cost, and a long-life turbine blade free from stress corrosion cracking and delayed fracture cracking is provided.
[Brief description of the drawings]
FIG. 1 is a thermal history characteristic diagram showing a method for erosion shielding a turbine blade according to the present invention.
FIG. 2 is an enlarged sectional view showing a main part of the steam turbine engine.
FIG. 3 is a schematic external view showing a turbine rotor blade having an erosion shield portion.
FIG. 4 is a cross-sectional view showing a turbine rotor blade having an erosion shield portion.
FIG. 5 is a characteristic diagram schematically showing a volume change due to a phase transformation.
FIG. 6 is a characteristic diagram showing a change in material length when subjected to a heat cycle of heating and cooling.
FIG. 7 is a characteristic diagram showing a correlation between hardness and stress corrosion cracking depth in various materials.
FIG. 8 is a characteristic diagram showing a correlation between hardness and delayed fracture strength ratio in various materials.
[Explanation of symbols]
8… Static wing,
9 ... rotor blade,
10 ... turbine blades,
91 ... wing body,
92 ... platform,
93 ... fixed part,
96: erosion shield part (cured part),

Claims (5)

An erosion shield method for a turbine blade for improving the resistance of the turbine blade to erosion,
A standard heat treatment step after solution-treating the entire turbine blade made of precipitation hardening martensitic stainless steel,
After the standard heat treatment step, only the blade tip portion of the turbine blade is locally heated to a point of Ar ₁ or more, held for a predetermined time, rapidly cooled to room temperature, and subsequently aged at a point of Ar ₁ or less to compress the surface layer of the blade tip portion. A local hardening heat treatment step to cause residual stress of
An erosion shield method for a turbine blade, comprising: The method according to claim 1, wherein in the local hardening heat treatment step, the blade tip portion is locally heated using a high-frequency heating method, a laser heating method, or a burner heating method. 2. The method according to claim 1, wherein in the local hardening heat treatment step, a cooling rate of a blade tip portion after heating and holding is set to be higher than that of a surrounding portion. It is made of precipitation hardening type martensitic stainless steel. After solution treatment, after the whole is aged, only the blade tip portion is locally heated to _one or more points of Ar, kept for a predetermined time, quenched to room temperature, and then cooled to room temperature. An erosion-shielded turbine blade having a hardened portion in which a residual stress of compression exists on a surface layer of the blade tip portion by aging at _one point or less. The turbine blade according to claim 4, wherein the precipitation hardening type martensitic stainless steel is made of 17-4PH steel (SUS630).

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