JP2000282803A - Moving blade castings for steam turbine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To extremely improve tensile characteristic and shock characteristic which are necessary for a steam turbine moving blade, as well as a forging material by applying hot static water pressure treatment after cast molding out of stainless steel or heat resistant steel. SOLUTION: A moving blade casting for a steam turbine is cast-molded with stainless steel or heat resistant steel of a martensite base. In the case of a casting material, there may be cases where entrainment of gas, micro- porosity, micro-shrinkage exist in a casting, and frequently the casting material can be broken by comparatively low stress. In the moving blade casting for the steam turbine which is casting-molded, hot static water pressure treatment is applied. In the hot static water pressure treatment, the stress is loaded on a material which is softened in a high temperature, since inner defect is easily broken with low pressure. Load pressure is set as static water pressure, and deformation of casting is suppressed to a minimum. In a medium used for pressure load, an inert gas such as Ar is used for suppressing high temperature oxidized corrosion of a material. After that, heat treatment such as quenching and tempering is carried out, as necessary.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a moving blade for a steam turbine.

[0002]

2. Description of the Related Art Generally, stainless steel or heat-resistant steel, which is hardly oxidized and corroded, is used for a rotor blade for a steam turbine because it is exposed to steam for a long time. Further, since the rotor blade is a rotating body, it is required to have a strength capable of withstanding high-speed rotation and an impact characteristic capable of withstanding the impact at the start and stop of rotation. For this reason, conventionally, forging materials have generally been formed by machining.

[0003]

Conventionally, in order to obtain the characteristics required for a moving blade for a steam turbine, it is common to use a forged material as described above. There is a problem that the man-hour is very large and the material yield is low, so that the cost is high. In addition, when molding is performed using stamping forging, there is also a problem that the shape that can be molded is limited depending on whether or not the mold is removed.

[0004] The molding by precision casting is advantageous in that the number of machining steps is significantly reduced as compared with a forged product, so that it can be produced at low cost and that there is almost no restriction on the shape. However, in the case of a stainless steel or heat-resistant steel casting as-cast or a material subjected to only conventional heat treatment,
There is a problem that the mechanical properties required for the rotor blade cannot be satisfied. Even if it is a casting, it is possible to obtain the required characteristics as long as it is a super heat-resistant alloy such as Ni-base, but these alloys are still not practical because of high material cost. SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems of high cost and shape limitation with respect to a moving blade for a steam turbine, and to further provide a moving blade for a steam turbine having mechanical characteristics as a moving blade.

[0005]

SUMMARY OF THE INVENTION The present inventor has studied the problem of high cost in forming a moving blade for a steam turbine, and has conducted a hot isostatic pressure treatment on a stainless steel or heat-resistant steel casting to thereby obtain a mechanical strength of the casting. The inventors have found that the characteristics can be greatly improved, and have reached the present invention.

[0006] That is, the present invention is a moving blade casting for a steam turbine, characterized in that a hot isostatic pressure treatment is performed after forming the stainless steel or the heat-resistant steel. A moving blade casting for a steam turbine, which is a martensite system, is formed by precision casting, and is subjected to a hot isostatic pressure treatment after the forming.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, an important feature of the present invention is that a steam turbine rotor blade cast and formed of stainless steel or heat-resistant steel is subjected to hot isostatic pressure treatment, and then to a suitable Heat treatment is performed to improve the mechanical properties of the blade to the same level as that of a forged material.

In the case of a cast material, gas entrainment due to turbulence of the molten metal during casting, microporosity resulting from the reaction between the molten metal and the mold material, and microshrinkage generated during solidification shrinkage may be present inside the casting. . When stress is applied to the material having such casting defects from outside,
In many cases, these defects serve as starting points and break at relatively low stress. In the case of a forged material, these defects are crushed when pressure is applied to the material from the outside, and thus generally show higher strength than a cast material. That is, in order to obtain characteristics similar to those of a forged material in a cast material, it is necessary to remove the above-described microporosity and microshrinkage by some method.

As a method for preventing the turbulence of the molten metal during casting, vacuum casting or the like in which a metal material is melted and poured in a vacuum is effective. Further, in order to prevent the reaction between the molten metal and the mold, there is a method in which a refractory such as zirconia or yttria which is thermodynamically stable is used for the mold material. However, these casting methods and refractories are not practical because of extremely high working and material costs. In addition, it is possible to prevent the reaction with the molten metal by die casting, but there is a problem in the durability of the die in die casting, which is not practical.

From the above, it is practically very difficult to completely prevent casting defects, and the present inventor has proposed a method of physically removing the generated defects instead of preventing the occurrence of the defects. investigated. As a result, it has been found that hot isostatic pressure treatment in which a pressure is uniformly applied to the casting material from outside at a high temperature to crush the defects is effective.

In the hot isostatic pressure treatment, it is desirable to apply a stress to a material softened at a high temperature of 900 ° C. or higher in order to easily crush internal defects at a pressure as low as possible. Also,
The load pressure was set to be a hydrostatic pressure in order to minimize the deformation of the casting. Further, it is desirable to use an inert gas such as Ar for the medium used for the pressure load in order to suppress high-temperature oxidation corrosion of the material.

Generally, a moving blade for a steam turbine is generally used by forming the blades one by one and assembling them into a ring shape. For this reason, the stainless steel or heat-resistant steel used is a ferrite-based material having a small coefficient of thermal expansion or a martensitic-based material having a small coefficient of thermal expansion and excellent strength compared to an austenitic material having a large coefficient of thermal expansion. Many are used. Martensitic materials are usually used after heat treatment such as quenching and tempering.However, materials that have been subjected to hot isostatic pressure treatment alone may not be able to obtain the characteristics required as a moving blade. After the hot isostatic pressure treatment, appropriate quenching and tempering treatment must be performed as necessary to obtain the required characteristics. The present invention
Although a martensite-based material was used, a ferrite-based material may be used.

[0013]

EXAMPLES As examples, martensitic heat-resistant steels SUH600 and JIS G4303 specified in JIS G4311, which have been conventionally used for moving blades for steam turbines.
Martensitic stainless steel SUS403 specified in
The tensile properties and the impact properties of the two types of cast materials were compared with those of the forged material after the hot isostatic pressure treatment and without the hot isostatic pressure treatment.

The casting material is a refractory containing silica, alumina, and zirconia as main components. A molten metal melted in the air is cast into a mold in which holes of 11 mm × 90 mm and 11 mm × 11 mm × 55 mm are formed at several places. After casting and removing the refractory, the test piece material was cut off from the riser. Half of the collected materials were subjected to a hot isostatic pressure treatment of applying a hydrostatic pressure of 1000 atm in an Ar atmosphere furnace at 1180 ° C. After that, SUH600 became JIS G4311,
SUS403 is quenching specified in JIS G4303,
Tempering heat treatment was performed. The material after the heat treatment is
6.35mm diameter of parallel part by machining, gauge length 3
5.0 mm tensile test specimen, and 10 mm x 10 mm x
SUH600 is a U-shaped notch with a depth of 2mm to 55mm, S
US403 was processed into a Charpy impact test specimen having a V-shaped notch having a depth of 2 mm, and subjected to a tensile test and a Charpy impact test at room temperature.

The forged material was prepared by forging and elongating a melted 10 kg master ingot into a 30 mm square bar and then removing the flaws. SUH600 was JIS G4311, SUS40.
Sample No. 3 was subjected to a heat treatment specified in JIS G4303, cut out into a tensile test piece and a Charpy impact test piece having the same shape as that of the cast material, and manufactured a test piece by machining. The test specimen was subjected to a tensile test and a Charpy impact test at room temperature in the same manner as the cast material.

[0016]

[Table 1]

[0017]

[Table 2]

Table 1 summarizes the results of the room temperature tensile test. For both materials SUH600 and SUS403, the proof stress, tensile strength and elongation were improved by performing the hot isostatic pressure treatment, and values almost equivalent to those of the forged material were obtained. Table 2
Is a summary of the results of the room temperature Charpy impact test, and the Charpy impact value is also set for materials SUH600 and SUS40.
All of the samples No. 3 were improved by performing the hot isostatic pressure treatment, and showed values comparable to those of forged materials. Further, the material subjected to the hot isostatic pressure treatment has a smaller value variation than the untreated material.

[0019]

According to the present invention, by subjecting a cast material to hot isostatic pressure treatment, the tensile properties and impact properties required for a moving blade for a steam turbine can be remarkably improved to the level of a forged material. This is an indispensable technology for practical use of turbine bucket castings.

Claims (3)

[Claims] 1. A moving blade casting for a steam turbine, wherein a stainless steel or heat-resistant steel is subjected to hot isostatic pressure treatment after forming. 2. The moving blade casting for a steam turbine according to claim 1, wherein the stainless steel or the heat-resistant steel is martensitic. 3. The bucket casting for a steam turbine according to claim 1, wherein the bucket casting for a steam turbine is formed by a precision casting method.