Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-12-04
2004-08-10
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C416S24100B
Reexamination Certificate
active
06773519
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to turbine rotors and in particular it relates to high pressure and low pressure integrated type turbine rotors used in steam turbines employed in thermal electric power generation.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Conventionally, as one type of turbine rotor for steam turbines for thermal electric power generation, high pressure and low pressure integrated turbine rotors utilizing integrated materials from the high pressure part to the low-pressure part have been known. The steam turbine is exposed to high-temperature and high-pressure steam on the side of its steam inlet. As the end portion is being approached, the temperature and pressure of steam decrease, so that the steam turbine is exposed to steam that has a highly expanded volume. Therefore, in the high-pressure part, the turbine blades are short in length and the stress applied to the turbine rotor is relatively small, and thus the diameter of the turbine rotor may be small. On the other hand, in the low pressure part, to receive the force exerted by a larger amount of steam, the length of the turbine blades must be large and the diameter of the turbine rotor must be large, resulting in a large stress being applied to the turbine rotor. Therefore, the characteristics required for the high pressure and low pressure integrated type rotors are high temperature strength, in particular excellent creep strength at the high-pressure part, and on the other hand, at the low pressure part, mechanical strength and excellent toughness at ordinary temperature.
Conventionally, as examples of heat-resistant steels for use in high pressure and low pressure integrated type turbine rotors, CrMoV steels, which belong to low-alloys, and 12Cr steels, which belong to high-Cr steels, have been exclusively used (see Japanese Patent Applications, First Publications (Kokai), Nos. Sho 60-165359 and Sho 62-103345). A process for obtaining a turbine rotor having creep properties and toughness simultaneously has been proposed, in which a CrMoV based steel species is processed into a turbine rotor member and the high-pressure and low-pressure parts of a single turbine rotor are separately heat treated under different conditions. For example, Japanese Patent Application, First Publication (Kokai), No. Hei 5-195068 discloses a process for obtaining a high pressure and low pressure integrated type turbine rotor having creep strength at high temperatures and toughness simultaneously, in which the high pressure part of a rotor member is quenched after heating at a temperature higher than the low pressure part and then the whole rotor member is tempered at a predetermined temperature. Japanese Patent Application, First Publication (Kokai) No. Hei 8-176671 discloses a process for obtaining a high pressure and low pressure integrated turbine rotor having excellent creep properties at high temperatures and toughness simultaneously, in which a rotor member is normalizing-treated at 1100 to 1150° C. and pearlite-transformed, further normalizing-treated at 920 to 950° C., the high pressure part and low pressure part are quenched at different temperatures, and then the whole rotor member is tempered.
However, in recent years, further improvement in the energy efficiency has been desired, and there has been a trend that the temperature and the amount of steam introduced into turbines is increased, resulting in much stricter characteristics being required for turbine rotors. Therefore, rotors of a conventional type are insufficient in mechanical properties at high temperatures, particularly in terms of creep strength, at their high-pressure parts. Accordingly, the need for developing a material that is durable in use at higher steam temperatures has been growing. On the other hand, for low-pressure parts, developing a material that is durable to stronger stresses and has increased toughness has become necessary.
Conventionally, a CrMoV steel is used after quenching the CrMoV steel heated to a temperature of about 950° C. A higher heating temperature before quenching results in a higher strength of the material because precipitation of a pro-eutectoid ferrite phase, which is soft, is inhibited, and dissolution of the strengthening elements in a solid solution is promoted. However, another problem arises in that a higher heating temperature before quenching causes creep embrittlement of the material. Therefore, the heating temperature before quenching cannot be raised. Although attempts have been made in which various alloy elements were additionally used and heat treatments have been devised in order to inhibit the creep embrittlement, a satisfactory material has not yet been obtained.
A higher temperature before quenching causes a problem that coarsening of crystal grains is promoted and thus the toughness of the material deteriorates. In view of this, the temperature before quenching could not be elevated to 1000° C. or more. Thus, to satisfy the high temperature strength and brittleness of a CrMoV steel simultaneously involves the difficulty that inconsistent heat treatment conditions are used in the production of the steel. As a result, no satisfactory turbine rotor suitable for large volume steam turbines for use at high temperatures has been obtained.
BRIEF SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a heat-resistant steel which can be quenched after heating to a higher temperature, has a toughness equivalent to or higher than that of a conventional CrMoV steel, and has excellent creep properties at high temperature such as a high creep rupture property, according to a creep test on an unnotched test piece, and inhibition of creep embrittlement. Another object of the present invention is to provide a turbine rotor comprising this novel heat-resistant steel.
In order to achieve the above objects, the present inventors have diligently carried out research, and found that impurities greatly affect the properties of a steel at high temperatures, particularly the creep embrittlement resistance. As a result, the present inventors found that a high pressure and low pressure integrated type turbine rotor which can be quenched after heating to a high temperature between 980° C. and 1100° C., and having excellent creep strength at its high pressure part, such as not being subject to creep embrittlement, and a high toughness at its low pressure part can be obtained not only by mixing alloy components with predetermined proportions, but also by minimizing the amount of trace impurity elements which are harmful, such as phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony. The present inventors have thus achieved the present invention.
The high-pressure part of the high pressure and low pressure integrated type turbine rotor has excellent high temperature properties with a creep rupture time of 3000 hours or longer, according to a creep test on an unnotched test piece, under specific conditions of a temperature of 600° C. and a stress of 147 MPa, and a creep rupture time of 10000 hours or longer, according to a creep test on a notched test piece, under the same conditions as described above. The low-pressure part of the high pressure and low pressure integrated type turbine rotor has an excellent toughness of 0.2% yield strength of 686 MPa or more, and Charpy impact absorbed energy of 98 J or more. The high pressure and low pressure integrated type turbine rotor of the present invention has excellent creep properties at the high-pressure part and excellent toughness at the low-pressure part simultaneously.
The process for producing a high pressure and low pressure integrated type turbine rotor of the present invention is a method in which a rotor member made of an alloy steel having a specific composition is subjected to different heat treatments at its high pressure and low pressure parts, respectively. More particularly, the high pressure and low pressure integrated ty
Fujita Akitsugu
Kamada Masatomo
Mitsubishi Heavy Industries Ltd.
Yee Deborah
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