Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-03-04
2004-06-29
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S660000, C420S109000, C416S24100B
Reexamination Certificate
active
06755920
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to low-alloy heat-resistant steels which exhibit excellent performance as large turbine rotor members, heat resistant components for generating plants, and components for devices which are subjected to high temperatures, and relates to heat treatment methods for the low-alloy heat-resistant steels and turbine rotors comprising the low-alloy heat-resistant steels.
2. Description of the Related Art
Conventionally, as heat resistant steels for high temperature turbine rotor members for use in steam turbine plants for thermoelectric power generation, 12Cr steels, which belong to high-Cr steels (see Japanese Patent Applications, First Publications (Kokai), Nos. Sho 60-165359, and Sho 62-103345) and CrMoV steels, which belong to low-alloys (see Japanese Patent Application, First Publication (Kokai), No. Sho 60-70125), have been exclusively used. 12Cr steels have superior high temperature strength and can hence be used in plants having a steam temperature up to 600° C. However, 12Cr steels are disadvantageous in that the production of the material is difficult and costly. Of these, the use of CrMoV steel is restricted to plants having a steam temperature up to 566° C. because of its limited high temperature strength. Moreover, cooling of the rotor may be required depending on the steam temperature, which is disadvantageous in that it complicates the plant.
However, in recent years, further improvement in energy efficiency has been desired, and if it is desired to raise the operational temperature of a steam turbine, a steel of a conventional type is insufficient in mechanical properties at high temperatures, particularly in terms of creep strength. Accordingly, the demand to develop a material which is durable in use at higher steam temperatures has been growing. 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 elements such as cobalt, niobium, and tantalum, were additionally used in order to inhibit the precipitation of a pro-eutectoid ferrite phase, a satisfactory material has not yet been obtained.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a low-alloy heat-resistant steel in which when it is used to manufacturing a large element, which have uniform superior high temperature properties through a surface layer to a center part. In particular, an object of the present invention is to provide a low-alloy heat-resistant steel which has high creep embrittlement resistance.
Another object of the present invention is to provide a heat treatment method for preparing the novel low-alloy heat-resistant steels.
Another object of the present invention is to provide a turbine rotor comprising the novel heat resistant steel.
In order to achieve the above objects, the present inventors have diligently carried out research and have discovered that the sizes of crystal grains comprising the matrix greatly affects the properties of a steel at high temperatures, particularly the creep embrittlement resistance. That is, the following was discovered. When large elements, such as turbine rotors are cooled in a conventional method, in a center part thereof, since a suitable amount of a pro-eutectaid ferrite phase is easily precipitated, the crystal grains are relatively fine, a high toughness can be obtained, and the creep embrittlement does not occur. In contrast, in the surface layer thereof, since a pro-eutectaid ferrite phase is hard to be precipitated and the crystal grain easily becomes large, the Charpy impact absorbed energy decreases. Thereby, there is the possibility that the material in the surface layer is embrittled and that creep embrittlement occurs.
Consequently, it was also found that the low-alloy heat-resistant steels can be obtained, which have high toughness, excellent high temperatures properties, in particular, in which the creep embrittlement does not occur, and which are suitable for large elements, such as turbine rotors, not only by mixing alloy components in predetermined proportions and minimizing the amount of minor impurity elements which are harmful, but also by adjusting the crystal grain size of the matrix.
First, the measuring method for crystal grain size will be described below. As a measuring method for crystal grain size, Japanese Industrial Standard JIS G 0551 (1998) defines an austenitic grain size determination for steel and JIS G 0552 (1998) defines a ferritic grain size determination for steel. The low-alloy heat-resistant steels of the present invention comprise a metallic structure containing a ferrite phase and a bainite phase which is prepared by quenching from high temperatures, such as an austenitic phase stabilized temperature range, and thereby a pro-eutectoid ferrite phase is precipitated. Therefore, in the present invention, crystal grain size of the composite containing a ferritic phase and a bainite phase is specified. Specifically, in the present invention, the boundary between a bainite phase and a pro-eutectoid ferrite phase, the boundary between pro-eutectoid ferrite phases, and the boundary between austenitic particles which are to be transformed into a bainite phase are defined as the crystal grain boundary, and the size of the area surrounded by the crystal grain boundary is defined as the crystal grain size.
In the present invention, the ferritic grain size determination for steel comprising mixed crystal grains, which is defined by JIS G 0552 (1998), is adopted. In other words, the measuring method is one, in which a photograph of crystal grains which appears at a corroded surface of a test piece is taken using a microscope, and the sizes of the crystal grains are measured by a cross segment method. Moreover, when the crystal grain number is lower, the crystal grain size is larger.
Next, of the high temperature properties, the creep rupture strength of a creep test on a notched test piece (abbreviated as “notched creep test” below) will be described. Since a turbine rotor is subjected to high temperatures for a long time under stress during operation, deterioration in the strength of the material with age is a concern. The quality of turbine rotor members has been hitherto evaluated only by high temperature unnotched creep tests, as defined by the Japanese Industrial Standards or the like. However, the present inventors have discovered a method of evaluating high temperature strength properties of the material, particularly the creep embrittlement resistance, in a high temperature creep test on a notched test piece.
When a stress is applied to a steel product at a high temperature, even if the stress is relatively small, the steel product plastically deforms very gradually to become elongated, and finally the elongation proceeds, rapidly narrowing a part of the steel product, which results in rupture of the steel product. This phenomenon is called “the creep” or “the creep rupture phenomenon”. In addition, the cross area ratio between the test piece and the ruptured test piece is called “the creep rupture ratio”. In a high temperature creep test, a constant static load is applied to a material for a long time at a high temperature, and the time elapsed before rupture is measured. As a test piece, a round bar having a constant cross section is used. The measuring method is defined by JIS Z-2271. The measuring methods defined by the JIS standards are for creep tests on unnotched test pieces, and test pieces which are finished by smoothly shaving between gauge marks i
Fujita Akitsugu
Kamada Masatomo
Okamura Yoshihiro
Ooba Yoshiyuki
Yamaguchi Makoto
Mitsubishi Heavy Industries Ltd.
Yee Deborah
LandOfFree
Low-alloy heat-resistant steel, heat treatment method... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Low-alloy heat-resistant steel, heat treatment method..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low-alloy heat-resistant steel, heat treatment method... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3364767