Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
1999-01-20
2001-09-18
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C420S038000, C420S039000, C420S061000, C420S064000, C420S090000, C420S106000
Reexamination Certificate
active
06290904
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to welding materials for high-Cr steels which are suitable for use in boilers and turbines for electric power generation, chemical plants, and the like.
2.Description of the Related Art
When welding materials for high-Cr steels which have been developed up to now are used for welding purposes, the toughness of the weld metal is reduced. Consequently, in order to secure high toughness, welded joints formed by gas tungsten-arc welding (hereinafter abbreviated as GTAW) or gas metal-arc welding (hereinafter abbreviated as GMAW) must be subjected to a postweld heat treatment at 740-760° C.
However, where a high-Cr steel is used in combination with a low-Cr steel material (e.g., 2¼Cr-1Mo steel) for which a postweld heat treatment at a high temperature of 740-760° C. cannot be employed, it is conventional practice to first subject the high-Cr steel to a postweld heat treatment at 740-760° C. and then subject the 2¼Cr-1Mo steel, together with the high-Cr steel, to a postweld heat treatment at 700-730° C.
Accordingly, in order to overcome the disadvantage of requiring two-stage heat treatment, the present applicant has proposed, in Japanese Patent Provisional Publication No. 8-290290, a high-toughness welding material for high-Cr steels which permits high-Cr steels welded by GTAW and GMAW to be subjected to a postweld heat treatment at 700-730° C. similarly to 2¼Cr-1Mo steel, and thereby makes it possible to simplify the production process and save thermal energy.
Specifically, the welding material proposed in the aforementioned patent is a high-toughness welding material for high-Cr steels which is suitable for use in the gas tungsten-arc welding or gas metal-arc welding of high-Cr steels. Its chemical composition contains, on a weight percentage basis, up to 0.1% C, up to 0.3% Si, 0.2 to 1.5% Mn, up to 0.02% P, up to 0.01% S, 8 to 13% Cr, up to 0.75% Ni, 0.5 to 3% Mo, 0.18 to 0.25% V, 0.05 to 0.3% Ta, and 0.002 to 0.005% B, the balance being Fe.
In this welding material, Nb which has been contained in conventional welding materials for the purpose of enhancing creep rupture strength is replaced with Ta, and an appropriate amount of B is added, so as to strengthen grain boundaries and enhance long-time creep rupture strength and toughness. This makes it possible to perform the postweld heat treatment in a single stage instead of performing it in two stages using different temperatures, and thereby decrease the number of process steps and afford a saving of energy.
Recently, in order to improve the thermal efficiency of thermal electric power plants, materials having better temperature characteristics are being developed by using the composition of a conventional high-Cr steel as a basic composition and adding W, N and/or Co thereto, and are being put to practical use.
However, when such a homologous welding material using the composition of a high-Cr steel as a basic composition or a quasi-homologous welding materials obtained by replacing Nb contained in the basis composition with Ta and adding an appropriate amount of B is used for welding purposes, the toughness of the weld metal is markedly reduced and, therefore, cannot be satisfactorily restored by a postweld heat treatment at 700-730° C. Accordingly, where it is desired to use a high-Cr steel is in combination with 2¼Cr-1Mo steel, a structure of high-Cr steel is subjected to a postweld heat treatment at 740-760° C., while a structure of 2¼Cr-1Mo steel is subjected to a postweld heat treatment at 700-730° C. Thereafter, the aforesaid structure of high-Cr steel and the aforesaid structure of 2¼Cr-1Mo steel are welded together, and the resulting heterogeneous welded joint is locally subjected to a postweld heat treatment at 700-730° C. Alternatively, the aforesaid structure of high-Cr steel is subjected to a postweld heat treatment at 740-760° C. and then welded to a structure of 2¼Cr-1Mo steel. Thereafter, the resulting integral structure consisting of the structure of high-Cr steel and the structure of 2¼Cr-1Mo steel is subjected to a postweld heat treatment at 700-730° C. As used herein, the terms “homologous welding material” and “quasi-homologous welding material” mean welding materials in which the principal components are the same as those of the base metal.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide welding materials for high-Cr steels which make it possible to achieve a sufficient improvement in the toughness of welded joints of high-Cr steels as described above, by subjecting them to a single postweld heat treatment (i.e., a heat treatment at 700-730° C. for 2¼Cr-1Mo steel). Thus, after a structure of high-Cr steel as described above and a structure of 2¼Cr-1Mo steel are united together by using such a welding material, the resulting integral structure may be converted into a sound one by subjecting it to a single heat treatment at 700-730° C.
In order to obtain high-strength and high-toughness welded joints in 9Cr to 12Cr steel materials which are high-temperature and high-strength steel materials, it is effective to suppress the precipitation of ferrite and make crystal grains finer. As a result of extensive investigations conducted from this point of view, it has been discovered that the aforesaid purpose of suppressing the precipitation of ferrite and making crystal grains finer can be accomplished by adding appropriate amounts of Cu and Ta, and minimizing the addition of N. The present invention has been completed on the basis of this discovery.
That is, the present invention provides the following three welding materials for high-Cr steels.
(1) A welding material for high-Cr steels which contains, on a weight percentage basis, 0.03 to 0.12% C, up to 0.3% Si, 0.2 to 1.5% Mn, up to 0.02% P, up to 0.01% S, 8 to 13% Cr, 0.5 to 3% Mo, up to 0.75% Ni, 0.15 to 0.3% V, up to 0.01% Nb, 0.05 to 0.3% Ta, 0.1 to 2.5% W, 0.01 to 0.75% Cu, up to 0.03% Al, 0.002 to 0.005% B, up to 0.015% N, and up to 0.01% O, the balance being Fe and incidental impurities. (This welding material will hereinafter be referred to as the inventive material 1.)
(2) A welding material for high-Cr steels as described above in (1) wherein no W is positively added thereto and W is present in an amount introduced as an incidental impurity. (This welding material will hereinafter be referred to as the inventive material 2.) Accordingly, the content of W is on the same level as that of an incidental impurity.
(3) A welding material for high-Cr steels as described above in (1) and (2) wherein, on a weight percentage basis, 0.1 to 3% Co is further added thereto. (This welding material will hereinafter be referred to as the inventive material 3.)
Now, the action and effects of various components contained in the inventive materials 1 to 3 and the reasons for the restriction of their contents are described below. In the following description, all percentages are by weight unless otherwise specified.
(Inventive Material 1)
C: 0.03 to 0.12%
In order to maintain strength and secure hardenability, the lower limit of the C content is fixed at 0.03%. Since unduly high C contents will deteriorate weldability, an upper limit of 0.12% is placed. Accordingly, the content of C should be in the range of 0.03 to 0.12% and preferably 0.06 to 0.09%.
Si: up to 0.03%
Si is an element added as a deoxidizer. However, unduly high Si contents will cause a reduction in toughness. Accordingly, the content of Si should be up to 0.03% and preferably in the range of 0.15 to 0.25%.
Mn: 0.2 to 1.5%
Mn is a component which has a deoxidizing effect and is also necessary for the maintenance of strength. If its content is less than 0.2%, no sufficient effect will be produced. On the other hand, if its content is greater than 1.5%, a reduction in toughness will result. Accordingly, the content of Mn should be in the range of 0.20 to 1.5% and preferably 0.30 to 1.00%.
P: up to 0.02%
P is an impurity which i
Inami Takashi
Ishihara Iwami
Ishikawa Kaneyasu
Kawano Takayuki
Kumou Mitsushige
Mitsubishi Heavy Industries Ltd.
Myers Bigel Sibley & Sajovec P.A.
Yee Deborah
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