Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
2000-09-01
2004-02-24
King, Roy (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C148S326000
Reexamination Certificate
active
06696016
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-Cr containing ferrite heat resistant steel. In further detail, the invention according to the present application relates to a high-Cr ferrite heat resistant steel having not only an excellent long-term creep strength at a high temperature exceeding 650° C., but also an improved oxidation resistance.
BACKGROUND OF THE INVENTION
Conventionally, the creep strength of a ferrite based, heat resistant steel has been improved heretofore by converting the ferritic texture into a tempered martensitic texture having a higher creep strength.
However, a tempered martensitic texture is unstable at high temperatures because it undergoes textural change and becomes heterogeneous. This decreases the creep strength. Furthermore, dislocations present in the martensite accelerates the long term creep deformation. Thus, the texture is changed influenced by the heat applied at welding as to impair the creep strength at the welded portion
Although Cr (chromium) is known as an element effective for improving the oxidation resistance of a steel, the incorporation of Cr at a higher concentration of 12% by weight or more results in the generation of a &dgr;-ferrite phase which decreases the creep strength and the toughness. Accordingly, austenite stabilizing agents such as Ni, Cu and Co, have been added to the ferritic heat resistant steel known heretofore in order to suppress the generation of &dgr;-ferrite phase.
However, the addition of Ni or Cu lowers the transformation temperatures of austenite and ferrite. To achieve long term stability of the high-temperature strength, it is advantageous to set the tempering temperature higher after the normalization; however, the addition of Ni or Cu results in a lower tempering temperature because it thus lowers the transformation temperature of austenite and ferrite. Accordingly, it is practically unfeasible to add Cr at a quantity exceeding a concentration of 12% by weight.
DISCLOSURE OF THE INVENTION
The invention according to the present application has been made in the light of the aforementioned circumstances, and an object thereof is to provide a high-Cr ferrite heat resistant steel having excellent long-term creep strength at a high temperature exceeding 650° C., and yet having an improved oxidation resistance.
As described above, a conventional ferritic heat resistant steel based on the tempered martensitic texture suffers an abrupt drop in creep strength because it undergoes a heterogeneous textural change in the vicinity of the grain boundaries when subjected to higher temperatures over 650° C. for a long duration of time because of the unstable texture.
Accordingly, the inventors of the present invension extensively studied a means for achieving textural stability at higher temperatures. As a result, it has been found that the ferritic heat resistant steel having a greatly improved long term creep strength at high temperatures can be obtained by realizing a texture based on a ferritic phase and precipitating therein an intermetallic compound of a Laves phase, a &mgr; phase, a &sgr; phase, or a compound represented by Ni
3
X, where X is Al or Ti. The present invention has been accomplished based on these findings.
More specifically, in accordance with the first aspect of the invention of the present application, there is provided a heat resistant high-chromium containing ferrite steel containing 13% by weight or more of chromium and based on ferritic phase and containing precipitates of intermetallic compounds.
Furthermore, according to a second aspect of the invention of the present application, there is provided, a heat resistant high-chromium containing ferrite steel above, wherein the intermetallic compound is at least one type of precipitates selected from the group consisting of a Laves phase, a &mgr; phase, a &sgr; phase, or a compound represented by Ni
3
X, where X is Al or Ti.
REFERENCES:
patent: 3746586 (1973-07-01), Buchanan et al.
patent: 4218268 (1980-08-01), Hirishi et al.
patent: 4640722 (1987-02-01), Gorman
patent: 4902472 (1990-02-01), Isobe et al.
patent: 4985091 (1991-01-01), Culling
patent: 5061440 (1991-10-01), Watanabe et al.
patent: 5772956 (1998-06-01), Hasegawa et al.
patent: 6299704 (2001-10-01), Igarashi et al.
patent: 2002/0020473 (2002-02-01), Sawaragi et al.
patent: 48-023618 (1973-03-01), None
patent: 53-035617 (1978-04-01), None
patent: 07-048654 (1995-02-01), None
Smith, William, “Stainless Steels” from Principles of Materials Science and Engineering, McGraw-Hill Book Company, 1986, pp. 499-504.*
Kimura, “Creep Strength Properties of Precipitation Hardened High Cr Ferritic Steels”, Current Advances in Materials and Processes, vol. 12 (1999) No. 6, p. 1318 (The Iron and Steel Institute of Japan).
Japan as represented by Director General of National Research In
King Roy
Wenderoth , Lind & Ponack, L.L.P.
Wilkins, III Harry D.
LandOfFree
High-chromium containing ferrite based heat resistant steel does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High-chromium containing ferrite based heat resistant steel, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High-chromium containing ferrite based heat resistant steel will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3285282