Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Patent
1996-10-11
1998-06-30
Yee, Deborah
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
420 41, 420 69, 148325, C22C 3822, C22C 3830
Patent
active
057729565
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a ferritic heat-resistant steel. More particularly, the present invention relates to a ferritic heat-resistant steel for use in high temperature and high pressure environments, which has improved creep rupture strength and improved resistance to intermetallic compound precipitation-induced embrittlement.
BACKGROUND ART
In recent years, there has been a tendency for thermal power generation boilers to be operated under higher temperature and higher pressure conditions and operation under conditions of 566.degree. C. and 316 bar is planed. In the future, operation at temperatures up to 649.degree. C. and pressures up to 352 bar is expected. In this case, the materials used will be exposed to very severe conditions.
Heat resistant materials used in thermal power plants are exposed to different environments depending upon the sites where the materials are used. For sites where the temperature of the atmosphere is high, such as the so-called "superheater tube" and "reheater tube," austenitic materials having particularly improved corrosion resistance and strength at high temperatures, or 9-12% Cr-containing martensitic materials, when steam oxidation resistance and thermal conductivity are taken into consideration, have been used in many cases.
In recent years, novel heat resistant materials, to which W has been added in order to improve the high temperature strength, have been researched and developed and put into practical use, greatly contributing to an increase in efficiency of power generation plants. Japanese Unexamined Patent Publications (Kokai) No. 63-89644, No. 61-231139, and No. 62-297435 disclose ferritic heat resistant steels which, by taking advantage of W as a solid solution strengthening element, provide much higher creep strength than the conventional ferritic heat resistant steel with Mo added thereto. In many cases, these materials have a single phase structure of tempered martensite, and, by virtue of superior steam oxidation resistance of the ferritic steel in combination with high strength properties, are expected to be used as advanced materials for use under high temperature and high pressure environments. For example, Japanese Unexamined Patent Publications (Kokai) No. 5-263196, No. 5-311342, No. 5-311343, No. 5-311344, No. 5-311345, and No. 5-311346 disclose 12% Cr steels having improved high temperature creep strength.
The high temperature strength of ferritic heat resistant steels is governed by solid solution strengthening and precipitation strengthening. According to recent techniques, incorporation of the solid solution strengthening and precipitation strengthening in a well balanced manner could successfully increase the high temperature creep strength, and it has been confirmed that W and Mo are useful for solid solution strengthening, while Nb and V and carbides or nitrides thereof are useful to increase creep rupture strength by utilizing precipitation strengthening. The only practical problem of these additive elements useful for increase in strength is that, since all the additive elements are ferrite stabilizers, they enhance the Cr equivalent of the material, resulting in the formation of a dual phase structure of delta ferrite-tempered martensite rather than a single phase structure of martensite. The dual phase structure has properties different from the single phase structure, and, when homogeneous properties are required as material properties, the use thereof is avoided in many cases. Further, interphase partition or distribution of individual elements occurs, posing a problem in the case of materials having unsatisfactory corrosion resistance.
For this reason, among the ferritic heat resistant steels, those wherein high strength is attained by providing a tempered martensitic structure are required to have a single phase structure. For this reason, it is common practice to conduct constituent design in such a manner that a certain amount of an austenite stabilizer is added as a constituent to the material to form a
REFERENCES:
patent: 4957701 (1990-09-01), Masuyama et al.
patent: 5069870 (1991-12-01), Iseda et al.
patent: 5413754 (1995-05-01), Yazawa et al.
Hasegawa Yasushi
Naoi Hisashi
Ohgami Masahiro
Nippon Steel Corporation
Yee Deborah
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