Metal treatment – Stock – Ferrous
Patent
1996-07-22
1998-06-16
Yee, Deborah
Metal treatment
Stock
Ferrous
148334, 148547, C22C 3822, C22D 800, C22D 900
Patent
active
057663769
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a ferritic heat-resistant steel. More particularly, the present invention relates to a ferritic heat-resistant steel which is used in a high temperature and high pressure environment, has high creep rupture strength and has excellent HAZ softening resistance characteristics. The present invention particularly relates to an improvement in the strength and the toughness by controlling the change resulting from thermal influences on constituent elements of carbides.
BACKGROUND ART
A temperature and a pressure in the operation condition of thermal power boilers have become remarkably higher in recent years, and some boilers have been operated at 566.degree. C. and 316 bar. An operating condition of up to 649.degree. C. and 352 bar is expected in future, and extremely severe conditions will be imposed on the materials used.
Heat-resistant steels used for thermal power plants are exposed to different environments depending on the positions at which they are used. Austenitic materials having high temperature corrosion resistance and particularly high strength or ferritic materials containing 9 to 12% of Cr have been widely used for portions having a high metal temperature, such as so-called "superheater pipes" and "reheater pipes".
Recently, novel heat-resistant steels to which W is added afresh so as to contribute to the improvement in the high temperature strength have been developed and put into practical application, and have made a great contribution to the accomplishment of high-efficiency of electric power plants. For example, Japanese Unexamined Patent Publications (Kokai) No. 63-89644, No. 61-231139, No. 62-297435, and so forth, disclose a ferritic heat-resistant steel capable of drastically higher creep rupture strength in comparison with Mo-containing ferritic heat-resistant steels according to the prior art, by using W as a solid solution reinforcement element. The structure of these steels is in most cases a tempered martensite mono-phase. Due to the combination of the superiorities of ferritic steels having excellent steam oxidation resistance characteristics and high strength characteristics, they are expected as the materials of the next generation to be used in a high temperature and a high pressure environment.
Because a higher pressure of the thermal power plants has been accomplished, the operating conditions for the portions for which the operating temperatures have been relatively low so far, such as wall pipes of fire furnaces, heat-exchangers, steam generators, main steam pipes, have become also severer, and low Cr content ferritic heat-resistant steels such as so-called "1Cr steel", "1.25Cr steel", "2.25Cr steel", etc, that are stipulated by the industrial standards, cannot cope with such an operation condition.
To cope with such a trend, a large number of steels which improve the high temperature strength by positively adding W or Mo to these low strength materials have been proposed. In other words, Japanese Unexamined Patent Publication (Kokai) Nos. 63-18038 and 4-268040 and Japanese Examined Patent Publications (Kokoku) No. 6-2926 and No. 6-2927 propose a steel which improves the high temperature strength of 1 to 3%Cr steel by adding W as a main reinforcement element. Any of these steels have higher high-temperature strength than the conventional low Cr steels.
On the other hand, the ferritic heat-resistant steels utilize the high strength of ferritic structures such as the martensite structure, the bainite structure, etc, or their tempered structures, that contain large quantities of dislocation generated as a result of the supercooling phenomenon exhibited by the phase transformation from the austenite mono-phase region to the ferrite plus carbide precipitation phase occurring during the cooling process of the heat-treatment. Therefore, when this structure receives the thermal hysteresis of being again re-heated to the austenite mono-phase region such as when it is affected by welding heat, the high density dislocation is again released,
REFERENCES:
patent: 5211909 (1993-05-01), Iseda et al.
Fujita Toshio
Hasegawa Yasushi
Naoi Hisashi
Sato Takashi
Tamura Kohji
Babcock-Hitachi Kabushiki Kaisha
Fujita Toshio
Nippon Steel Corporation
Yee Deborah
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