Metal treatment – Stock – Ferrous
Patent
1999-11-12
2000-12-19
Yee, Deborah
Metal treatment
Stock
Ferrous
148328, 148648, 148649, 148654, C22C 3822, C22C 3828, C21D 713
Patent
active
061623075
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to ferritic heat resistant steel. More particularly, it relates to a ferritic heat-resistant steel that is excellent in creep rupture strength when used in a high temperature high-pressure environment, is excellent also in HAZ softening resistance, and can omit heat-treatment after welding.
BACKGROUND ART
Recently, temperatures and pressures in thermal power-plant boilers have been elevated remarkably. Operation at 566.degree. C. and 316 bar is planned for some plants, and future operating condition of up to 649.degree. C. and 352 bar is expected. Thus, the requirements for the plant materials have become more severer.
The heat-resistant materials used for thermal power plants are exposed to different environments depending on the portions at which the materials are employed. Materials having high corrosion resistance and strength at high temperatures, typified by austenite type materials, are used for portions exposed to a high atmospheric temperatures such as so-called "superheater pipes" and "reheater pipes", while martensite type materials containing 9 to 12% of Cr are used for portions where excellent steam oxidation resistance and thermal conductivity are required.
Recently, novel heat-resistant materials that contain W in order to improve the high temperature strength have been developed and put into practical application, and have made great contributions to the achievement of higher efficiency in power generation plants. For example, Japanese Unexamined Patent Publication (Kokai) Nos. 63-89644, 61-231139 and 62-297435 describe ferritic heat-resistant steels capable of achieving much higher creep strength in comparison with Mo-addition type ferrite steels according to the prior art by using W as a solid solution strengthening element. Most of these materials have a structure of a tempered martensite single phase, and are expected to be used, as the materials of the next generation, in a high temperature/high pressure environments due to superiority of steam oxidation resistance in combination with high strength of the ferrite steel.
As higher temperature and higher pressure have been achieved in the thermal power plants, severe operating conditions have been imposed on those portions which have so far been exposed to relatively low temperatures and pressures, such as furnace wall pipes, economizers, steam generators, main steam pipes, and so forth. In consequence, the application of low Cr ferritic heat-resistant steels stipulated by the industrial standard, such as so-called 1.25 Cr steel and 2.25 Cr steel, has become gradually impossible.
To cope with such a trend, a large number of steels that have improved the high temperature strength by positive addition W or Mo, have been proposed for such low strength materials, too.
Japanese Unexamined Patent Publication (Kokai) Nos. 63-18038 and 4-2680040 and Japanese Examined Patent Publication (Kokoku) Nos. 6-2926 and 6-2927 propose 1% to 3% Cr-containing steels that contain W as a principal strengthening element and have improved high temperature strength. All of them have higher high-temperature strength than the conventional low Cr containing steels.
On the other hand, the ferritic heat-resistant materials utilize a property of the steel in that the phase transformation from the austenite single phase region to the ferrite+carbide precipitation phase, that occurs with cooling at the time of heat-treatment, exhibits a super-cooling phenomenon. These materials utilize also the high strength of the resulting martensite or bainite structure involving large quantities of transition, or its tempered structure. Therefore, when this structure receives the thermal history such that it is again heated back to the austenite single phase region, such as when it is affected by welding heat, the high density transition is again released. In consequence, the drop of strength occurs locally in the welding heat affected zone. Among the portions re-heated to a temperature higher than the ferrite-austenite transformation point
Hasegawa Yasushi
Muraki Taro
Nippon Steel Corporation
Yee Deborah
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