Metal treatment – Stock – Ferrous
Patent
1996-11-21
1998-10-06
Yee, Deborah
Metal treatment
Stock
Ferrous
148326, 420 38, C22C 3852, C22C 3844
Patent
active
058171924
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to heat-resisting steels for use in large-sized forged products such as the high-pressure and intermediate-pressure rotors of steam turbines and the rotors of gas turbines. More particularly, it relates to heat-resisting steels which are suitable for use in the high-pressure and intermediate-pressure rotors of steam turbines operated at a steam temperature of 593.degree. C. or higher and which have high creep rupture strength at high temperatures within the range of 550.degree. to 650.degree. C. and excellent toughness at room temperature.
BACKGROUND ART
In thermal electric power plants, higher temperatures and higher pressures are recently aimed at from the viewpoint of efficiency improvement. The goal of the steam temperature of steam turbines is being raised from the current maximum level of 593.degree. C. to 600.degree. C. and ultimately to 650.degree. C. In order to raise the steam temperature, a heat-resisting material having more excellent high-temperature strength than conventionally used ferritic heat-resisting steels is required. One measure for this purpose is to use an austenitic heat-resisting alloy. To be sure, some austenitic heat-resisting alloys have excellent heat-resisting strength. Under the present situation, however, they cannot be put to practical use, for example, because they have poor thermal fatigue owing to their high coefficients of thermal expansion, they are expensive, and they are problematic from the viewpoint of design and fabrication.
Meanwhile, the so-called Cr--Mo--V steels and 12% Cr heat-resisting steels as described in Japanese Patent Publication 40-4137 and the like have conventionally been used for the high-pressure and intermediate-pressure rotors of large-sized steam turbines.
In the case of Cr--Mo--V steels, they have low strength at high temperatures and fail to provide various properties stably. Consequently, the resulting rotors are cooled with low-temperature steam. However, their limit of use will be exceeded under the above-described steam conditions being planned at present, so that Cr--Mo--V steels cannot be used for high-temperature rotors under this plan.
In the case of 12% Cr heat-resisting steels, they have higher strength at high temperatures than Cr--Mo--V steels. However, they show a reduction in long-time creep rupture strength at a steam temperature of 593.degree. C. or above, so that their limit of use will be exceeded.
For these reasons, a large number of new heat-resisting steels showing an improvement in long-time creep rupture strength have been proposed in recent years. Examples of such heat-resisting steels include those disclosed in Japanese Patent Laid-Open No. 62-103345, Japanese Patent Laid-Open No. 61-69948, Japanese Patent Laid-Open No. 57-207161, Japanese Patent Publication No. 57-25629, Japanese Patent Laid-Open No. 4-147948 and Japanese Patent Laid-Open No. 7-34202. Another heat-resisting steel over which the present invention has made an improvement is that disclosed in Japanese Patent Laid-Open No. 7-216513. Among them, 12% Cr heat-resisting steels containing Co are proposed by Japanese Patent Laid-Open No. 4-147948 and Japanese Patent Laid-Open No. 7-34202.
The steel of the former Japanese Patent Laid-Open No. 4-147948 is a heat-resisting steel in which Co is added in a relatively larger amount than in conventional alloys of the same type and in which Mo and W are added concurrently, but importance is attached to W rather than Mo in that W is added in a larger amount than conventional. When this alloy composition is compared with that of the present invention, they differ from each other, especially in the contents of Mo and W. Accordingly, this steel is considered to differ in material characteristics from the steel of the present invention. In the examples which will be given later, a steel analogous to the steel of Japanese Patent Laid-Open No. 4-147948 is used as a comparative alloy for the purpose of comparison with the steel of the present invention. According to the results thu
REFERENCES:
patent: 5310431 (1994-05-01), Buck
patent: 5415706 (1995-05-01), Scarlin et al.
patent: 5560788 (1996-10-01), Yamada et al.
Kadoya Yoshikuni
Kawai Hisataka
Magoshi Ryotarou
Sakon Toshio
Tsuji Ichirou
Mitsubishi Jukogyo Kabushiki Kaisha
Yee Deborah
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