Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-05-27
2001-04-17
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S909000, C148S590000
Reexamination Certificate
active
06217676
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a steel having corrosion resistance to carbon dioxide and/or seawater environments. The steel is useful as an oil well pipe, especially a seamless pipe.
BACKGROUND OF THE INVENTION
Recently, so-called sweet oil wells containing carbon dioxide (referred to as CO
2
hereafter) have been exploited because of increasing energy demand and a shortage of high quality oil resources that can be easily exploited. In addition, exploitation of rather small-scale oil wells, which have a short production life up to about 10 years because of relatively small reserves, is increasing. When the production efficiency of an oil well decreases, deaired (degassed) seawater is injected into the pipe in order to recover the oil production efficiency.
In the situation as mentioned above, an oil well pipe having high corrosion resistance to both CO
2
and seawater, which contains small amounts of dissolved oxygen of about 500 ppb, is required. The seawater containing a small amount of dissolved oxygen as mentioned above, is referred to as “seawater” in this specification.
Conventionally an inhibitor is used to suppress corrosion of carbon steel pipes, when the pipe is used for both oil production and seawater injection. The inhibitor, however, not only increases production cost but also induces pollution. Therefore, there is a need in the art for an oil well pipe of steel which has sufficient corrosion resistance to eliminate the inhibitor.
It is known from the publications by A. Ikeda, M. Ueda and S. Mukai “Corrosion/83” NACE Houston, Paper No. 45, 1983, and Masakatsu Ueda and A. Ikeda “Corrosion/96” NACE Houston, Paper No. 13, 1996 that the corrosion rate of steel in CO
2
environments decreases and resistance to general corrosion is improved, according to an increase of Cr content. In fact, the JIS SUS 410 series steels, which contain 12 to 13% of Cr (“%” for content of alloy elements means weight % in this specification) have already been utilized for oil well pipe.
However, the SUS 410 series steels are expensive because of the high Cr content thereof. In addition, such high Cr steels have a disadvantage in that they suffer localized corrosion (pitting) in seawater containing little dissolved oxygen.
A steel containing smaller amounts of Cr and cheaper than the 12 to 13% Cr steel is desired for an oil well pipe used for short life wells as described above. Furthermore, considering seawater injection, a steel having resistance to localized and general corrosion in seawater, i.e., a seawater resistant steel, is necessary.
Japanese Examined Patent Application 53-38687 discloses a low alloy seawater resistant steel containing 1.0 to 6.0% Cr and 0.1 to 3.0% Al. However, this steel is not for an oil well pipe, and the CO
2
corrosion resistance thereof is not known.
Japanese Laid-Open Patent Publication No. 57-5846 discloses a steel containing 0.5-5% Cr and having resistance to sweet corrosion. While this reference states that such steel has good corrosion resistance in seawater containing CO
2
, the resistance is merely the general corrosion resistance, which has been estimated by corrosion weight loss. In addition, the microstructure thereof cannot be determined because the producing method of the steel is not disclosed.
Japanese Examined Patent Application No. 57-37667 proposes a wet CO
2
resistant steel for line pipes, which contains more than 3.0% to 12.0% Cr. This steel's resistance against localized corrosion is improved in specific areas such as the welded portion, where the heat treatment history is different from other areas. The steel, however, cannot have a single phase martensite microstructure because of its low C content. Therefore, its tensile strength is low and its resistance to localized corrosion when used as a pipe is not sufficient.
Japanese Laid-Open Patent Publication No. 5-112844 discloses a steel pipe, which has good CO
2
corrosion resistance and can be used for oil well pipes. However, the Cr content of this steel pipe is as low as 0.25-1.0%. Further, the pipe was not designed to improve the seawater corrosion resistance. In addition, the CO
2
corrosion resistance of this pipe is improved mainly by a decarburized layer of more than 100 &mgr;m thickness, which is formed in the inner surface of the pipe.
As mentioned above, it is already well known that increasing the Cr content improves the general corrosion resistance of the steel in CO
2
environments. However, it is uneconomical to use steel having more than 10% Cr for short life oil wells such as 10 years or less. In addition, steel containing such a high content of Cr has the disadvantage of localized corrosion (pitting) in seawater of low dissolved oxygen. The oil well pipe becomes useless after suffering localized corrosion, which passes through the pipe wall, even if it has good general corrosion resistance. This means that not only general corrosion resistance but also localized corrosion resistance is remarkably important in a steel for an oil well pipe.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a steel that can exhibit one or more of the following properties:
1) Yield strength not less than 552 MPa (yield strength of API 80 grade or more) in a heat-treated condition by quenching-tempering or normalizing-tempering;
2) Superior resistance to localized corrosion in wet CO
2
environments and seawater of low dissolved oxygen; and
3) Superior resistance to general corrosion in seawater of low dissolved oxygen.
Another objective of the present invention is to provide a comparatively inexpensive seamless oil well pipe made of the above mentioned steel.
The inventors have investigated the means to improve the resistance of steel for an oil well pipe to localized corrosion in CO
2
environments and corrosion in seawater. The inventors thereby have found the fact that the resistance not only to localized corrosion in CO
2
environments, but also to the corrosion in seawater can be remarkably improved by making the microstructure substantially of single phase martensite in a condition as quenched or as normalized.
It is known that localized corrosion resistance to wet CO
2
environments of Cr-free carbon steel depends on the microstructure, and it is also known that the ferrite - pearlite duplex (dual-phase) structure is better than the single homogeneous martensite structure for localized corrosion resistance. However, according to the investigation by the present inventors, in steel containing Cr, the single phase martensitic structure has superior resistance to localized corrosion in wet CO
2
environments.
This invention provides, on the basis of the foregoing finding, a steel for an oil well pipe, which can have the following characteristics.
(a) Chemical Composition:
The steel consists essentially of, in weight %, more than 0.10 to 0.30% of C, 0.10 to 1.0% of Si, 0.1 to 3.0% of Mn, 2.0 to 9.0% of Cr and 0.01 to 0.10% of Al, and the balance of Fe and incidental impurities including not more than 0.03% P and not more than 0.01% S. Furthermore, 0.05 to 0.5% of Cu, as an alloy element, may also be contained in the steel.
(b) Microstructure:
The microstructure is substantially single phase martensite in the as-quenched or as-normalized condition. The terminology “substantially single phase martensite” denotes a microstructure in which about 95% or more, in the cross-sectional area ratio, is martensite. In addition to martensite, less than about 5% in total of ferrite, bainite and/or pearlite can be allowed in the microstructure.
(c) Strength:
The yield strength is not lower than 552 MPa after heat treatment of “quenching-tempering” or “normalizing-tempering”.
The present invention also provides a seamless oil well pipe, which is made of the above-mentioned steel and has excellent resistance to wet CO
2
corrosion and seawater corrosion.
REFERENCES:
patent: 3684493 (1972-08-01), Kubota et al.
patent: 5049210 (1991-09-01), Miyasaka et al.
patent: 2756191 (1978-07-01), None
patent: 1568616 (1980-06-01), None
patent: 49-52117 (1974-05-01)
Takabe Hideki
Ueda Masakatsu
Burns Doane , Swecker, Mathis LLP
Sumitomo Metal Industries Ltd.
Yee Deborah
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