Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-08-27
2001-10-09
King, Roy (Department: 1742)
Metal treatment
Stock
Ferrous
C148S328000, C148S333000, C420S034000, C420S104000
Reexamination Certificate
active
06299704
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to heat resisting steel. More particularly, the present invention relates to heat resisting steel excellent in creep strength at over 650° C. and with little deterioration of steam oxidation resistance, which is useful for a material for apparatuses operated under high pressure and high temperature over 630° C. which has been thought to be a critical temperature of conventional ferritic steel. The heat resisting steel of the present invention is especially useful for steel pipes provided for heat exchanging boilers, steel sheets for pressure vessels, or a turbine material.
DESCRIPTION OF THE PRIOR ART
Boilers and turbines for electric power generation, nuclear power plants, or chemical industrial apparatuses are used under high temperature and high pressure for a long time. Heat resisting steel adopted to these apparatuses is required to be excellent in strength, corrosion resistance, oxidation resistance under high temperature and ductility under room temperature. For such heat resisting steel, austenitic stainless steel such as JIS-SUS321H and JIS-SUS347H, low alloy steel such as JIS-STBA24(2·1/4Cr-1Mo) and high chromium ferritic steel such as JIS-STBA26(9Cr-1Mo) have been utilized. High chromium ferritic steel is more excellent than low alloy steel in strength and corrosion resistance at temperature range of 500~650° C. High chromium ferritic steel also has several strong points compared with austenitic stainless steel, that is, lower price, higher coefficient of heat conductivity, and smaller coefficient of thermal expansion. Besides, high chromium ferritic steel is excellent in thermal stress resistance and has attractive properties that scale peeling is not likely to occur and that stress corrosion cracking does not happen.
With regard to thermal power generation, increase of both steam temperature and pressure of a boiler has been promoted in order to improve thermal efficiency of a boiler. Conversion from a supercritical pressure condition of 538° C. and 246 atmosphere to an extra supercritical pressure condition of 650° C. and 350 atmosphere is now being planned. According to the change of a steam condition, required properties for boiler steel pipes are severer and therefore it has been difficult for high chromium ferritic steel to meet several requirements such as long time creep strength, oxidation resistance, and, especially, steam oxidation resistance. Steam oxidation is an oxidizing phenomenon at the surface of boiler pipes exposed to steam with high temperature and high pressure. When oxide films as a scale are formed, they peels off according to the change of boiler temperature. Since a scale after peeling clogs steel pipes, suppression of steam oxidation is one of the most important problems to be solved.
Some austenitic stainless steel which meets the requirement only for steam oxidation resistance has been developed, but this is not available to steam plumbing with large diameter and large thickness because the austenitic stainless steel has several defects in ductility and in both thermal stress resistance and thermal shock resistance against thermal distortion based on the operating schedule of the power plant. Other scientific efforts have been made, in which critical temperature is increased by improving properties of high chromium ferritic steel.
As a result of them, heat resisting steel has been obtained by adding tungsten(W) to the conventional high chromium ferritic steel. The Japanese patent provisional publication 3-97832 discloses that large amount of W is added to high chromium ferritic steel and that copper(Cu) is also added to improve oxidation resistance at high temperature. The Japanese patent provisional publications 4-371551 and 4-371552 propose high chromium ferritic steel with both increased strength at high temperature and ductility by adding W and molybdenum(Mo) in suitable ratios together with cobalt(Co) and boron(B).
These kinds of high chromium ferritic steel above-mentioned include so large amount of W that they are excellent in creep strength at high temperature. W as well as Mo and chromium(Cr) is, however, one of the constituent elements for ferrite. When the amount of W is excessively large, &dgr;-ferrite is formed in steel, resulting in deterioration of ductility.
A martensite single phase is effective for preventing from deteriorating ductility. The Japanese patent provisional publication 5-263196 discloses that a martensite single phase is formed in heat resisting steel with small amount of Cr. The Japanese patent provisional publications 5-311342, 5-311343, 5-311344, 5-311345, and 5-311346 propose high chromium ferritic steel with increased ductility, which is realized by adding constituent elements for austenite such as nickel(Ni), Cu, and Co.
The high chromium ferritic steel disclosed in the Japanese patent provisional publication 5-263196, however, has a defect in that steam oxidation resistance is not sufficient because Mo and Ni destroy fine and stable scale layers formed on the surface of steel which consist of Cr
2
O
3
characterized as a corundum type. With regard to the high chromium ferritic steel disclosed such as in the Japanese patent provisional publication 5-311342, since the steel includes a large amount of Ni and Cu, both A
1
and A
3
transformation temperatures are low and therefore resistance to temper softening is so small that long time creep strength is low. Inclusion of Ni and Cu also changes the structure of oxides such as Cr
2
O
3
, this leading to deterioration of steam oxidation resistance of high chromium ferritic steel.
As above-mentioned, the conventional high chromium ferritic steel has a serious defect in that long time creep strength at over 600° C. is low. This is primarily because strengthening mechanisms brought by a ferritic matrix phase, a carbide of M
23
C
6
or M
6
C, a carbo-nitride of MX, and an intermetallic compound such as a Laves phase, which are precipitated in a final stable structure, are deteriorated at high temperature.
Strengthening in a martensite lath grain, or strengthening both a former austenite grain boundary and a martensite lath interface is considered in order to improve creep strength of ferritic steel with the structure above-mentioned. It has been thought that stabilization by MX is effective for the former and that M
23
C
6
and Laves phase are effective for the latter. According to the idea, several alloys have been designed. However, no steel with remarkably increased creep resistance at high temperature has been actually obtained up to now.
The present invention has an object to overcome the limit of the conventional technology and to provide heat resisting steel excellent in a long time creep property even at over 650° C.
This and other objects, features and advantages of the invention will become more apparent upon a reading of the following detailed specification and drawings, in which:
REFERENCES:
patent: 4244754 (1981-01-01), Masumoto et al.
patent: 3-97832 (1991-04-01), None
patent: 4-371552 (1992-12-01), None
patent: 4-371551 (1992-12-01), None
patent: 5-263196 (1993-10-01), None
patent: 5-311343 (1993-11-01), None
patent: 5-311344 (1993-11-01), None
patent: 5-311345 (1993-11-01), None
patent: 5-311342 (1993-11-01), None
Igarashi Masaaki
Muneki Seiichi
Coy Nicole
Japan as represented by Director General of National Research In
King Roy
Wenderoth , Lind & Ponack, L.L.P.
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