Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
1999-04-07
2001-01-09
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C420S053000, C148S327000
Reexamination Certificate
active
06171547
ABSTRACT:
TECHNICAL FIELD
The entire disclosure of the International application No. PCT/JP98/03567, filed on Aug. 10, 1998 including specification, claims and summary, are incorporated herein by reference in its entirety.
The present invention relates to an austenitic stainless steel which has excellent sulfuric acid corrosion resistance and excellent workability. In particular, the present invention relates to an austenitic stainless steel which has excellent resistance against sulfuric acid dew-point corrosion, a problem characteristic to a variety of materials for heat exchangers, flues and chimneys used for thermal power plants and industrial use boilers, as well as structural materials including those for flue gas desulfurization equipment used in various industries and for facilities used in a sulfuric acid environment. And also, the present invention relates to an austenitic stainless steel which has excellent workability, especially excellent hot workability.
TECHNICAL BACKGROUND
So-called “fossil fuels” such as petroleum and coal, which are used as fuel for thermal power plants and industrial boilers, contain sulfur (S). Therefore, combustion of fossil fuels produces sulfur oxides (SO
x
) in the exhaust gas. When the temperature of the exhaust gas drops, SO
x
reacts with water in the gas to produce sulfuric acid, which is condensed on a material surface having a temperature lower than a dew-point, permitting occurrence of sulfuric acid dew-point corrosion. Similarly, in flue gas desulfurization equipment used in various industries, reduction of gas temperature causes sulfuric acid dew-point corrosion, if an SO
x
-containing gas flows in the equipment. Hereinafter in this specification, for the sake of simplicity, the SO
x
-containing gas is referred to as exhaust gas.
Because of the above-mentioned phenomenon, in heat exchangers and other equipment used for exhaust gas systems, the exhaust gas temperature has been maintained at 150° C. or higher so that sulfuric acid does not form dew condensation on the material surface.
However, in view of the recent increase of energy demand, and also from the viewpoint of the effective use of energy, recycling of heat energy is desired to be as effective as possible. For example, attempts have been made to lower the exhaust gas temperature of a heat exchanger to a point lower than the dew-point of sulfuric acid. Thus, materials having resistance against sulfuric acid have been demanded.
Unless the exhaust gas temperature is maintained at 150° C. or higher, an exhaust gas of a typical composition and having a temperature of about 140° C. permits dew condensation of about 80% concentrated sulfuric acid on the material surface. For such environment, various so-called “low alloy steels” have been used as steel stocks for structural use. This is because low alloy steels have higher levels of resistance against a high-temperature and high-concentration sulfuric acid than do general-purpose stainless steels.
Boshoku Gijutsu
(vol. 26 (1977), p. 731 to 740) describes that sulfuric acid corrosion accelerates in a temperature range of 20 to 60° C. lower than a sulfuric acid dew-point. This is because that the amount of condensed sulfuric acid reaches a maximum in the above-described temperature range. For this reason, unless the exhaust gas is maintained at 150° C. or higher, generally, resistance against corrosion is most required in a temperature range in the vicinity of 100° C, where the concentration of sulfuric acid becomes about 70%. However, in this temperature range, to say nothing of general-purpose stainless steels, even low alloy steels cannot be used because of high corrosion.
Patent Application Laid-Open (Kokai) Nos. 56-93860, 2-170946, 4-346638 and 5-156410 disclose that specific corrosion resistance materials are usable for a sulfuric acid environment.
Patent Application Laid-Open (Kokai) No. 6-128699 discloses a highly alloyed austenitic stainless steel which has excellent corrosion resistance in an environment containing sulfate ion, halide ion and oxidizing metal ion simultaneously. Patent Application Laid-Open (Kokai) No. 64-21038 discloses an austenitic stainless steel which has excellent pitting corrosion resistance, crevice corrosion resistance, stress corrosion cracking resistance and acid resistance. And Patent Application Laid-Open (Kokai) No. 58-52463 discloses a stainless steel which exhibits corrosion resistance in an environment containing hydrogen sulfide, and moreover, has excellent mechanical properties.
DISCLOSURE OF THE INVENTION
Of the materials proposed as having sulfuric acid corrosion resistance, Patent Application Laid-Open (Kokai) No. 56-93860 discloses “an anti-sulfuric acid corrosion alloy”, which exhibits excellent corrosion resistance in a sulfuric acid environment of about 100° C. in temperature and 95% or higher in concentration. However, because the alloy disclosed in this publication has a Cu content of as low as 0.5 to 3.0%, the alloy has poor corrosion resistance in, for example, the aforementioned sulfuric acid environment of about 100° C., where sulfuric acid concentration is about 70%. The above-mentioned alloy contains Si in an amount of 1.5% or higher, imparting to the alloy high corrosion resistance in the above-described sulfuric acid environment (temperature: about 100° C., sulfuric acid concentration: 95% or higher) . For this reason, in order to improve corrosion resistance in the environment to which the present invention is directed (for example, temperature: about 100° C., sulfuric acid concentration: about 70%), mere incorporation of a great amount of Cu into the above-described alloy, as a base alloy, results in extremely poor hot workability.
Patent Application Laid-Open (Kokai) No. 2-170946 discloses “a highly alloyed stainless steel for flues, chimneys and desulfurization equipment having excellent corrosion resistance”, which exhibits corrosion resistance in an environment where 1000 ppm Fe
3+
and 1000 ppm Cl
−
are added to 50% sulfuric acid in concentration. However, because the stainless steel disclosed in this publication has a low Cu content, i.e., from 0.5 to 2.0 wt. % Cu, the steel has poor sulfuric acid corrosion resistance in, for example, the above-stated environment where the temperature is about 100° C. and the sulfuric acid concentration is about 70%.
Patent Application Laid-Open (Kokai) No. 4-346638 discloses “a sulfuric acid dew-point corrosion-resistant stainless steel having excellent hot workability”, which contains 0.05 wt. % or more N (nitrogen) in order to stabilize austenitic structure and obtain corrosion resistance. However, the present inventors' investigation reveals that incorporation of 0.05 wt. % or more N reduces sulfuric acid corrosion resistance of austenitic stainless steels to which Cu, Cr and Mo have been added in combination. Moreover, the investigation reveals that in the case of N content of 0.05 wt. % or higher, increase of Cu content to improve sulfuric acid corrosion resistance results in an extreme reduction of hot workability in a temperature range of lower than 1000° C.
“A stainless steel for high-temperature and high-concentration sulfuric acid”, which is disclosed in Patent Application Laid-Open (Kokai) No. 5-156410, has no Cu in its chemical composition. So, the stainless steel has poor corrosion resistance in, for example, the above-mentioned environment where the temperature is about 100° C. and the sulfuric acid concentration is about 70%.
Patent Application Laid-Open (Kokai) No. 6-128699 entitled “a highly alloyed austenitic stainless steel having excellent hot workability and excellent localized corrosion resistance” discloses techniques for obtaining corrosion resistance, especially localized corrosion resistance for flue gas scrubbing equipment of an incineration system for urban garbage and so on. Therefore, the steel has excellent localized corrosion resistance in an environment where sulfate ion, halide ion and oxidizing metal ion exist simultaneously. However, in the above-described environment where the t
Azuma Shigeki
Kajimura Haruhiko
Sagara Masayuki
Clark & Brody
Sumitomo Metal Industries Ltd.
Yee Deborah
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