Metal treatment – Stock – Ferrous
Patent
1993-12-08
1995-06-27
Yee, Deborah
Metal treatment
Stock
Ferrous
420 68, 420 69, C22C 3826, C22C 3828
Patent
active
054276349
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a ferrite system stainless steel which is used as a high temperature member of an exhaust pipe and a catalyst outer cylinder of automobiles, an exhaust duct of a power generation plant, etc, has an excellent high temperature strength and is particularly excellent in a NaCl-induced hot corrosion resistance.
BACKGROUND ART
Improvements in fuel-to-cost performance and power output of automobiles have been desired in recent years, and a reduction in the weight of automobile materials has been strongly desired. Also, to strengthen a regulation of pollution, purification of an exhaust gas has also been strongly desired. With such a background, as exhaust system materials for automobiles, SUS430LX and AISI409, as existing ferrite system stainless steels, have been used to reduce the weight of exhaust system materials for automobiles and to accomplish a lower thermal capacity of the components. With the improvement in fuel-to-cost performance and a higher output, the highest temperature of the exhaust gas now reaches about 900.degree. C., and the temperature is about 900.degree. C. in the vicinity of an exhaust manifold and about 600.degree. C. in the vicinity of a front pipe. For these reasons, heat-resistant materials used for these thin sheet structures must satisfy the following material properties:
(1) The materials have a high high-temperature strength and high-temperature strength during use.
(2) The materials have excellent thermal fatigue characteristics and high temperature fatigue characteristics.
(3) The materials have excellent normal temperature workability.
(4) The materials have excellent NaCl-induced hot corrosion resistance.
(5) The materials have excellent oxidation resistance.
(6) The materials form a weld bead shape which does not invite stress concentration.
It is expected that if the high temperature strength is improved and a high temperature strength during use can be secured, a high temperature fatigue as well as thermal fatigue characteristics can be improved. In other words, when the characteristics (1) are satisfied, the characteristics (2) can be accomplished, too. The structure described is welded structure made of thin sheets. Therefore, weldability and thermal fatigue characteristics at the weld portions are important. Accordingly, the item (6) is also very important.
Further, the NaCl-induced hot corrosion resistance (4) has drawn a particular attention in exhaust system components of automobiles in which the thickness of said component has been considerably reduced. When automobiles run on the road on which an antifreezing mixture (consisting primarily of salt) is scattered in the cold season, this antifreezing mixture adheres to exhaust system components of the automobile, particularly to an exhaust manifold and a front pipe, which are heated to about 600.degree. C. by an exhaust gas. Then, the surface of these components is corroded and reduction of the thickness takes place until at last breakage of the components occurs and a critical accident is invited.
Accordingly, an improvement in the high temperature tensile strength and NaCl-induced hot corrosion resistance of the materials of the exhaust system of the automobiles due to the increase in the exhaust gas temperature has become a serious problem.
As technologies for improving the various characteristics described above, the following prior art references are known.
Japanese Unexamined Patent Publication (Kokai) Nos. 64-8254, 3-274245 and 4-74852 describe technology relating to the exhaust manifold. Japanese Unexamined Patent Publication (Kokai) No. 64-8254 discloses a composition in which at least 17% of Cr is added in order to improve the oxidation resistance, which uses Nb as an essential component and Mo as a selective element. However, this reference does not add an element (e.g. Ti.) having higher affinity with C and N than Nb and Mo. Therefore, this reference does not consider how to secure the high temperature strength during use because Nb is in a state where it is lik
REFERENCES:
patent: 4286986 (1981-09-01), Borneman
"Localized Corrosion Resistance of High Alloy Austenitic Stainless Steels and Welds", N. Suutala, et al., Stainless Steel '84, Gotebrog, Sweden, pp. 240-247.
Fujita Nobuhiro
Ohmura Keiichi
Sato Eiji
Nippon Steel Corporation
Yee Deborah
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