Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Reexamination Certificate
2000-08-18
2002-03-05
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
C420S052000, C420S049000, C148S327000
Reexamination Certificate
active
06352670
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
FEDERALLY SPONSORED RESEARCH
Not Applicable
TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION
The present invention relates to an oxidation and corrosion resistant austenitic stainless steel. More particularly, the present invention relates to an austenitic stainless steel adapted for use in high temperature and corrosive environments, such as, for example, use in automotive exhaust system components. The austenitic stainless steel of the invention finds particular application in components exposed to temperatures up to 1800° F. and to corrosive environments, such as, for example, chloride-rich waters.
DESCRIPTION OF THE INVENTION BACKGROUND
In the manufacture of automotive exhaust system components, concurrent goals are to minimize both cost and weight, while also maintaining the integrity of the system. Typically, automobile components for these applications are fabricated from thin stainless steel stock in order to minimize the weight of the components and, therefore, the components' resistance to corrosive attack must be high to prevent failure by perforation or other means. Corrosion resistance is complicated by the fact that components used for certain automotive exhaust system applications are exposed to severely corrosive chemical environments at elevated temperatures. In particular, automotive exhaust system components and other automotive engine components are exposed to contamination from road deicing salts under conditions of elevated temperature due to the hot exhaust gases. The stainless steel and other metal components subjected to these conditions are susceptible to a complex mode of corrosive attack known as hot salt corrosion.
Typically, at higher temperatures, stainless steel components undergo oxidation on surfaces exposed to air to form a protective metal oxide layer. The oxide layer protects the underlying metal and reduces further oxidation and other forms of corrosion. However, road deicing salt deposits may attack and degrade this protective oxide layer. As the protective oxide layer is degraded, the underlying metal may be exposed and become susceptible to severe corrosion.
Thus, metal alloys selected for automotive exhaust system components are exposed to a range of demanding conditions. Durability of automotive exhaust system components is critical because extended lifetimes are demanded by consumers, by federal regulations, and also under manufacturers' warranty requirements. To further complicate alloy selection for automotive exhaust system components, a recent development in these applications is the use of metallic flexible connectors, which act as compliant joints between two fixed exhaust system components. Flexible connectors may be used to mitigate problems associated with the use of welded, slip, and other joints. A material chosen for use in a flexible connector is subjected to a high temperature corrosive environment and must be both formable and have resistance to hot salt corrosion and various other corrosion types, such as, for example, intermediate temperature oxidation, general corrosion, and chloride stress corrosion cracking.
Alloys for use in automotive exhaust system flexible connectors often experience conditions in which elevated temperature exposure occurs after the alloy has been exposed to contaminants such as road deicing salts. Halide salts can act as fluxing agents, removing the protective oxide scales which normally form on the connectors at elevated temperatures. Degradation of the connectors may be quite rapid under such conditions. Therefore, simple air oxidation testing may be inadequate to reveal true resistance to corrosive degradation in service.
The automotive industry uses several alloys for manufacturing automotive exhaust system components. These alloys range from low cost materials with moderate corrosion resistance to high cost, highly alloyed materials with much greater corrosion resistance. A relatively low cost alloy with moderate corrosion resistance is AISI Type 316Ti (UNS Designation S31635). Type 316Ti stainless steel corrodes more rapidly when exposed to elevated temperatures and, therefore, is not generally used in automotive exhaust system flexible connectors when temperatures are greater than approximately 1200° F. Type 316Ti is typically only used for automotive exhaust system components which do not develop high exhaust temperatures.
Higher cost, more highly alloyed materials are commonly used to fabricate flexible connectors for automotive exhaust systems exposed to higher temperatures. A typical alloy used in the manufacture of flexible connectors that are subjected to elevated temperature corrosive environments is the austenitic nickel-base superalloy of UNS Designation N06625, which is sold commercially as, for example, ALLEGHENY LUDLUM ALTEMP® 625 (hereinafter “AL 625”). AL 625 is an austenitic nickel-based superalloy possessing excellent resistance to oxidation and corrosion over a broad range of corrosive conditions and displaying excellent formability and strength. Alloys of UNS Designation N06625 generally comprise, by weight, approximately 20-25% chromium, approximately 8-12% molybdenum, approximately 3.5% niobium, and 4% iron. Although alloys of this type are excellent choices for automotive exhaust system flexible connectors, they are quite expensive compared to Type 316Ti alloys and other iron-based alloys.
Automotive exhaust system component manufacturers may use other alloys for constructing exhaust system flexible connectors. However, none of those alloys provide high corrosion resistance, especially when exposed to elevated temperatures and corrosive contaminants such as road deicing salts.
Thus, there exists a need for a corrosion resistant material for use in high temperature corrosive environments that is not as highly alloyed as, for example, alloys of UNS Designation N06625 and which, therefore, is less costly to produce than such superalloys. More particularly, there exist a need for an iron-base alloy which may be formed into, for example, light-weight flexible connectors and other components for automotive exhaust systems and which will resist corrosion from corrosive substances such as salt deposits and other road deicing products at elevated temperatures.
SUMMARY OF THE INVENTION
The present invention addresses the above described needs by providing an austenitic stainless steel comprising, by weight, 17 to 23% chromium, 19 to 23% nickel, and 1 to 6% molybdenum. The addition of molybdenum to the iron-base alloys increases their resistance to corrosion at high temperatures.
The present invention also provides an austenitic stainless steel consisting essentially of, by weight, 17 to 23% chromium, 19 to 23% nickel, 1 to 6% molybdenum, 0 to 0.1% carbon, 0 to 1.5% manganese, 0 to 0.05% phosphorus, 0 to 0.02% sulfur, 0 to 1.0% silicon, 0.15 to 0.6% titanium, 0.15 to 0.6% aluminum, 0 to 0.75% copper, iron, and incidental impurities.
Austenitic stainless steels according to the present invention exhibit enhanced resistance corrosion by salt at a broad temperature range up to at least 1500° F. Articles of manufacture of the austenitic stainless steel as described above are also provided by the present invention. Thus, the stainless steel of the present invention would find broad application as, for example, automotive components and, more particularly, as automotive exhaust system components and flexible connectors, as well as in other applications in which corrosion resistance is desired. The alloy of the present invention exhibits excellent oxidation resistance at elevated temperatures and, therefore, finds broad application in high temperature applications, such as for heating element sheaths. The present invention also provides methods of fabricating an article of manufacture from the austenitic stainless steels comprising, by weight, 17 to 23% chromium, 19 to 23% nickel, and 1 to 6% molybdenum.
The reader will appreciate the foregoing details and advantages of the present invention, as well as o
ATI Properties Inc.
Viccaro P. J.
Yee Deborah
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