At high temperatures, currently available cast stainless steel alloys used for engine component applications do not have the long-term stability of their original castings, and are lacking in their ability to resist deformation and cracking from extreme temperature changes. There is a need for high-strength, oxidation-resistant, and crack-resistant cast alloys for use in internal combustion engine components such as exhaust manifolds and turbocharger housings, gas-turbine engine components such as combustor housings, and other components that must function in extreme environments for prolonged periods of time.
Current materials used for these applications are limited by oxidation and corrosion resistance as well as by strength at high temperatures and detrimental effects of aging. Specifically, current exhaust manifold materials — such as high silicon and molybdenum cast ductile iron (Hi–Si–Mo), and austenitic ductile iron (Ni-resist) — must be replaced by cast stainless steels when used for more severe applications such as higher operating temperatures, or when longer operating lifetimes are demanded due to increased warranty coverage.
The currently commercially available cast stainless steels include ferritic stainless steels such as NHSR-F5N, or austenitic stainless steels such as NHSR-A3N, CF8C, and CN-12. These currently available cast stainless steels are deficient in terms of tensile and creep strength at temperatures exceeding 600 °C, do not provide adequate cyclic oxidation resistance for temperatures exceeding 700 °C, do not provide sufficient room temperature ductility either as-cast or after service exposure and aging, do not have the requisite long-term stability of the original microstructure, and lack long-term resistance to cracking during severe thermal cycling.
The corrosion-resistant grade of cast austenitic stainless steel, CN-12, is in commercial use for automotive applications, but is not optimized for extended-service applications (e.g. diesel applications). Currently commercially available CN-12 austenitic stainless steel includes about 25 wt.% chromium, 13 wt.% nickel, and smaller amounts of carbon, nitrogen, niobium, silicon, manganese, molybdenum, and sulfur. The addition of sulfur is considered essential or desirable for machinability from the cast material. The amount of added sulfur ranges from 0.11 wt.% to 0.15 wt.%.
The present invention may be characterized as a heat-resistant and cast, corrosion-resistant austenitic stainless steel alloy that comprises from about 0.2 wt.% to about 0.5 wt.% carbon, from about 2.0 wt.% to about 10 wt.% manganese, and less than about 0.03 wt.% sulfur.
At a cost comparable to currently available alloys, the enhanced properties of these new alloys will enable manufacturers to produce parts that can operate at high temperatures and extreme temperature changes while providing superior performance, reliability, and durability. These alloys will also help manufacturers meet emission regulations for diesel, turbine, and gasoline engine applications. A significant benefit to consumers is that manufacturers will be able to reduce the weight of engines by using designs that are thinner than those used for conventional materials, resulting in increased fuel efficiency. In addition, although these alloys are aimed at the production of engine components, they could be used for other applications as well.