Heat and corrosion resistant cast austenitic stainless steel alloy with improved high temperature strength

Abstract

A heat and corrosion resistant cast austenitic stainless steel alloy which contains less than about 15% nickel. The alloy has a completely austenitic microstructure in an as-cast state, and a creep rupture life exceeding 20,000 hrs at a stress of 35 MPa and a temperature of 850° C., when creep tested in the as-cast state under ASTM E139 test conditions.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of U.S. patent application Ser. No. 10 / 195,724 filed Jul. 15, 2002, the disclosure of which is incorporated by reference herein.[0002] This invention was made with government support under the terms of Contract No. DE-AC05-960R2264 awarded by the U.S. Department of Energy. The government may have certain rights in this invention.TECHNICAL FIELD [0003] This disclosure relates generally to cast austenitic stainless steel alloys of the CF8C type with improved high temperature strength. More particularly, this disclosure relates to CF8C type stainless steel alloys and articles cast therefrom having excellent high temperature creep strength, and aging resistance, and which exhibit a stable austenitic microstructure with substantially no delta ferrite after casting and high temperature aging. BACKGROUND [0004] There is a need for high strength, oxidation resistant and crack resistant cast alloys for ...

AI Technical Summary

Benefits of technology

[0011] In one aspect, the present disclosure is directed to a heat and corrosion resistant cast austenitic stainless steel alloy which contains less than about 15% nickel. The alloy has a completely austenit

Problems solved by technology

However, it is being pushed beyond its high temperature strength and corrosion limitations.

However, the cost of nickel makes nickel based super alloys expensive, and turbine manufacturers are considering lower-cost alternatives for casings and large structural components.

These material issues are not unique to diesel engines and combustion turbines.

A significant factor in the cost of producing a casting is the post casting stress relief or solution heat treatment typically required for stainless steel castings.

However, standard practice is to solution treat CF8C castings at 1050° C., which like discussed above, may increase its cost for some applications.

CF8C is a niobium stabilized grade of austenitic stainless steel most suitable for applications at temperatures below 500° C. In the standard form CF8C has poor strength at temperatures above 600° C. It also does not provide adequate cyclic oxidation resistance at temperatures exceeding 700° C., does not provide sufficient ductility, does not have the requisite long-term stability of the original microstructure after high temperature aging, and lacks long-term resistance to cracking during severe thermal cycling.

As described earlier, presence of ferrite and sigma phases deteriorates the properties of steels exposed to high temperatures.

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