Thermostable and corrosion-resistant cast nickel-chromium alloy

Abstract

A nickel-chromium casting alloy comprising up to 0.8% of carbon, up to 1% of silicon, up to 0.2% of manganese, 15 to 40% of chromium, 0.5 to 13% of iron, 1.5 to 7% of aluminum, up to 2.5% of niobium, up to 1.5% of titanium, 0.01 to 0.4% of zirconium, up to 0.06% of nitrogen, up to 12% of cobalt, up to 5% of molybdenum, up to 6% of tungsten and from 0.01 to 0.1% of yttrium, remainder nickel, has a high resistance to carburization and oxidation even at temperatures of over 1130° C. in a carburizing and oxidizing atmosphere, as well as a high thermal stability, in particular creep rupture strength.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS [0001] This application is a continuation of prior filed copending PCT International application no. PCT / EP2004 / 000504, filed Jan. 22, 2004, which designated the United States and on which priority is claimed under 35 U.S.C. §120, the disclosure of which is hereby incorporated by reference, and which claims the priority of German Patent Application, Serial No. 103 02 989.3, filed Jan. 25, 2003, pursuant to 35 U.S.C. 119(a)-(d).BACKGROUND OF THE INVENTION [0002] The present invention relates to a thermostable and corrosion-resistant cast nickel-chromium alloy. [0003] Nothing in the following discussion of the state of the art is to be construed as an admission of prior art. [0004] High-temperature processes, for example those used in the petrochemical industry, require materials which are not only heat-resistant but also sufficiently corrosion-resistant and in particular are able to withstand the loads imposed by hot product and combustion gas...

AI Technical Summary

Benefits of technology

[0017] The high creep rupture strength of the alloy according to the invention, for example a service life of 2000 hours under a load of from 4 to 6 MPa and a temperature of 1200° C., guarantees that a continuous, securely bonded oxidic barrier layer is retained in the form of an Al2O3 layer which has the effect of preventing carburization and oxidation, results from the high aluminum content of the alloy and continues to top itself up or grow. As tests have shown, this layer comprises α-Al2O3 and contains at most isolated spots of mixed oxides, which do not alter the essential nature of the α-Al2O3 layer; at higher temperatures, in particular over 1050° C., in v...

Problems solved by technology

The consequence of this is internal stresses resulting from the increase in volume associated with the carbide formation or transformation and a decrease in the strength and ductility of the tube material.

Furthermore, graphite or dissociation carbon may form in the interior of the tube material, which can, in combination with internal stresses, lead to the formation of cracks, which in turn cause more carbon to diffuse into the tube material.

However, at temperatures over 1050° C., the Cr2O3 becomes volatile, and consequently the protective action of the covering layer is rapidly lost.

The resistance to carburization and oxidation is further put at risk by the limited creep rupture strength and ductility of the conventional nickel-chromium alloys, which lead to the formation of creep cracks in the chromium oxide covering layer and to the penetration of carbon and oxygen into the tube material via the cracks.

In particular in the event of a cyclical temperature loading, covering layer cracks may form and also the covering layer may become partially detached.

Tests have revealed that microstructural phase reactions, in particular at higher silicon contents, for example of over 2.5%, evidently lead to a loss of ductility and to a reduction in the short-time strength.

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