A new wrought alloy has been developed for use as furnace tubes in ethylene pyrolysis plants. This alloy has an excellent carburization resistance due to uniform formation of protective A1203 oxide scale on the metal surface. At temperatures between 1000°C and 1150°C, laboratory corrosion tests have been Carried out to evaluate carburization resistance of the developed alloy. Commercial austenitic stainless steels used for furnace tubes were tested for comparison. In a simulated carburizing environment of 15vol%CH4-3%CO2-82%H2 gas mixture, the developed alloy has a three times better carburization resistance than the conventional austenitic stainless steels containing more than 25mass% chromium and high silicon at evaluated temperatures.
Another laboratory test has been conducted to clarify carburization and coking resistance under conditions of cyclic carburization and oxidation environments. Carbon ingress and coke deposition of conventional alloys forming chromium oxide scale increased with increasing heat cycles, whereas the developed alloy remained unchanged. Based on these results, carburization and coking behavior of alloys used as ethylene pyrolysis furnace tubes has been explained.
Ethylene pyrolysis in the petrochemical industries is considered the most important process for producing chemical products. Some furnace tubes used for ethylene pyrolysis suffer severe material damage since outside tube surfaces are heated up to approx. 1100°C in contact with steam-hydrocarbon gas mixtures inside ~ -3. Deterioration of mechanical properties due to coking (carbon deposition on the inside tube surface) and carburization can result in a failure of tube materials 4-6. Studies on carburization 7 showed that Ni, Cr, Si and A1 are effective alloying elements to increase the carburization resistance for high temperature alloys. Carbon-stabilizing elements such as Mo, Ti and Nb have been considered beneficial as well. On this basis, carburization-resistant alloys such as alloy HK40 containing high Cr, and alloy HP containing high Cr, Ni and Si have been applied as pyrolysis furnace tubes 8'9, and alloys containing higher Cr and Ni are under development ~°. However, excess addition of these elements to alloys does not increase the carburization resistance significantly; instead, it results in reducing malleability and weldability. Moreover, protective chromium oxide formed on the tube inside metal surface can react to form chromium carbides such as Cr7C3 and Cr2C3 at temperatures above 1030-1040°C, which anticipated according to a thermodynamic analysis calculating equilibrium P02 of the inside tube gas atmosphere of actual ethylene pyrolysis furnaces ~. Hence, in these temperatures, protection by Cr203 oxide scale is not realistic.
In recent studies, alloys containing high aluminum in a nickel base alloy ~2, an oxide dispersion strengthened (ODS) alloy ~3, and an intermetallic alloy ~°, were reported. Surface-modified alloys coated by aluminum or its compounds have been tested in some ethylene pyrolysis furnaces 14'15. These alloys are characterized by the formation of protective aluminum oxide film on metal surface to prevent carbon ingress and to minimize coking, but due to inherent lack of ductility for ODS and intermetallic alloys, welding and bending are usually difficult and troublesome upon manufacturing tube bundles for constructing ethylene pyrolysis furnaces.
A new carburization-resistant alloy with excellent carburization resistance to the ethylene pyrolysis environments has been developed. This alloy has good malleability, high temperature creep strength, and weldability for use as furnace tube in pyrolysis plants. This study was intended