Alloy Development for High Temperature Operation and Reduced Coking

摘要

In 2003 S+C launched a new High Performance Alloy named "Centralloy~® 60HT" for commercial application. Compared to our standard Centralloy~® products, the resulting properties of Centralloy~® 60HT are: Thermal and mechanical resistance up to 1200℃ tube wall temperature, due to an unique balance of strengthening elements which acts as the "backbone" in order to achieve application temperatures beyond 1150℃. The special metallurgy of Centralloy® 60 HT reduces the formation of catalytic coke significantly. The Aluminum content in the alloy forms a tight Aluminum oxide scale and thus provides a barrier between the active Nickel and Iron catalyst of the bare tube and the hydrocarbon. This reduces catalytic coke formation and lengthens the time a furnace can operate between decokes. Aluminiumoxide scales are known to be thermodynamically, the most stable protective scales of metallic materials. The slow growth rate of the scale ensures the materials survival in long-term service applications up to temperatures well above 1150℃. The resistance of conventional materials against carburization is provided by the Ni-content in the alloys matrix and the presence of protective oxide layers (Chromium oxide layer and a Silicon oxide subscale). Therefore, modern materials have a high Nickel content (well above 40%), Chromium contents of a minimum of 25-30% and contain 1,5-2,5%Si. In carburizing atmospheres at temperatures above 1100℃, the protective chromia scale fails by conversion to carbides, allowing ingress of carbon (carburization). Centralloy~® 60 HT excludes carbon ingress almost completely from the alloy by forming a stable (up to > 1200℃) Aluminumoxide scale. Additionally, the high Nickel content reduces the diffusion rate of carbon into the alloy matrix. Despite the highest degree of "metallurgically active" alloying, the material can be machined, bent (by induction bending) and welded like any other Centralloy~® product. A wide range of field tests, started in 2003 in the Petrochemical Industry, show very promising results and the first full furnace applications have been installed in steam crackers during 2004.