Abstract

A new modified 9 wt% Cr steel has been developed in order to improve the resistance against coke deposition on the internal surface of refinery tube furnaces, in comparison to widely used grades, such as Grade 5 (5 wt% Cr) or Grade 9 (9 wt% Cr). The new grade has an improved composition, based on Cr and Si and further additions of Cu and Ni. This optimal chemical analysis has been specified after extensive laboratory testing on different laboratory and industrial heats. Thermogravimetric analyses have been performed to benchmark various materials (ferritic and austenitic grades) in terms of coking rate. Specimens of these alloys have been exposed to this coking atmosphere in a wide temperature range. The new modified 9Cr steel exhibits an almost 10 times lower coking rates than typical Grade 9 steel. The new 9Cr steel shows allowable stress levels up to 90% higher than Grade 9 at temperatures below 500°C (time independent regime) and up to 7% higher stress levels at temperatures above 500°C (creep regime). The industrial feasibility of production of elbows has been successfully implemented and a welding solution using a commercially available filler material has been established.

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