Abstract

Carburization is a failure mechanism common to the petrochemical industry in Catalytic Reforming Units (CRU's) where atmospheres containing hydrocarbons and/or carbon monoxide are prominent. Elevated fuel prices cause refineries to run with low excess oxygen to generate cost savings. The resulting atmosphere at elevated temperatures creates an environment where carbon is favorably transferred to iron and low alloy steels, forming a hardened layer of carbides that reduce the life of the steel tubing and vessels.

Ceramic coatings have previously been applied in CRUs to increase radiant efficiency. A dual functionality was hypothesized for select materials to aid in the prevention of carburization. To evaluate this potential, ceramic coatings were applied to a commonly used low-alloy steel tube material and exposed to a low-oxygen, high-temperature, carbon-rich environment. Chemical etching, optical microscopy, and microhardness evaluations were completed to determine the degree of carburization on the ceramic-coated and uncoated control tube surfaces. Results indicate that some ceramic coating materials are highly effective at the prevention of carburization of the treated tube surface when compared to the uncoated control. The implementation of cladding with this class of materials would be optimal for not just improving heat transfer but additionally extending the serviceable life of CRU units in low excess oxygen environments.

Introduction

Common materials employed in catalytic reforming unit tubes are typically resistant to carburization due to protective chromium oxide films, but under low excess oxygen conditions can become compromised and allow carbon penetration and carbide formation at the exposed surface. Embrittlement and material wastage as a result of these mechanisms causes premature failures, with production loss, in addition to shutdown maintenance and replacement costs. Carburization in this environment is simulated in this paper through a pack carburizing method designed to create an environment optimal for diffusing carbon in an ASTM 335 9Cr-1Mo tube material.

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