Seminar Series

Carburized Ethylene Pyrolysis Tubes – An Analysis of the Relationship between Microstructure, Creep Performance, and Magnetic Response


Amy McLeod


UC Mechanical Engineering PhD candidate

Time & Place

Tue, 29 Mar 2016 09:00:00 NZDT in Erskine 101


Ethylene is used extensively in the production of plastics, cabling, and automotive products. It is typically produced by the thermal cracking (pyrolysis) or more complex hydrocarbons, such as naphtha or ethane, at 850 1100 C inside radiant tubes within an ethylene pyrolysis furnace. The radiant tubes are typically either HP (25%Cr-35%Ni) or ET45 (35%Cr-45%Ni) alloys - the latest in a series of high alloy, heat resisting stainless steels developed in order to provide the strength, ductility, corrosion, and oxidation resistance necessary in the carbon, hydrogen, and oxygen rich environment typical of ethylene pyrolysis furnaces.

The main contributor to the failure of ethylene pyrolysis tubes is carburization of the tubes, which causes an increase in internal volume, a loss in weldability, and a reduced ability to withstand thermal cycles. Plant operators prefer to replace tubes at planned outages in order to minimize downtime, but there is currently no reliable end-of-life indicator by which tube replacement decisions can be made. Knowing the level of carburization of an in-situ tube can assist in remaining life estimates based on finite element analysis modelling, thermography, and fracture mechanics. Due to changes in the microstructure and magnetic properties of the tubes over their service life, the level of carburization can be detected non-destructively using eddy current probes. However, the eddy current system requires calibration on ex-service tubes that have had their microstructure, mechanical properties, and magnetic response characterized. In order to improve the interpretation of the eddy current non-destructive testing results and improve the accuracy of the material property data necessary for the FEA models, fundamental research of carburized tubes is necessary. In the present work, detailed characterization of carburized microstructures has been performed, and the effects of carburized microstructures on the mechanical properties and magnetic response analysed in order to better understand the relationship between the factors that influence the remaining life estimates of ethylene pyrolysis tubes.