Investigation of carburization extent from the inner diameter (ID) of ethylene furnace tubes using non-destructive examination (NDE) applied on the outer diameter (OD)

Abstract

This research project is to investigate the possibility of measuring the depth and location of carburization and metal dusting on the inside of ethylene pyrolysis tubes using nondestructive tools applied to the outer surface. A carburization test was conducted in a specially designed and manufactured HK-40 alloy tube. Ethylene pyrolysis is considered the most important process in the petrochemical industry for producing petrochemical products. Some furnace tubes used for ethylene pyrolysis often suffer severe high-temperature corrosion since the tube surfaces are heated up to about 1,100 0C in contact with steam-hydrocarbon gas mixtures. Simultaneous carbon deposition on the tube surfaces (i.e. coking) can deteriorate the mechanical properties of the tube materials and may result in a failure of the furnace tubes in the form of carburization or metal dusting. This investigation consists of four separate experimental practices to detect and assess the carburization and metal dusting damage. It involves a) impulse testing to study changes in natural elastic wave frequencies, b) metallographic evaluation, c) resonant ultrasound spectroscopy (RUS), and d) computational modeling to simulate guided wave and ultrasonic phased array technique. The collaboration of the results of these experiments demonstrated the possibility that ultrasonic phased array can offer an efficient and economical practice to determine the remaining services life of the ethylene furnace tubes. The elastic responses (frequency shift and mode shape changes) of the HK-40 alloy tube was studied due to changes in material properties that result from carburization and metal dusting, a severe form of carburization. Natural frequency analysis was conducted as measured at different locations of the HK-40 alloy tube before and after the carburization test. A shift in natural frequencies was observed and is understood to be due to a reduction in shear wave speed in the carburized samples. This shift seems to correlate to the extent of carburization and forms the basis for a new nondestructive evaluation method. Extensive metallographic examination of transverse and longitudinal cross section samples cut out of the HK-40 alloy tube after carburization using optical macrograph, scanning electron microscopy and energy dispersive X-ray were used to quantify the carburization extent as compared to the corresponding shift in natural frequencies. Computational modeling using a finite element analysis program (COMSOL) was used to simulate different scenarios of alternating the material properties, depth and shape of carburization and metal dusting. The simulations revealed strong reflections from relatively high defect depth to tube thickness ratio based on exciting a guided wave. Relatively small defects were very weak in terms of wave reflection amplitude. Hence, phased array technique was utilized in the simulation to enhance the use of guided wave by amplifying the low peak amplitude of the reflected wave from defects. The research reported provides a new approach to advanced nondestructive testing to evaluate the remaining life of ethylene furnace tubes and other application tubes suffering carburization and metal dusting.