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Abstract: This aim of this study is to identify failure mechanism of ethylene pyrolysis furnace tube after five year of operation. The tubes were manufactured from centrifugally cast heat resistant steel HK 40. Failure analysis of the radiant tubes was performed by careful visual inspection of the failed tubes, scanning electron microscopy observation of crack region samples, hardness and micro-hardness measurements. Selected specimens were prepared from the four samples, measurements was carried out at inner, middle and outer sides of the samples. The experimental results showed that the mode of tube failure was a combination of high temperature carburization attack and creep damage leading to intergranular cracking. Maximum hardness is associated with internally carburized zone where the amount of carbides is maximum in this region. The hardness of the radiant tubes decreases as the distance moves from the inner surface to the middle section. The metallurgical background of the combined action of carburization and creep ductility exhaustion have been investigated and are explained. Pyrolysis tube failures can be prevented by a combination of proper furnace operation, materials choice, regular inspections and good design.
Keywords: ethylene pyrolysis; furnace; tube; damage; inspection
INTRODUCTION
Ethylene is a key product in the petrochemical industry, and it is produced by the thermal cracking of complex hydrocarbons in pyrolysis furnaces. The furnace coils are formed from the tubes by welding. The tubes used in thermal cracking furnaces in an ethylene manufacturing process are usually made of HP and HK grades of heat resistant stainless steel. These alloys exhibit excellent properties in terms of oxidation resistance, carburization resistance, high temperature creep and thermal expansion. These tubes are exposed to high temperature (approximately 1100 °C and internal pressure of about 1 bar). Normal service life of these coiled tubes is approximately 100.000 h, but does depends on the service condition and could vary from 30.000 to 180.000 h [4].
Ethylene pyrolysis furnace tubes which are made of high Cr-Ni alloys often become difficult to weld after few years in service due to carburization and creep damage. The presence of carburization, often attempted to detect by magnetic permeability, can escape detection due to high Cr-Ni content of the alloy. Based on optical microstructural analysis and supported by scanning electron microscopy, this paper establishes...