Abstract
This paper presents a specific kind of failure in ethylene pyrolysis furnace tubes. It considers the case in which the tubes made of 35Cr-45Ni-Nb high temperature alloy failed to carburization, causing creep damage. The investigation found that used tubes became difficult to weld repair due to internal carburized layers of the tube. The microstructure and geochemical component of crystallized carbide at grain boundary of tube specimens were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) with back-scattered electrons mode (BSE), and energy dispersive X-ray spectroscopy (EDS). Micro-hardness tests was performed to determine the hardness of the matrix and the compounds of new and used tube material. The testing result indicated that used tubes exhibited a higher hardness and higher degree of carburization compared to those of new tubes. The microstructure of used tubes also revealed coarse chromium carbide precipitation and a continuous carbide lattice at austenite grain boundaries. However, thermal heat treatment applied for developing tube weld repair could result in dissolving or breaking up chromium carbide with a decrease in hardness value. This procedure is recommended to improve the weldability of the 35Cr-45Ni-Nb used tubes alloy.
Highlights
Ethylene (C2 H4 ) can be generated by the thermal cracking of ethane (C2 H6 ), which is passed through a coil of reaction tubes externally heated to a temperature of 1000–1150 ̋ C in pyrolysis furnaces
1.8% C, 1.7% Si, 1% Nb and 1% Mn) was used as a material. This alloy does not belong to the standardized ASME-II Part B code (Non-ferrous metal); it was typically used in carburizing applications
The aim of this work was to find an suitable heat treatment process to decrease the hardness value and/or reduce the chromium carbide precipitation at grain boundary of material due to the major problem in welding repair of ethylene pyrolysis tubes, a dissimilar material between used tubes and new tubes. This experiment confirmed the carburization in the 35Cr-45Ni-Nb alloy tube after long-term service at high temperature in an ethylene pyrolysis furnace
Summary
Ethylene (C2 H4 ) can be generated by the thermal cracking of ethane (C2 H6 ), which is passed through a coil of reaction tubes externally heated to a temperature of 1000–1150 ̋ C in pyrolysis furnaces. The decomposition of ethane into ethylene is represented by the reaction below (1) [1]. The internal carbide formation can occur after a corrosion phenomenon called “carburization”, which reduces the Metals 2016, 6, 26; doi:10.3390/met6010026 www.mdpi.com/journal/metals mechanical properties of materials and causes damages [2,3]. The alloy material must be suitable properties of materials causes damages These [2,3]. The selected alloy material must suitable to to accommodate the highand process temperature
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