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Research Papers: Materials and Fabrication

Carburization of Ethylene Pyrolysis Furnace Tube in a Petrochemical Plant

[+] Author and Article Information
Tao Chen, Xuedong Chen, Chunjiao Liu, Juan Ye, Defu Nie

National Engineering and Technical Research
Center on Pressure Vessels and Piping Safety,
Hefei General Machinery Research Institute,
Hefei 230031, China

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 30, 2014; final manuscript received March 3, 2015; published online May 20, 2015. Assoc. Editor: David L. Rudland.

J. Pressure Vessel Technol 137(6), 061403 (Dec 01, 2015) (11 pages) Paper No: PVT-14-1118; doi: 10.1115/1.4029974 History: Received July 30, 2014; Revised March 03, 2015; Online May 20, 2015

Carburization phenomena of the ethylene pyrolysis furnace tube made of 25Cr35NiNb alloy were investigated after service for approximate 30,000 h in a petrochemical plant. Three different zones, namely, oxidation zone, carburization zone, and aging product zone, were observed in the cross section of furnace tube by microscopic analysis. The oxidation zone near the inner surface has a thickness of about 700 μm and possesses the characterization of many loose pores and voids. The inner surface has a continuous thin Cr2O3 layer and the grain boundaries near the inner wall are mainly composed of mixed oxides of Cr2O3 and SiO2. In addition, segregation of impurities S and P was detected at grain boundaries. The dark gray SiO2 distributes in the front of the oxidation zone. Carbides distribute at grain boundaries in the carburization zone with lots of voids and micro cracks. The width of grain boundary is broadened. According to the experimental results, the initiation of cracks may be promoted by carbides at grain boundaries. The uneven distribution of the Cr element was found in the carburization zone, and direct experimental evidence of Cr element diffusion was detected. In the aging product zone, the carbides evolution was observed. The chromium carbides are composed of Cr7C3 and Cr23C6. The microstructure and composition evolutions of the furnace tube and their effects on service life of ethylene pyrolysis furnace tube were also discussed.

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References

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Figures

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Fig. 1

Image of tubing etched to show uneven degree of carburization

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Fig. 2

SEM image of (a) the inner wall, and (b)–(e) EDS analysis of the oxides

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Fig. 3

SEM image of the front zone of oxides

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Fig. 4

SEM image of (a) the inner wall, and (b)–(d) EDS map scanning analysis

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Fig. 5

SEM image of oxides near the inner wall, and (b)–(h) EDS line scanning analysis maps

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Fig. 6

(a) SEM image of oxides near the inner wall, and (b)–(e) EDS line scanning analysis maps

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Fig. 7

SEM image of oxides near the inner wall

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Fig. 8

SEM image of oxides near the inner wall, and (b)–(g) EDS line scanning analysis maps

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Fig. 9

SEM image of the outer wall

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Fig. 10

SEM images of carburized zone

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Fig. 11

SEM images of zones near the outer wall

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Fig. 12

SEM image of carburized zone near the inner wall and EDS analysis points

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Fig. 13

XRD pattern of the central material of the radiant tube

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Fig. 14

The sketch map of carburization process of the ethylene pyrolysis furnace tubes

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