Page 357 - Failure Analysis Case Studies II

342

by rapid oxidation. This thick oxide scale was usually cracked because the heat expansion coefficient
of the oxide was different from that of the tube metal on which the scale is attached. Through the
opening of the oxide crack, fresh tube metal which was located beneath the oxide crack tip suffered
repeated oxidation resulting in small cracks initiating in the tube metal. Tube failure finally
occurred as a result of propagation of these small cracks to the outer surface of the tube. The
failure could be prevented by maintaining the temperature of the tube at the flame side of the
burner, that is, in the vicinity of supporting guide A in Fig. 1, below 1000°C by improving the
existing combustion system or by modifying the burner tips.

Acknowledgements

The authors are grateful for the support provided by a grant from the KOSEF (Korea Science
and Engineering Foundation) through Safety and Structural Integrity Research Center in Sung
Kyun Kwan University. The authors also would like to thank POSCO (Pohang Iron and Steel
Co.) for providing samples.

References

[I] Williamson J, Shipley M. Life assessment and monitoring of furnace heaters, improving reliability in petroleum
refineries and chemical and natural gas plants. Houston, TX, USA, Novembcr 9-12, 1992.
[2] Walter M, Schutze M, Rahmel A. Oxidation of Metals, 1993;40:37.
[3] Life prediction of tubes for steam reformer and cracker. Document for Information Document No. 85, KHK, 1983.
[4] Lai GY. High temperature corrosion of engineering alloy, ASM International, 1990.