"Failure of HP-Nb Modified tubes for Ethylene Cracking Furnaces"

Question:

"In an Ethylene cracking furnace some tubes have been failed due to overheating or thermal shock beyond 1200deg C. The tubes have fractured circumferentially implying that the tubes did not fail due to hoop stress by internal pressure but due to tensile stresses. The following are my queries: 1) How to calculate approximate temperature upto which the tubes were overheated? (For carbon/low alloy steels there is an empirical relationship for calulating the skin temperature upto which the tubes were overheated from the thickness of oxide scale) 2) How to assess the remaining life of the tubes? API530 may not be suitable because data for HP-Nb alloys is not given in API 530 moreover the resistance to carburisation is more important in the present case. I am examining the microstructures, however I have found that the hardness has been reduced to around 75HRB against 85-90HRB. Under what conditions the hardness will reduce so much can you give some indication of temperature and/or time upto which the tubes were heated before failure?"

Answer:

Accurately estimating tube-metal-temperatures (TMT) in HP alloys is difficult owing to the fact that they do not undergo a phase change from room temperature to melting. One method used to estimate TMT's is by the morphology of the primary and seconday carbides. Another method is microscopically observe for evidence of melting. The above two methods require prior experience with observing high temperature cast alloys. A good reference is "Microstructure of Heat-Resisting Casting alloys" by the American Casting Institute (ACI). The remaining life of ethylene cracking tubes us usually determined by their degree of carburization and ability to perform weld repairs.

 

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