Page 750 - Corrosion Engineering Principles and Practice

700 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 701

However, carburization is more common in the petrochemical
processing industry. A notable problem area has been the radiant and
shield sections of ethylene cracking furnaces, due to high tube
temperatures up to 1150°C. Apart from temperature, an increase in
carbon potential of the gas mix is responsible for a higher severity of
damage. High carbon potentials are associated with the ethane,
propane, naphtha, and other hydrocarbons as reactants that are
cracked. Carburization has been identified as the most frequent
failure mechanism of ethylene furnace tubes.
Less severe and frequent carburization damage has been
reported in reforming operations and in other processes handling
hydrocarbon streams or certain ratios of CO/CO /H gas mixtures
2
2
at high temperature [8]. As it is the case with oxidation and
sulfidation, chromium is considered to impart the greatest
resistance to carburization [3]. Other beneficial elements include
nickel, silicon, columbium, titanium, tungsten, aluminum, and
molybdenum. The most important characteristic of a successful
alloy is its ability to form and maintain a stable, protective oxide
film. Aluminum and silicon alloying additions can contribute
positively to this requirement. Unfortunately, the addition of
aluminum or silicon to the heat-resistant alloys in quantities to
develop full protection involves metallurgical trade-offs in
strength, ductility, and/or weldability. Considering fabrication
requirements and mechanical properties, viable alloys are generally
restricted to about two percent of either element. This is helpful
but not a total solution.
The tubes of ethylene-cracking furnaces were originally largely
manufactured out of the cast HK-40 alloy (Fe-25Cr-20Ni). Since the
mid-1980s, more resistant HP alloys have been introduced, but
carburization problems have not been eliminated, probably due to
more severe operating conditions at higher temperatures. Some
operators have implemented a 35Cr-45Ni cast alloy, with various
additions, to combat these conditions. For short residence-time
furnaces with small tubes, wrought alloys including HK4M and
HPM, Alloy 803, and Alloy 800H have been used. Other wrought
alloys (e.g., 85H and HR-160, both with high silicon) have been
applied to combat carburization of trays, retorts, and other components
used in carburizing heat treatments. However, their limited
fabricability precludes broad use in the refining or petrochemical
industry [8].
Carburization causes the normally nonmagnetic wrought and
cast heat-resistant alloys to become magnetic. The resulting magnetic
permeability provides a means for monitoring the extent of
carburization damage. Measurement devices range from simple
handheld magnets to advanced multifrequency eddy current
instruments. Carburization patterns can also reveal uneven
temperature distributions that might otherwise have gone undetected.