Carburization can occur when metals are exposed
to carbonaceous material or carburizing environment at elevated
temperatures. Carbon from the environment combines with
carbide-forming elements such as Cr, Mo, Nb, W, Mo, Ti, and Fe in the
alloy to form internal carbides. These carbides precipitate at the
grain boundaries of the alloy or inside the grains. As a rule,
carburization problems only occur for Cr-Mo alloys at temperatures
above 1100°F, and 1500°F (815°C) for austenitic alloys. In
refining operations, carburization damage is sometimes found in
ferritic heater tubes in catalytic reformers and coker units. The
effect of carburization is to reduce the ambient temperature
ductility, toughness, and weldability of the alloy. Carburization
also reduces oxidation resistance by tying up chromium in the form of
stable chromium-rich carbides. The creep strength of the alloy may
also be adversely affected as the result of the reduced ductility of
the alloy due to the carbide precipitation within in the grains and
at the grain boundary.
In
petrochemical operations, carburization is typically found in
austenitic heater tubes in ethylene pyrolysis and steam reformer
furnaces where significant carburization can occur during decoking
cycles. Carburization has been identified as the most frequent
failure mechanism of ethylene furnace tubes. Experience has
indicated that the severity of carburization damage in ethylene
cracking is process dependent. Some important factors identified
include the following:
- Steam dilution, which tends to decrease the rate of damage
- The use of lighter feeds versus heavier feeds, the former having a higher carbon potential
- The frequency and nature of decoking operations; decoking is thought to be a major contributor to carburization damage
Carburization causes the normally nonmagnetic
wrought and cast heat-resistant alloys to become magnetic.
Measurement devices range from simple hand-held magnets to advanced
multi-frequency eddy current instruments. Carburization patterns can
also reveal uneven temperature distributions that might otherwise
have gone undetected. Most alloys tend to have more carburization
penetration with increasing temperatures.
As in the case of oxidation and sulfidation,
chromium is considered to impart the greatest resistance to
carburization. Aluminum and silicon alloying additions can also
contribute positively to resistance to carburization. It should be
noted that 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 2 percent for each
element. Other approaches to reducing the potential for
carburization damage includes reducing the carbon activity of the
environment through lower temperatures and higher oxygen/sulfur
partial pressures. Also, the addition of H2S in the process stream
inhibits carburization in steam/gas cracking in olefin and thermal
hydrodealkylation units
Originally, tubes in ethylene cracking furnaces
were manufactured out of cast HK-40 alloy (Fe-25Cr-20Ni). Since the
mid-1980s, more resistant HP alloys have been utilized, but
carburization problems have not been eliminated as the result of more
severe operating conditions in the form of 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, Alloy 800H have been used.
No comments:
Post a Comment