Lo Que Sea

4.4.3 Carburization
4.4.3.1 Description of Damage
Carbon is absorbed into a material at elevated temperature while in contact with a carbonaceous material or
carburizing environment.
4.4.3.2 Affected Materials
Carbon steel and low alloy steels, 300 Series SS and 400 Series SS, cast stainless steels, nickel base alloys
with significant iron content (e.g., Alloys 600 and 800) and HK/HPalloys.
4.4.3.3 Critical Factors
a) Three conditions must be satisfied:
i) Exposure to a carburizing environment or carbonaceous material.
ii) Temperature high enough to allow diffusion of carbon into the metal [typically above 1100oF
(593oC)].
iii) Susceptible material.
b) Conditions favoring carburization include a high gas phase carbon activity (hydrocarbons, coke, gases
rich in CO, CO2,methane, ethane) and low oxygen potential (minimal O2 or steam).
c) Initially, carbon diffuses into the component at a high rate and then tapers off as the depth of
carburization increases.
d) In carbon steels and low alloy steels, carbon reacts to form a hard, brittle structure at the surface that
may crack or spall upon cooling.
e) 300 Series SS are more resistant than carbon steel and the lowalloy steels due to higher chromium and
nickel content.
f) Carburization can result in the loss of high temperature creep ductility, loss of ambient temperature
mechanical properties (specifically toughness/ductility), loss of weldability, and corrosion resistance.
4.4.3.4 Affected Units or Equipment
a) Fired heater tubes are the most common type of equipment susceptible to carburization inthe
environments mentioned earlier.
b) Coke deposits are a source of carbon that may promote carburization, particularly during decoke cycles
where temperatures exceed the normal operating temperatures, accelerating the carburization.
c) Carburization is sometimes found in heater tubes in catalytic reformers and coker units or other heaters
where steam/air decoking is performed.
d)Carburization is also encountered in ethylene pyrolysis and steam reformer furnaces. Significant
carburization occurs during decoking cycles.
4.4.3.5 Appearance or Morphology of Damage
a) The depth of carburization can be confirmed by metallography.
b) Carburization can be confirmed by substantial increases in hardness and loss in ductility.
c) In a more advanced stage, there may be a volumetricincrease in the affected component.
d) A change (increase) in the level of ferromagnetism can occur in some alloys.
e) Carburization results in the formation of metal carbides depleting the surrounding matrix of the carbideforming
element.
Copyright American Petroleum Institute
Reproduced by IHS under license with API Licensee=Ecopetrol/5915281003
No reproduction or networking permitted withoutlicense from IHS Not for Resale, 12/14/2004 04:08:55 MST
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4-114 API Recommended Practice 571 December 2003
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4.4.3.6 Prevention / Mitigation
a) Select alloys with adequate resistance to carburization including alloys with a strong surface oxide orsulfide film formers (silicon and aluminum).
b) Reduce the carbon activity of the environment through lower temperatures and higher oxygen/sulfur
partial pressures. Sulfur inhibits carburization and is often added in the process stream in small amounts
in steam/gas cracking in olefin and thermal hydrodealkylation units.
4.4.3.7 Inspection and Monitoring
a) Inspection for carburization in theinitial stages of attack is difficult. If the process side surfaces are
accessible, hardness testing and field metallography can be used. Destructive sampling and magnetic
based techniques (Eddy Current) have also been used.
b) Inspection techniques based on determining increased levels of ferromagnetism (magnetic permeability)
are also useful for alloys that are paramagnetic when initially...