3.13 Carburization Flashcards
Carbon is absorbed into a material at elevated temperature while in contact with___________________. Carburización also can reduce (or eliminate) wall thickness and may reduce the corrosion resistance of _____________
carbonaceous material or carburizing environment ; stainless steel
Carburized steel is brittle and may
spall or crack (astillarse)
Affected Materials are:
Carbon steel and low-alloy steels, 300 and 400 series SS, cast stainless steels, nickel base alloys with significant iron content (e.g. Alloys 600 and 800), and HK/HP alloys
Three factors must be satisfied
- exposure to a carburizing environment or carbonaceous material.
- temperature high to allow diffusion of carbon [typically above 1100 °F (595 °C)];
- susceptible material.
Othres conditions that favoring carburization include are:
- high gas phase carbon activity
- low oxygen potencial.
300 series SS are ______________ than carbon steel and low-alloy steels due to their higher chromium and nickel content.
more resistant
Carburization can result in the loss of:
- high-temperature creep ductility,
- ambient temperature mechanical properties (specifically toughness/ductility),
- weldability,
- corrosion resistance.
the most common type of equipment susceptible to carburization are ______________. However it can also be found in _______________________
Fired heater tubes ;
Coke deposits, heater tubes in catalytic reformers and coker and ethylene pyrolysis and steam reformer furnaces
The depth of carburization can be confirmed by:
metallography
Carburization can be confirmed by substantial increases in ___________ and loss in __________.
hardness ; ductility
Carburization can cause __________ in the level of ferromagnetism
Increase
Severe cases may also result ______________________________________. Cracking can also have a _________________ appearance.
in bulges, heavy scale, thin-line brittle cracking, and/or “thick-lip” tube failures;
crow’s feet
For prevention/Mitigation :
- Select alloys with adequate resistance to carburization commonly achieved using an alumina-forming coating (Alonizing)
- Reduce the carbon activity of the process environment.
- Add low levels of a reactive sulfur compound to the process. Less than 10 ppm are needed to prevent carburization.
VT and A-scan UT are ________________ in detecting carburization and determining carburized thickness
Ineffectives techniques.
Carburization can be determined accurately by destructive sampling, i.e.
sampling for chemical and/or physical testing.
Inspection for carburization in the initial stages of attack is difficult. If the process-side surfaces are accessible, ___________________ can be used. Destructive sampling and ____________________ have also been used.
hardness testing and field metallography (replication) ; electromagnetic based techniques (eddy current)
Carburization causes the normally nonmagnetic wrought and in the other hand causes in cast heat-resistant alloys to ________________. Inspection techniques based on determining increased levels of ferromagnetism (________________) are also useful for alloys that are paramagnetic when initially installed (austenitic alloys).
become magnetic ; magnetic permeability
Measurement devices range from simple hand-held magnets to advanced multi-frequency eddy current instruments. However, ______________ may interfere with the results.
surface oxides
The techniques for inspecciones In the advanced stages of carburization where cracking has initiated, _____, ______, and some magnetic techniques, which can be combined for effectiveness, may be used.
RT, UT
Techniche used to determine the case depth of carburization but requires trained technicians for application and interpretation.
TOFD
Cast austenitic stainless steel tubes and Cr-Mo ferritic alloy tubes in fired heater/boiler service should not be tested by ____________ when tubes are heavily carburized.
hammer test
