Certain types of deterioration
experienced in heater tubes result from some change in metallurgical structure.
The more common types of deterioration are carburization, decarburization, the
initial stages of external stress corrosion cracking, creep, fatigue cracking,
and some forms of hydrogen attack.
It is possible to detect most of these types of
deterioration in the field by visual inspection, nondestructive testing, in-situ
metallography, or replication. Carburization and decarburization can be
determined accurately by a chemical or physical test. Most of the testing must
be done by specially trained personnel. Damage that results from some
metallurgical changes can be determined by a wide range of NDT techniques
designed for the characterization of material degradation e.g., ultrasonic,
magnetic-particle, and liquid penetrant testing. In-situ
metallography/replication is rarely used alone for evaluation of these
deterioration mechanisms. It is best
used in combination with other NDE techniques.
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