Unexpected tube failure is the major factor causing unreliability in utility boilers. The
first step in analyzing tube failures is to identify the active damage mechanisms. Three tube
damage mechanisms related to overheating are presented and possible root causes are discussed to
resolve these tube failures. Damage mechanisms can be recognized by metallurgical evaluation,
comprising a combination of both visual and microexamination, complemented by chemical
analyses of tube or fireside deposits, as appropriate. Characterizing the degree of microstructural
degradation can also help to confirm and separate various potential high temperature tube damage
modes, such as long or short term overheating, as well fuel ash attack. Carbon steel is the standard
tube material for high pressure boilers (typically up to 625 psig steam) and has a normal design
temperature limit of about 440 °C. However, microstructural changes occurring as a result of higher
temperature exposures in service can include carbide spheroidization, graphitization and other
transformations. Metallography is powerful tool for evaluating overheated failures and also for fire
damage assessment. Evaluating overheated steel microstructures utilizes the principles of steel heat
treatment and application of the iron-iron carbide equilibrium diagram.
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