- Report #: NT MAT 002
- Approved: November 2001
This NORDTEST method describes how oxide thickness measurements and/or metallographic investigations (changes in carbide distribution) can be used to assess the temperature history of superheater and reheater tubes. The method is actually composed of three mutually complementary and independent sub-methods. During service an oxide layer will grow on the inner surface of the boiler tubes. This growing oxide layer acts as a thermal insulation and consequently the temperature of the tube material will rise even if the surrounding gas temperature remains constant.
This oxide induced temperature rise will of course reduce the lifetime of the tube. If the oxide thickness is known at the beginning and at the end of a service time interval, it is possible to estimate the rise in temperature and thus to take into account the lifetime reduction due to this phenomenon.
The application of this method is described. Metallurgical degradation due to carbide growth in the tube material is a time and temperature dependent process. By metallographic measurement of the size of the carbides at the beginning and at the end of a service period the corresponding effective temperature can be estimated. This sub-method is also described below.
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