Abstract: It is of great significance to analyze the material status of high-temperature parts in service as well as its security status and life loss. It is good to the safety operation, reasonable arrangements for unit maintenance cycle and maintenance cost reduction, etc. In order to investigate the safety status and service life of equipment, elongation tests, microstructure analysis and high-temperature lasting strength tests on the final superheater and final reheater tubes made of X20CrMoV121 steel were conducted in one thermal power plant after 12 000-h service. The results show that compared with the original tubes, the strength of the tubes in service is getting lower and the crystal grain of precipitation phases is becoming larger and rounder. In addition the substructure appears in martensite laths and the dislocation density declines apparently. However the high-temperature permanent strength is still higher than the designed value. Comprehensive analysis concludes that although certain aging occurs to some extents after 12 000-h operation, the tubes can still satisfy the requirements of safe operation under the designed working conditions.

Key words: 12Cr steel, final superheater tube, high-temperature reheater tube, heating surface tube, mechanical property, microstructure, permanent strength, state analysis