In order to understand the applicability of Sanicro 25 boiler tubes in high-temperature heating surfaces of ultra-supercritical thermal power units with steam temperature above 630 ℃

sampling and testing were carried out periodically on Sanicro 25 boiler tubes installed on the 700 ℃ key components demonstraion test rig of Huaneng Nanjing Power Plant. Sanicro 25 tubes with different operating times(0 h

10 523 h

24 014 h

32 796 h) were tested

compared and analyzed. The results show that

Sanicro 25 had good mechanical stability. Although the plasticity and toughness at room temperature decreased after operation

they still maintained good level at high temperature.M_(23)C_6 particles precipitated at the grain boundary after long time operation

fine and dispersed Z phase

MX phase

M_(23)C_6 phase and Cu-rich phase precipitated within the grain. The Z phase degraded during the operation at high temperature and formed a M_(23)C_6 “wall” with a thickness of about 50 nm around it. With the prolonging of operation time

the amount of cubic M_(23)C_6 near the grain boundary increased

while the Z phase

MX phase

M_(23)C_6 phase and Cu-rich phase did not grow significantly. Sanicro 25 alloy had good steam oxidation resistance and flue gas corrosion resistance at 651～665 ℃.