Discussion on Upper Temperature Limit of 12Cr2MoWVTiB Steel Tube in Superheater for Power Plant

A superheater in power plant was operated at equivalent temperature of 585 ℃ for 140 000 h; Macromorphology, chemical composition, microstructure and mechanical properties at room temperature and elevated temperature of the heat surface tube of 12Cr2MoWVTiB steel from the superheater were studied, and upper temperature limit of the steel tube was discussed. The results show that no obvious swelling and thinning were observed in the steel tube. No creep holes were found in the microstructure and the aging grade of microstructure was grade 3. The mechanical properties at room temperature were slightly lower than those of unoperated steel tube, and still met the standard requirements. After service for about 140 000 h, the steel tube still had good high-temperature creep resistance; the stress-rupture strength was only 12.1 MPa lower than the standard requirement. Considering the running situation of heat surface tube in superheator of the power plant, it was suggested that 12Cr2MoWVTiB steel heat surface tube can be reliably operated at temperatures no higher than 585 ℃; this conflicts with the 575 ℃ upper temperature limit specified in DL/T 715-2015.