650°C Long-Term Structure Stability Study on 18Cr10NiNb Heat-Resistant Steel

The Nb-contained austenitic heat-resistant steel 18Cr10NiNb(TP347H) has been widely used as super-heater and re-heater tube material for modern ultra-super-critical (USC) power plants in the world. High temperature structure stability is considered to be one of the most important factors for long-term service. Long-term aging at 650 degrees C for this steel was conducted till 10,000 hours. Effect of aging time on microstructure was studied by means of SEM and TEM. Tensile tests were carried out after aging at 650 degrees C for different times. Experimental results show that MX phase and M23C6 carbides are major strengthening precipitates in this steel. With increasing of aging time, fine nano-size MX particles precipitate in grains and its size keeps about 50 nm till 10,000 h at 650 degrees C. Carbide M23C6 mainly precipitates at grain boundaries but coarsens quickly. Investigation results show that MX phase plays the most important strengthening effect in grains. The amount of MX phase increases with increasing of Nb and C contents. The effects of Nb and C contents on mole fractions of MX phase in 18Cr10NiNb steel have been calculated by using Thermo-Calc software.