Study on the properties and microstructural stability of steel T92 during long-term exposure to high temperature

T92 steels, first developed by Nippon Steel in 1991 with a designation of NF616, are currently among the most important candidate materials use as tubes in ultra-supercritical power plants. By an addition of 1.8%W and a reduction of Mo content from 1% to 0.5% in steel T91, steel T92 offers a further increase in creep rupture strength, making it suitable for much higher temperature up to 620°C. Development and manufacture of T92 tubes have been carried out in Bao Steel under a certain development program in the past years. This paper will present part of the evaluation results relating to those T92 tubes, including the stability of microstructure, mechanical and creep properties. The studied T92 steel was hot-rolled tube with a dimension of 73mm in diameter and 11mm in wall thickness. After the standard heat treatment of normalizing and tempering, T92 steel exhibits a lath martensitic structure with high dislocation density and precipitates of carbides, nitrides or carbonitrides, which leads to significant hardening of the steel. Nevertheless, changes in microstructure take place inevitably in steel during exposure to high temperature. Detailed microstructrual analysis of steel mainly revealed three simultaneous processes during long-term aging at a temperature of 650°C, i.e. reduction of dislocation density, increase in sub-grain width and evolution of precipitates. The lath morphology of martensitic structure in T92 steel almost retains after aging up to 10000h but partially transforms to equiaxial sub-grains after creep testing. Rapid coarsening of M23C6 precipitates as well as formation and coarsening of Laves phase in the investigated T92 were found to occur in the first 3000h of duration at 650°C. The influence of microstructure changes at high temperature on the mechanical properties and creep resistance of steel are discussed later on.