Effect Of W Content On Microstructure And Properties Of Sanicro25 Steel

With global environment pressure on carbon dioxide controlling, increasing the efficiency of coal-fired power station to reduce the coal consumption and reduce carbon dioxide emission has become the key issue of coal-fired power technology. High steam parameter ultra supercritical(USC) power station is the effective way to reducing coal consumption and carbon dioxide emission. Sanicro25 steel is a new type austenitic heat resistant steel used to the USC boiler superheater or reheater fabrication. W element in Sanicro25 not only plays the role of solid solution strengthening,but also affects the precipitation behavior and thermal deformation behavior. The paper mainly investigated the effect of W element in Sanicro25 steel.With the thermo-calc thermodynamic software and microstructure observation, the important precipitated phases and the content of each phase was investigated. And the effect of chemical composition on precipitate behavior was discussed. M23C6 carbide, Laves phase, a phase and Z phase precipitated at the temperature of 700℃. M23C6 carbide is mainly composed of Cr, Fe, C, W, Co, N, in which Cr is the main alloy element, and the content of Cr is about 70.8%, followed by W, about 14.8%. The Z phase is mainly composed of Nb, Cr, N, which accounted for 26.5% Cr and 58.2% Nb, and 11.7% N. a phase is mainly composed of Fe, Ni, W, Co and Cr. Cr is the main alloying element, the Cr content is about 43.3%, followed by Fe content, and the content of Fe is 33.3%.With W and C content increasing, the resolving temperature and the precipitated amount of M23C6 increases. And the content of Cr and Co don’t affect the precipitated amount of M23C6 carbide obviously. With lower Cr content and higher W content, the resolving temperature and amount of Laves phase increases. The lower content of Cr, N and the higher content of Nb, the higher amount of MX phase precipitates. With the higher content of Nb, the lower content of N, the higher amount of Z phase precipitates. With higher W and Cr content, the higher amount of a phase precipitates. And also the resolving temperature of σ phase increases.Using scanning electron microscopy and transmission electron microscopy, the precipitate was investigated. The results show that the thermodynamic calculationsresult are agree with the microstructure analysis results. The main precipitate phase in Sanicro25 steel is M23C6 carbide, MX phase, Z phase, Laves phase and σ phase.The hot deformation behavior of Sanicro25 steel was investigated, especially the effect of W content on hot workability of Sanicro25 steel bearing 1.81%W and 3.87%W respectively.The results show that with the increasing of deformation rate of each steel, the peak stress increasing obviously, and with increasing of deformation temperature, the peak stress decreasing significantly, high W content of the experimental steels has a higher peak stress than the lower W content steel, high W content experimental steel has a higher thermal deformation resistance than the lower W content steel. High W content of the experimental steels has higher deformation activation energy than lower W content steel. The hot deformation equations and dynamic recrystallization diagram of Sanicro25 steel were established. Hot processing maps of Sanicro25 steel with different W content were obtained. The suggested processing window was given.