The Study Of The Tissue Deterioration Mechanism Of 9CR-1Mo Ferritic Heat-resistant Steel

SA213 T91, Which acts as the typical representative of the 9Cr-1Mo ferritic heat-resistant steel, is widely used in the hating plates of the super /ultra-supercritical unit steam superheater with temperature of 600? and the repeated heat pipes. Currently, the research of the microstructure deterioration of the T91 steel focused on the aging accelerated under the high-temperature. However, a little research has been done in the change of microstructure and mechanical properties under real service conditions.The pipes of the T91(original state), 26280h(after service), 87600h(after service) and blasting tube(after two accidents) have been studied, and showed the change of the T91-steel's mechanical properties and microstructure under the change of service time and over-temperature. The research indicated :1 ? After normalizing at 1050?and high temperature tempering at 790?, the microstructure of the T91-steel is the tempered lath martensite with subgrain. The high density of dislocations and a few dispersed MX carbides inside existed. At the interface the larger size of M23C6 type carbide appeared. After long-time service, the width of lath martensite has increased and the recovery of the subgrain and dislocation density decreased.There is no significant change in the number and size distribution of the MX type carbonitrides. While M23C6 carbide coarsening increase significantly, and after long-time service a new Laves phase intermetallic compound at the interface has been generated,which show the rough phase velocity faster than M23C6. The two over-temperature of the over-temperature tube are higher than the A1 line and ? phase has been generated. While the changes of the number and coarsening of the M23C6 carbide are not obvious for the short-time over-temperature tube, but the number and coarsening has increased significantly for the long over-temperature blass, both of which showed the reduction in dislocation and linear.2?After long service, the hardness and the room temperature tensile properties of the T91 decline. Mechanical properties of two samples of burst pipes were significantly reduced or lower than the standard requirements, and a significant increase in hardness at the short-term over-temperature burst mouth is not accordance with its microstructure,which is attributed to the hardening because of the high deformation.