Research And Application Of Improved Domestic T/P91 Steels On The Technology And Microstructure And Mechanical Properties

T91 and P91 are heat resistant steels developed during 1970s in US for boiler components such as steam piping, steam head, reheater, etc in fossil power plants. The steels have higher performance in terms of allowable stress, rupture strength, creep-resistant ability, fatigue strength, and heat conductivity. The steels also have better weld ability and corrosion-resistant ability with a moderate price. Up to now, T91 and P91 have been widely used in fossil plants of the advanced countries and regions such as US, Europe and Japan with good economic benefits achieved. The steels were introduced in China during the 10th 5-year plan period, and the researches on its localization and the productions of the steels have since then been initiated. The 10Cr9MolVNb is a newly localized type of steel imitative of T91 and P91. But there have been many problems with 10Cr9MolVNb in its research and production. The quality of tube made of 10Cr9MolVNb is still substantially lower than that of tube made in advanced countries with as to be applied in power plant. In order to fill the performance gap between 10Cr9MolVNb and T91/P91, this paper carries out investigations on theoretical understanding and practice concerning production process; tube quality and engineering applications of domestically produced T91 and P91 steel. The paper also conducts application researches on theories of its structure, strengthening mechanism, alloying mechanism and optimization of production process. The optimized process has been used in a tube factory for verification need. The tubes manufactured with optimized process are then assessed statistically and comprehensively upon their structures, mechanic properties and fractures etc. The performance properties of the 10Cr9MolVNb under 566C temperature are discussed in detail in this paper. The paper also gives the application experience and testing results of the 10Cr9Mol VNb under operating condition.The paper indicates definitely in theory that the microstructure of the 10Cr9Mo1VNb is tempered martensitic lath when normalized and tempered under high temperature. It is proved by the experiments in this paper that lath martensite only appears high temperature recovery other than recrystallization when tempered below the temperature of A1. This is caused by the lack of driving force of recrystallization due to the elevation of recovery and recrystallization temperature owing to the solid solution of alloying element Cr and Mo, the release of strain energy stored during the phase transformation of martensite and the formation of dislocation network pining. When recovered under high temperature, the inner structure of martensitic lam breaks into several sub-grains. The high-density dislocation transforms into metastable type. The carbide precipitated are mainly type M23C6 and type MC, the former with elliptic shape appearing on grain boundary of martensite lath and austenite, while the latter with fine-short shape precipitated inside of the martensite lath. A1N also participates together with carbide in dispersed precipitation strengthening of steel. Besides dispersed precipitation strengthening, the steel acquires heat-resistant property through some composite strengthening mechanisms such as solution treatment, martensitic strengthening, fined interface surface strengthening, etc. The solution strengthening and martensitic strengthening are realized with the use of Cr and Mo. The precipitation strengthening is acquired through the precipitation of carbide M23C6 due to the elevation of the temperature of recovery and recrystallization. However, Mo and Cr can easily migrate from solid solution matrix to compound M23C6 under high temperature causing the Ostwald ripening of M23C6, thus, V, Nb and solid C are applied to obstruct the migration of Mo and Cr toM23Q5. Ostwald ripening of M23C6 is a significant mark indicating the heat stability of the steel. The interfacing surface energy is widely recognized as the driving force of the Ostwald ripening. However the author of this paper thinks that the...