The Research On Microstructure And Failure Mechanism Of T/P91 Heat-resistant Alloy Steel After Long-term Service At Elevated Temperature

T/P91 are heat resistant steels developed during 1970s in US for boiler components such as steam piping, steam head, reheated, 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, T/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-yeat plan period, and the researches on its localization and the productions of the steels have since then been initiated. Through structural analyzing and testing mechanical properties of samples after heat treatment with various technological parameters, the key parameters effected on T/P91 performance are found out. There search results show that, tempering temperature and tempering time affect ultimately structure and distributing of carbide, thereby result in difference on mechanical propertiesTEM, SEM and OM were used to observe and analyze the fracture and microstructure. The results show that the structure of the heat-resistant steel mainly consists of lath martensite and carbide precipitation phases. The substructures of martensites are high density dislocations and the precipitated phases are mainly consisted by MC, M23C6. The second phases in the grain boundary and lath of martensite hinder the movement of dislocations. Dispersed compound MX (M is metal element. Such as V. Nb. X is C. N. ect) in the grain precipitate strengthened T/P91. Because of the strength of second phase and the solid solution harden, the heat-resistant steel have good toughness and high creep rupture strength. On the base of macro and micro observation of impact fracture, the morphology of the fracture is dimple. In this paper the rupture time of T/P91 steel under different test conditions was calculated, using constrained cavity growth model. Results show that the ratio of calculated time to measured value is less than 2. After the modification of constrained cavity growth model, the calculated rupture time further approaches the measured value. Therefore, it can be applied to estimate the rupture lifetime of T91steel under high temperature service condition.