| With the development of the thermal power plant towards large capacity and highparameters, propertys requirement to steels used in the power plant become more exigent.Presently, SA-213T91steel is a primary material used in subcritical and supercritical unit,however some issues about T91are still controversial, such as the original microstructureassessment, degradation law of microstructure during serviced time and the growthmechanism of inner oxide film. Therefore, it is necessary to further research and discuss theabove-mentioned issues.In this paper, the as-received and aged tubes after the practically operating with differenttime and the high temperature aging tests of T91were concentrated. The influence of heattreatment parameters including normalizing/tempering temperature and time onmicrostructures and hardness of T91was discussed. Besides, the metallurgical structure ofT91at the delivery state was assessed. Then the carbides in the running tubes and acceleratedaging tubes were extracted using a electrolytic extraction method. The chemical composition,phase structure and morphology of the carbides were analyzed by means of XRD and SEM.The formation mechanism of the oxide film on steam contacting side were studied.Firstly, we discussed the influence of different normalizing and tempering treatments onthe microstructures and hardness of T91. The results showed that with increasing thenormalizing temperature, the lath martensite grew up. Carbides and alloy elements weregradually integrated into the matrix. When the sample was normalized at1050℃, thehardness reached up to apeak of380Hv. The normalizing time showed slightly effect on themicrostructure and hardness of T91. Recovery, recrystallization and phase transition wereobservered as the tempering temperature. It was found that790℃was the recrystallizingtemperature and820℃was the critical phase transform point for T91. Besides, temperingtime showed great effect on normalized microstructure. The martensite would decompose asthe tempering time increased, and the hardness decreased with the increase of temperingtemperature and time. In summary, the optimal normalizing and tempering parameters were1050℃-40min and760℃-2h, respectively. It is more reasonable to judge themetallurgical structure of T91as the tempered martensite at the delivery state.Secondly, we analyzed the effect of serviced time, stress and temperature on themicrostructure and hardness of T91. The XRD and SEM analysis on the extracted carbidesindicated that the lath of martensite disappeared, while the carbides which turned to gather and spheroidize. The hardness of T91decreased with increasing running time, stress andtemperature. At the same time, we found that the relative content of Mo increased tomaximum extent, and Cr followed, while the content of Fe decreased gradually.Finally, the formation mechanism of oxide film on the surface of T91contacted steamside was discussed. The oxide flim formation was resulted from two simultaneous processes,where the Fe atoms incline to diffuse outward whears the O atoms prefer to diffuse inward.The final oxide layer could be divided into three parts, where the outer layer of Fe2O3/Fe3O4,the inner (Fe，Cr)3O4layer and the transition layer with Cr rich phase. |
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