Abstract: The burst cause of a platen superheater tube of T91 steel in certain supercritical units was ana-lyzed by macroscopical morphology, chemical composition, hardness test,OM and SEM observation using the speci-mens exploded platen superheater tube.Results show a low hardness of the failed tube which is related to the micro-structure transformation.The metallographic structure of T91 steel is tempered martensite, while the metallographic struc-ture of the exploded tube doesn't show any martensite at all.The martensite lath is evolved into ferrite and coarsen carbides precipitated at grain boundry.The overheat opera-tion of superheater tube causes the microstructure degrada-tion and the decrease of mechanical properties, and finally causes the leakage of platen superheater tube under the condition of high temperature and pressure steam.

Key words: supercritical units, platen superheater tube, T91 steel, burst tube