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Abstract
The microstructure change of TP304H steel after long-term service and aging at high temperatures is investigated through XRD, OM, SEM, TEM, SAED and EDS. The results indicate that after long-term service, α’-martensite and ε-martensite are found in TP304H steel. α’-martensite distributes in stripes along grain boundaries; the substructure of α’-martensite is high-density tangling dislocations. ε-martensite distributes in parallel, a little further from grain boundaries than α’-martensite; the substructure is stacking fault. After long-term high-temperature aging, α’-martensite is found in TP304H steel. With the extension of aging time, the amount of α’-martensite increases and the lath morphology becomes clearer. The increase of martensitic transformation start temperature besides grain boundaries due to the decrease of Cr content in this area is the main reason for the formation of martensites in TP304H steel after long-term service and being aged at high temperature.