Substructure and strengthening mechanisms in 2.25 Cr-1 Mo steel at elevated temperatures

The substructures of thermally aged, creep deformed and fatigued 2.25 Cr-1 Mo steel have been studied using optical and transmission electron microscopy. In agreement with earlier work, the substructure of the proeutectoid ferrite was found to be very stable when exposed to thermal aging or creep deformation. This stability is explained based on the tendency of molybdenum atoms to form pairs in the ferrite matrix. Nucleation and growth of additional carbide particles during creep testing was not observed. The results of these creep tests and those of Klueh have been interpreted on the basis of Mo pair stability and the affinity between molybdenum and carbon. Fatigue tests at 866 K, however, did produce a fine Mo_u2C precipitate which contributed to secondary cyclic hardening in tests lasting longer than 200 h. The alloy was found to undergo early cyclic hardening followed by abrupt softening within the first tens of cycles.