[en] The material studied is Fe9Cr1Mo modified by the addition of small amounts of V and Nb. The major changes detected by the neutrons are alterations in size, number and composition of the carbides. SANS measurements of samples aged 5000 h at various temperatures showed that aging at 538⁰C produces a maximum in the scattering cross section. Aging at this temperature also maximizes hardness. Size distributions of the carbides derived from inversion of the SANS data show that 538⁰C results in the largest number of small carbides. Higher aging temperatures lead to appreciable carbide coarsening. SANS measurements of samples deformed at 649⁰C showed that microstructural changes can be detected after only a few hours of fatigue. High voltage electron microscopy studies have shown that the major microstructural changes produced by high temperature exposure are carbide coarsening, subgrain development, and a drastic decrease in dislocation density