Martensitic-ferritic steels of the P91/T91 type have been widely used in applications to structural components foreseen for working at high temperature (up to 600 ºC), and requiring for this reason an exigent combination of properties such as oxidation resistance and high creep strength. Both steels contain approximately 9%Cr, 1%Mo and minor amounts of Nb, V and N; their properties and applications have already been extensively reviewed, see for instance Orr and Burton (1992) or Foldyna et al. (1996).