Abstract
	Microstructural studies have been carried out on two casts of the 12CrMoVNb steel which exhibited sigmoidal creep rupture behaviour when tested at 550°C. It has been demonstrated that the sigmoidal inflexion can be related to the dissolution of fine nitrogen rich M2X and MX  phases due to precipitation of the thermodynamically more stable phases: Z-phase and M6X. Coarse particles of these phases do not contribute to the precipitation strengthening. This phenomenon and coarsening of M23C6 particles result in the creep rupture strength of the steel progressively changing during creep from being initially controlled by precipitation strengthening to being dependent in the long-term on solid solution hardening. The results of  investigations on precipitation reactions have been discussed in terms of equilibrium calculations using MTDATA program. The noticeable effect of nickel content in the 12CrMoVNb steels on microstructural degradation during long-term exposure at 550°C has been demonstrated.