Microstructural evolution of the martensitic cast steel GX12CrMoVNbN9-1 during long-term annealing and creep

The microstructure of four specimens of the martensitic cast steel GX12CrMoVNbN9-1 crept to fracture at 873 K for 220-33027 h was quantified by electron microscopy with regard to large M23C6-particles, small V- and Nb-containing particles, dislocations, pores, and oxidation layers. The laws of time-dependent coarsening of particles and strain-controlled growth of subgrains are consistent with those previously established for X20(22)CrMoV12-1. Using these data the microstructural model of deformation developed for X20(22)CrMoV12-1 can explain the creep behaviour of GX12CrMoVNbN9-1. The minimum in creep rate is related to the coarsening of particles and subgrains. At low stresses (< 140 MPa) the fraction of small slowly coarsening V- and Nb-containing particles leads to improved creep resistance. The material resists against pore formation; fracture occurs by necking. (orig.)