Microstructure-based creep modelling of a 9%Cr martensitic steel

Martensitic 9 –12%Cr steels can undergo significant changes in their microstructure during thermal/mechanical exposure. Four elements of their microstructure seem to be of particular importance: (i) strain-dependent coarsening of subgrains within the initial tempered martensitic lath microstructure; (ii) an accompanying proportionate decrease in the density of subgrain network dislocations; (iii) Ostwald ripening of MX carbo-nitrides within the subgrains; and (iv) depletion of subgrain matrix-strengthening solid solution elements (Mo/W) due to low density precipitation of large, strength-benign particles of Laves phase. A body of data now exists within the literature on the evolution kinetics of each of these processes but this can only be utilised for life prediction by one or two of the many creep models developed over the decades. In the present work, a microstructure-based Continuum Creep Damage Mechanics (CDM) model has been used to generate strain trajectories by incorporating previously published evolution kinetics for (iii) and (iv) into the kinetic creep equation as well as introducing strain-dependent coarsening of subgrains and network dislocations in a novel way. Using data specific to 9Cr –1Mo–V,Nb steel, the CDM calculations demonstrate that although subgrain-coarsening dominates tertiary creep trajectories (as Blum has long suggested), lifetimes may be significantly reduced further both by solid-solution depletion of Mo and coarsening of MX carbo-nitrides, depending upon stress/temperature.