Laves-phase precipitates in a low-carbon 9% Cr martensitic steel during aging and creep at 923Â K

The precipitation of the Laves-phase in a low-carbon 9% Cr heat resistant steel was studied under conditions of aging and creep at 923 K. The Laves-phase of Fe2(W,Mo)-type with a ratio of Mo:W of 1:5 was observed on various boundaries in a tempered martensite structure after a long period of aging/creep. The Laves-phase precipitations provided effective stabilization of the tempered martensite lath structure and therefore promoted creep resistance, although their effect on the dislocation substructure was negligibly small. The mean size of the Laves-phase particles (D) gradually increased with aging/creep time (τ) and could be expressed as D4−D04=Kτ, were K=5×10−34 m4/s for aging and K=13×10−34 m4/s for creep. The large Laves-phase particles, which coarsened during the creep tests, served as preferential sites for pore and crack nucleation leading to fracture.