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The size and distribution of Cr(V,Nb)N (Z-phase) particles in a 9Cr-3Co-2W-0.6Mo-0.1Ni-0.2V-0.06Nb-0.1C-0.05N-0.005B steel subjected to creep rupture test during 11,151 h at 650°C under an applied stress of 100 MPa were studied. The replacement of V-rich (V,Nb)(C,N) carbonitrides by Z-phase was accelerated by plastic flow as suggested by a comparative analysis of these particles in different portions of crept specimen, namely, the grip section, the portion of uniform elongation, and the necked portions. Coarse "hybrid" Z-phase particles evolved throughout the crept specimen by an in situ transformation mechanism. The strain-induced Z-phase nucleated on the V-rich (V,Nb)(C,N)/ferrite interfaces, leading to the formation of numerous Z-phase particles with dimensions less than 50 nm in the necked portion.

The contributions from the martensitic laths, dislocations, secondary phase particles, and supersaturated solid solutions to the overall strength of the 12%Cr-3%Co-2.5%W creep-resistant steel with low N and high B contents were calculated after various heat treatments consisting of normalizing followed by medium-temperature tempering. An increase in the normalizing temperature from 1050 to 1150°C led to an increase in the average size of the prior austenitic grains from 44 to 68 lm, but the d-ferrite fraction did not significantly change. Medium-temperature tempering in the range of 750-800°C ensured the formation of a tempered martensite lath structure with an average martensitic lath/subgrain size of 0.23-0.34 lm, along with a high dislocation density inside the laths/subgrains, fine secondary phase particles such as M 23 C 6 carbides along the boundaries of the prior austenite grains, packets, blocks, and martensitic laths/subgrains, and (Ta,Nb)X carbonitrides uniformly distributed inside the matrix. After medium-temperature tempering in the range of 750-800°C, the ferritic matrix was supersaturated with substitutional elements such as Cr, W, Mo, and Cu. An increase in the tempering temperature from 750 to 800°C led to decreases in the yield strength and ultimate tensile strength by 16.2% and 10.5%, respectively, as well as an increase in the elongation of 43.8%. The main contributions to the overall strengthening of the steel investigated after the different heat treatment regimens produced solid solution strengthening and precipitation hardening, which were independent of the tempering temperature, as well as lath boundary and dislocation strengthening, which was strongly dependent on the tempering temperature. Different approaches for evaluating the strengthening mechanisms and their contributions to the yield strength were applied, and the results are discussed.