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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.

Five Co-modified P92-type steels with different contents of Cr, W, Mo, B, N, and Re have been examined to evaluate the effect of the chemical composition on the evolution of Laves phase during creep at 650 °C. The creep tests have been carried out at 650 °C under various applied initial stresses ranging from 80 to 200 MPa until rupture. An increase in the B and Cr contents leads to a decrease in the size and volume fraction of M23C6 carbides precipitated during tempering and an increase in their number particle density along the boundaries. In turns, this affects the amount of the nucleation sites for Laves phase during creep. The (W+Mo) content determines the diffusion growth and coarsening of Laves phase during creep. Susceptibility of Laves phase to coarsening with a high rate is caused by the large difference in Gibbs energy between fine and large particles located at the low-angle and high-angle boundaries, respectively, and can cause the creep strength breakdown. The addition of Re to the 10%Cr steel with low N and high B contents provides the slowest coarsening of Laves phase among the steels studied.